WO2015000066A1

WO2015000066A1 - Solvent addition to improve efficiency of hydrocarbon production

Info

Publication number: WO2015000066A1
Application number: PCT/CA2014/000547
Authority: WO
Inventors: Mohammed Taha AL-MURAYRI; Thomas Harding; Brij Bhooshan Maini; Javad OSKOUEI
Original assignee: Nexen Energy Ulc
Priority date: 2013-07-05
Filing date: 2014-07-07
Publication date: 2015-01-08
Also published as: CA2917263A1; CA2917263C; WO2015000065A1; CA2952864A1; CA2952864C; CN105473811A; CA2917260A1; CN105518250A

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, etc. Thus, methods to improve SAGD efficiency are sought after in the industry.

SUMMARY

[0004] In one aspect, the present invention provides 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: while injecting a drive fluid into the injection well for conducting of the drive fluid to the oil sands reservoir 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, and recovering the mobilized bitumen through the production well, wherein the drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; effecting a reduction in the ratio of the moles of production phase fluid solvent to the moles of steam within the drive fluid being injected.

[0005] In another aspect, the present invention provides 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: injecting a first drive fluid into the injection well for conducting of the injected first drive 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 to the production well, wherein the first drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; recovering the first mobilized bitumen through the production well; injecting a second drive fluid into the injection well for conducting of the injected second drive 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 to the production well, wherein the second drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; and recovering the second mobilized bitumen through the production well; wherein the ratio of moles of production phase fluid solvent to steam is greater within the first drive fluid relative to that within the second drive fluid.

[0006] In yet another aspect, the present invention provides 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: while injecting a drive fluid into the injection well for conducting of the injected drive to the oil sands reservoir 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, and recovering the mobilized bitumen through the production well, wherein the drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; effecting a reduction in the density of the production phase fluid solvent being injected.

[0007] In yet still another aspect, the present invention provides 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: while injecting a drive fluid into the injection well for conducting of the injected drive to the oil sands reservoir 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, and recovering the mobilized bitumen through the production well, wherein the drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; effecting a reduction in the weight average molecular weight of the production phase fluid solvent being injected.

[0008] In still yet another aspect, the invention provides 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: injecting a first drive fluid into the injection well for conducting of the injected first drive 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 to the production well, wherein the first drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; recovering the first mobilized bitumen through the production well; injecting a second drive fluid into the injection well for conducting of the injected second drive 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 to the production well, wherein the second drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; and recovering the second mobilized bitumen through the production well; wherein the density of the production phase fluid solvent is greater within the first drive fluid relative to that within the second drive fluid.

[0009] In yet still another aspect, the present invention provides 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: injecting a first drive fluid into the injection well for conducting of the injected first drive 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 to the production well, wherein the first drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; recovering the first mobilized bitumen through the production well; injecting a second drive fluid into the injection well for conducting of the injected second drive 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 to the production well, wherein the second drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; and recovering the second mobilized bitumen through the production well; wherein the weight average molecular weight of the production phase fluid solvent is greater within the first drive fluid relative to that within the second drive fluid.

[0010] In yet still another aspect, the present invention provides 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: injecting a first drive fluid into the injection well for conducting of the injected first drive 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 to the production well, wherein the first drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; recovering the first mobilized bitumen through the production well; injecting a second drive fluid into the injection well for conducting of the injected second drive 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 to the production well, wherein the second drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; and recovering the second mobilized bitumen through the production well; wherein at least 70 mol % of the production phase fluid solvent, of the first drive fluid, consists of heavy hydrocarbon material, based on the total number of moles of the injected production phase solvent, wherein the heavy hydrocarbon material consists of one or more heavy hydrocarbons; and wherein at least 70 mol % of the production phase fluid solvent, of the second drive fluid, consists of light hydrocarbon material, based on the total number of moles of the injected production phase solvent, wherein the light hydrocarbon material consists of one or more light hydrocarbons.

[0011] In yet still another aspect, the present invention provides a process for establishing fluid communication between an injection well and a production well within an oil sands reservoir, comprising: supplying, to an interwell region disposed between the injection well and the production well, a mobilization-initating fluid, the mobilization-initating fluid including steam and a start-up phase fluid solvent, wherein the start-up phase fluid solvent consists of one or more start-up phase solvent hydrocarbons.

[0012] In yet still another aspect, the present invention provides a process for producing bitumen from an oil sands reservoir, comprising: establishing fluid communication between an injection well and a production well by supplying, to an interwell region disposed between the injection well and the production well, steam and a start-up phase fluid solvent, wherein the start-up phase fluid solvent consists of one or more start-up phase solvent hydrocarbons; after the fluid communication between the injection well and the production well has been established, injecting a drive fluid to the injection well for conducting of the injected mobilization-initiating fluid to the oil sands formation for effecting mobilization of bitumen within the oil sands formation such that the mobilized bitumen is conducted through the interwell region to the production well; and recovering the mobilized bitumen through the production well.

[0013] In yet still another aspect, the present invention provides 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 injecting a mobilization-initiating fluid into the injection well for conducting of the injected mobilization-initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir, wherein the mobilization-initiating fluid includes steam and a start-up phase fluid solvent, the start-up phase fluid solvent consisting of one or more start-up phase solvent hydrocarbons; after the fluid communication has been established, injecting a drive fluid into the injection well for conducting of the injected drive 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 to the production well, wherein the drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; and recovering the mobilized bitumen through the production well; wherein the density of the start-up phase fluid solvent within the mobilization-initiating fluid is greater than the density of the production phase fluid solvent within the drive fluid.

[0014] In yet still another aspect, the present invention provides 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 injecting a mobilization-initiating fluid into the injection well for conducting of the injected mobilization-initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir, wherein the mobilization-initiating fluid includes steam and a start-up phase fluid solvent, the start-up phase fluid solvent consisting of one or more start-up phase solvent hydrocarbons; after the fluid communication has been established, injecting a drive fluid into the injection well for conducting of the injected drive 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 to the production well, wherein the drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; and recovering the mobilized bitumen through the production well; wherein the weight average molecular weight of the start-up phase fluid solvent within the mobilization-initiating fluid is greater than the weight average molecular of the production phase fluid solvent within the drive fluid.

[0015] In yet still another aspect, the present invention provides 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 injecting a mobilization-initiating fluid into the injection well for conducting of the injected mobilization-initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir, wherein the mobilization-initiating fluid includes steam and a start-up phase fluid solvent, the start-up phase fluid solvent consisting of one or more start-up phase solvent hydrocarbons; after the fluid communication has been established, injecting a drive fluid into the injection well for conducting of the injected drive 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 to the production well, wherein the drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; and recovering the mobilized bitumen through the production well; wherein at least 70 mol % of the start-up phase fluid solvent, of the mobilization-initiating fluid, consists of heavy hydrocarbon material, based on the total number of moles of the start-up phase fluid solvent being injected, wherein the heavy hydrocarbon material consists of one or more heavy hydrocarbons, and wherein at least 70 mol % of the production phase fluid solvent, of the drive fluid, consists of light hydrocarbon material, based on the total number of moles of the production phase fluid solvent being injected, wherein the light hydrocarbon material consists of one or more light hydrocarbons.

