Method for enhancing recovery of heavy crude oil by in-situ combustion in the presence of strong aquifers

Abstract

 A method for enhancing recovery of crude oil from a crude oil containing formation (1), which is located adjacent to a strong aquifer (2) comprises:
- producing crude oil through at least one crude oil production well (5) having an inflow zone which traverses the crude oil containing formation (1) and in which zone a fluid pressure (or bottomhole pressure or BHP) is lower than a fluid pressure in the aquifer (2);
- injecting an oxidant into the crude oil containing formation through one or more oxidant injection wells (3,4) and igniting a mixture of oxidant and crude oil to heat the hydrocarbon containing formation (1) such that the viscosity of at least a substantial fraction of unburned crude oil in the crude oil containing formation (1) is reduced to such a level, preferably to less than 50 cP, that influx of water from the aquifer (2) into the inflow zone of the crude oil production well (5) is reduced; and
- allowing the aquifer (2) to sweep the oil into the inflow zone of the production well (5) after oxidant injection has ceased, the viscosity if the produced crude oil being substantially reduced by heating of the reservoir (1).

Description

BACKGROUND OF THE INVENTION

[0001] Primary recovery of medium-heavy oil reservoirs with a strong bottom aquifer is generally poor. The strong aquifer provides an almost constant pressure source that drives the oil to the producers. As people skillful in the art of heavy oil production would immediately recognize, oil recovery will depend strongly on the mobility ratio between the water and oil. In heavy oils with viscosities between few hundred and few thousand centipoises the mobility ratio is very high and oil displacement by water is unstable. Water finds its way to the producers at the expense of oil production, which results in low oil recoveries and high water cuts early in the life of the field.

[0002] The introduction of horizontal wells that are drilled at the top of the oil column has resulted in somewhat improved recovery. However, even horizontal wells suffer from fast water breakthrough and subsequent oil production at high water cut. Given the low primary recoveries, these fields are targets for Enhanced Oil Recovery (EOR). One way to enhance recovery is to reduce the viscosity of the oil to improve the effectiveness of the aquifer drive.

[0003] In-Situ Combustion, a thermal EOR technology to reduce the viscosity of oil, has been practiced for many years in a spectrum of reservoir and oil types.

[0004] In-Situ Combustion (ISC) as known from US patent 5,211,230 is a displacement process generally applied to medium-heavy and heavy oil reservoirs to increase oil production by reducing the oil viscosity.

[0005] In In-Situ Combustion air is injected into an oil-bearing formation to burn a fraction of the oil and generate steam and hot gases to heat the rock and fluids downstream of the combustion zone and drive the oil to the producers . Because of the much lower density of air compared with the density of both oil and water present in the reservoir, air overrides the oil column resulting in very poor sweep efficiency. For this reason In-Situ Combustion has been applied in relatively thin and dipping formations where gravity drainage of the heated oil can contribute to production.

[0006] European Patent 1112956 introduced a method of injecting a lower-than-oil density fluid, such as air to displace and burn oil in the upper section of an oil-bearing reservoir and injecting a higher-than-oil density fluid, such as water, in the lower part of the reservoir to displace the fluids in an upward direction.

[0007] Oil reservoirs with strong bottom aquifers are typically developed with horizontal wells or vertical wells both drilled or perforated at the top of the oil column. In oil reservoirs with more than 20 meters of oil column and strong bottom aquifers the recovery of a process consisting of injection of air at the top of the oil column will be severely compromised by a) early oxygen breakthrough at horizontal producers drilled next to the water-oil contact, or b) by air override at producers completed or drilled at the upper part of the oil column.

[0008] A new method is required to use ISC in relatively thick oil formations with strong aquifers to increase recovery efficiency.

[0009] It is an objective of the invention to provide a method of enhancing crude oil production by an In-Situ Combustion (ISC) process, wherein influx of water from an adjacent aquifer is inhibited.

SUMMARY OF THE INVENTION

[0010] In accordance with the invention there is provided a method for enhancing recovery of crude oil from a crude oil containing formation which overlies a water containing formation, the method comprising:

• producing crude oil through at least one crude oil production well having an inflow zone, which traverses the crude oil containing formation and in which a fluid pressure is lower than a fluid pressure in the water containing formation; and
• injecting an oxidant into the crude oil containing formation and igniting a mixture of oxidant and crude oil to heat the hydrocarbon containing formation such that the viscosity of at least a substantial fraction of uncombusted crude oil in the crude oil containing formation is reduced to such a level that fractional flow of water from the water containing formation into the inflow zone is reduced.

