WO2016004501A1

WO2016004501A1 - Solvent addition to improve efficiency of hydrocarbon production

Info

Publication number: WO2016004501A1
Application number: PCT/CA2014/000884
Authority: WO
Inventors: Mohammed Taha AL-MURAYRI; Thomas Harding; Brij Bhooshan Maini; Javad OSKOUEI
Original assignee: Nexen Energy Ulc
Priority date: 2014-07-07
Filing date: 2014-12-12
Publication date: 2016-01-14

Abstract

Solvent is co-injected with steam during the start-up phase of a steam assisted gravity drainage ("SAGD") operation. As well solvent is co-injected with steam during the production phase of a SAGD operation. Both processes improve efficiencies for recovering bitumen from oil sands.

Description

SOLVENT ADDITION TO IMPROVE EFFICIENCY OF HYDROCARBON

PRODUCTION

FIELD

[0001] The present disclosure relates to recovery of hydrocarbons from hydrocarbon- containing reservoirs, and the use of solvents to improve efficiencies of such recovery.

BACKGROUND

[0002] Steam- Assisted Gravity Drainage (SAGD) is an enhanced oil recovery technology for producing heavy crude oil and bitumen. However, in spite of its success in recovering highly viscous bitumen, SAGD remains an expensive technique that requires large energy input in the form of steam for each barrel of produced oil. This entails consuming large quantities of water and natural gas, resulting in considerable greenhouse gas emissions and costly post-production water treatment procedures.

[0003] Many modifications to SAGD continue to evolve to achieve higher energy efficiency and environmental sustainability while maintaining economic viability. Such efforts include the use of solvents along with steam to reduce bitumen viscosity simultaneously through thermal diffusion and dilution. However, many of these techniques still suffer from poor efficiencies due to, for example, the use of excessive amounts of solvent, the need to use excessive amounts of steam, losses of solvent, failure to produce a suitable steam to oil ratio, the high cost of solvents, etc. Thus, methods to improve SAGD efficiency are sought after in the industry.

SUMMARY

[0004] In one aspect, there is provided a process for producing bitumen from an oil sands reservoir through a production well that is disposed in fluid communication with an injection well via an interwell region, comprising: supplying a production-initiating fluid into the oil sands reservoir via the injection well for effecting mobilization of bitumen within the oil sands reservoir such that the mobilized bitumen is conducted through the interwell region to the production well; wherein the production-initiating fluid includes steam and a production phase solvent, and the production phase solvent includes heavy hydrocarbon material and light hydrocarbon material; and wherein the heavy hydrocarbon material includes one or more heavy hydrocarbons, wherein each one of the one or more heavier hydrocarbons, independently, is a hydrocarbon that includes a total number of carbons five (5) or more; and wherein the light hydrocarbon material includes one or more light hydrocarbons, wherein each one of the one or more light hydrocarbons, independently, is a hydrocarbon that includes a total number of carbons of four (4) or less.

[0005] In another aspect, there is provided a process for producing bitumen from an oil sands reservoir: establishing fluid communication between an injection well and a production well via an interwell region within an oil sands reservoir, including: supplying a start-up phase fluid into the oil sands reservoir via the injection well such that thermal communication between the startup phase fluid and the bitumen within the interwell region is effected; wherein the start-up phase fluid includes steam and a start-up phase solvent, and the start-up phase solvent includes heavy hydrocarbon material and light hydrocarbon material; and wherein the heavy hydrocarbon material includes one or more heavy hydrocarbons, wherein each one of the one or more heavy hydrocarbons, independently, is a hydrocarbon that includes a total number of carbons of five (5) or more; and wherein the light hydrocarbon material includes one or more light hydrocarbons, wherein each one of the one or more light hydrocarbons, independently, is a hydrocarbon that includes a total number of carbons of four (4) or less; and after the fluid communication has been established, producing bitumen from the oil sands reservoir via the production well.

[0006] In another aspect, there is provided a process for producing bitumen from an oil sands reservoir through a production well that is disposed in fluid communication with an injection well via an interwell region, comprising: selecting a production-initiating fluid, including steam and a production phase solvent, such that the production phase solvent is disposed, or substantially disposed, in a vapour state when supplied to the oil sands reservoir, wherein the selection is based upon information embodied in a multicomponent phase diagram for the components of the production-initiating fluid; and supplying the production-initiating fluid into the oil sands reservoir via the injection well for effecting mobilization of bitumen within the oil sands reservoir such that the mobilized bitumen is conducted through the interwell region to the production well. [0007] In another aspect, there is provided a process for producing bitumen from an oil sands reservoir through a production well that is disposed in fluid communication with an injection well via an interwell region, comprising: establishing fluid communication between an injection well and a production well via an interwell region within an oil sands reservoir, wherein the establishing includes: selecting a start-up phase fluid composition, including steam and a start-up phase solvent, such that the start-up phase solvent is disposed, or substantially disposed, in a vapour state when supplied to the oil sands reservoir, wherein the selection is based upon information embodied in a multicomponent phase diagram for the components of the start-up phase fluid; and supplying the start-up phase fluid into the oil sands reservoir via the injection well or the production well such that the start-up phase fluid becomes disposed in thermal communication with bitumen within the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir; and after the fluid communication has been established, producing bitumen from the oil sands reservoir via the production well.

[0008] In another aspect, there is provided a process for producing bitumen from an oil sands reservoir through a production well that is disposed in fluid communication with an injection well via an interwell region, comprising: during a SAGD production phase, supplying a production-initiating fluid to the oil sands reservoir via the injection well such that mobilization of bitumen within the oil sands reservoir is effected, and such that the mobilized bitumen is conducted to the production well and produced via the production well, wherein the production- initiating fluid includes steam and solvent that includes hydrocarbon material; suspending the supplying of the production-initiating fluid to the oil sands reservoir via the injection well, wherein, prior to the suspension of the production-initiating fluid to the oil sands reservoir via the injection well, and while the production- initiating fluid is being supplied to the oil sands reservoir via the injection well, the reservoir is disposed at a pre-SAGD production phase suspension pressure; after the supplying of the production-initiating fluid to the oil sands reservoir via the injection well has been suspended, and after the pressure within the oil sands reservoir has been reduced from a pre-SAGD production phase suspension pressure, and while the oil sands reservoir is being vented via at least one of the injection well and the production well, collecting gaseous material that is being conducted via the at least one of the injection well and the production well such that at least some of the supplied solvent is recovered. [0009] In another aspect, there is provided a process for producing bitumen from an oil sands reservoir through a production well that is disposed in fluid communication with an injection well via an interwell region, comprising: establishing fluid communication, through the interwell region, between the injection well and the production well, wherein the establishing fluid communication includes supplying a start-up phase fluid via the injection well or the production well such that thermal communication between the start-up phase fluid and the bitumen within the interwell region is effected, wherein the start-up phase fluid includes steam; after the fluid communication has been established, during a SAGD production phase, supplying a production- initiating fluid to the oil sands reservoir via the injection well such that mobilization of bitumen within the oil sands reservoir is effected, and such that the mobilized bitumen is conducted to the production well and produced via the production well, wherein the production-initiating fluid includes steam; wherein at least one of the start-up phase fluid and the production-initiating fluid includes solvent that includes hydrocarbon material; suspending the supplying of the production- initiating fluid to the oil sands reservoir via the injection well, wherein, prior to the suspension of the production-initiating fluid to the oil sands reservoir via the injection well, and while the production-initiating fluid is being supplied to the oil sands reservoir via the injection well, the reservoir is disposed at a pre-SAGD production phase suspension pressure; after the supplying of the production-initiating fluid to the oil sands reservoir via the injection well has been suspended, and after the pressure within the oil sands reservoir has been reduced from a pre- SAGD production phase suspension pressure, and while the oil sands reservoir is being vented via at least one of the injection well and the production well, collecting gaseous material that is being conducted via the at least one of the injection well and the production well such as at least some of the supplied solvent is recovered.

[0010] In another aspect, there is provided a process for producing bitumen from an oil sands reservoir: establishing fluid communication between an injection well and a production well via an interwell region within an oil sands reservoir, including: supplying a start-up phase fluid into the oil sands reservoir via the injection well such that thermal communication between the startup phase fluid and the bitumen within the interwell region is effected, wherein the start-up phase fluid includes steam and a start-up phase solvent, and after the fluid communication has been established, producing bitumen from the oil sands reservoir via the production well. [0011] In another aspect, there is provided a process for producing bitumen from an oil sands reservoir through a production well that is disposed in fluid communication with an injection well via an interwell region, comprising: during a first stage of the SAGD production phase, supplying a first production-initiating fluid into the oil sands reservoir via the injection well for effecting mobilization of bitumen within the oil sands reservoir such that the mobilized bitumen is conducted through the interwell region to the production well, wherein the first production- initiating fluid includes steam and a production phase solvent consisting of one or more production phase solvent hydrocarbons; suspending the supplying of the first production initiating fluid; and after the suspending of the supplying of the first production initiating fluid, during a second stage of the SAGD production phase, supplying a second production initiating fluid is injected into the injection well for effecting mobilization of bitumen within the oil sands reservoir such that a second mobilized bitumen is conducted through the interwell region to the production well, wherein the second production initiating fluid includes steam and a production phase solvent consisting of one or more production phase solvent hydrocarbons; wherein the ratio of moles of start-up phase solvent to steam within the start-up phase fluid is greater than the ratio of moles of production phase solvent to steam within the first production initiating fluid, and the ratio of moles of production phase solvent to steam is greater within the first production initiating fluid relative to that within the second production initiating fluid.

