Improving crude oil production from a layered oil reservoir

Abstract

 A method of improving the oil-productivity and recovery of a layered oil reservoir (1) having an upper layer (2) that is more permeable than a lower layer (3) of the oil reservoir (1), wherein:
- a viscosified aqueous liquid is injected through an injection well (5) into the upper layer (2)
- a non-aqueous fluid that is miscible with the oil is injected into the lower layer (3),
- oil is recovered from the layered formation through a production well (10),
- the viscosity of the aqueous liquid is so selected that the pressure gradient in the injected aqueous liquid in the upper section (2) is larger than or equal to the pressure gradient in the injected non-aqueous fluid in the lower section (3) to confine the no-aqueous fluid in the lower section (3), or
the pressure gradient in the upper layer (2) enhances cross flow of the aqueous liquid into the lower layer (3) or the pressure gradient in the upper layer (2) is lower than that of the lower layer (3) to allow limited cross flow of the non-aqueous fluid into the upper layer (2), and
- at least one of the injection and production wells (5,10) comprises a permeable fluid transfer section with a substantially horizontal orientation.

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method of improving the productivity and recovery of oil reservoirs. The method comprises injecting viscosified aqueous liquid and miscible gas in reservoirs of high permeability contrast using vertical and/or horizontal wells and for a range of aqueous liquid viscosity and aqueous liquid/gaseous fluid ratio's.
The invention presents a novel solution to reservoirs of different levels of heterogeneity, both lateral and vertical heterogeneity, and different wettability characteristics (mixed-wet to oil-wet).

[0002] Fluid flow in porous media is governed by the interaction of viscous, gravity and capillary forces. It is well recognized that cross-flow has significant impact on sweep efficiency of immiscible displacement in layered reservoirs. Both gravity and viscous forces have been extensively studied in the literature. However, the impact of capillary forces is generally under-estimated in field simulation studies especially for carbonate reservoirs.

[0003] In layered reservoirs where a high-permeability zone is located above a low-permeability zone, gravity leads to cross flow of injected water from the top to the bottom zone of the reservoir. Moreover, in water-wet reservoirs, capillary forces align with gravity (i.e. act in the same direction, helping to flood the Lower reservoir) and result in very good sweep efficiency of the low permeability lower zone. However, for non-water-wet layered reservoirs capillary forces will act opposite to gravity and that results in a barrier which retards water from moving downwards, limits cross-flow between the different zones and leads to poor sweep efficiency of the lower zone.

[0004] A similar phenomenon happens in reservoirs or zones where high frequency cycles of high and low-permeability layers exist. In this case the low permeability layers within such zones will be by-passed as capillary forces will act as barrier to cross-flow.

[0005] The current invention addresses both kind of heterogeneity (upper and lower zone permeability contrast and high/low permeability layers within the upper zone).

[0006] US patent 4,715,444 discloses a method of recovering hydrocarbons from an underground hydrocarbon-containing formation penetrated by at least an injection well and a production well, which method comprises the steps of:

(a) injecting a gaseous stream into the formation near the bottom of the formation through the injection well;

(b) injecting an aqueous stream into the formation near the top of the formation through the injection well; and

(c) recovering hydrocarbons from the formation through the production well.

[0007] The gaseous stream used in the known method can include carbon dioxide, nitrogen, light hydrocarbon gases or mixtures thereof, and the aqueous stream can be water or brine.

[0008] As the streams move through the formation, the aqueous stream tends to move towards the lower part of the formation, and the gaseous stream tends to move to the upper part of the formation. As the streams move to the production well, an interference zone is created in which the aqueous stream and the gaseous stream mix. The mixture moves through the formation and provides a good sweep of the formation and a good recovery of oil from the formation.

[0009] The known method is applied in a reservoir consisting of a single layer. The invention, however, relates to a method of improving the productivity of a layered oil reservoir, which has an upper reservoir section that is more permeable to fluid flow than the lower reservoir section.