BRIEF DESCRIPTION OF DRAWINGS

[0016] The preferred embodiments will now be described with the following accompanying drawings, in which:

[0017] 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 steam-assisted gravity drainage process;

[0018] Figure 2 shows a graph of cumulative oil production versus cumulative injected steam when various concentrations of cracked naphtha are injected into a model system according to one embodiment;

[0019] Figure 3 shows a graph of oil drainage rate versus time when various concentrations of cracked naphtha are injected into a model system according to one embodiment;

[0020] Figures 4 and 5 illustrate data upon which to determine the amount of solvent that can be used for co-injection with steam, while keeping the solvent in the vapour phase;

[0021] Figure 6 shows a graph of cumulative oil production versus cumulative injected steam when various concentrations of gas condensate are injected into a model system according to one embodiment; and

[0022] Figure 7 shows a graph of oil drainage rate versus time when various concentrations of gas condensate are injected into a model system according to one embodiment.

DETAILED DESCRIPTION

[0023] 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.

[0024] 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.

[0025] In a steam assisted gravity drainage ("SAGD") operation, the wells 104, 106 are vertically spaced from one another, such that the well 104 is vertically higher than the well 106. Being vertically higher, the well 104 functions, during the production phase a SAGD operation, to inject a drive 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, resulting in gravity drainage of the mobilized reservoir hydrocarbons to the well 106. 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 drained by gravity to the well 106) 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.

[0026] The production fluid may subsequently be conducted to a processing facility 1 10. At the processing facility 1 10, 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 1 10, with or without further treatment, and used to generate the steam used for supply to the well 104.

[0027] 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 drive fluid 1 16 (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. This results in the creation of a fluid passage, including a steam chamber, between the wells 104, 106 as the locally entrained reservoir hydrocarbons escape the interwell region 108. In some embodiments, for example, the heat is supplied to the interwell region 108 by circulating a mobilization- initiating fluid 1 18 (such as steam, or a fluid including steam) through one or both of the wells 104, 106.

[0028] The production phase includes ramp-up. plateau and wind-down. During ramp-up, bitumen production rates are still increasing. During plateau, the rate have peaked. During wind-down, the rates are declining.

Solvent Addition During the Production Phase of a SAGD Operation

[0029] During the production phase of the SAGD operation, in parallel with the injection of steam into the reservoir 102, production phase fluid solvent may also be injected into the reservoir 102. In some embodiments, for example, a drive fluid 1 16 may be injected into the reservoir 102, the drive fluid including a mixture of steam and the production phase fluid solvent. In this respect, in some embodiments, for example, the production phase fluid solvent is co-injected with the steam through the injection well 104. [0030] By injecting production phase fluid solvent, and thereby supplementing the injected steam, mobilization of the reservoir hydrocarbons, and their drainage to the production well 106, is accelerated.

[0031] Once disposed in the formation, 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 fluid 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 the reservoir hydrocarbons. The condensed production phase fluid solvent dissolves into the reservoir hydrocarbons and, in this respect, in conjunction with the heat received from the steam, decreases the viscosity, and thereby increases the mobility of the reservoir hydrocarbons. As the reservoir hydrocarbons drain, a new interface emerges for interaction with the steam and the production phase fluid solvent. In this respect, with the supplementary production phase fluid 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.

[0032] The use of production phase fluid 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 04, 106. This is because the provision of the production phase fluid 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.

[0033] The production phase fluid 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.

[0034] 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.

[0035] In some embodiments, for example, the production phase solvent hydrocarbons are selected such that they are in substantially a vapour state at the conditions within the steam chamber.

[0036] In some embodiments, for example, the drive fluid may include between 0.1 and 30 mol % of production phase fluid solvent, based on the total moles of the drive fluid. The total amount of production phase fluid solvent used is based on oil viscosity at initial conditions, operating pressure, the formation permeability and the composition of the production phase fluid solvent.

[0037] The ratio of the vapor pressure of the production phase fluid solvent at steam temperature to the total pressure of the system determines the maximum amount of the production phase fluid 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 fluid solvent that should be used, as using additional production phase fluid solvent will not result in additional benefits. Viewed another way, once the bitumen becomes saturated with production phase fluid solvent, there are only small incremental improvements that may come from injecting additional production phase fluid solvent into the reservoir.

[0038] The production phase fluid 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 useful to use production phase fluid solvent which is an on-site diluent as this can reduce blending requirements for facilitating transport, by pipeline, to a refinery.

[0039] The production phase fluid solvent can be a single or multi-component fluid. Multi- component production phase fluid solvent allow for operational flexibility, as pressure changes can more easily be absorbed compared to single-component solvents. 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. In this context, a heavy hydrocarbon is a hydrocarbon having a total number of carbon atoms of five or more, and a light hydrocarbon is a hydrocarbon having a total number of carbons of four or less. Exemplary hydrocarbons include aromatics, xylene, hexane, gasoline, kersosene, naphtha, gas condensates, diesel, benzene, toluene, distallates, butane, methane, pentane.

[0040] An example of a multi-component production phase fluid 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. Typically, cracked naphtha is high in olefins.

[0041] Another suitable multi-component production phase fluid 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] In some embodiments, for example, it may be suitable to reduce the amount of production phase fluid solvent being injected during latter stages of the production phase of a SAGD operation.

[0043] In this respect, in some embodiments, for example, while injecting the drive fluid into the injection well 104 for conducting of the drive fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir such that the mobilized bitumen is conducted through the interwell region 108 to the production well 106, and recovering the mobilized bitumen through the production well 106, wherein the drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons, the process includes effecting a reduction in the ratio of the moles of production phase fluid solvent to the moles of steam within the drive fluid being injected. In some embodiments, for example, the effected reduction is at least a 10% reduction. In some embodiments, for example, the effected reduction is at least a 20% reduction. In some embodiments, for example, the effected reduction is at least a 30% reduction.