[0012] In another aspect, there is provided a process for producing bitumen from an oil sands reservoir through a production well that is disposed in fluid communication with an injection well via an interwell region, comprising: during a first stage of the SAGD production phase, supplying a first production-initiating fluid into the oil sands reservoir via the injection well for effecting mobilization of bitumen within the oil sands reservoir such that the mobilized bitumen is conducted through the interwell region to the production well, wherein the first production- initiating fluid includes steam and a production phase solvent consisting of one or more production phase solvent hydrocarbons; suspending the supplying of the first production initiating fluid; and after the suspending of the supplying of the first production initiating fluid, during a second stage of the SAGD production phase, supplying a second production initiating fluid is injected into the injection well for effecting mobilization of bitumen within the oil sands reservoir such that a second mobilized bitumen is conducted through the interwell region to the production well, wherein the second production initiating fluid includes steam and a production phase solvent consisting of one or more production phase solvent hydrocarbons; wherein at least 70 mol % of the production phase solvent, of the first production initiating fluid, consists of heavy hydrocarbon material, based on the total number of moles of the production phase solvent being injected, and less than 35 mol % of the production phase solvent, of the second production- initiating fluid, consists of light hydrocarbon material, based on the total number moles of the production phase solvent being injected, wherein the production phase solvent of the first production-initiating fluid has a higher molar concentration of heavy hydrocarbon material than the production phase solvent of the second production-initiating fluid.

BRIEF DESCRIPTION OF DRAWINGS

[0013] The preferred embodiments will now be described with the following accompanying drawings, in which:

[0014] Figure 1 is a schematic illustration of a system including an injection well and a production well within an oil sands reservoir for carrying out a SAGD process;

[0015] Figure 2 is a schematic illustration of the phase during SAGD production when the steam chamber has grown such that the steam chamber has reached the cap rock; and

[0016] Figure 3 is table illustrating the composition of an embodiment of a solvent.

DETAILED DESCRIPTION

[0017] The present disclosure relates to the use of solvents during the start-up phase of a steam-assisted gravity drainage ("SAGD") operation, as well as the use of solvents during the production phase of a SAGD operation.

[0018] As used herein, the following terms have the following meanings:

[0019] "Hydrocarbon" is an organic compound consisting primarily of hydrogen and carbon, and, in some instances, may also contain heteroatoms such as sulfur, nitrogen and oxygen.

[0020] "Hydrocarbon material" is material that consists of one or more hydrocarbons. [0021] "Heavy hydrocarbon" is a hydrocarbon having a total number of carbon atoms of five (5) or more.

[0022] "Heavy hydrocarbon material" is a material that consists of one or more heavy hydrocarbons.

[0023] "Light hydrocarbon" is a hydrocarbon having a total number of carbon atoms of four (4) or less.

[0024] "Light hydrocarbon material" is material that consists of one or more light hydrocarbons.

[0025] Referring to Figure 1, there is provided a system 100 for carrying out a process for producing a hydrocarbon from a hydrocarbon-containing reservoir 102. In some embodiments, for example, the hydrocarbon-containing reservoir is an oil sands reservoir, and the hydrocarbons includes heavy oil, such as bitumen.

[0026] The system 100 includes a pair of wells 104, 106. An interwell region 108, of the reservoir 102, is disposed between the wells 104, 106.

[0027] In a SAGD operation, each one of the wells 104, 106 includes a horizontal portion, and the horizontal portions are vertically spaced from one another, such that the horizontal portion of the well 104 is vertically higher than the well 106. During the production phase a SAGD operation, the well 104 functions to inject a production-initiating fluid 116 (such as steam, or a fluid including steam) into the reservoir 102, and thereby mobilize the hydrocarbons (the "reservoir hydrocarbons") within the interwell region 108. Upon mobilization, the bitumen is conducted to the horizontal portion of the well 106 by gravity drainage through a steam chamber (that has been established earlier during a start-up phase of SAGD, by circulation of the steam within one or both of the wells 104, 106: see below). In parallel, during the production phase of the SAGD operation, the well 106 functions to receive the mobilized reservoir hydrocarbons, as well as some of the condensed water, (which has also drained by gravity to the well 106 though the established steam chamber) and produce a production fluid 112, including the received reservoir hydrocarbons and the condensed water. In this respect, the well 104 may be referred to as the injection well 104, and the well 106 may be referred to as the production well 106.

[0028] The production fluid may subsequently be conducted to a processing facility 110. At the processing facility 110, various processing operations can occur but generally, the water and the reservoir hydrocarbons can be separated, with the reservoir hydrocarbons 114 sent on for further refining. Water from the separation may be recycled to a steam generation unit within the facility 110, with or without further treatment, and used to generate the steam used for supply to the well 104.

[0029] The production phase of a SAGD operation is able to occur when fluid communication between the wells 104, 106, within the interwell region 108, has been established. In some embodiments, for example, initially, the reservoir 102 has relatively low fluid mobility. In order to enable the injected production-initiating fluid 116 (being injected through the injection well 104) to promote the conduction of the reservoir hydrocarbons, within the reservoir 102, to the production well 106, fluid communication must be established within the interwell region 108 between the wells 104, 106. The fluid communication may be established during a "start-up" phase of the SAGD operation. During the start-up phase, the interwell region 108 is heated. The heat that is supplied to the interwell region 108 effects mobilization of the reservoir hydrocarbons within the interwell region 108 by reducing the viscosity of the reservoir hydrocarbons. The mobilized bitumen drains to the production well 106, resulting in the creation of a fluid passage, for enabling the locally entrained reservoir hydrocarbons, including bitumen, to escape the interwell region 108. As more bitumen is mobilized and drains to the production well 106, the fluid passage grows and eventually effects fluid communication between the injection well 104 and the production well 106. In some embodiments, for example, the heat is supplied to the interwell region 108 by circulating a startup phase fluid 118 (such as steam, or a fluid including steam) through one or both of the wells 104, 106.

[0030] The production phase includes ramp-up. plateau and wind-down. During ramp-up, bitumen production rates are still increasing. During plateau, the rates have peaked and remain stable or decline slowly. During wind-down, the rates are declining. [0031] Solvent may be added during the start-up phase of SAGD, during the production phase of SAGD, or both. In this context, the term "solvent" is intended to refer to material that, when disposed in the liquid state, is able to, at least to some extent, dissolve in bitumen.

(A) Solvent Addition During Start-Up Phase of SA GD Operation

[0032] In some embodiments, for example, the addition of solvent during the start-up phase of a SAGD operation (i.e. prior to establishment, or substantial establishment, of interwell fluid communication) accelerates the mobilization of bitumen in the interwell region, and promotes the rapid formation of a steam chamber. The ability to establish good interwell communication during the start-up phase in turn allows the subsequent phases of the SAGD operation to perform more effectively. Once good communication is established, there is continued development and growth of the steam chamber, and the entirety of the SAGD operation is enhanced. Establishing good communication early on in a SAGD operation allows for much better ramp-up and much better overall SAGD performance. The time required to switch between the start-up phase of a SAGD operation to the ramp-up phase of a SAGD-mode of operation is diminished when solvent is added during the start-up phase of a SAGD operation. The more rapid and/or enhanced mobilization of bitumen is due to the combined effects of conduction, convective heating and dilution by solvent on viscosity of the bitumen in the inter-well zone, and all of these effects are particularly pronounced when solvent is added early in a SAGD operation.

[0033] In this respect, in some embodiments, for example, during the start-up phase of a SAGD operation, a start-up phase fluid 118 is supplied via either one of the injection well 104 and the production well 106, such as by circulation within the respective well, such that thermal communication is effected between the start-up phase fluid 118 and the bitumen within the interwell region disposed between the injection well 104 and the production well 106. The effected thermal communication is such that the bitumen within the interwell region is mobilized. The mobilized bitumen drains to the production well 106, resulting in the creation of a fluid passage, for enabling the locally entrained reservoir hydrocarbons, including bitumen, to escape the interwell region 108. As more bitumen is mobilized and drains to the production well 106, the fluid passage grows and eventually effects fluid communication between the injection well 104 and the production well 106. [0034] The start-up phase fluid includes steam and a start-up phase solvent. In this respect, in some embodiments, for example, the start-up phase solvent is co-injected with the steam.

[0035] The start-up phase solvent consists of one or more start-up phase solvent hydrocarbons. A variety of hydrocarbons can be used. In some embodiments, for example, the hydrocarbon is chosen based on miscibility in bitumen, availability, cost and thermo-physical properties.

[0036] The function of the start-up phase solvent hydrocarbons includes, amongst other things, is to dissolve into the reservoir hydrocarbons, and effect a reduction in viscosity of the reservoir hydrocarbons.

[0037] In some embodiments, for example, the start-up phase fluid may include between 0.1 and 30 mol % (such as, for example, between 3 and 30 mol %) of the start-up phase solvent, based on the total number of moles of the start-up phase fluid. The total amount of start-up phase solvent used is based on oil viscosity at initial conditions, operating pressure, the formation permeability and the composition of the start-up phase solvent.

[0038] The start-up phase solvent may be recovered from the produced production fluid in the facility 1 10 and re-used for injection into the oil sands reservoir. In some embodiments, for example, it is useful to use start-up phase solvent which is an on-site diluent as this can reduce blending requirements for facilitating transport, by pipeline, to a refinery.

[0039] The start-up phase solvent may be a single or multi-component fluid. Multi- component production phase solvent allow for operational flexibility, as the functionality of the solvent may be preserved over a wider range of operating conditions. In some embodiments, for example, the one or more start-up phase solvent hydrocarbons may include a hydrocarbon having a total number of 1 to 30 carbon atoms. In this respect, in some embodiments, for example, the one or more start-up phase solvent hydrocarbons may include heavy hydrocarbons and/or light hydrocarbons. Exemplary hydrocarbons include aromatics, xylene, hexane, gasoline, kersosene, naphtha, gas condensates, diesel, benzene, toluene, distallates, butane, methane, and pentane.

[0040] An example of a multi-component start-up phase solvent that may be used is cracked naphtha. "Cracked naphtha" generally refers to naphthas that come from refinery processes such as catalytic or thermal cracking or visbreaking. There are a number of suitable cracked naphtha compositions.