[0010] It is an object of the present invention to provide a method of improving the oil-productivity and oil recovery of a layered oil reservoir, wherein the mobility of the aqueous stream and the ratio of the injection rates for aqueous liquid and non-aqueous fluid are controlled in order to either prevent mixing of the two streams or enhance cross flow between the two zones such that both sweep efficiency and oil recovery from both zones are improved. The invention is particularly suited for layered oil reservoirs having a zone that is more permeable than the other zones and where there is an impediment for water to flow from the upper to the lower reservoir section due to e.g. (vertical) permeability reduction at the interface or a capillary pressure barrier. It is also applicable for improving oil recovery from the low permeable layers inter-bedded within the more permeable reservoir unit.

[0011] The current invention aims to provide a method that is stable and robust to reservoir lateral and vertical heterogeneity using both vertical and horizontal well technology. As the main reason for the low recovery factor of oil and the poor sweep efficiency of the lower zone is the lack of cross flow of water from the upper zone to the lower zone by either vertical permeability reduction at the interface and/or capillary pressure barrier, improved recovery can be achieved by either:

1-Control the mobility of the injected fluids in the upper and lower zones in order to prevent mixing of the two streams such that the injected gaseous fluid is confined in the lower zone leading to enhanced oil recovery. This is achieved by keeping the upper zone pressurized by continuous water injection and simultaneously injecting gas into the lower zone. A lateral pressure gradient is maintained in the Upper zone, providing Lower zone gas confinement.

2-Enhance cross-flow of the aqueous phase from the upper to the lower zone by increasing the viscous force vertical component to overcome the barrier to flow (either permeability reduction or capillary forces). This is achieved by reducing the mobility of the fluid in the upper zone through, for example, polymer solution, polymer-surfactant solution or alkaline-polymer surfactant, foam, hence forcing it to cross-flow into the lower zone and to the low permeable layers within the upper zone.

3-A combination of the above two mechanisms, i.e., any combination of viscosified water or foam and (miscible) gas injection ratios, from complete confinement of gas in the lower zone (no cross flow) or complete viscous dominated flow (viscosified aqueous liquid alone) across the entire reservoir interval.

[0012] In addition, the current invention has the following aspects:

1- Improves oil recovery from the low permeable layers inter-bedded within the high permeable layers in the upper part of the reservoir by enhancing cross flow with the upper zone.

2- Improves frontal advance of the injected fluids and sweep of oil by using horizontal well technology. This increases the reservoir area that is in direct contact with injected fluids and thus enhances the recovery of un-swept oil.

3- In the current invention the required increase in aqueous liquid viscosity depends on the actual permeability contrast between the upper and lower zone, in most cases only a modest increase in viscosity is required. Viscous cross-flow of viscosified aqueous liquid from high permeable layers to low permeable layers improves the recovery in both upper and lower zones.

4-The current invention provides high sweep efficiency for all cases of aqueous liquid/gaseous fluid injection ratio, up to 100% of aqueous liquid.

5- In the current invention an extra increase in oil recovery is achieved by reducing residual oil saturation because of the low interfacial tension between the injected fluids and the reservoir oil. This occurs in cases of injecting surfactant, surfactant-polymer or alkaline-surfactant-polymer solution in the upper zone or by allowing cross flow of the injected miscible gaseous fluid to the upper zone.

6-The above shows that the process is robust against some interruptions in execution, it is robust to some injection interruptions and injection ratio's and it is not critically dependent on maintaining an optimal aqueous liquid viscosity nor is it very sensitive to injection rates.

SUMMARY OF THE INVENTION

[0013] In accordance with the invention there is provided a method of improving the oil-productivity and recovery of a layered oil reservoir having an upper layer that is more permeable than a lower layer of the oil reservoir, which method comprises:

• injecting through an injection well a viscosified aqueous liquid into the upper layer;
• injecting a non-aqueous fluid that is miscible with the reservoir oil into the lower layer; and

• recovering oil from a production well; wherein:
• the viscosity of the aqueous liquid is so selected that the pressure gradient in the injected aqueous liquid in the upper section is larger than or equal to the pressure gradient in the injected non-aqueous fluid in the lower section or that the pressure gradient in the upper layer allows limited cross flow of the gaseous fluid from the lower into the upper layer; and
• at least one of the injection and production wells comprises a permeable fluid transfer section with a substantially horizontal orientation.