[0044] In another 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. The process includes injecting a first drive fluid into the injection well 104 for conducting of the injected drive 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 drive fluid includes steam and a production phase fluid solvent, the production phase fluid 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 drive fluid is suspended, and a second drive fluid is injected into the injection well 104 for conducting of the injected drive 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 drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons. The second mobilized bitumen is recovered through the production well 106. The ratio of moles of production phase fluid solvent to steam is greater within the first drive fluid relative to that within the second drive fluid. In some embodiments, for example, the ratio of moles of production phase fluid solvent to steam is at least 10% greater within the first drive fluid relative to the ratio of moles of production phase fluid solvent to steam within the second drive fluid. In some embodiments, for example, the ratio of moles of production phase fluid solvent to steam is at least 20% greater within the first drive fluid relative to the ratio of moles of production phase fluid solvent to steam within the second drive fluid. In some embodiments, for example, the ratio of moles of production phase fluid solvent to steam is at least 30% greater within the first drive fluid relative to the ratio of moles of production phase fluid solvent to steam within the second drive fluid.

[0045] Solvent recovery can be increased significantly if the rate of solvent injection is reduced in later stages of the production phase of the SAGD operation, and, in some embodiments, later stopped at some time prior to the end of the production life of a SAGD well pair. As the steam chamber matures, the oil becomes mobile enough by heating, As well, by the later stages of the production phase, the injected solvent tends to rise rather than move outwardly. The combined effect reduces the relative usefulness of solvent co-injection to further increase oil mobility. Tapering down, or reducing, the concentration of the co-injected solvent as the SAGD process matures improves solvent recovery while maintaining a favorable level of energy efficiency and hydrocarbon production rate. The concentration of the co-injected solvent is preferably reduced as the instantaneous steam-to-oil ratio (SOR) increases until no more solvent is co-injected, which, in some embodiments, marks the point at which non-condensable gas co- injection with steam can be initiated. In some embodiments, for example, the drive fluid includes up to 5 mol % (such as, for example from about 1 mol % to about 5 mol %, such as, for example, from about 2 mol % to about 4 mol %, such as, for example, about 3 mol %) of non-condensable gas, based on the total number of moles of drive fluid that is being injected.

[0046] In some embodiments, for example, during the blow-down phase, only non-condensable gas is injected, then cold water is circulated and well pair is shut in in order to maintain the pressure within the formation so that adjacent well pairs are not affected by a pressure sink.

[0047] "Non-Condensable Gas" refers to gases which do not condense into a liquid phase under the operating conditions of the hydrocarbon recovery process. Examples include hydrogen, nitrogen, helium, oxygen, air, methane, ethane, propane, butane, carbon dioxide, carbon monoxide, combustion gases, flue gases, or any combination thereof.

[0048] Non-condensable gas injection improves the energy efficiency of the SAGD process due to the accumulation of non-condensable gas underneath the overburden, thereby reducing heat loss from the steam chamber. Non-condensable gas injection can also improve the drainage of mobile hydrocarbons into the SAGD production well 106 and can help maintain pressure within the hydrocarbon- depleted zone while reducing steam consumption. Movement of non-condensable gas ahead of the steam can reduce water mobility in high water saturation zones due to three phase relative permeability effects.

[0049] In some embodiments, for example, the ratio of non-condensable gas to steam can increase during later stages of the production phase of SAGD production. During the wind-down phase of the SAGD operation, the rate of injection of steam into the oil reservoir is decreased, and, in parallel, the rate of injection of non-condensable gas is increased, so as to maintain constant or substantially constant pressure within the reservoir, and thereby enable satisfactory production rates of bitumen.

[0050] In some embodiments, for example, the injection of the non-compressible gas is configured such that, after completion of the SAGD operation, the injected non-compressible gas remains within the oil sands reservoir such that the oil sands reservoir is disposed in a pressurized state. By creating conditions such that the oil sands reservoir is disposed in a pressurized state after the completion of the SAGD operation, the creation of a pressure sink, that could attract steam and fluid from a neighboring SAGD operation, or other undesirable ingress of material resulting from an adverse geological event, is mitigated. [0051] In some embodiments, after completion of the SAGD operation, and while at least a fraction of the remaining non-compressible gas remains is being conducted to the surface (for example, for the purpose of recover thermal energy of the non-compressible gas, such as by using such thermal energy to generate steam), cold water is injected into the oil sands reservoir to compensate for the transfer of the non-compressible gas from the oil sands reservoir (and thereby mitigate any loss of pressure).

[0052] In some embodiments, for example, it may be suitable to transition from a heavier production phase fluid solvent to a lighter production phase fluid solvent at some point in time during the production phase of a SAGD operation.

[0053] In some embodiments, for example, at some point in time during the production phase of a SAGD operation, a heavier production phase fluid solvent may, relative to a lighter production phase fluid 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 fluid solvent, in order to improve efficiencies in mobilizing bitumen within the oil sands reservoir.

[0054] Also, in some embodiments, for example, it may be suitable, during later stages of the production phase of a SAGD operation, to increase the pressure of the drive fluid being injected into the oil sands reservoir, so as to better enable penetration of the drive fluid into the oil sands reservoir, and the transition to a lighter production phase fluid solvent dovetails with this increase in pressure, as a heavier production phase fluid solvent would have a greater tendency to condense within the oil sands reservoir and drain to the production well 106 without interacting with bitumen at the outer edges of the steam chamber.

[0055] In this respect, in some embodiments, for example, while injecting a drive fluid into the injection well 104 for conducting of the injected drive to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir such that the mobilized bitumen is conducted through the interweli region 108 to the production well 106, and recovering the mobilized bitumen through the production well 106, wherein the drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons, the process further includes effecting a reduction in the density of the production phase fluid solvent being injected. In some embodiments, for example, the effected reduction is at least a 10% reduction, such as, for example, at least a 20% reduction, such as, for example, at least a 30% reduction. In some embodiments, for example, prior to the effected reduction, at least 70 mol % (such as, for example, at least 80 mol %, such as, for example, at least 90 mol%) of the production phase fluid solvent consists of heavy hydrocarbon material, based on the total number of moles of the production phase fluid solvent being injected, and after the effected reduction, at least 70 mol % (such as, for example, at least 80 mol %, such as, for example, at least 90 mol %) of the production phase fluid solvent consists of light hydrocarbon material, based on the total number moles of the production phase fluid solvent being injected. The heavy hydrocarbon material consists of one or more heavy hydrocarbons, and the light hydrocarbon material consists of one or more light hydrocarbons. In some embodiments, for example, the pressure of the drive fluid during a time interval after the effected reduction is greater than the pressure of the drive fluid during a time interval prior to the reduction. In some embodiments, for example, the pressure of the drive fluid during a time interval after the effected reduction is greater than the pressure of the drive fluid during a time interval prior to the reduction by at least 10%, such as, for example, at least 20%, such as, for example, at least 30%.