[0041] Another suitable multi-component start-up phase solvent is natural gas condensate. Natural gas condensate may have a variety of compositions depending on the source, but generally has a specific gravity ranging from 0.5 to 0.8 and is composed of hydrocarbons such as propane, butane, pentane, hexane, etc. Gas condensate generally has very low viscosity and is frequently used as a diluent to dilute heavier oils to meet pipeline specifications.

[0042] Some of the benefits of injection of a mixture of steam and the start-up phase solvent during the start-up phase of a SAGD operation includes:

• oil production rates are accelerated and the SOR is reduced;

• solvent injection with steam improves the dehydration of produced emulsions and post- production water handling;

• when solvents having higher molecular weight hydrocarbons, such as gas condensate, are used, the amount of asphaltene precipitation is minimized;

• solvent recovery is improved;

• steam chamber growth rate is faster when solvent is added during the start-up phase of SAGD, allowing the optimization of the later stages of a SAGD operation; and

• starting solvent injection earlier extends the solvent-bitumen contact time and consequently increases the solvent penetration depth into the bitumen.

(B) Solvent Addition During the Production Phase of a SA GD Operation

[0043] During the production phase of the SAGD operation, in parallel with the injection of steam into the reservoir 102, production phase solvent may also be injected into the reservoir 102. In some embodiments, for example, a production-initiating fluid 116 may be injected into the reservoir 102, the production-initiating fluid including a mixture of steam and the production phase solvent. In this respect, in some embodiments, for example, the production phase solvent is co-injected with the steam through the injection well 104.

[0044] By injecting production phase solvent, and thereby supplementing the injected steam, mobilization of the reservoir hydrocarbons, and their drainage to the production well 106, is accelerated.

[0045] Once disposed within the reservoir, the injected steam condenses within the steam chamber that has been developed within the reservoir 102, thereby transferring its latent heat to the reservoir 102, resulting in heating of the reservoir hydrocarbons, with a concomitant reduction in their viscosity. In parallel, the injected production phase solvent, in gaseous form, upon becoming disposed within the reservoir, also condenses within the reservoir 102 at the boundary of the steam chamber, liberating further heat to the reservoir 102 and thereby heating the reservoir hydrocarbons. The condensed production phase solvent also dissolves into the reservoir hydrocarbons and, in this respect, in conjunction with the heat received from the steam, decreases the viscosity, and thereby further increasing the mobility of the reservoir hydrocarbons. As the reservoir hydrocarbons drain, a new interface emerges for interaction with the steam and the production phase solvent. In this respect, with the supplementary production phase solvent injection, hydrocarbon recovery may be increased, and cumulative steam-to-oil ratio ("SOR") may be reduced, relative to the production phase of a SAGD operation without any solvent injection.

[0046] The use of production phase solvent, in conjunction with steam, during the production phase of a SAGD operation, may also enable more uniform conduction of mobilized hydrocarbons along the length of the wells 104, 106. This is because the provision of the production phase solvent, in those well segments that are being heated to lower temperatures, compensates for these local "cold spots", by enabling mobilization of the reservoir hydrocarbons, notwithstanding the lower temperatures in these segments. This is due to the fact that, generally, solvents have greater solubility at lower temperatures. Accordingly, in cold spots, more solvent will be dissolved, partly compensating for the higher oil viscosity caused by the lower temperature. [0047] The production phase solvent consists of one or more production phase solvent hydrocarbons. A variety of hydrocarbons can be used. In some embodiments, for example, the hydrocarbon is chosen based on miscibility in bitumen, availability, cost and thermo-physical properties.

[0048] The function of the production phase solvent hydrocarbons includes, amongst other things, to dissolve into the reservoir hydrocarbons, and effect a reduction in viscosity of the reservoir hydrocarbons.

[0049] In some embodiments, for example, the production-initiating fluid may include between 0.1 and 30 mol % (such as, for example, between 3 and 30 mol %) of production phase solvent, based on the total moles of the production-initiating fluid. The total amount of production phase solvent used is based on oil viscosity at initial conditions, operating pressure, the formation permeability and the composition of the production phase solvent.

[0050] The ratio of the vapor pressure of the production phase solvent at steam temperature to the total pressure of the system determines the maximum amount of the production phase solvent that can be kept in the vapor phase within the steam chamber at specific SAGD operating conditions. This ratio also represents the maximum amount of production phase solvent that should be used, as using additional production phase solvent may not result in additional benefits. Viewed another way, once the bitumen becomes saturated with production phase solvent, there are only small incremental improvements that may come from injecting additional production phase solvent into the reservoir. Also, due to increasing partial pressure of the solvent in the vapour phase, as solvent concentration increases, there will be a greater tendency for solvent to condense into the liquid phase such that its conduction to the edge of the vapour chamber is curtailed.

[0051] The production phase solvent may be recovered from the produced production fluid in the facility 110 and re-used for injection into the oil sands reservoir. In some embodiments, for example, it is useiul to use production phase solvent which is an on-site diluent as this can reduce blending requirements for facilitating transport, by pipeline, to a refinery. [0052] The production phase solvent can be a single or multi-component fluid. Multi- component production phase solvents allow for operational flexibility, as the functionality of the solvent may be preserved over a wider range of operating conditions. In some embodiments, for example, the one or more production phase solvent hydrocarbons may include a hydrocarbon having a total number of 1 to 30 carbon atoms. In this respect, in some embodiments, for example, the one or more production phase solvent hydrocarbons may include heavy hydrocarbons and/or light hydrocarbons. Exemplary hydrocarbons include aromatics, xylene, hexane, gasoline, kersosene, naphtha, gas condensates, diesel, benzene, toluene, distallates, butane, methane, and pentane.

[0053] An example of a multi-component production phase solvent that may be used is cracked naphtha. "Cracked naphtha" generally refers to naphthas that come from refinery processes such as catalytic or thermal cracking or visbreaking. There are a number of suitable cracked naphtha compositions.

[0054] Another suitable multi-component production phase solvent is natural gas condensate. Natural gas condensate may have a variety of compositions depending on the source, but generally has a specific gravity ranging from 0.5 to 0.8 and is composed of hydrocarbons such as propane, butane, pentane, hexane, etc. Gas condensate generally has very low viscosity and is frequently used as a diluent to dilute heavier oils to meet pipeline specifications.

(i) Reducing solvent content being added in a later stage of the SAGD production phase

[0055] In some embodiments, for example, it may be suitable to reduce the amount of production phase solvent being injected during later stages of the production phase of a SAGD operation. In this way, less residual solvent may remain within the oil sands reservoir after completion of SAGD.

[0056] In this respect, in some embodiments, for example, a process for producing bitumen from an oil sands reservoir through a production well 106 that is disposed in fluid communication with an injection well 104 via an interwell region 108 is provided. The process includes, during a first stage of the SAGD production phase, injecting a first production initiating fluid into the injection well 104 for conducting of the injected production initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir such that a first mobilized bitumen is conducted through the interwell region 108 to the production well 106. The first production initiating fluid includes steam and a production phase solvent, the production phase solvent consisting of one or more production phase solvent hydrocarbons. The first mobilized bitumen is recovered through the production well 106. Subsequently, the injecting of the first production initiating fluid is suspended, such that the first stage of the SAGD production phase is completed. After completion of the first stage of the SAGD production phase, the second stage of the SAGD production phase is effected (i.e. the first stage is an earlier stage, and the second stage is a later stage). During the second stage of the SAGD production phase, a second production initiating fluid is injected into the injection well 104 for conducting of the injected production initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir such that a second mobilized bitumen is conducted through the interwell region 108 to the production well 106. The second production initiating fluid includes steam and a production phase solvent, the production phase solvent consisting of one or more production phase solvent hydrocarbons. The second mobilized bitumen is recovered through the production well 106. Subsequently, the injecting of the second production initiating fluid is suspended, such that the second stage of the SAGD production phase is completed. The ratio of moles of production phase solvent to steam is greater (such as, for example, 10% greater, such as, for example, 25% greater, such as, for example, 50% greater) within the first production initiating fluid relative to that within the second production initiating fluid. The ratio of moles of production phase solvent to steam is greater (such as, for example, 10% greater, such as, for example, 25% greater, such as, for example, 50% greater) within the second production initiating fluid relative to that within the third production initiating fluid. In some embodiments, for example, the rate of production of bitumen, via the production well, during the first stage of the SAGD production phase is greater than the rate of production of bitumen, via the production well, during the second stage of the SAGD production phase, such as, for example, by at least 25%, such as, for example, by at least 50%, such as, for example, by at least 100%. In some embodiments, for example, the first stage of the SAGD production phase occurs prior to the steam chamber reaching the cap rock 300 (see Figure 2), and the second stage of the SAGD production phase occurs after the steam chamber has reached the cap rock 300. In some embodiments, for example, the first stage may include at least a portion of a "steam chamber initialization" phase, at least a portion of a "plateau phase", or at least portions of both. In some embodiments, for example, the second stage may include a "lateral growth only" phase, a "winding down" phase, or at least portions of both.

(ii) Transitioning from heavier to lighter solvents in later stages of the SAGD production phase

[0057] In some embodiments, for example, it may be suitable to transition from a heavier production phase solvent to a lighter production phase solvent at some point in time during the production phase of a SAGD operation. In some embodiments, for example, at some point in time during the production phase of a SAGD operation, a heavier production phase solvent may, relative to a lighter production phase solvent, have a greater tendency to condense prior to reaching the interface between the bitumen, that is entrained within the oil sands reservoir, and the steam chamber, and thereby fail to mobilize the bitumen. This may dictate the switching over to a lighter production phase solvent, in order to improve efficiencies in mobilizing bitumen within the oil sands reservoir.