[0011] Preferably oxidant injection is terminated when the combustion front approaches the inflow zone of the crude oil production well so that the fluid pressure of the water in the water containing formation (also called aquifer) sweeps the heated oil to the inflow zone of the crude oil production well.

[0012] Preferably the viscosity of said at least substantial fraction of crude oil in the crude oil containing formation is reduced to less than 50 centipoise (50 cP, or 0.05 Pa.s), more preferably to less than 30 centipoise (30 cP or 0.03 Pa.s), most preferably to less than 10 centipoise (10 cP or 0.01 Pa.s).

[0013] These and other features, aspects, embodiments and advantages of the method according to the invention are described in the accompanying claims, abstract and the following detailed description of preferred embodiments of the invention in which reference is made to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] 

FIG. 1 shows a crude oil reservoir in which the in situ combustion (ISC) method according to the invention is applied;

FIG.2 depicts a computer simulation which indicates a vertical cross section of the viscosity distribution (centipoises or cP) of oil at the end of the air injection or ISC step; and

FIG.3 depicts the results of the computer simulation, which illustrates the rate of heated oil production and the magnitude of water influx or watercut reduction by use of the ISC process according to the invention.

DETAILED DESCRIPTION OF THE DEPICTED EMBODIMENTS

[0015] This invention provides an In-Situ Combustion (ISC) method to significantly increase the oil recovery from oil-bearing reservoirs with in-situ oil viscosities ranging from a few hundred to a few thousand centipoises (cP) and with strong bottom aquifers.

[0016] Typically ISC is applied after a first production phase is completed of an aquifer-assisted production utilizing production wells drilled or perforated at the top of a crude oil containing formation 1 as shown in FIG 1.

[0017] As illustrated in FIG.1 in accordance with a preferred embodiment of this invention horizontal producers 3 and 4 are drilled before ISC is applied to drain the formation until the produced water fraction from the water containing formation or aquifer 2 becomes excessive. Then an additional crude oil production well or producer 5 is drilledcloser to the water oil contact (WOC), for example at a vertical elevation above the WOC of about h = 1/3 H, where H is the total height of the oil column or reservoir 1. The total height H may typically be between 20 and 60 m.

[0018] The optimum vertical location h of the inflow zone of the production well 5 in the oil column depends amongst others, on the initial oil viscosity, aquifer strength and production schedules. In most crude oil containing formations h will be between 0.2 and 0.5 H, preferably between 0.3 and 0.4 H above the water oil contact (WOC), measured when ISC enhanced crude oil production commences and before the water oil contact (WOC) starts to rise due to the ISC enhanced production of crude oil from the crude oil containing formation 1.

[0019] A second phase consisting of an ISC process is then commenced.

[0020] It consists of two steps.

1) During the first step air is injected by employing vertical or horizontal injectors, which are either specially drilled at the top of the oil column 1 or converted from existing wells 3 and 4 employed in the first phase of aquifer-assisted primary production.
Air is initially injected at high rates to result in a good burn of the oil. This is clearly indicated by the CO₂ concentration in the effluent exhaust gases at the producers. As expected in ISC recovery the flue gas front moves much faster than the combustion front, where most of the oxygen is consumed. The well arrangement illustrated in FIG.1 results in a heated zone at the top of the reservoir 1 that expands down toward the middle through heat convected by the hot combustion gases in their way to the producer 5, as well as by thermal conduction through the rock and fluids. When the combustion front approaches the producer 5 the oxygen concentration in the produced gas starts to rise. A threshold concentration below the flammability limit of the hydrocarbons in the well, typically around 3 mole % to 5 mole% oxygen concentration in the produced gas, is not to be exceeded. When this limit is reached the air injection rate can be significantly reduced and continued for a period similar to the high-injection-rate period, or terminated.

2) Step two commences by shutting-in the air injection well or wells 3 and 4 and letting the aquifer 2 sweep the heated oil to the inflow region of the crude oil production well 5.