[0056] 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, while injecting a drive fluid into the injection well 104 for conducting of the injected drive to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir such that the mobilized bitumen is conducted through the interwell region 108 to the production well 106, and recovering the mobilized bitumen through the production well 106, wherein the drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons, effecting a reduction in the weight average molecular weight of the production phase fluid solvent being injected. In some embodiments, for example, the effected reduction is at least a 10% reduction (such as, for example, at least a 20% reduction, such as, for example, at least a 30% reduction). In some embodiments, for example, prior to the effected reduction, at least 70 mol % (such as, for example, at least 80 mol %, such as, for example, at least 90 mol %) of the production phase fluid solvent consists of heavy hydrocarbon material, based on the total number of moles of the production phase fluid solvent being injected, and after the effected reduction, at least 70 mol % (such as, for example, at least 80 mol %, such as, for example, at least 90 mol%) of the production phase fluid solvent consists of light hydrocarbon material, based on the total number moles of the production phase fluid solvent being injected. The heavy hydrocarbon material consists of one or more heavy hydrocarbons, and the light hydrocarbon material consists of one or more light hydrocarbons. In some embodiments, for example, the pressure of the drive fluid during a time interval after the effected reduction is greater than the pressure of the drive fluid during a time interval prior to the reduction. In some embodiments, for example, the pressure of the drive fluid during a time interval after the effected reduction is greater than the pressure of the drive fluid during a time interval prior to the reduction by at least 10%, such as, for example, at least 20%, such as, for example, at least 30%.

[0057] 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 injecting a first drive fluid into the injection well 104 for conducting of the injected first drive 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 drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons. The first mobilized bitumen is recovered through the production well 106. The injecting of the first drive fluid is suspended, and a second drive fluid is then injected into the injection well 104 for conducting of the injected second drive 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 drive fluid includes steam and a production phase fluid solvent, the production phase fluid 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 fluid solvent is greater within the first drive fluid relative to that within the second drive fluid. In some embodiments, for example, the density of the production phase fluid solvent is at least 10% (such as, for example, at least 20%, such as, for example, at least 30%) greater within the first drive fluid relative to , the density of the production phase fluid solvent within the second drive 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 fluid solvent, of the first drive fluid, consists of heavy hydrocarbon material, based on the total number of moles of the production phase fluid solvent being injected, and at least 70 mol % (such as, for example, at least 80 mol %, such as, for example, at least 90 mol %) of the production phase fluid solvent, of the second drive fluid, consists of light hydrocarbon material, based on the total number moles of the production phase fluid solvent being injected. The heavy hydrocarbon material consists of one or more heavy hydrocarbons, and the light hydrocarbon material consists of one or more light hydrocarbons. In some embodiments, for example, the pressure of the second drive fluid is greater than the pressure of the first drive 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%

[0058] 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 injecting a first drive fluid into the injection well 104 for conducting of the injected first drive 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 drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons. The first mobilized bitumen is recovered through the production well 106. The injecting of the first drive fluid is suspended, and a second drive fluid is then injected into the injection well 104 for conducting of the injected second drive 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 drive fluid includes steam and a production phase fluid solvent, the production phase fluid 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 fluid solvent is greater within the first drive fluid relative to that within the second drive fluid. In some embodiments, for example, the weight average molecular weight of the production phase fluid solvent is at least 10% (such as, for example, at least 20%, such as, for example, at least 30%) greater within the first drive fluid relative to the weight average molecular weight of the production phase fluid solvent within the second drive 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 fluid solvent, of the first drive fluid, consists of heavy hydrocarbon material, based on the total number of moles of the production phase fluid solvent being injected, and at least 70 mol % (such as, for example, at least 80 mol %, such as, for example, at least 90 mol %) of the production phase fluid solvent, of the second drive fluid, consists of light hydrocarbon material, based on the total number moles of the production phase fluid solvent being injected. The heavy hydrocarbon material consists of one or more heavy hydrocarbons, and the light hydrocarbon material consists of one or more light hydrocarbons. In some embodiments, for example, the pressure of the second drive fluid is greater than the pressure of the first drive 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%.

[0059] 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 injecting a first drive fluid into the injection well 104 for conducting of the injected first drive 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 drive fluid includes steam and a production phase fluid solvent, and the production phase fluid solvent consists of one or more production phase solvent hydrocarbons. The first mobilized bitumen is recovered through the production well 106. Injecting of the first drive fluid is suspended, and a second drive fluid is injected into the injection well 104 for conducting of the injected second drive 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 drive fluid includes steam and a production phase fluid solvent. The production phase fluid 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, 80 mol %, such as, for example, 90 mol %) of the production phase fluid solvent, of the first drive fluid, consists of heavy hydrocarbon material, based on the total number of moles of the injected production phase solvent, wherein the heavy hydrocarbon material consists of one or more heavy hydrocarbons. At least 70 mol %(such as, for example, 80 mol %, such as, for example, 90 mol %) of the production phase fluid solvent, of the second drive fluid, consists of light hydrocarbon material, based on the total number of moles of the injected production phase solvent, wherein the light hydrocarbon material consists of one or more light hydrocarbons. In some embodiments, for example, the pressure of the second drive fluid is greater than the pressure of the first drive fluid. In some of these embodiments, for example, the pressure of the second drive fluid is greater than the pressure of the first drive fluid by at least 10%.

Solvent Addition During Start-Up Phase of SAGD Operation

[0060] In some embodiments, for example, the addition during the start-up phase of a SAGD operation (i.e. prior to establishment, or substantial establishment, of inter-well fluid communication) accelerates the mobilization of bitumen in the inter-well region, and promotes the rapid formation of a steam chamber. The ability to establish good inter-well 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.

[0061] In this respect, in some embodiments, for example, during the start-up phase of a SAGD operation, a mobilization-initiating fluid 1 18 is supplied to the interwell region 108 disposed between the injection well 104 and the production well 106. The mobilization-initiating fluid includes steam and a startup phase fluid solvent. The start-up phase fluid solvent consists of one or more start-up phase solvent hydrocarbons. In this respect, in some embodiments, for example, the start-up phase fluid solvent is co- injected with the steam.

[0062] As mentioned above, the start-up phase fluid 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.

[0063] The function of the start-up phase solvent hydrocarbons includes, amongst other things, dissolving into the reservoir hydrocarbons, and effect a reduction in viscosity of the reservoir hydrocarbons. [0064] In some embodiments, for example, the mobilization-initiating fluid may include between 0.1 and 30 mol % of the start-up phase fluid solvent, based on the total moles of the mobilization-initiating fluid. The total amount of start-up phase fluid solvent used is based on oil viscosity at initial conditions, operating pressure, the formation permeability and the composition of the start-up phase fluid solvent.

[0065] The start-up phase fluid 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 useful to use start-up phase fluid solvent which is an on-site diluent as this can reduce blending requirements for facilitating transport, by pipeline, to a refinery.