[0058] In one respect, in some embodiments, for example, there is provided a process for producing bitumen from an oil sands reservoir through a production well 106 that is disposed in fluid communication with an injection well 104 via an interwell region 108. The process includes, during a first stage of the SAGD production phase, injecting a first production initiating fluid into the injection well 104 for conducting of the injected first production initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir such that a first mobilized bitumen is conducted through the interwell region 108 to the production well 106, wherein the first production initiating fluid includes steam and a production phase solvent, the production phase solvent consisting of one or more production phase solvent hydrocarbons. The first mobilized bitumen is recovered through the production well 106. Subsequently, the injecting of the first production initiating fluid is suspended, such that the first stage of the SAGD production phase is completed. After completion of the first stage of the SAGD production phase, the second stage of the SAGD production phase is effected. During the second stage of the SAGD production phase, a second production initiating fluid is injected into the injection well 104 for conducting of the injected second production initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir such that a second mobilized bitumen is conducted through the interwell region 108 to the production well 106, wherein the second production initiating fluid includes steam and a production phase solvent, the production phase solvent consisting of one or more production phase solvent hydrocarbons. The second mobilized bitumen is recovered through the production well 106. The density of the production phase solvent is greater within the first production initiating fluid relative to that within the second production initiating fluid. In some embodiments, for example, the density of the production phase solvent is at least 10% (such as, for example, at least 20%, such as, for example, at least 30%) greater within the first production initiating fluid relative to, the density of the production phase solvent within the second production initiating fluid. In some embodiments, for example, at least 70 mol % (such as, for example, at least 80 mol %, such as, for example, at least 90 mol %) of the production phase solvent, of the first production initiating fluid, consists of heavy hydrocarbon material, based on the total number of moles of the production phase solvent being injected, and less than 35 mol % (such as, for example, less than 25 mol %, such as, for example, less than 15 mol %) of the production phase solvent, of the second production-initiating fluid, consists of light hydrocarbon material, based on the total number moles of the production phase solvent being injected, wherein the production phase solvent of the first production-initiating fluid has a higher molar concentration of heavy hydrocarbon material than the production phase solvent of the second production-initiating fluid. In some embodiments, for example, the vapour pressure of the second production initiating fluid is greater than the vapour pressure of the first production initiating fluid, such as, for example, by at least 10%, such as, for example, by at least 20%, such as, for example, by at least 30%. In some embodiments, for example, the rate of production of bitumen, via the production well, during the first stage of the SAGD production phase is greater than the rate of production of bitumen, via the production well, during the second stage of the SAGD production phase, such as, for example, by at least 25%, such as, for example, by at least 50%, such as, for example, by at least 100%. In some embodiments, for example, the first stage of the SAGD production phase occurs prior to the steam chamber reaching the cap rock 300 (see Figure 2), and the second stage of the SAGD production phase occurs after the steam chamber has reached the cap rock 300. In some embodiments, for example, the first stage may include at least a portion of a "steam chamber initialization" phase, at least a portion of a "plateau phase", or at least portions of both. In some embodiments, for example, the second stage may include a "lateral growth only" phase, a "winding down" phase, or at least portions of both.

[0059] In another respect, in some embodiments, for example, there is provided a process for producing bitumen from an oil sands reservoir through a production well 106 that is disposed in fluid communication with an injection well 104 via an interwell region 108. The process includes, during a first stage of the SAGD production phase, injecting a first production initiating fluid into the injection well 104 for conducting of the injected first production initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir such that a first mobilized bitumen is conducted through the interwell region 108 to the production well 106, wherein the first production initiating fluid includes steam and a production phase solvent, the production phase solvent consisting of one or more production phase solvent hydrocarbons. The first mobilized bitumen is recovered through the production well 106. Subsequently, the injecting of the first production initiating fluid is suspended, such that the first stage of the SAGD production phase is completed. After completion of the first stage of the SAGD production phase, the second stage of the SAGD production phase is effected. During the second stage of the SAGD production phase, a second production initiating fluid is injected into the injection well 104 for conducting of the injected second production initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir such that a second mobilized bitumen is conducted through the interwell region 108 to the production well 106, wherein the second production initiating fluid includes steam and a production phase solvent, the production phase solvent consisting of one or more production phase solvent hydrocarbons. The second mobilized bitumen is recovered through the production well 106. The weight average molecular weight of the production phase solvent is greater within the first production initiating fluid relative to that within the second production initiating fluid. In some embodiments, for example, the weight average molecular weight of the production phase solvent is at least 10% (such as, for example, at least 20%, such as, for example, at least 30%) greater within the first production initiating fluid relative to the weight average molecular weight of the production phase solvent within the second production initiating fluid. In some embodiments, for example, at least 70 mol % (such as, for example, at least 80 mol %, such as, for example, at least 90 mol %) of the production phase solvent, of the first production initiating fluid, consists of heavy hydrocarbon material, based on the total number of moles of the production phase solvent being injected, and less than 35 mol % (such as, for example, less than 25 mol %, such as, for example, less than 15 mol %) of the production phase solvent, of the second production- initiating fluid, consists of light hydrocarbon material, based on the total number moles of the production phase solvent being injected, wherein the production phase solvent of the first production-initiating fluid has a higher molar concentration of heavy hydrocarbon material than the production phase solvent of the second production-initiating fluid. In some embodiments, for example, the vapour pressure of the second production initiating fluid is greater than the vapour pressure of the first production initiating fluid, such as, for example, by at least 10%, such as, for example, by at least 20%, such as, for example, by at least 30%. In some embodiments, for example, the rate of production of bitumen, via the production well, during the first stage of the SAGD production phase is greater than the rate of production of bitumen, via the production well, during the second stage of the SAGD production phase, such as, for example, by at least 25%, such as, for example, by at least 50%, such as, for example, by at least 100%. In some embodiments, for example, the first stage of the SAGD production phase occurs prior to the steam chamber reaching the cap rock 300 (see Figure 2), and the second stage of the SAGD production phase occurs after the steam chamber has reached the cap rock 300. In some embodiments, for example, the first stage may include at least a portion of a "steam chamber initialization" phase, at least a portion of a "plateau phase", or at least portions of both. In some embodiments, for example, the second stage may include a "lateral growth only" phase, a "winding down" phase, or at least portions of both.

[0060] In another respect, in some embodiments, for example, there is provided another process for producing bitumen from an oil sands reservoir through a production well 106 that is disposed in fluid communication with an injection well 104 via an interwell region 108. The process includes, during a first stage of the SAGD production phase, injecting a first production initiating fluid into the injection well 104 for conducting of the injected first production initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir such that a first mobilized bitumen is conducted through the interwell region 108 to the production well 106. The first production initiating fluid includes steam and a production phase solvent, and the production phase solvent consists of one or more production phase solvent hydrocarbons. The first mobilized bitumen is recovered through the production well 106. Subsequently, the injecting of the first production initiating fluid is suspended, such that the first stage of the SAGD production phase is completed. After completion of the first stage of the SAGD production phase, the second stage of the SAGD production phase is effected. During the second stage of the SAGD production phase, a second production initiating fluid is injected into the injection well 104 for conducting of the injected second production initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir such that a second mobilized bitumen is conducted through the interwell region 108 to the production well 106. The second production initiating fluid includes steam and a production phase solvent. The production phase solvent consists of one or more production phase solvent hydrocarbons. The second mobilized bitumen is recovered through the production well 106. At least 70 mol % (such as, for example, at least 80 mol %, such as, for example, at least 90 mol %) of the production phase solvent, of the first production initiating fluid, consists of heavy hydrocarbon material, based on the total number of moles of the production phase solvent being injected, and less than 35 mol % (such as, for example, less than 25 mol %, such as, for example, less than 15 mol %) of the production phase solvent, of the second production-initiating fluid, consists of light hydrocarbon material, based on the total number moles of the production phase solvent being injected, wherein the production phase solvent of the first production-initiating fluid has a higher molar concentration of heavy hydrocarbon material than the production phase solvent of the second production-initiating fluid. In some embodiments, for example, the vapour pressure of the second production initiating fluid is greater than the vapour pressure of the first production initiating fluid. In some of these embodiments, for example, the vapour pressure of the second production initiating fluid is greater than the vapour pressure of the first production initiating fluid by at least 10%. In some embodiments, for example, the rate of production of bitumen, via the production well, during the first stage of the SAGD production phase is greater than the rate of production of bitumen, via the production well, during the second stage of the SAGD production phase, such as, for example, by at least 25%, such as, for example, by at least 50%, such as, for example, by at least 100%. In some embodiments, for example, the first stage of the SAGD production phase occurs prior to the steam chamber reaching the cap rock 300 (see Figure 2), and the second stage of the SAGD production phase occurs after the steam chamber has reached the cap rock 300. In some embodiments, for example, the first stage may include at least a portion of a "steam chamber initialization" phase, at least a portion of a "plateau phase", or at least portions of both. In some embodiments, for example, the second stage may include a "lateral growth only" phase, a "winding down" phase, or at least portions of both.

(C) Varying Solvent Content in Added Solvent as between Solvent Addition During the Start-up Phase and Solvent Addition During Various Stages of the SAGD Production Phase

[0061] In some embodiments, for example, a process for producing bitumen from an oil sands reservoir through a production well 106 that is disposed in fluid communication with an injection well 104 via an interwell region 108. The process includes establishing fluid communication, through the interwell region 108, between the injection well 104 and the production well 106. The establishing fluid communication includes injecting a start-up phase fluid 118 into the injection well 104 for conducting of the injected start-up phase fluid to the oil sands reservoir such that thermal communication between the start-up phase fluid and the bitumen within the interwell region is effected. The effected thermal communication is such that the bitumen within the interwell region is mobilized. The mobilized bitumen drains to the production well 106, resulting in the creation of a fluid passage, for enabling the locally entrained reservoir hydrocarbons, including bitumen, to escape the interwell region 108. As more bitumen is mobilized and drains to the production well 106, the fluid passage grows and eventually effects fluid communication between the injection well 104 and the production well 106. The start-up phase fluid includes steam and a start-up phase solvent, the start-up phase solvent consisting of one or more start-up phase solvent hydrocarbons. After the fluid communication has been established, a SAGD production phase is operated. The process further includes, during a first stage of the SAGD production phase, injecting a first production initiating fluid into the injection well 104 for conducting of the injected production initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir such that a first mobilized bitumen is conducted through the interwell region 108 to the production well 106. The first production initiating fluid includes steam and a production phase solvent, the production phase solvent consisting of one or more production phase solvent hydrocarbons. The first mobilized bitumen is recovered through the production well 106. Subsequently, the injecting of the first production initiating fluid is suspended, such that the first stage of the SAGD production phase is completed. The ratio of moles of start-up phase solvent to steam within the start-up phase fluid is greater (such as, for example, 10% greater, such as, for example, 25% greater, such as, for example, 50% greater) than the ratio of moles of production phase solvent to steam within the first production initiating fluid.