[0021] The method of this invention intends to heat the crude oil containing formation 1 such that the viscosity of at least a substantial fraction of crude oil in the region of the crude oil containing formation 1 between the burned zone and the water containing formation, or aquifer, 2 is reduced to such a level that fractional flow of water into the producer zone from the water containing formation 2 through the crude oil is significantly reduced and the fractional flow of lower-viscosity oil significantly increased over the non-ISC case.

[0022] Numerical simulation models of the method according to the present invention provide examples of the viscosity reduction effect due to the deployment of the heat bank by ISC.

[0023] FIG. 2 is one of such examples and clearly illustrates the reduction of oil viscosity from around 300 cP in the area close to the Water-Oil Contact WOC to a range between 2 to 10 cP in the center part of the oil column or reservoir 1.

[0024] In accordance with the invention a reservoir 1 with an oil bearing column with oil viscosities in the few hundred to few thousand centipoises range and a strong aquifer is developed by drilling long horizontal wells at the top of the oil column or reservoir 1.

[0025] In FIG.1 reference numeral 1 represents the oil column or reservoir, reference numeral 2 the underlying aquifer, and reference numerals 3 and 4 two horizontal crude oil production wells or producers that have a substantially horizontal inflow region drilled at the top of the oil column, reference numeral 5 the horizontal producer which is located at a height of about 1/3 of the height H of the crude oil column above the initial Water Oil Contact (WOC).

[0026] The horizontal distance between two adjacent horizontal producer wells 3,4 varies from case to case but it is typically up to a few hundred meters.

[0027] For the application of the In-Situ Combustion (ISC) method according to this invention one infill producer, as 5 in FIG. 1, is drilled closer to the Water Oil Contact (WOC) than to the top 6 of the reservoir or oil column 1 so that a significant amount of heat can be rapidly deployed in the middle and upper sections of the reservoir 1. Subsequently, the aquifer 2 is used to sweep the oil through the heated zone towards the producers, wherein the viscosity of the heated crude oil is sufficiently low so that water fractional flow into the producer 5 is significantly reduced.

[0028] It will be understood that the Fast Heat Deployment employing the ISC concept according to the invention may also be carried out with vertical wells. In such case the vertical wells may be placed on production until the water cut renders the well uneconomic. A vertical infill producer is then drilled and perforated closer to the water-oil contact WOC than to the top 6 of the crude oil reservoir 1.

[0029] One or more of the initial aquifer assisted production phase producer wells 3,4 may be converted into injectors or two new injectors may be drilled in their vicinity to inject air at high rates to result in a high-temperature oxidation process.

[0030] A generic reservoir model was used to prove the viability of the production method according to the invention.

[0031] Model properties are:

Pay Thickness = 50 m;

Initial Oil Saturation = 80%,

Initial Water Saturation Sw = 20% ;

Oil viscosity ∼ 300 centiPoises (cP);

Crude oil comprises 2 components: Maltenes and asphaltenes;

Well spacing Injector-Producer = 100 meter in horizontal direction; 2 Dimensional Grid 100 x 1 x 60 (1.0 x 1.0 x 1.0 m);

Dynamic gridding based on temperature and saturation gradients;

Aquifer blocks + analytic aquifer;

Reactions and kinetics are based on a published model for high temperature oxidation reactions.

[0032] Computer simulations with the above parameters showed that it is feasible to enhance crude oil recovery(EOR) employing In-Situ Combustion in reservoirs with strong bottom aquifers.

[0033] Computer simulations further indicate that high oil recovery factors can be obtained in reservoirs with strong bottom aquifers, which use top injectors (horizontal or vertical) flanked by horizontal producers closer to the water-oil contact.

[0034] Computer simulations further confirm the experimental evidence that air injectivity at high injection rates results in combustion of the burning fuel in the so called high temperature oxidation mode that is continued until the oxygen concentration in the produced gas is above a safety limit of 3 to 5 mole%, above which air injection is either stopped or reduced to avoid exceeding this limit.

[0035] Crude oil in the unheated formation may have a viscosity between 200 and 2000 centipoises and the water in the aquifer may have a viscosity of between 0.4 and 1 centipoises.

