CA2898897A1

CA2898897A1 - Partial height steam chamber sagd

Info

Publication number: CA2898897A1
Application number: CA2898897A
Authority: CA
Inventors: Cathal Joseph Tunney; Haibo Huang; Yoshiaki Ito
Original assignee: Alberta Innovates Technology Futures
Current assignee: Alberta Innovates Technology Futures
Priority date: 2015-07-29
Filing date: 2015-07-29
Publication date: 2017-01-29

Abstract

A method for thermal recovery of heavy hydrocarbons from a subterranean reservoir.
More particularly, the present disclosure relates to thermal recovery of heavy hydrocarbons by an adaptation of Steam Assisted Gravity Drainage (SAGD) utilizing a partial height steam chamber.

Description

PARTIAL HEIGHT STEAM CHAMBER SAGD
FIELD
[0001] The present disclosure relates generally to thermal recovery of heavy hydrocarbons from a subterranean reservoir. More particularly, the present disclosure relates to thermal recovery of heavy hydrocarbons by an adaptation of Steam Assisted Gravity Drainage (SAGD).
BACKGROUND

[0002] The first field testing and demonstration of the SAGD process was conducted during the 1980s in an Athabasca oil sand reservoir by the Alberta Oil Sand Technology Research Authority (AOSTRA) in Alberta, Canada. First commercial implementations of SAGD by industry commenced in the early 2000s and most of these early SAGD
projects are still in operation. Consequently, the accumulated experience on actual full life cycle performance of SAGD is as yet limited and there remains a need to review and test the validity of the conventional engineering descriptions of the SAGD process against such field experience.

[0003] A typical SAGD well configuration, also known as a SAGD well pair, is illustrated in cross-section in Figure 1. A horizontal production well, located near the bottom of a target oil bearing interval, also referred to herein as a reservoir, is designed to accommodate fluid inflow over its full length. A horizontal steam injection well is located about 5m above and parallel to the injection well and is designed to accommodate steam outflow along its full length. Before steam injection proceeds, the zone around and between the wells of a SAGD well pair must be heated to reduce the viscosity of the contained bitumen to enable it to flow toward the production well. Steam is injected via the injection well and oil is produced via the production well, and the steam chamber expands upward and laterally, until the steam chamber reaches a zone of reduced permeability, for example an upper cap-rock, and then the steam chamber expands laterally.

[0004] The conventional understanding of the SAGD process is that heated mobilized oil that drains downward under gravity is displaced by injected steam, producing a so called steam chamber wherein the volume of the pore space once occupied by the drained oil becomes occupied by an equal volume of steam. Stated otherwise, this conceptualization requires that any incremental oil production is matched by an equivalent incremental expansion of the pore volume of the steam chamber. Further, conventional SAGD
theory teaches that the steam chamber should be expanded preferentially upward to the top of the target oil bearing interval first and thereafter expanded laterally. This prescription is understood to maximize the hydraulic head driving gravity drainage of liquids to the production well and is intended to produce maximized instantaneous oil production rates early in the SAGD operating cycle. The expected oil production rate peaks early, coincident with the steam chamber reaching the top of the target oil bearing interval.
The corresponding steam to oil ratio (SOR) profile, the typical parameter used to track the energy efficiency of the process, is predicted to continually increase after the steam chamber has grown to the top of the target oil bearing interval, since thereafter the area of contact between the hot steam chamber and the over-burden continually increases as the steam chamber expands laterally, which causes increasing heat loss to the overburden.

[0005] Roger Butler, who is credited with the invention of SAGD, developed an analytical model of the SAGD process that is consistent with the foregoing description.
Commercially available numerical simulators, such as STARS TM by CMG of Calgary, Alberta, Canada also encode the above analytical description of the SAGD process, including the prediction that the steam chamber will continue to grow vertically upward unless and until it encounters a layer in the target oil bearing interval, also referred to herein as the reservoir, where the vertical permeability is sufficiently low.

[0006] Typically, field implementations of SAGD prescribe that the steam operating pressure be as high as permitted for an initial period that is estimated to be required to grow the steam chamber vertically to the top of the reservoir as quickly as possible. Thereafter, the operating pressure is reduced to about half the initial value, which is adequate to drive lateral expansion of the steam chamber while limiting the steam chamber temperature and thereby conductive heat loss to the overburden.

[0007] This steam injection strategy is consistent with the understanding, supported by field experience, that the rate of vertical growth of a steam chamber is positively correlated with steam operating pressure.