[0062] In some embodiments, for example, after completion of the first stage of the SAGD production phase, a second stage of the SAGD production phase is effected. During the second stage of the SAGD production phase, a second production initiating fluid is injected into the injection well 104 for conducting of the injected production initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir such that a second mobilized bitumen is conducted through the interwell region 108 to the production well 106. The second production initiating fluid includes steam and a production phase solvent, the production phase solvent consisting of one or more production phase solvent hydrocarbons. The second mobilized bitumen is recovered through the production well 106. Subsequently, the injecting of the second production initiating fluid is suspended, such that the second stage of the SAGD production phase is completed. The ratio of moles of production phase solvent to steam is greater (such as, for example, 10% greater, such as, for example, 25% greater, such as, for example, 50% greater) within the first production initiating fluid relative to that within the second production initiating fluid.

[0063] In those embodiments where the SAGD production phase includes two stages, for example, the first stage of the SAGD production phase occurs prior to the steam chamber reaching the cap rock 300 (see Figure 2), and the second stage of the SAGD production phase occurs after the steam chamber has reached the cap rock 300. In some embodiments, for example, the first stage may include at least a portion of a "steam chamber initialization" phase, at least a portion of a "plateau phase", or at least portions of both. In some embodiments, for example, the second stage may include a "lateral growth only" phase, a "winding down" phase, or at least portions of both.

(D) Varying Solvent Composition as Between Solvent Addition During the Start-up Phase and Solvent Addition During the Production Phase

[0064] In some embodiments, for example, it may be suitable to add heavier solvents during the start-up phase of the SAGD operation, and to switch to lighter solvents following the start-up phase (such as during the production phase), once inter-well communication has been established.

[0065] The ability to choose the appropriate solvent type allows one to minimize solvent losses to the reservoir. Using heavier solvents early in the SAGD operation may be beneficial because heavier solvents would be able to fall towards the production well 106 at early stages when there is very little inter-well communication. At later stages of the SAGD operation, after inter-well communication has been established (such as during the production phase), it may be beneficial to switch to lighter solvents. Lighter solvents would tend not to condense as early as heavier solvents, and would stay in the vapour phase within the steam chamber.

[0066] At later stages of a SAGD operation (e.g. during the production phase), if the solvent selected is too heavy, solvent losses may be greater due to retention of condensed solvent in the depleted zone. In the start-up phase of a SAGD operation, it should be possible to use heavier solvents, as solvent short-circuiting to the production well 106 actually promotes communication between the two wells.

[0067] In this respect, in some embodiments, for example there is provided a process for producing bitumen from an oil sands reservoir through a production well 106 that is disposed in fluid communication with an injection well 104 via an interwell region 108. The process includes establishing fluid communication, through the interwell region 108, between the injection well 104 and the production well 106. The establishing fluid communication includes injecting a start-up phase fluid 118 into the injection well 104 for conducting of the injected start-up phase fluid to the oil sands reservoir such that thermal communication between the startup phase fluid and the bitumen within the interwell region is effected. The effected thermal communication is such that the bitumen within the interwell region is mobilized. The mobilized bitumen drains to the production well 106, resulting in the creation of a fluid passage, for enabling the locally entrained reservoir hydrocarbons, including bitumen, to escape the interwell region 108. As more bitumen is mobilized and drains to the production well 106, the fluid passage grows and eventually effects fluid communication between the injection well 104 and the production well 106. The start-up phase fluid includes steam and a start-up phase solvent, the start-up phase solvent consisting of one or more start-up phase solvent hydrocarbons. After the fluid communication has been established, a production-initiating fluid 116 is injected into the injection well 104 for conducting of the injected production-initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir such that mobilized bitumen is conducted through the interwell region 108 to the production well 106. The production-initiating fluid includes steam and a production phase solvent, the production phase solvent consisting of one or more production phase solvent hydrocarbons. The mobilized bitumen is recovered through the production well 106. The density of the start-up phase solvent within the start-up phase fluid is greater than the density of the production phase solvent within the production-initiating fluid. In some embodiments, for example, the density of the start-up phase solvent within the start-up phase fluid is at least 10% greater than the density of the production phase solvent within the production-initiating fluid. In some embodiments, for example, the density of the start-up phase solvent within the start-up phase fluid is at least 20% greater than the density of the production phase solvent within the production-initiating fluid. In some embodiments, for example, the density of the start-up phase solvent within the start-up phase fluid is at least 30% greater than the density of the production phase solvent within the production-initiating fluid. In some embodiments, for example, at least 70 mol % (such as, for example, at least 80 mol %, such as, for example, at least 90 mol %) of the start-up phase solvent, of the start-up phase fluid, consists of heavy hydrocarbon material, based on the total number of moles of the start-up phase solvent being injected, and less than 35 mol % (such as, for example, less than 25 mol %, such as, for example, less than 15 mol %) of the production phase solvent, of the production-initiating fluid, consists of light hydrocarbon material, based on the total number moles of the production phase solvent being injected, wherein the production phase solvent has a lower molar concentration of heavy hydrocarbon material than the start-up phase solvent. In some embodiments, for example, the vapour pressure of the production- initiating fluid is greater than the vapour pressure of the start-up phase fluid. In some of these embodiments, for example, the vapour pressure of the production-initiating fluid is greater than the vapour pressure of the start-up phase fluid by at least 10%, such as, for example, at least 20%, such as, for example, at least 30%.

[0068] In another respect, there is provided another process for producing bitumen from an oil sands reservoir through a production well 106 that is disposed in fluid communication with an injection well 104 via an interwell region 108. The process includes establishing fluid communication, through the interwell region 108, between the injection well 104 and the production well 106. The establishing fluid communication includes injecting a start-up phase fluid 118 into the injection well 104 for conducting of the injected start-up phase fluid to the oil sands reservoir such that thermal communication between the start-up phase fluid and the bitumen within the interwell region is effected. The effected thermal communication is such that the bitumen within the interwell region is mobilized. The mobilized bitumen drains to the production well 106, resulting in the creation of a fluid passage, for enabling the locally entrained reservoir hydrocarbons, including bitumen, to escape the interwell region 108. As more bitumen is mobilized and drains to the production well 106, the fluid passage grows and eventually effects fluid communication between the injection well 104 and the production well 106. The start-up phase fluid includes steam and a start-up phase solvent, the start-up phase solvent consisting of one or more start-up phase solvent hydrocarbons. After the fluid communication has been established, a production-initiating fluid is injected into the injection well 104 for conducting of the injected producti on-initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir such that mobilized bitumen is conducted through the interwell region 108 to the production well 106. The production-initiating fluid includes steam and a production phase solvent, the production phase solvent consisting of one or more production phase solvent hydrocarbons. The mobilized bitumen is recovered through the production well 106. The weight average molecular weight of the start-up phase solvent within the start-up phase fluid is greater than the weight average molecular of the production phase solvent within the production-initiating fluid. In some embodiments, for example, the weight average molecular weight of the start-up phase solvent within the start-up phase fluid is at least 10% greater than the weight average molecular weight of the production phase solvent within the production-initiating fluid. In some embodiments, for example, the weight average molecular weight of the start-up phase solvent within the start-up phase fluid is at least 20% greater than the weight average molecular weight of the production phase solvent within the production-initiating fluid. In some embodiments, for example, the weight average molecular weight of the start-up phase solvent within the start-up phase fluid is at least 30% greater than the weight average molecular weight of the production phase solvent within the production-initiating fluid. In some embodiments, for example, at least 70 mol % (such as, for example, at least 80 mol %, such as, for example, at least 90 mol %) of the start-up phase solvent, of the start-up phase fluid, consists of heavy hydrocarbon material, based on the total number of moles of the start-up phase solvent being injected, and less than 35 mol % (such as, for example, less than 25 mol %, such as, for example, less than 15 mol %) of the production phase solvent, of the production-initiating fluid, consists of light hydrocarbon material, based on the total number moles of the production phase solvent being injected, wherein the production phase solvent has a lower molar concentration of heavy hydrocarbon material than the start-up phase solvent. In some embodiments, for example, the vapour pressure of the production- initiating fluid is greater than the vapour pressure of the start-up phase fluid. In some of these embodiments, for example, the vapour pressure of the production-initiating fluid is greater than the vapour pressure of the start-up phase fluid by at least 10%, such as, for example, at least 20%, such as, for example, at least 30%.

[0069] In another respect, there is provided another process for producing bitumen from an oil sands reservoir through a production well 106 that is disposed in fluid communication with an injection well 104 via an interwell region 108. The process includes establishing fluid communication, through the interwell region 108, between the injection well 104 and the production well 106. The establishing fluid communication includes injecting a start-up phase fluid 118 into the injection well 104 for conducting of the injected start-up phase fluid to the oil sands reservoir such that thermal communication between the start-up phase fluid and the bitumen within the interwell region is effected. The effected thermal communication is such that the bitumen within the interwell region is mobilized. The mobilized bitumen drains to the production well 106, resulting in the creation of a fluid passage, for enabling the locally entrained reservoir hydrocarbons, including bitumen, to escape the interwell region 108. As more bitumen is mobilized and drains to the production well 106, the fluid passage grows and eventually effects fluid communication between the injection well 104 and the production well 106. The start-up phase fluid includes steam and a start-up phase solvent. The start-up phase solvent consisting of one or more start-up phase solvent hydrocarbons. After the fluid communication has been established, a production-initiating fluid is injected into the injection well 104 for conducting of the injected production-initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir such that mobilized bitumen is conducted through the interwell region 108 to the production well 106. The production-initiating fluid includes steam and a production phase solvent. The production phase solvent consists of one or more production phase solvent hydrocarbons. The mobilized bitumen is recovered through the production well 106. At least 70 mol % (such as, for example, at least 80 mol %, such as, for example, at least 90 mol %) of the start-up phase solvent, of the start-up phase fluid, consists of heavy hydrocarbon material, based on the total number of moles of the start-up phase solvent being injected, and less than 35 mol % (such as, for example, less than 25 mol %, such as, for example, less than 15 mol %) of the production phase solvent, of the production-initiating fluid, consists of light hydrocarbon material, based on the total number of moles of the production phase solvent being injected, wherein the production phase solvent has a lower molar concentration of heavy hydrocarbon material than the start-up phase solvent. In some embodiments, for example, the vapour pressure of the production-initiating fluid is greater than the vapour pressure of the start-up phase fluid. In some of these embodiments, for example, the vapour pressure of the production-initiating fluid is greater than the vapour pressure of the startup phase fluid by at least 10%, such as, for example, at least 20%, such as, for example, at least 30%.