[0014] Optionally the viscosity of the aqueous liquid is selected such that the pressure gradient in the injected aqueous liquid in the upper section forces cross flow of the injected fluids from the upper zone to the lower zone.

[0015] Optionally, the viscosified aqueous liquid is injected through a first injection well and non-aqueous fluid is injected through a second injection well with a substantially horizontal permeable fluid transfer section through which the non-aqueous fluid is injected into the lower layer. The first and second injection wells may be formed by substantially horizontal branches of a branched multilateral injection well.

[0016] The production well may also have a substantially horizontal fluid transfer section, which is oriented substantially parallel to the substantially horizontal fluid transfer section of the second injection well.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The invention will now be described by way of example in more detail with reference to the accompanying drawings, wherein:

Figure 1 shows schematically a cross-section of a subsurface two-layer oil reservoir;

Figure 2 shows a comparison between the method according to the present invention and a typical water flood;

Figure 3 is a schematic three-dimensional view of a layered oil reservoir which is traversed by and a production well having a substantially horizontal inflow section and two injection wells of which one has a substantially horizontal permeable lower fluid transfer section; and

Figure 4 is a schematic three-dimensional view of a layered oil reservoir which is traversed by a branched fluid injection well having two substantially horizontal fluid injection branches and a production well having a substantially horizontal inflow section.

DETAILED DESCRIPTION OF THE DEPICTED EMBODIMENTS

[0018] Reference is now made to Figure 1.
Figure 1 shows schematically a layered subsurface oil reservoir 1. For the sake of clarity the layers above and below the reservoir 1 are not shown. The layered oil reservoir 1 comprises an upper layer 2 and a lower layer 3. The upper layer 2 of the oil reservoir 1 is more permeable than the lower layer 3. The layered reservoir is penetrated by a first injection well 5 and a second injection well 7, and by a production well 10 having a substantially horizontal perforated lower inflow section 10A.

[0019] Through the first injection well 5 a viscosified aqueous liquid is injected into the upper layer 2 of the layered reservoir 1. Simultaneously a non-aqueous fluid that is miscible with the oil is injected through the second injection well 7 into the lower layer 3. Produced oil is recovered from the layered oil reservoir 1 through the perforated lower inflow section 10A of the production well 10.

[0020] Instead of injecting the aqueous and gaseous streams through two wells 5 and 7 with perforated intervals in the upper and lower section of the reservoir respectively, at a close distance, the streams can be injected through two strings in one well (not shown).

[0021] In accordance with the invention, the viscosity of the aqueous liquid is so selected that

1- The pressure gradient in the upper reservoir layer 2 is larger than or equal to the pressure gradient in the lower reservoir layer 3. In this case the injected fluids are confined to their respective zones and minimum or no mixing of the two streams occur.

2- The pressure gradient and the mobility of the aqueous liquid in the upper layer 2 enhance the cross flow to the lower layer 3 and improve sweep efficiency and oil recovery from the lower layer 3.

3- The pressure gradient in the upper layer 2 allows limited cross flow of the gaseous fluid to the upper zone which leads to improving oil recovery in the lower layer 3 (as most of the gas is still confined there) and from the upper layer 2 due to reduction in residual oil saturation because of the low interfacial tension between the gas and the oil reservoir in the upper layer 2.

[0022] The required viscosity for the 3 cases described above can be calculated based on permeability contrast and characteristics of the injected gaseous fluid. For example the required viscosity for the case 1 above (pressure gradient in the upper layer 2 is equal or larger than that of the lower layer 3) can be put as follows in an equation:

wherein the variables are listed in Table 1.
This condition can also be written as

[0023] In accordance with the invention (and case 1 above) the ratio of the injection rates for aqueous liquid and non-aqueous fluid is suitably so selected that the rate of advance of the aqueous liquid (arrow 11) in the upper layer 2 is larger than or equal to the rate of advance of the non-aqueous fluid (arrow 12) in the lower layer 3. In this way the injected non-aqueous fluid is prevented from entering the upper layer 2. However, downstream of the fronts 13 and 14, the flow rate of the hydrocarbons (arrow 15) flowing through the more permeable upper layer 2 is much larger than the flow rate of the hydrocarbons (arrow 16) flowing through the less permeable lower layer 3.