[0066] The start-up phase fluid solvent can be a single or multi-component fluid. Multi-component production phase fluid solvent allows for operational flexibility, as pressure changes can more easily be absorbed compared to single-component solvents. 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. In this context, a heavy hydrocarbon is a hydrocarbon having a total number of carbon atoms of five or more, and a light hydrocarbon is a hydrocarbon having a total number of carbons of four or less. Exemplary hydrocarbons include aromatics, xylene, hexane, gasoline, kersosene, naphtha, gas condensates, diesel, benzene, toluene, distallates, butane, methane, pentane.

[0067] An example of a multi-component start-up phase fluid 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. Typically, cracked naphtha is high in olefins.

[0068] Another suitable multi-component start-up phase fluid 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.

[0069] Some of the benefits of injection of a mixture of steam and the start-up phase fluid 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 such as cracked naphtha and 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.

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

[0070] 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.

[0071] 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, once 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.

[0072] 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.

[0073] 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 mobilization-initiating fluid 118 into the injection well 104 for conducting of the injected first drive fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir. The mobilization-initiating fluid includes steam and a start-up phase fluid solvent, the start-up phase fluid solvent consisting of one or more start-up phase solvent hydrocarbons. After the fluid communication has been established, a drive fluid 116 is injected into the injection well 104 for conducting of the injected drive 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 drive fluid includes steam and a production phase fluid solvent, the production phase fluid 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 fluid solvent within the mobilization-initiating fluid is greater than the density of the production phase fluid solvent within the drive fluid. In some embodiments, for example, the density of the start-up phase fluid solvent within the mobilization-initiating fluid is at least 10% greater than the density of the production phase fluid solvent within the drive fluid. In some embodiments, for example, the density of the start-up phase fluid solvent within the mobilization-initiating fluid is at least 20% greater than the density of the production phase fluid solvent within the drive fluid. In some embodiments, for example, the density of the start-up phase fluid solvent within the mobilization-initiating fluid is at least 30% greater than the density of the production phase fluid solvent within the drive 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 fluid solvent, of the mobilization-initiating fluid, consists of heavy hydrocarbon material, based on the total number of moles of the start-up phase fluid solvent being injected, and at least 70 mol % (such as, for example, at least 80 mol %, such as, for example, at least 90 mol %) of the production phase fluid solvent, of the drive fluid, consists of light hydrocarbon material, based on the total number moles of the production phase fluid solvent being injected. In this context, the heavy hydrocarbon material consists of one or more heavy hydrocarbons, and the light hydrocarbon material consists of one or more light hydrocarbons. In some embodiments, for example, the pressure of the drive fluid is greater than the pressure of the mobilization-initiating fluid. In some of these embodiments, for example, the pressure of the drive fluid is greater than the pressure of the mobilization-initiating fluid by at least 10%, such as, for example, at least 20%, such as, for example, at least 30%.

[0074] 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 mobilization-initiating fluid into the injection well 104 for conducting of the injected mobilization-initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir. The mobilization-initiating fluid includes steam and a start-up phase fluid solvent, the start-up phase fluid solvent consisting of one or more start-up phase solvent hydrocarbons. After the fluid communication has been established, a drive fluid is injected into the injection well 104 for conducting of the injected drive 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 drive fluid includes steam and a production phase fluid solvent, the production phase fluid 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 startup phase fluid solvent within the mobilization-initiating fluid is greater than the weight average molecular of the production phase fluid solvent within the drive fluid. In some embodiments, for example, the weight average molecular weight of the start-up phase fluid solvent within the mobilization-initiating fluid is at least 10% greater than the weight average molecular weight of the production phase fluid solvent within the drive fluid. In some embodiments, for example, the weight average molecular weight of the start-up phase fluid solvent within the mobilization-initiating fluid is at least 20% greater than the weight average molecular weight of the production phase fluid solvent within the drive fluid. In some embodiments, for example, the weight average molecular weight of the start-up phase fluid solvent within the mobilization- initiating fluid is at least 30% greater than the weight average molecular weight of the production phase fluid solvent within the drive 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 fluid solvent, of the mobilization-initiating fluid, consists of heavy hydrocarbon material, based on the total number of moles of the start-up phase fluid solvent being injected, and at least 70 mol % (such as, for example, at least 80 mol %, such as, for example, at least 90 mol %) of the production phase fluid solvent, of the drive fluid, consists of light hydrocarbon material, based on the total number moles of the production phase fluid solvent being injected. In this context, the heavy hydrocarbon material consists of one or more heavy hydrocarbons, and the light hydrocarbon material consists of one or more light hydrocarbons. In some embodiments, for example, the pressure of the drive fluid is greater than the pressure of the mobilization-initiating fluid. In some of these embodiments, for example, the pressure of the drive fluid is greater than the pressure of the mobilization-initiating fluid by at least 10%, such as, for example, at least 20%, such as, for example, at least 30%.

[0075] 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, wherein the establishing fluid communication includes injecting a mobilization-initiating fluid into the injection well 104 for conducting of the injected mobilization-initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir. The mobilization-initiating fluid includes steam and a start-up phase fluid solvent. The start-up phase fluid solvent consisting of one or more start-up phase solvent hydrocarbons. After the fluid communication has been established, a drive fluid is injected into the injection well 104 for conducting of the injected drive 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 drive fluid includes steam and a production phase fluid solvent. The production phase fluid 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 fluid solvent, of the mobilization- initiating fluid, consists of heavy hydrocarbon material, based on the total number of moles of the start-up phase fluid solvent being injected, wherein the heavy hydrocarbon material consists of one or more heavy hydrocarbons. At least 70 mol % (such as, for example, at least 80 mol %, such as, for example, at least 90 mol %) of the production phase fluid solvent, of the drive fluid, consists of light hydrocarbon material, based on the total number of moles of the production phase fluid solvent being injected, wherein the light hydrocarbon material consists of one or more light hydrocarbons. In some embodiments, for example, the pressure of the drive fluid is greater than the pressure of the mobilization-initiating fluid. In some of these embodiments, for example, the pressure of the drive fluid is greater than the pressure of the mobilization-initiating fluid by at least 10%.

[0076] The below examples illustrate some of the inventive features of the invention but are not intended to limit the scope of the invention.

[0077] Example 1: Solvent Co-injection with Steam using Cracked Naphtha

[0078] A series of high pressure experiments was conducted using a highly instrumented physical model to accurately measure pressure, temperature and fluid flow rates using oil samples from the field and typical SAGD operating conditions. Upon completion of each experiment, extensive produced fluid and porous media analyses were conducted to evaluate the impact of solvent injection on SAGD performance. The lab results demonstrated that co-injecting solvent with steam increases oil production rates and reduces SOR.