(D) Multicomponent solvents for use within the start-up phase solvent or the production phase solvent

[0070] As mentioned above, either one, or both of, the start-up phase solvent and the production phase solvent may be a single or multi-component solvent. Multi-component solvents allow for operational flexibility, as the functionality of the solvent may be preserved over a wider range of operating conditions.

[0071] Additionally, in producing a solvent, by combining two or more hydrocarbon compounds, advantageously, the characteristics of the resulting produced solvent may be more favourable to effecting mobilization of bitumen, relative to each one of the constituent hydrocarbons, in isolation. In some embodiments, for example, at least one of vapour pressure, solubility, and viscosity of the produced solvent may be sufficiently different from that of each one of the constituent hydrocarbons, in isolation, such that the resulting produced solvent may be more favourable to effecting mobilization of bitumen, relative to each one of the constituent hydrocarbons, in isolation. [0072] In this respect, the solvent includes heavy hydrocarbon material and light hydrocarbon. A suitable solvent composition is illustrated in Figure 3.

[0073] In some embodiments, for example, the solvent includes at least 50 mole % of heavy hydrocarbon material having a total number of carbon atoms of seven (7) or more, based on the total number of moles of the solvent. In some of these embodiments, for example, the solvent includes at least 60 mole % of heavy hydrocarbon material having a total number of carbon atoms of seven (7) or more, based on the total number of moles of the solvent. In some of these embodiments, for example, the solvent includes at least 70 mole % of heavy hydrocarbon material having a total number of carbon atoms of seven (7) or more, based on the total number of moles of the solvent.

[0074] Heavy hydrocarbons are favourable to bitumen mobilization, in some respects, as heavy hydrocarbons are more soluble within bitumen relative to light hydrocarbons. The dissolving of the heavy hydrocarbon within the bitumen produces a fluid having a reduced viscosity, relative to that of bitumen, in isolation, thereby improving the mobility of bitumen within the reservoir. However, generally speaking, heavy hydrocarbons, relative to light hydrocarbons, are more viscous. Accordingly, the effective viscosity reduction, effected by the dissolution of a heavy hydrocarbon within the bitumen, may not be sufficiently significant to effect a sufficiently appreciable increase in the mobility of bitumen within the reservoir. As well, generally speaking, heavy hydrocarbons, relative to light hydrocarbons, have lower vapour pressures and will, therefore, condense at higher temperatures, which means that the use of heavy hydrocarbons as a solvent is more sensitive to temperature decreases.

[0075] Combining the heavy hydrocarbon material with the light hydrocarbon material produces a solvent having a lower viscosity relative to the heavy hydrocarbon material, in isolation, and also having a higher vapour pressure relative to the heavy hydrocarbon material, in isolation. In this respect, such combination mitigates at least some of the characteristics of the heavy hydrocarbon material that are detrimental to bitumen production, while still benefiting from the favourable solubility characteristics of the heavy hydrocarbon material within bitumen.

[0076] In some embodiments, for example, the start-up phase fluid includes 0.1 to 30 mole % of heavy hydrocarbon material of the start-up phase solvent within the start-up phase fluid, based on the total number of moles of the start-up phase fluid. In some of these embodiments, for example, the start-up phase fluid includes 15 to 25 mole % of heavy hydrocarbon material of the start-up phase solvent within the start-up phase fluid, based on the total number of moles of the start-up phase fluid.

[0077] In some embodiments, for example, the production-initiating fluid includes 0.1 to 30 mole % of heavy hydrocarbon material of the production phase solvent within the production- initiating fluid, based on the total number of moles of the production-initiating fluid. In some embodiments, for example, the production-initiating fluid includes 15 to 25 mole % of heavy hydrocarbon material of the production phase solvent within the production-initiating fluid, based on the total number of moles of the production-initiating fluid.

(E) Selecting composition of the start-up phase fluid or the production-initiating fluid such that the admixed solvent is in a vapour state when supplied to the reservoir

[0078] In some embodiments, for example, the composition of the start-up phase fluid is selected such that the start-up phase solvent is disposed, or substantially disposed, in a vapour state when supplied to the oil sands reservoir. After the supplying, the start-up phase fluid becomes disposed in thermal communication with bitumen within the interwell region 108, and transfers heat to the bitumen such that condensation of at least a fraction of the start-up phase solvent is effected to produce condensed start-up phase solvent. After the condensing, the condensed start-up phase solvent mixes with the bitumen within the oil sands reservoir such that dissolution of the produced condensed start-up phase solvent into the bitumen is effected to produce a bitumen-comprising mixture. The bitumen-comprising mixture is then drained, by gravity, to the production well 106, thereby contributing to the establishment of interwell communication (such as in the form of a steam chamber) between the injection well 104 and the production well 106.

[0079] The selection is based upon information embodied in a multicomponent phase diagram for the components of the start-up phase fluid. In some embodiments, for example, the selected composition of the start-up phase fluid is disposed within the vapour region of the multicomponent phase diagram at the temperature and pressure of the interwell region 108 of the oil sands reservoir to which the start-up phase fluid is being supplied, by either one, or both of, the injection well 104 and the production well 106, during the start-up phase of SAGD. It is understood that information embodied in a multicomponent phase diagram is not limited by the manner such information is presented, such as by a graphical illustration, and extends, for example, to any electronic or digital form of such presentation.

[0080] In some embodiments, for example, the start-up phase solvent of the start-up phase fluid is a single component solvent (i.e. one hydrocarbon) or a multicomponent solvent (i.e. two or more hydrocarbons).

[0081] In some embodiments, for example, the composition of the production-initiating fluid is selected such that the production phase solvent is disposed in a vapour state when supplied to the oil sands reservoir. After the supplying, the production-initiating fluid becomes disposed in thermal communication with bitumen within the oil sands region, and transfers heat to the bitumen such that condensation of at least a fraction of the production phase solvent is effected to produce condensed production phase solvent. After the condensing, the condensed production phase solvent mixes with the bitumen within the oil sands reservoir such that dissolution of the bitumen by the produced condensed production phase solvent is effected to produce a bitumen- comprising mixture. The bitumen-comprising mixture is then drained, by gravity, to the production well, and is then produced via the production well.

[0082] In some embodiments, for example, the selection is based upon a multicomponent phase diagram for the components of the production-initiating fluid. In some embodiments, for example, the selected composition of the production-initiating fluid is disposed within the vapour region of the multicomponent phase diagram at the temperature and pressure of the oil sands reservoir to which the production-initiating fluid is being supplied by the injection well during the production phase of SAGD. It is understood that information embodied in a multicomponent phase diagram is not limited by the manner such information is presented, such as by a graphical illustration, and extends, for example, to any electronic or digital form of such presentation.

[0083] In some embodiments, for example, the production phase solvent of the production- initiating fluid is a single-component solvent or a multicomponent solvent.

Blowdown after completion of SAGD [0084] In some embodiments, for example, upon the completion of the production phase of SAGD, where, during SAGD, solvent has been co-injected with steam, sufficiently significant amounts of residual solvent may remain within the oil sands reservoir, and it may be desirable to recover such residual solvent, such as, for example, for purposes of recycle and re-use.

[0085] In this respect, in some embodiments, for example, a process is provided for producing bitumen from an oil sands reservoir through a production well that is disposed in fluid communication with an injection well via an interwell region. The process includes establishing fluid communication, through the interwell region, between the injection well and the production well. The establishing fluid communication includes supplying a start-up phase fluid via the injection well or the production well such that thermal communication between the start-up phase fluid and the bitumen within the interwell region is effected. The start-up phase fluid includes steam. After the fluid communication has been established, during a SAGD production phase, a production-initiating fluid is supplied to the oil sands reservoir via the injection well such that mobilization of bitumen within the oil sands reservoir is effected, and such that the mobilized bitumen is conducted to the production well and produced via the production well. The production-initiating fluid includes steam

[0086] At least one of the start-up phase fluid and the production-initiating fluid also includes solvent that includes hydrocarbon material. In some embodiments, for example, both of the start-up phase fluid and the production-initiating fluid includes solvent that includes hydrocarbon material. In some embodiments, for example, where the start-up phase fluid includes steam and solvent that includes hydrocarbon material, the start-up phase fluid may include between 0.1 and 30 mol % (such as, for example, between 3 and 30 mol %) of the startup phase solvent, based on the total number of moles of the start-up phase fluid. In some embodiments, for example, where the production-initiating fluid includes steam and solvent that includes hydrocarbon material, the production-initiating fluid may include between 0.1 and 30 mol % (such as, for example, between 3 and 30 mol %) of production phase solvent, based on the total moles of the production-initiating fluid.