[0008] Typically some amount of non-condensable gas occurs in a SAGD
operation.
This non-condensable gas is comprised of a combination of solution gas and in-situ generated gas. Solution gas, primarily pre-existing methane dissolved in the oil under virgin reservoir conditions is liberated from the oil as the reservoir is heated during SAGD. In-situ generated gas, primarily carbon-dioxide, is generated by chemical reactions occurring in the reservoir as it is heated by injected steam.

[0009] There is a lot of uncertainty in assessing and predicting the impact of non-condensable gas on a SAGD operation because it is difficult to accurately predict in-situ generated gas volumes and also to account for possible leak-off of non-condensable gas away from the steam chamber. Significant volumes of non-condensable gas can also be produced, along with condensed steam and mobilized oil, to surface through SAGD
production wells. However, the mechanisms by which such gas production occurs may aggravate the problem of non-uniform steam chamber development and oil production over the full intended producing length of the SAGD wells.

[0010] It is known that the presence of non-condensable gas can slow the rate of expansion, including the rate of vertical expansion, of a SAGD steam chamber.
Therefore, it is generally understood that the presence of non-condensable gas can have a negative impact on SAGD performance.

[0011] It is also known that in particular circumstances deliberate injection of non-condensable gas can have a positive impact on the SOR performance of a SAGD
operation.
In such circumstances it is understood that the injected non-condensable gas creates an insulating blanket effect at the top of a SAGD steam chamber, thereby reducing heat losses to the overburden.

[0012] Yoshiaki Ito has analyzed the historical performance of many SAGD
operations undertaken in the Athabasca oil sands and has undertaken several numerical simulation studies, some of which have been published, of both early stage and maturing SAGD operations with a particular focus on attempting to match the recorded temperature profiles in observation wells located close to SAGD well pairs. These analyses indicate that several recurring observations, although typically not all occurring in the same project, from actual historical SAGD performance are not explained by the conventional conceptualization of the SAGD process, including:

[0013] 1. Observations of declining or constant SOR for maturing SAGD
projects that deviate significantly from predictions based on the conventional conceptualization of SAGD, even when such predictions include adjustments for reduced steam injection rates in the later stages of SAGD operation.

[0014] 2. Observations of a slow decline in oil production rates after the initial peak rate is achieved are not consistent with the more rapid rate of decline predicted by conventional SAGD theory.

[0015] 3. Observations of continuing strong oil production rates after steam injection has been stopped ("shut in") do not match the immediate and significant production rate reductions predicted by conventional SAGD theory.

[0016] 4. Estimates, derived from history matched simulations of observation well temperature data, of the pore volume of steam chambers that do not match the corresponding cumulative oil production volumes, conflict with conventional SAGD theory.

[0017] 5. Instances where the temperature profile measured in observation wells shows that the steam chamber does not extend to the top of the target pay interval, even where no apparent vertical permeability barrier exists, are not consistent with conventional SAGD theory.

[0018] 6. Observations of SAGD infill well (also known as "wedge well") performance showing better oil production rates and lower apparent SOR than predicted by conventional SAGD theory.

[0019] 7. Observations of commingling times, the period of SAGD
operation after which flow communication is established between adjacent SAGD well pairs that are significantly shorter than predicted by conventional SAGD theory, e.g. 2 to 3 years versus 5 to 6 years.

[0020] Ito's conceptualization to explain the foregoing anomalous observations is that the rise of a SAGD steam chamber can be significantly slowed, if not completely terminated, by some combination of the effects of steam injection pressure that is inadequate to induce vertical steam fingering and the accumulation of non-condensable gas within the target oil bearing interval above the steam chamber. Then, as the steam chamber expands laterally, the oil remaining above it is heated by conduction and is gradually displaced downward toward the production well by non-condensable gas that is produced as the reservoir is heated or is deliberately injected. Details of this modified conceptualization of the SAGD
process and of a technique whereby it can be numerically simulated are presented in Y.Ito and J.Chen, "Numerical History Match of the Burnt Lake SAGD Process", JCPT May 2010.

[0021] It is, therefore, desirable to provide a method for thermal recovery of hydrocarbons from a subterranean reservoir.

SUMMARY

[0022] It is an object of the present disclosure to obviate or mitigate at least one disadvantage of previous recovery methods.