[0036] Computer simulations indicate that after air injection is discontinued the water influx from the aquifer into the production well(s) is significantly reduced. Computer simulations of results of the method according to the invention are illustrated in FIG. 3. They show that;

a) crude oil is produced as illustrated by line 11 at lower water cut 12 after air has been injected as illustrated by line 13 compared to the cold case (60 - 80 % water cut versus 95% water cut);

b) oil production rates 11 and therefore ultimate oil recoveries increase significantly compared with the non-thermal case illustrated by line 11 before air injection or ISC 13 commences.

[0037] Computer simulations also indicate that it is relevant to reduce the viscosity of the crude oil in the region between the air injection wells 3 and 4 and the crude oil production well 5 in order to enhance oil production and to reduce fractional flow of water and that the crude oil containing formation 1 should preferably be heated such that the viscosity of at least a substantial fraction of the crude oil in the region between the air injection wells 3,4 and the oil producer 5 is less than 50 centipoise (50 cP, or 0.05 Pa.s), more preferably to less than 30 centipoise (30 cP or 0.03 Pa.s), most preferably to less than 10 centipoise (10 cP or 0.01 Pa.s) such that the difference between the viscosities and mobility ratios of crude oil and water is significantly reduced.

Claims

1. A method for enhancing recovery of crude oil from a crude oil containing formation which overlies a water containing formation, the method comprising:

- producing crude oil through at least one crude oil production well having an inflow zone, which traverses the crude oil containing formation and in which a fluid pressure is lower than a fluid pressure in the water containing formation; and

- injecting an oxidant into the crude oil containing formation and igniting a mixture of oxidant and crude oil to heat the hydrocarbon containing formation such that the viscosity of at least a substantial fraction of uncombusted crude oil in the the crude oil containing formation is reduced to such a level that fractional flow of water from the water containing formation into the inflow zone is reduced.

2. The method of claim 1, wherein the viscosity of said at least substantial fraction of uncombusted crude oil is reduced to less than 50 centipoise (50 cP, or 0.05 Pa.s).

3. The method of claim 1, wherein the viscosity of said at least substantial fraction of uncombusted crude oil is reduced to less than 30 centipoise (30 cP or 0.03 Pa.s).

4. The method of claim 1, wherein the viscosity of said at least substantial fraction of uncombusted crude oil is reduced to less than 10 centipoise (10 cP or 0.01 Pa.s).

5. The method of claim 1, wherein the oxidant is injected through an oxidant injection well which has an oxidant injection zone traversing the crude oil containing formation at a selected distance from the inflow zone of the crude oil production well.

6. The method of claim 1, wherein flue gases stemming from the combustion of crude oil are vented to surface through the crude oil production well.

7. The method of claim 1, wherein oxidant injection into the crude oil containing formation is interrupted when the produced crude oil comprises a gaseous fraction comprising more than 3 mole %, or optionally more than 5 mole % of oxygen.

8. The method of claim 1, wherein the oxidant comprises air, oxygen or oxygen enriched air.

9. The method of claim 1, wherein the oxidant injection zone is located at a selected horizontaland vertical distance from the inflow zone of the crude oil production well.The method of claim 9, wherein oxidant is injected into the crude oil containing formation to heat the hydrocarbon containing formation such that the viscosity of at least a substantial fraction of uncombusted crude oil in the region of the crude oil containing formation between the inflow zone of the production well and the Water Oil Contact (WOC)is reduced to less than 30 cP, preferably to less than 10 cP.

10. The method of claim 1, wherein the inflow zone of the crude oil production well has a substantially horizontal orientation and wherein the water containing formation is located below the crude oil containing formation.

11. The method of claim 11, wherein at least a substantial part of the crude oil containing formation has a height H above the water containing formation and at least a substantial part of the inflow zone of the crude oil production well is located at a height of between 0.2 H and 0.5 H above the water containing formation when oxidant injection is started.

12. The method of claim 12, wherein at least a substantial part of the inflow zone of the crude oil production well is located at a height between 0.3 H and 0.4 H above the water containing formation when oxidant injection is started.

13. The method of claim 1, wherein oxidant injection is terminated when the combustion front approaches the inflow zone of the crude oil production well, whereupon the fluid pressure of the water in the water containing formation sweeps the heated oil to the inflow zone of the crude oil production well.

Drawing