[0079] Experiments 3, 4 and 5 were conducted to investigate the impact of co-injecting different amounts of cracked naphtha with steam on SAGD performance. Cracked naphtha is a multi-component solvent that is readily available as a product of the Nexen Long Lake upgrader.

[0080] Experiment 2 refers to an experiment set-up where only steam was injected using the SAGD injection well to establish a SAGD baseline that could be compared with subsequent SCIS experiments.

[0081] Experiment 3 refers to the experiment set-up where 10 vol% cracked naphtha combined with steam was used to enhance SAGD performance.

[0082] Experiment 4 refers to the experiment set-up where 15 vol% cracked naphtha combined with steam is used to enhance SAGD performance.

[0083] Experiment 5 refers to the experiment set-up where 5 vol% cracked naphtha combined with steam is used to enhance SAGD performance.

[0084] All of Experiments 2 to 5 were commenced by injecting steam and cracked naphtha into the injector well in the model set-up. The cracked naphtha concentration (per total steam and solvent volume on a cold liquid equivalent basis) in the injected steam-solvent mixture ranges from 5-15% volume. Low injection rates were used initially then steam-solvent injection was increased. The steam-solvent injection pressure was fixed at 2,100 kPa and the steam and solvent injection rates were maintained such that the pressure difference between the injection and production wells remained at a reasonable level throughout the life of the process. The injectant was pre-heated to saturated steam temperatures before passing through the steam generator to be further heated to superheated steam conditions.

[0085] The cracked naphtha used in Experiments 2 to 5 had the following composition:

[0086] The produced fluids from Experiments 2, 3, 4 and 5 were analyzed extensively to evaluate the impact of injecting different amounts of cracked naphtha on SAGD performance.

[0087] Figure 2 shows cumulative produced oil when various concentrations of cracked naphtha are injected with steam using a model experimental set-up. Oil production rates peak earlier, and are consistently higher when cracked naphtha is injected along with steam during the start-up phase of the SAGD operation. These results indicate that solvent injection during the start-up phase of a SAGD operation allows for more rapid establishment of oil production rates, and that establishing good communication early allows for overall better performance of a SAGD operation.

[0088] As shown in Figure 3, co-injecting cracked naphtha at the early stages of the SAGD process can potentially accelerate the start-up phase even when only 5 vol% of cracked naphtha is used. The SAGD wells are normally switched from a circulation mode of operation to a SAGD mode of operation when the viscosity in the inter-well region is between 600-1200 cp, and this can be achieved sooner by co-injecting cracked naphtha with steam due to the synergy of heat and mass transfer processes. The experiments also showed that the injection of cracked naphtha with steam allows more oil to be drained using lower amounts of steam. The best performance was achieved using 10 vol% of cracked naphtha. [0089] Figure 3 also shows that the impact of solvent addition is more pronounced in the beginning of the drainage process. In fact, the slopes of the cumulative produced oil versus cumulative injected steam were nearly the same after 4000 ml of steam injection. Therefore, solvent injection is most effective when initiated early in the process. Figure 2 shows cumulative oil produced versus cumulative injected steam. The results also suggested that co-injecting cracked naphtha can extend the economic window of a SAGD operation, by allowing oil drainage to continue at lower steam oil ratios, particularly in the presence steam thief zones such as top water underneath the reservoir overburden. The energy efficiency of the baseline SAGD and cracked naphtha SCIS cases deteriorated with time.

[0090] Figures 4 and 5 demonstrate how to determine the amount of solvent to use while keeping the solvent in the vapour phase.

[0091] The experiments showed that cumulative steam oil ratios (cSOR) when cracked naphtha was used were lower compared to experiments where no cracked naphtha was added to the steam. Even with only 5 vol% of cracked naphtha, SCIS had a lower cSOR than SAGD without cracked naphtha addition.

[0092] Example 2: Solvent Co-injection with Steam using Gas Condensate

[0093] The same experimental set-up as used for the cracked naphtha experiments was used to test the effects of gas condensate added with steam on SAGD operation. Gas condensate is a multicomponent solvent that is often used to blend produced bitumen to make it suitable for pipeline transportation.

[0094] Experiment 6 refers to the experiment set-up where 5 vol% gas condensate combined with steam was used to enhance SAGD performance.

[0095] Experiment 7 refers to the experiment set-up where 10 vol% gas condensate combined with steam was used to enhance SAGD performance.

[0096] Experiment 8 refers to the experiment set-up where 15 vol% gas condensate combined with steam was used to enhance SAGD performance.

[0097] Water and gas condensate were mixed before passing through the pre-heater and steam generator toward the heel of the SAGD injection well. The steam and gas condensate mixture were co- injected into the reservoir using the SAGD injection well. Steam and solvent injection was increased gradually.

[0098] Gas condensate concentration in the steam-solvent injection ranged from 5-15 vol% of the liquid stream. The produced fluids from Experiments 6, 7 and 8 were analyzed extensively to evaluate the impact of injecting different amounts of gas condensate on SAGD performance. [0099] Figure 6 shows oil production rate when various concentrations of gas condensate are added during the start-up phase of a SAGD operation as modelled in an experimental set-up. These results indicate that solvent injection during the start-up phase of a SAGD operation allows for more rapid establishment of oil production rates, and that establishing good communication early allows for overall better performance of a SAGD operation.

[00100] The injection of gas condensate with steam accelerates the reduction of bitumen viscosity and facilitates inter-well communication between the SAGD wells. Figure 7 shows that that co-injecting gas condensate with steam accelerated the start-up phase of the SAGD process even when only 5 vol% of gas condensate was used. The experiments showed that using the same amount of steam, Experiment 8 (15 % volume gas condensate) produced more oil relative to baseline SAGD and Experiments 6 (5% volume gas condensate) and 7 (10% volume gas condensate). As with the experiments using cracked naphtha, the results show that co-injecting gas condensate can extend the economic window for SAGD, by allowing oil drainage to continue at lower steam oil ratios.

[00101] The energy efficiency of SAGD and gas condensate SCIS cases deteriorated with time. Cumulative steam oil ratios (cSOR) for all experiments in which gas condensate was added were lower than SAGD operations having no gas condensate addition. cSOR decreased progressively as the amount of co-injected gas condensate increased. The lowest cSOR was achieved in where 15 vol% of gas condensate was used.

[00102] The results of this study show that the injection of cracked naphtha and gas condensate with steam can enhance SAGD performance. Using the same amount of steam, SAGD with cracked naphtha or gas condensate can increase oil recovery and reduce cumulative steam oil ratio significantly relative to SAGD. This can reduce greenhouse gas emissions while making SAGD more energy-efficient and cost- effective.

[00103] 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; while injecting a drive fluid into the injection well for conducting of the drive fluid to the oil sands reservoir 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, and recovering the mobilized bitumen through the production well, wherein the drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; effecting a reduction in the ratio of the moles of production phase fluid solvent to the moles of steam within the drive fluid being injected.