[0023] The present disclosure relates to increasing the energy efficiency of the Steam Assisted Gravity Drainage (SAGD) process for recovery of heavy oil or bitumen. In particular, the present disclosure provides oil recovery methods that include control of the vertical extent of a SAGD steam chamber to a specified fraction of the total thickness of the target oil bearing interval located above a SAGD production well while producing oil from the oil bearing zone above the SAGD steam chamber in addition to the oil that is produced by drainage from the SAGD steam chamber

[0024] By controlling, over the life of a SAGD operation, the vertical extent of the steam chamber to a pre-determined fraction of the total thickness of the target oil bearing interval above the production well, that the performance of the SAGD operation can be increased, especially with respect to energy efficiency.

[0025] Therefore, the energy efficiency of a SAGD operation may be increased by the follow method.

[0026] Identify the steam chamber "optimum height" that minimizes the predicted cumulative SOR to achieve a target recovery factor by using a representative numerical model of the target reservoir to run a series of SAGD simulations with varying pre-set steam chamber height. Operate the SAGD process to achieve the aforesaid optimum height steam chamber by tracking the location of the top of the steam chamber and controlling it by adjusting the steam injection rate or the rate of accumulation of non-condensable gas within the target reservoir.

[0027] The SAGD simulations used to identify the optimum height steam chamber should explore the sensitivity of predicted cumulative SOR performance to the variation in the occurrence of non-condensable gas during the SAGD operating life. This sensitivity analysis should bracket the expected occurrence of non-condensable gas for the target reservoir as predicted from a combination of:

[0028] Initial occurrence of solution gas as measured from representative reservoir core samples;

[0029] Measured non-condensable gas generation from representative reservoir core samples exposed to steam over the target operating temperature range; and

[0030] Measured leak-off rates for injected non-condensable gas in one or more wells located in representative portions of the target reservoir.

[0031] Where the predicted occurrence of non-condensable gas is so high as to make the rate of steam chamber development non-viable it may be desirable to provide a means whereby non-condensable gas may be vented to surface. For this purpose, dedicated vertical or horizontal vent wells may be installed near the top of the target reservoir.

[0032] Where the top of the target reservoir is immediately overlain by a well-defined cap-rock, such vent wells should be located within the target reservoir but below the sealing cap-rock. Where the target reservoir is immediately overlain by a reservoir interval of reduced quality such vent wells should be located at about the location of the interface between the target reservoir and the overlying reservoir interval of reduced quality. Although the primary purpose of vent wells is to remove surplus non-condensable gas to surface they may also be used to preferentially inject non-condensable gas into the top of the target reservoir or the interval immediately above it. One or more separate vent wells may be associated with each SAGD well-pair or one or more vent wells may be used to service several SAGD well-pairs concurrently. The use of vent wells may be particularly important where the SAGD production wells are equipped with inflow control devices that are designed to greatly limit if not completely shut off the inflow of gas, specifically targeting live steam but also including non-condensable gas.

[0033] Effective implementation of SAGD requires that the target reservoir be overlain by a sealing cap-rock that exhibits effectively zero permeability and prevents loss of steam to the overburden. Where the top of the target reservoir is immediately overlain by a well-defined cap-rock, it is adequate that the SAGD simulations used to identify the optimum height steam chamber use a reservoir model that extends vertically only to the interface with the cap-rock. Where the target reservoir is immediately overlain by a reservoir interval of reduced quality, typically exhibiting reduced oil saturation and/or reduced vertical permeability, the SAGD simulations used to identify the optimum height steam chamber should use a reservoir model that extends vertically to include such reservoir interval of reduced quality.

[0034] As steam injection proceeds, the location of the top of the steam chamber may be monitored using the vertical temperature profile from observation wells located close to the SAGD well-pair. Preferably, each SAGD well-pair will have at least one adjacent observation well equipped to record the vertical temperature profile over the SAGD operating life. Where the reservoir geology and occurrence of non-condensable gas is expected to be reasonably uniform over the target reservoir and the planned SAGD operating conditions are the same across well-pairs, it may be adequate to estimate the location of the top of the steam chamber for all SAGD well-pairs using temperature profiles from only a few observation wells. In such cases the measured rate of vertical growth with elapsed time from the commencement of steam injection is assumed to be a common characteristic of all well-pairs.

[0035] In cases where separate vent wells are installed in the top of the reservoir and these vent wells are equipped with temperature sensors, the location of the top of the steam chamber can be estimated by comparing the measured temperature rise in the vent wells to the temperature rise predicted by numerical simulation of an approaching steam chamber. A
particularly advantageous arrangement involves a separate vent well located approximately directly above and longitudinally coextensive with each SAGD well-pair, since this allows direct monitoring of likely non-uniform steam chamber height along each well-pair.