[0087] Completion of the SAGD production phase is defined by the suspension of the supplying of the production-initiating fluid to the oil sands reservoir via the injection well. Prior to the suspension of the production-initiating fluid to the oil sands reservoir via the injection well, and while the production-initiating fluid is being supplied to the oil sands reservoir via the injection well, the reservoir is disposed at a pre-SAGD production phase suspension pressure. After the supplying of the production-initiating fluid to the oil sands reservoir via the injection well has been suspended, and after the pressure within the oil sands reservoir has been reduced from a pre-SAGD production phase suspension pressure, and while the oil sands reservoir is being vented via at least one of the injection well and the production well, collecting gaseous material that is being conducted via the at least one of the injection well and the production well such as at least some of the supplied solvent is recovered. In some embodiments, for example, the pressure reduction is at least a 25% pressure reduction. In some embodiments, for example, the pressure reduction is at least a 50% pressure reduction. In some embodiments, for example, the pressure reduction is such that the oil sands reservoir is disposed at reservoir pressure.

[0088] In the above description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the present disclosure. However, it will be apparent to one skilled in the art that these specific details are not required in order to practice the present disclosure. Although certain dimensions and materials are described for implementing the disclosed example embodiments, other suitable dimensions and/or materials may be used within the scope of this disclosure. All such modifications and variations, including all suitable current and future changes in technology, are believed to be within the sphere and scope of the present disclosure. All references mentioned are hereby incorporated by reference in their entirety.

Claims

1. A process for producing bitumen from an oil sands reservoir through a production well that is disposed in fluid communication with an injection well via an interwell region, comprising: supplying a production-initiating fluid into the oil sands reservoir via the injection well for effecting mobilization of bitumen within the oil sands reservoir such that the mobilized bitumen is conducted through the interwell region to the production well; wherein the production-initiating fluid includes steam and a production phase solvent, and the production phase solvent includes heavy hydrocarbon material and light hydrocarbon material; and wherein the heavy hydrocarbon material includes one or more heavy hydrocarbons, wherein each one of the one or more heavier hydrocarbons, independently, is a hydrocarbon that includes a total number of carbons five (5) or more; and wherein the light hydrocarbon material includes one or more light hydrocarbons, wherein each one of the one or more light hydrocarbons, independently, is a hydrocarbon that includes a total number of carbons of four (4) or less.

2. The process as claimed in claim 1 ; wherein the production-initiating fluid includes at least 50 mole % of heavy hydrocarbon material having a total number of carbon atoms of seven (7) or more, based on the total number of moles of the solvent.

3. The process as claimed in claim 1 or 2; wherein the production-initiating fluid includes 0.1 to 30 mole % of the heavy hydrocarbon material of the production phase solvent within the production-initiating fluid, based on the total number of moles of the production-initiating fluid.

4. The process as claimed in claim 1 or 2; wherein the production-initiating fluid includes 15 to 25 mole % of heavy hydrocarbon material of the production phase solvent within the production-initiating fluid, based on the total number of moles of the production-initiating fluid.

5 A process for producing bitumen from an oil sands reservoir: establishing fluid communication between an injection well and a production well via an interwell region within an oil sands reservoir, including: supplying a start-up phase fluid into the oil sands reservoir via the injection well such that thermal communication between the start-up phase fluid and the bitumen within the interwell region is effected; wherein the start-up phase fluid includes steam and a start-up phase solvent, and the startup phase solvent includes heavy hydrocarbon material and light hydrocarbon material; and wherein the heavy hydrocarbon material includes one or more heavy hydrocarbons, wherein each one of the one or more heavy hydrocarbons, independently, is a hydrocarbon that includes a total number of carbons of five (5) or more; and wherein the light hydrocarbon material includes one or more light hydrocarbons, wherein each one of the one or more light hydrocarbons, independently, is a hydrocarbon that includes a total number of carbons of four (4) or less; and after the fluid communication has been established, producing bitumen from the oil sands reservoir via the production well.

6 The process as claimed in claim 5; wherein the start-up phase fluid includes at least 50 mole % of heavy hydrocarbon material having a total number of carbon atoms of seven (7) or more, based on the total number of moles of the solvent.

7 The process as claimed in claim 5 or 6; wherein the production-initiating fluid includes 0.1 to 30 mole % of the heavy hydrocarbon material of the production phase solvent within the production-initiating fluid, based on the total number of moles of the production- initiating fluid.

8. The process as claimed in claim 5 or 6; wherein the production-initiating fluid includes 15 to 25 mole % of heavy hydrocarbon material of the production phase solvent within the production-initiating fluid, based on the total number of moles of the production-initiating fluid.

9 The process as claimed in any one of claims 5 to 8; wherein the producing includes supplying a production-initiating fluid into the oil sands reservoir via the injection well for effecting mobilization of bitumen within the oil sands reservoir such that the mobilized bitumen is conducted through the interwell region to the production well.

10 The process as claimed in claim 9; wherein the production-initiating fluid includes steam and a production phase solvent, and the production phase solvent includes heavy hydrocarbon material and light hydrocarbon material; and wherein the heavy hydrocarbon material includes one or more heavy hydrocarbons, wherein each one of the one or more heavy hydrocarbons, independently, is a hydrocarbon that includes a total number of carbons of five (5) or more, and wherein the light hydrocarbon material includes one or more light hydrocarbons, wherein each one of the one or more light hydrocarbons, independently, is a hydrocarbon that includes a total number of carbons of four (4) or less.

11. The process as claimed in claim 10; wherein the production-initiating fluid includes at least 50 mole % of heavy hydrocarbon material having a total number of carbon atoms of seven (7) or more, based on the total number of moles of the solvent.

12 The process as claimed in claim 10 or 11; wherein the production-initiating fluid includes 0.1 to 30 mole % of the heavy hydrocarbon material of the production phase solvent within the production-initiating fluid, based on the total number of moles of the production-initiating fluid.

13 The process as claimed in claim 10 or 11 ; wherein the production-initiating fluid includes 15 to 25 mole % of heavy hydrocarbon material of the production phase solvent within the production-initiating fluid, based on the total number of moles of the production-initiating fluid.

14. A process for producing bitumen from an oil sands reservoir through a production well that is disposed in fluid communication with an injection well via an interwell region, comprising: selecting a production-initiating fluid, including steam and a production phase solvent, such that the production phase solvent is disposed, or substantially disposed, in a vapour state when supplied to the oil sands reservoir, wherein the selection is based upon information embodied in a multicomponent phase diagram for the components of the production-initiating fluid; and supplying the production-initiating fluid into the oil sands reservoir via the injection well for effecting mobilization of bitumen within the oil sands reservoir such that the mobilized bitumen is conducted through the interwell region to the production well.

15. The process as claimed in claim 14; wherein the production-initiating fluid is disposed within the vapour region of the multicomponent phase diagram at the temperature and pressure of the oil sands reservoir to which the production-initiating fluid is being supplied.

16 The process as claimed in claim 14 or 15; wherein the production phase solvent includes two or more hydrocarbons.

17. The process as claimed in any one of claims 14 to 16; wherein the supplying of the production-initiating fluid is such that the production phase solvent becomes disposed in thermal communication with bitumen within the oil sands reservoir, transfer heat to the bitumen such that condensation of at least a fraction of the production phase solvent is effected to produced condensed solvent, and such that the condensed solvent dissolves the bitumen to produce a dissolved bitumen mixture.

18 A process for producing bitumen from an oil sands reservoir through a production well that is disposed in fluid communication with an injection well via an interwell region, comprising: establishing fluid communication between an injection well and a production well via an interwell region within an oil sands reservoir, wherein the establishing includes: selecting a start-up phase fluid composition, including steam and a start-up phase solvent, such that the start-up phase solvent is disposed, or substantially disposed, in a vapour state when supplied to the oil sands reservoir, wherein the selection is based upon information embodied in a multicomponent phase diagram for the components of the start-up phase fluid; and supplying the start-up phase fluid into the oil sands reservoir via the injection well or the production well such that the start-up phase fluid becomes disposed in thermal communication with bitumen within the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir; and after the fluid communication has been established, producing bitumen from the oil sands reservoir via the production well.

19. The process as claimed in claim 18; wherein the start-up phase fluid is disposed within the vapour region of the multicomponent phase diagram at the temperature and pressure of the oil sands reservoir to which the start-up phase fluid is being supplied.

20. The process as claimed in claim 18 or 19; wherein the start-up phase solvent includes two or more hydrocarbons.

21. The process as claimed in any one of claims 18 to 20; wherein the supplying of the start-up phase fluid is such that the start-up phase solvent becomes disposed in thermal communication with bitumen within the oil sands reservoir, and such that heat is transferred to the bitumen such that condensation of at least a fraction of the start-up phase solvent is effected to produced condensed solvent, and such that the condensed solvent dissolves the bitumen to produce a dissolved bitumen mixture.

22. A process for producing bitumen from an oil sands reservoir through a production well that is disposed in fluid communication with an injection well via an interwell region, comprising: during a SAGD production phase, supplying a production-initiating fluid to the oil sands reservoir via the injection well such that mobilization of bitumen within the oil sands reservoir is effected, and such that the mobilized bitumen is conducted to the production well and produced via the production well, wherein the production-initiating fluid includes steam and solvent that includes hydrocarbon material; suspending the supplying of the production-initiating fluid to the oil sands reservoir via the injection well, wherein, prior to the suspension of the production-initiating fluid to the oil sands reservoir via the injection well, and while the production-initiating fluid is being supplied to the oil sands reservoir via the injection well, the reservoir is disposed at a pre-SAGD production phase suspension pressure; after the supplying of the production-initiating fluid to the oil sands reservoir via the injection well has been suspended, and after the pressure within the oil sands reservoir has been reduced from a pre-SAGD production phase suspension pressure, and while the oil sands reservoir is being vented via at least one of the injection well and the production well, collecting gaseous material that is being conducted via the at least one of the injection well and the production well such as at least some of the supplied solvent is recovered.