2. The process as claimed in claim 1 : wherein the reduction is at least a 10% reduction.

3 The process as claimed in claim 1 or 2, wherein the reduction is such that, after the effected reduction, supply of the production phase fluid solvent to the oil sands reservoir is suspended, or substantially suspended, and further comprising: after the suspension of the supplying of the production phase fluid solvent to the oil sands reservoir, injecting a later stage drive fluid mixture to the oil sands reservoir 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, and recovering the mobilized bitumen through the production well, wherein the later stage drive fluid mixture includes steam and a non-condensable gas.

4.. 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: injecting a first drive fluid into the injection well for conducting of the injected first drive 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 to the production well, wherein the first drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; recovering the first mobilized bitumen through the production well; injecting a second drive fluid into the injection well for conducting of the injected second drive 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 to the production well, wherein the second drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; and recovering the second mobilized bitumen through the production well; wherein the ratio of moles of production phase fluid solvent to steam is greater within the first drive fluid relative to that within the second drive fluid.

5. The process as claimed in claim 4: wherein the ratio of moles of production phase fluid solvent to steam is at least 10% greater within the first drive fluid relative to that within the second drive fluid.

6. The process as claimed in claim 4 or 5 further comprising: suspending the injecting of the second drive fluid; after the suspending of the injecting of the second drive fluid, injecting a third drive fluid into the injection well for conducting of the injected third drive fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir such that a third mobilized bitumen is conducted through the interwell region to the production well, wherein the third drive fluid includes steam and a non- condensable gas.

7. 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: while injecting a drive fluid into the injection well for conducting of the injected drive to the oil sands reservoir 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, and recovering the mobilized bitumen through the production well, wherein the drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; effecting a reduction in the density of the production phase fluid solvent being injected.

8. The process as claimed in claim 7: wherein the effected reduction is at least a 10% reduction.

9. The process as claimed in claim 7 or 8: wherein, prior to the effected reduction, at least 70 mol % of the production phase fluid solvent consists of heavy hydrocarbon material, based on the total number of moles of the production phase fluid solvent being injected, wherein the heavy hydrocarbon material consists of one or more heavy hydrocarbons; and wherein, after the effected reduction, at least 70 mol % of the production phase fluid solvent consists of light hydrocarbon material, based on the total number moles of the production phase fluid solvent being injected, wherein the light hydrocarbon material consists of one or more light hydrocarbons.

10. The process as claimed in claim in any one of claims 7 to 9: wherein the pressure of the drive fluid during a time interval after the effected reduction is greater than the pressure of the drive fluid during a time interval prior to the reduction.

11. The process as claimed in claim 10: wherein the pressure of the drive fluid during a time interval after the effected reduction is greater than the pressure of the drive fluid during a time interval prior to the reduction by at least 10%.

12. 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; while injecting a drive fluid into the injection well for conducting of the injected drive to the oil sands reservoir 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, and recovering the mobilized bitumen through the production well, wherein the drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; effecting a reduction in the weight average molecular weight of the production phase fluid solvent being injected.

13. The process as claimed in claim 12: wherein the effected reduction is at least a 10% reduction.

14. The process as claimed in claim 11 or 12: wherein, prior to the effected reduction, at least 70 mol % of the production phase fluid solvent consists of heavy hydrocarbon material, based on the total number of moles of the production phase fluid solvent being injected, wherein the heavy hydrocarbon material consists of one or more heavy hydrocarbons; and wherein, after the effected reduction, at least 70 mol % of the production phase fluid solvent consists of light hydrocarbon material, based on the total number moles of the production phase fluid solvent being injected, wherein the light hydrocarbon material consists of one or more light hydrocarbons.

15. The process as claimed in any one of claims 12 to 14: wherein the pressure of the drive fluid during a time interval after the effected reduction is greater than the pressure of the drive fluid during a time interval prior to the reduction.

16. The process as claimed in claim 15: wherein the pressure of the drive fluid during a time interval after the effected reduction is greater than the pressure of the drive fluid during a time interval prior to the reduction by at least 10%.

17. 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: injecting a first drive fluid into the injection well for conducting of the injected first drive 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 to the production well, wherein the first drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; recovering the first mobilized bitumen through the production well; injecting a second drive fluid into the injection well for conducting of the injected second drive 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 to the production well, wherein the second drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; and recovering the second mobilized bitumen through the production well; wherein the density of the production phase fluid solvent is greater within the first drive fluid relative to that within the second drive fluid.

18. The process as claimed in claim 17: wherein the density of the production phase fluid solvent is at least 10% greater within the first drive fluid relative to that within the second drive fluid.

19. The process as claimed in claim 17 or 18: wherein at least 70 mol % of the production phase fluid solvent, of the first drive fluid, consists of heavy hydrocarbon material, based on the total number of moles of the production phase fluid solvent being injected, wherein the heavy hydrocarbon material consists of one or more heavy hydrocarbons; and wherein at least 70 mol % of the production phase fluid solvent, of the second drive fluid, consists of light hydrocarbon material, based on the total number moles of the production phase fluid solvent being injected, wherein the light hydrocarbon material consists of one or more light hydrocarbons.

20. The process as claimed in any one of claims 17 to 19: wherein the pressure of the second drive fluid is greater than the pressure of the first drive fluid.

21. The process as claimed in claim 20: wherein the pressure of the second drive fluid is greater than the pressure of the first drive fluid by at least 10%

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: injecting a first drive fluid into the injection well for conducting of the injected first drive 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 to the production well, wherein the first drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; recovering the first mobilized bitumen through the production well; injecting a second drive fluid into the injection well for conducting of the injected second drive 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 to the production well, wherein the second drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; and recovering the second mobilized bitumen through the production well; wherein the weight average molecular weight of the production phase fluid solvent is greater within the first drive fluid relative to that within the second drive fluid.

23. The process as claimed in claim 22: wherein the weight average molecular weight of the production phase fluid solvent is at least 10% greater within the first drive fluid relative to that within the second drive fluid.

24. The process as claimed in claim 22 or 23: wherein at least 70 mol % of the production phase fluid solvent, of the first drive fluid, consists of heavy hydrocarbon material, based on the total number of moles of the production phase fluid solvent being injected, wherein the heavy hydrocarbon material consists of one or more heavy hydrocarbons; and wherein at least 70 mol % of the production phase fluid solvent, of the second drive fluid, consists of light hydrocarbon material, based on the total number moles of the production phase fluid solvent being injected, wherein the light hydrocarbon material consists of one or more light hydrocarbons.

25. The process as claimed in any one of claims 22 to 24: wherein the pressure of the second drive fluid is greater than the pressure of the first drive fluid.