[0036] When measured or predicted vertical temperature profiles indicate that the vertical growth of the steam chamber is approaching the "optimum height," SAGD
operating conditions are adjusted to maintain this "optimum height" by a combination of some or all of the following:

[0037] Reduce the steam injection rate;

[0038] Commence injection or increase the rate of injection of non-condensable gas;
or

[0039] Reduce the rate of venting of non-condensable gas.

[0040] Where injection of non-condensable gas is required, it may be co-injected with steam through the SAGD injection well. Alternatively, where separate vent wells are installed in or near the top of the target reservoir, these may be used for injection of non-condensable gas.

[0041] In a first aspect, the present disclosure provides a method of thermal recovery of heavy hydrocarbons from a subterranean target reservoir, including determining a target reservoir total thickness, determining a target height for a steam chamber, providing a horizontal production well, low in the target reservoir, providing an injection well, above the horizontal production well, establishing fluid communication between the injection well and the production well, injecting steam into the target reservoir, establishing a steam chamber, producing mobilized heavy hydrocarbons from the target reservoir, propagating and growing the steam chamber vertically, towards a target height, said target height being less than the target reservoir total thickness, at least periodically monitoring the height of the steam chamber, and selectively controlling the height of the steam chamber when the top of the steam chamber approaches the target height.

[0042] In an embodiment disclosed, the selectively controlling the height of the steam chamber includes, one or more, in combination of reducing the rate of steam injection or pressure or both, commencing injection or increasing the rate of injection of non-condensable gas, and reducing the rate of venting of non-condensable gas.

[0043] In an embodiment disclosed, the target height is selected through use of a representative numerical model of the target reservoir to run a series of SAGD
simulations with varying pre-set steam chamber target heights.

[0044] In an embodiment disclosed, the target height is about 70 percent of the total thickness of the target reservoir.

[0045] Other aspects and features of the present disclosure will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments in conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Embodiments of the present disclosure will now be described, by way of example only, with reference to the attached Figures.

[0047] Fig. 1 is an illustration typically used for depiction of a prior art SAGD
operation;

[0048] Fig. 2 is an illustration of an embodiment of the present disclosure, with the target oil bearing interval immediately overlain with a cap-rock; and

[0049] Fig. 3 is an illustration of an embodiment of the present disclosure, with the target oil bearing interval immediately overlain with an interval of reduced quality and the interval of reduced quality overlain with a cap-rock.
DETAILED DESCRIPTION

[0050] Generally, the present disclosure provides a method and system for thermal recovery of hydrocarbons from a subterranean reservoir.

[0051] Conventional SAGD

[0052] A typical SAGD well configuration, also known as a SAGD well pair, is illustrated in cross-section in Figure 1. A horizontal production well, located near the bottom of a target reservoir, is designed to accommodate fluid inflow over its full length. A horizontal steam injection well is typically located about 5m above and parallel to the production well and is designed to accommodate steam outflow along its full length. Below the target reservoir is a base, and above the target reservoir is a cap-rock.

[0053] Before steam injection proceeds at any substantial rate or pressure, the zone around and between the injection well and the production well are heated to reduce the viscosity of the contained bitumen to enable it to flow toward the production well.

[0054] Steam is injected via the injection well and oil is produced via the production well. Over time the steam chamber expands upward and laterally, until the steam chamber reaches a zone of reduced permeability, for example an upper cap rock, and then the steam chamber expands laterally.

[0055] Partial Height Steam Chamber SAGD with Cap-Rock Ceiling

[0056] Referring to Fig. 2, a typical SAGD well configuration, also known as a SAGD
well pair, is illustrated in cross-section. A horizontal production well, located near the bottom of a target reservoir, is designed to accommodate fluid inflow over its full length. A horizontal steam injection well is typically located about 5m above and parallel to the production well and is designed to accommodate steam outflow along its full length. Below the target reservoir is a base, and above the target reservoir is a reduced quality cap-rock

[0057] Before steam injection proceeds at any substantial rate or pressure, the zone around and between the injection well and the production well are heated to reduce the viscosity of the contained bitumen to enable it to flow toward the production well.

[0058] Steam is injected via the injection well and oil is produced via the production well, and the steam chamber expands upward and laterally.

[0059] A predetermined optimum height or target height of the steam chamber is selected. The target height may be selected that minimizes the predicted cumulative SOR to achieve a target recovery factor by using a representative numerical model of the target reservoir to run a series of SAGD simulations with varying pre-set steam chamber heights.
Thus, an optimum steam chamber target height may be selected.