23 The process as claimed in claim 22; wherein the pressure reduction is at least a 25% pressure reduction.

24 The process as claimed in claim 22; wherein the pressure reduction is at least a 50% pressure reduction.

25. A process for producing bitumen from an oil sands reservoir through a production well that is disposed in fluid communication with an injection well via an interwell region, comprising: establishing fluid communication, through the interwell region, between the injection well and the production well, wherein the establishing fluid communication includes supplying a start-up phase fluid via the injection well or the production well such that thermal communication between the start-up phase fluid and the bitumen within the interwell region is effected, wherein the start-up phase fluid includes steam; after the fluid communication has been established, during a SAGD production phase, supplying a production-initiating fluid to the oil sands reservoir via the injection well such that mobilization of bitumen within the oil sands reservoir is effected, and such that the mobilized bitumen is conducted to the production well and produced via the production well, wherein the production- initiating fluid includes steam; wherein at least one of the start-up phase fluid and the production-initiating fluid includes solvent that includes hydrocarbon material; suspending the supplying of the production-initiating fluid to the oil sands reservoir via the injection well, wherein, prior to the suspension of the production-initiating fluid to the oil sands reservoir via the injection well, and while the production-initiating fluid is being supplied to the oil sands reservoir via the injection well, the reservoir is disposed at a pre-SAGD production phase suspension pressure; after the supplying of the production-initiating fluid to the oil sands reservoir via the injection well has been suspended, and after the pressure within the oil sands reservoir has been reduced from a pre-SAGD production phase suspension pressure, and while the oil sands reservoir is being vented via at least one of the injection well and the production well, collecting gaseous material that is being conducted via the at least one of the injection well and the production well such as at least some of the supplied solvent is recovered.

26 The process as claimed in claim 25; wherein the pressure reduction is at least a 25% pressure reduction.

27 The process as claimed in claim 26; wherein the pressure reduction is at least a 50% pressure reduction.

28 The process as claimed in any one of claims 25 to 27; wherein both of the start-up phase fluid and the production-initiating fluid includes solvent that includes hydrocarbon material.

29. A process for producing bitumen from an oil sands reservoir: establishing fluid communication between an injection well and a production well via an interwell region within an oil sands reservoir, including: supplying a start-up phase fluid into the oil sands reservoir via the injection well such that thermal communication between the start-up phase fluid and the bitumen within the interwell region is effected, wherein the start-up phase fluid includes steam and a start-up phase solvent, and after the fluid communication has been established, producing bitumen from the oil sands reservoir via the production well.

30. The process as claimed in claim 29; wherein the producing includes supplying a production-initiating fluid into the oil sands reservoir via the injection well for effecting mobilization of bitumen within the oil sands reservoir such that the mobilized bitumen is conducted through the interwell region to the production well, wherein the production-initiating fluid includes steam and a production phase solvent

31. The process as claimed in claim 30; wherein the density of the start-up phase solvent within the start-up phase fluid is greater than the density of the production phase solvent within the production-initiating fluid.

32. The process as claimed in claim 31 : wherein the density of the start-up phase solvent within the start-up phase fluid is at least 10% greater than the density of the production phase solvent within the production-initiating fluid.

33. The process as claimed in claim 31 or 32: wherein at least 70 mol % of the start-up phase solvent, of the start-up phase fluid, consists of heavy hydrocarbon material, based on the total number of moles of the start-up phase solvent being injected, wherein the heavy hydrocarbon material consists of one or more heavy hydrocarbons; and wherein less than 35 mol % of the production phase solvent, of the production-initiating fluid, consists of light hydrocarbon material, based on the total number moles of the production phase solvent being injected, wherein the light hydrocarbon material consists of one or more light hydrocarbons; and wherein the production phase solvent has a lower molar concentration of heavy hydrocarbon material than the start-up phase solvent.

34. The process as claimed in claim 30 wherein the weight average molecular weight of the start-up phase solvent within the start-up phase fluid is greater than the weight average molecular of the production phase solvent within the production-initiating fluid.

35. The process as claimed in claim 34: wherein the weight average molecular weight of the start-up phase solvent within the start-up phase fluid is at least 10% greater than the density of the production phase solvent within the production-initiating fluid.

36. The process as claimed in claim 34 or 35; wherein at least 70 mol % of the start-up phase solvent, of the start-up phase fluid, consists of heavy hydrocarbon material, based on the total number of moles of the start-up phase solvent being injected, wherein the heavy hydrocarbon material consists of one or more heavy hydrocarbons; and wherein less than 35 mol % of the production phase solvent, of the production-initiating fluid, consists of light hydrocarbon material, based on the total number moles of the production phase solvent being injected, wherein the light hydrocarbon material consists of one or more light hydrocarbons; and wherein the production phase solvent has a lower molar concentration of heavy hydrocarbon material than the start-up phase solvent.

37. The process as claimed in claim 30 wherein at least 70 mol % of the start-up phase solvent, of the start-up phase fluid, consists of heavy hydrocarbon material, based on the total number of moles of the start-up phase solvent being injected, wherein the heavy hydrocarbon material consists of one or more heavy hydrocarbons; and wherein less than 35 mol % of the production phase solvent, of the production-initiating fluid, consists of light hydrocarbon material, based on the total number moles of the production phase solvent being injected, wherein the light hydrocarbon material consists of one or more light hydrocarbons; and wherein the production phase solvent has a lower molar concentration of heavy hydrocarbon material than the start-up phase solvent.

38. The process as claimed in claim 30; wherein the ratio of moles of start-up phase solvent to steam within the start-up phase fluid is greater than the ratio of moles of production phase solvent to steam within the production initiating fluid.

39. The process as claimed in claim 30; wherein the ratio of moles of start-up phase solvent to steam within the start-up phase fluid is greater than the ratio of moles of production phase solvent to steam within the production initiating fluid by at least 10%.

40. The process as claimed in claim 30; wherein the ratio of moles of start-up phase solvent to steam within the start-up phase fluid is greater than the ratio of moles of production phase solvent to steam within the production initiating fluid by at least 25%.

41. The process as claimed in claim 30; wherein the ratio of moles of start-up phase solvent to steam within the start-up phase fluid is greater than the ratio of moles of production phase solvent to steam within the production initiating fluid by at least 50%.

42. The process as claimed in claim 29; wherein the producing includes: during a first stage of the SAGD production phase, supplying a first production-initiating fluid into the oil sands reservoir via the injection well for effecting mobilization of bitumen within the oil sands reservoir such that the mobilized bitumen is conducted through the interwell region to the production well, wherein the first production-initiating fluid includes steam and a production phase solvent consisting of one or more production phase solvent hydrocarbons; suspending the supplying of the first production initiating fluid; and after the suspending of the supplying of the first production initiating fluid, during a second stage of the SAGD production phase, supplying a second production initiating fluid is injected into the oil sands reservoir via the injection well for effecting mobilization of bitumen within the oil sands reservoir such that a second mobilized bitumen is conducted through the interwell region to the production well, wherein the second production initiating fluid includes steam and a production phase solvent consisting of one or more production phase solvent hydrocarbons; wherein the ratio of moles of start-up phase solvent to steam within the start-up phase fluid is greater than the ratio of moles of production phase solvent to steam within the first production initiating fluid, and the ratio of moles of production phase solvent to steam is greater within the first production initiating fluid relative to that within the second production initiating fluid.

43. The process as claimed in claim 42; wherein the first stage of the SAGD production phase occurs prior to the steam chamber reaching the cap rock, and the second stage of the SAGD production phase occurs after the steam chamber has reached the cap rock.

44. A process for producing bitumen from an oil sands reservoir through a production well that is disposed in fluid communication with an injection well via an interwell region, comprising: during a first stage of the SAGD production phase, supplying a first production-initiating fluid into the oil sands reservoir via the injection well for effecting mobilization of bitumen within the oil sands reservoir such that the mobilized bitumen is conducted through the interwell region to the production well, wherein the first production-initiating fluid includes steam and a production phase solvent consisting of one or more production phase solvent hydrocarbons; suspending the supplying of the first production initiating fluid; and after the suspending of the supplying of the first production initiating fluid, during a second stage of the SAGD production phase, supplying a second production initiating fluid is injected into the injection well for effecting mobilization of bitumen within the oil sands reservoir such that a second mobilized bitumen is conducted through the interwell region to the production well, wherein the second production initiating fluid includes steam and a production phase solvent consisting of one or more production phase solvent hydrocarbons; wherein the ratio of moles of start-up phase solvent to steam within the start-up phase fluid is greater than the ratio of moles of production phase solvent to steam within the first production initiating fluid, and the ratio of moles of production phase solvent to steam is greater within the first production initiating fluid relative to that within the second production initiating fluid.

45. The process as claimed in claim 44; the first stage of the SAGD production phase occurs prior to the steam chamber reaching the cap rock, and the second stage of the SAGD production phase occurs after the steam chamber has reached the cap rock.

46. A process for producing bitumen from an oil sands reservoir through a production well that is disposed in fluid communication with an injection well via an interwell region, comprising: during a first stage of the SAGD production phase, supplying a first production-initiating fluid into the oil sands reservoir via the injection well for effecting mobilization of bitumen within the oil sands reservoir such that the mobilized bitumen is conducted through the interwell region to the production well, wherein the first production-initiating fluid includes steam and a production phase solvent consisting of one or more production phase solvent hydrocarbons; suspending the supplying of the first production initiating fluid; and after the suspending of the supplying of the first production initiating fluid, during a second stage of the SAGD production phase, supplying a second production initiating fluid is injected into the injection well for effecting mobilization of bitumen within the oil sands reservoir such that a second mobilized bitumen is conducted through the interwell region to the production well, wherein the second production initiating fluid includes steam and a production phase solvent consisting of one or more production phase solvent hydrocarbons; wherein at least 70 mol % of the production phase solvent, of the first production initiating fluid, consists of heavy hydrocarbon material, based on the total number of moles of the production phase solvent being injected, and less than 35 mol % of the production phase solvent, of the second production-initiating fluid, consists of light hydrocarbon material, based on the total number moles of the production phase solvent being injected, wherein the production phase solvent of the first production-initiating fluid has a higher molar concentration of heavy hydrocarbon material than the production phase solvent of the second production-initiating fluid.