26.. The process as claimed in claim 25: wherein the pressure of the second drive fluid is greater than the pressure of the first drive fluid by at least 10%.

27. 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: injecting a first drive fluid into the injection well for conducting of the injected first drive 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 to the production well, wherein the first drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; recovering the first mobilized bitumen through the production well; injecting a second drive fluid into the injection well for conducting of the injected second drive 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 to the production well, wherein the second drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; and recovering the second mobilized bitumen through the production well; wherein at least 70 mol % of the production phase fluid solvent, of the first drive fluid, consists of heavy hydrocarbon material, based on the total number of moles of the injected production phase solvent, wherein the heavy hydrocarbon material consists of one or more heavy hydrocarbons; and wherein at least 70 mol % of the production phase fluid solvent, of the second drive fluid, consists of light hydrocarbon material, based on the total number of moles of the injected production phase solvent, wherein the light hydrocarbon material consists of one or more light hydrocarbons.

28. The process as claimed in claim 27: wherein the pressure of the second drive fluid is greater than the pressure of the first drive fluid.

29. The process as claimed in claim 28: wherein the pressure of the second drive fluid is greater than the pressure of the first drive fluid by at least 10%.

30. A process for establishing fluid communication between an injection well and a production well within an oil sands reservoir, comprising: supplying, to an interwell region disposed between the injection well and the production well, a mobilization-initating fluid, the mobilization-initating fluid including steam and a start-up phase fluid solvent, wherein the start-up phase fluid solvent consists of one or more start-up phase solvent hydrocarbons.

31. A process for producing bitumen from an oil sands reservoir, comprising: establishing fluid communication between an injection well and a production well by supplying, to an interwell region disposed between the injection well and the production well, steam and a start-up phase fluid solvent, wherein the start-up phase fluid solvent consists of one or more start-up phase solvent hydrocarbons; after the fluid communication between the injection well and the production well has been established, injecting a drive fluid to the injection well for conducting of the injected mobilization-initiating fluid to the oil sands formation for effecting mobilization of bitumen within the oil sands formation such that the mobilized bitumen is conducted through the interwell region to the production well; and recovering the mobilized bitumen through the production well.

32. 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 injecting a mobilization-initiating fluid into the injection well for conducting of the injected mobilization-initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir, wherein the mobilization- initiating fluid includes steam and a start-up phase fluid solvent, the start-up phase fluid solvent consisting of one or more start-up phase solvent hydrocarbons; after the fluid communication has been established, injecting a drive fluid into the injection well for conducting of the injected drive 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 to the production well, wherein the drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; and recovering the mobilized bitumen through the production well; wherein the density of the start-up phase fluid solvent within the mobilization-initiating fluid is greater than the density of the production phase fluid solvent within the drive fluid.

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

34. The process as claimed in claim 32 or 33: wherein at least 70 mol % of the start-up phase fluid solvent, of the mobilization-initiating fluid, consists of heavy hydrocarbon material, based on the total number of moles of the start-up phase fluid solvent being injected, wherein the heavy hydrocarbon material consists of one or more heavy hydrocarbons; and wherein at least 70 mol % of the production phase fluid solvent, of the drive fluid, consists of light hydrocarbon material, based on the total number moles of the production phase fluid solvent being injected, wherein the light hydrocarbon material consists of one or more light hydrocarbons.

35. The process as claimed in any one of claims 32 to 34: wherein the pressure of the drive fluid is greater than the pressure of the mobilization-initiating fluid.

36. The process as claimed in claim 35: wherein the pressure of the drive fluid is greater than the pressure of the mobilization-initiating fluid by at least 10%.

37. 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 injecting a mobilization-initiating fluid into the injection well for conducting of the injected mobilization-initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir, wherein the mobilization- initiating fluid includes steam and a start-up phase fluid solvent, the start-up phase fluid solvent consisting of one or more start-up phase solvent hydrocarbons; after the fluid communication has been established, injecting a drive fluid into the injection well for conducting of the injected drive 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 to the production well, wherein the drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; and recovering the mobilized bitumen through the production well; wherein the weight average molecular weight of the start-up phase fluid solvent within the mobilization-initiating fluid is greater than the weight average molecular of the production phase fluid solvent within the drive fluid.

38. The process as claimed in claim 37: wherein the weight average molecular weight of the start-up phase fluid solvent within the mobilization-initiating fluid is at least 10% greater than the density of the production phase fluid solvent within the drive fluid.

39. The process as claimed in claim 37 or 38: wherein at least 70 mol % of the start-up phase fluid solvent, of the mobilization-initiating fluid, consists of heavy hydrocarbon material, based on the total number of moles of the start-up phase fluid solvent being injected, wherein the heavy hydrocarbon material consists of one or more heavy hydrocarbons; and wherein at least 70 mol % of the production phase fluid solvent, of the drive fluid, consists of light hydrocarbon material, based on the total number moles of the production phase fluid solvent being injected, wherein the light hydrocarbon material consists of one or more light hydrocarbons.

40. The process as claimed in any one of claims 37 to 39: wherein the pressure of the drive fluid is greater than the pressure of the mobilization-initiating fluid.

41. The process as claimed in claim 40: wherein the pressure of the drive fluid is greater than the pressure of the mobilization-initiating fluid by at least 10%.

42. 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 injecting a mobilization-initiating fluid into the injection well for conducting of the injected mobilization-initiating fluid to the oil sands reservoir for effecting mobilization of bitumen within the oil sands reservoir, wherein the mobilization- initiating fluid includes steam and a start-up phase fluid solvent, the start-up phase fluid solvent consisting of one or more start-up phase solvent hydrocarbons; after the fluid communication has been established, injecting a drive fluid into the injection well for conducting of the injected drive 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 to the production well, wherein the drive fluid includes steam and a production phase fluid solvent, the production phase fluid solvent consisting of one or more production phase solvent hydrocarbons; and recovering the mobilized bitumen through the production well; wherein at least 70 mol % of the start-up phase fluid solvent, of the mobilization-initiating fluid, consists of heavy hydrocarbon material, based on the total number of moles of the start-up phase fluid solvent being injected, wherein the heavy hydrocarbon material consists of one or more heavy hydrocarbons; and wherein at least 70 mol % of the production phase fluid solvent, of the drive fluid, consists of light hydrocarbon material, based on the total number of moles of the production phase fluid solvent being injected, wherein the light hydrocarbon material consists of one or more light hydrocarbons.

43. The process as claimed in claim 42: wherein the pressure of the drive fluid is greater than the pressure of the mobilization-initiating fluid.

44. The process as claimed in claim 43: wherein the pressure of the drive fluid is greater than the pressure of the mobilization-initiating fluid by at least 10%.