[0060] When measured or predicted vertical temperature profiles indicate that the vertical growth of the steam chamber is approaching the "optimum height", SAGD
operating conditions are adjusted to maintain this "optimum height". An oil bearing zone remains above the steam chamber, between the steam chamber and the top of the target reservoir. SAGD
operating conditions are adjusted to maintain the optimum height by a combination of some or all of the following:

[0061] 1. Reduce the steam injection rate;

[0062] 2. Commence injection or increase the rate of injection of non-condensable gas; or

[0063] 3. Reduce the rate of venting of non-condensable gas.

[0064] Where injection of non-condensable gas is required, it may be co-injected with steam through the SAGD injection well. Alternatively, where separate vent wells are installed at or near the top of the target reservoir, the vent wells may be used for injection of non-condensable gas. The vent well(s) may be horizontal or vertical or both.

[0065] Partial Height Steam Chamber SAGD with Reduced Quality Reservoir Ceiling

[0066] Referring to Fig. 3, a typical SAGD well configuration, also known as a SAGD
well pair, is illustrated in cross-section. A horizontal production well, located near the bottom of a target reservoir, is designed to accommodate fluid inflow over its full length. A horizontal steam injection well is typically located about 5m above and parallel to the production well and is designed to accommodate steam outflow along its full length. Below the target reservoir is a base, and above the target reservoir is an overlying reservoir interval of reduced quality. Above the reservoir interval of reduced quality is a cap-rock.

[0067] Before steam injection proceeds at any substantial rate or pressure, the zone around and between the injection well and the production well are heated to reduce the viscosity of the contained bitumen to enable it to flow toward the production well.

[0068] Steam is injected via the injection well and oil is produced via the production well, and the steam chamber expands upward and laterally.

[0069] A predetermined optimum height or target height of the steam chamber is selected. The target height may be selected that minimizes the predicted cumulative SOR to achieve a target recovery factor by using a representative numerical model of the target reservoir to run a series of SAGD simulations with varying pre-set steam chamber heights.
Thus, an optimum steam chamber target height may be selected.

[0070] When measured or predicted vertical temperature profiles indicate that the vertical growth of the steam chamber is approaching the "optimum height," SAGD
operating conditions are adjusted to maintain this "optimum height" by a combination of some or all of the following:

[0071] 1. Reduce the steam injection rate;

[0072] 2. Commence injection or increase the rate of injection of non-condensable gas; or

[0073] 3. Reduce the rate of venting of non-condensable gas.

[0074] Where injection of non-condensable gas is required, it may be co-injected with steam through the SAGD injection well. Alternatively, where separate vent wells are installed in or near the top of the target reservoir, these may be used for injection of non-condensable gas.

[0075] In the preceding description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the embodiments.
However, it will be apparent to one skilled in the art that these specific details are not required. In other instances, well-known structures are shown in block diagram form in order not to obscure the understanding.

[0076] The above-described embodiments are intended to be examples only.
Alterations, modifications and variations can be effected to the particular embodiments by those of skill in the art without departing from the scope, which is defined solely by the claims appended hereto.

Claims

WHAT IS CLAIMED IS:

1. A method of thermal recovery of heavy hydrocarbons from a subterranean target reservoir, comprising:
- determine a target reservoir total thickness;
- determine a target height for a steam chamber;
- provide a horizontal production well, low in the target reservoir;
- provide an injection well, above the horizontal production well;
- establish fluid communication between the injection well and the production well;
- injecting steam into the target reservoir;
- establish a steam chamber;
- produce mobilized heavy hydrocarbons from the target reservoir;
- propagate and grow the steam chamber vertically, towards a target height, said target height being less than the total thickness;
- at least periodically monitoring the height of the steam chamber;
- selectively controlling the height of the steam chamber when the top of the steam chamber approaches the target height.

2. The method of claim 1, the selectively controlling the height of the steam chamber comprising, one or more, in combination:
- reducing the rate of steam injection or pressure or both;
- commencing injection or increasing the rate of injection of non-condensable gas;
and - reducing the rate of venting of non-condensable gas.

3. The method of claim 1, wherein the target height is selected by using a representative numerical model of the target reservoir to run a series of SAGD simulations with varying pre-set steam chamber height to select the target height.

4. The method of claim 1, wherein the target height is about 70 percent of the total thickness of the target reservoir.