[0024] The rate of advance of a fluid in a formation is proportional to the storage capacity of the formation. A good approximation of the rate of advance of the aqueous liquid in the upper layer 2 is:

The rate of advance of the non-aqueous fluid in the lower layer 3 is approximated by

That the rate of advance of the aqueous liquid in the upper layer 2 is larger than or equal to the rate of advance of the non-aqueous fluid in the lower layer 3, is equivalent to stating that v_w ≥ v_g. With the two equations for the velocity, it follows that

Suitably,

wherein C is greater than or equal to 1.

[0025] If condition

is met then the requirement that the pressure gradient in the upper reservoir layer 2 is larger than or equal to the pressure gradient in the lower reservoir layer 3 can be written as:

Suitably,

wherein D is greater than or equal to 1.

[0026] Suitably, the aqueous liquid is water, seawater or brine. The viscosity of the aqueous liquid can be adjusted to the required amount by adding a suitable amount of a viscosifier to the aqueous liquid. Polymers and surfactants (polymer, surfactant, polymer-surfactant, alkaline-polymer-surfactant) are suitable viscosifiers.

[0027] Suitably, the gaseous fluid can include carbon dioxide, hydrogen sulphide and lower hydrocarbons. The gas can also be a first contact miscible or multi-contact miscible.

[0028] The invention will now be described with reference to the below example.

[0029] A numerical model that comprises the most salient characteristics of an actual reservoir will serve to illustrate the merits of the invention. There are two geological sections with a permeability contrast of up to 100:1, wherein the upper layer 2 has the higher permeability. About 60% of the original oil in place is in the lower layer 3. The non-aqueous, miscible injection fluid (supplied through the second injection well 7) has (in-situ) a viscosity of 0.06 cP. The injection rate of the non-aqueous, miscible fluid is 50% higher (subsurface volumes) than the injection rate of the aqueous liquid (supplied through the first injection well 5), and the viscosity of the aqueous liquid is adjusted to 4 cP. In this way the conditions of the invention have been complied with.

[0030] Reference is now made to Figure 2, which shows a comparison between the method according to the present invention and a typical water flood. On the horizontal axis is put the amount of liquid injected (both aqueous and gaseous) into the upper and lower layers 2&3 in pore volumes injected. On the left vertical axis we put the cumulative amount of oil produced (as a fraction of the original oil in place) and the water-cut (as a volume fraction of water in the mixture of water and oil), and on the right vertical axis we put the gas-oil-ratio (in Mcuft at standard pressure and temperature per barrel of oil). The forecasts were generated with a three dimensional numerical model, which simulated a line drive of horizontal injector(s)/producer(Solid line 20 shows the cumulative oil production for the method according to the present invention, and dashed line 21 shows the cumulative oil production for the water flood. Solid line 24 shows the water-cut for the method according to the present invention and dashed line 25 shows the water-cut for the water flood. Solid line 28 shows the gas-oil-ratio for the method according to the present invention and dashed line 29 shows the gas-oil-ratio for the water flood.

[0031] Figure 2 illustrates how oil production from a layered oil reservoir is enhanced by the method according to the present invention.

[0032] Figure 3 is a schematic three dimensional view of a layered oil reservoir 2,3 which is traversed at one side thereof by a production well 10 having a substantially horizontal inflow section 10A, and at an opposite side by a substantially vertical first injection well 5 and a second injection well 7, which second injection well 7 has a substantially horizontal permeable lower fluid transfer section 7A.

[0033] Figure 4 is a schematic three dimensional view of a layered oil reservoir 2,3 which is traversed by a production well 10 having a substantially horizontal inflow section 10A and a branched multilateral fluid injection well 5,7 having an upper substantially horizontal fluid injection branch 5A through which an aqueous liquid is injected into the upper layer, as illustrated by arrow H₂O, and a lower substantially horizontal fluid injection branch 7A, through which a non-aqueous fluid is injected into the lower layer 3 as illustrated by arrow CO2 + H2S.
The substantially vertical upper section (5,7) of the branched multilateral fluid injection well may comprise co-axial or parallel liquid and fluid injection conduits (not shown) through which aqueous liquid is supplied to the upper branch 5A and through which non-aqueous fluid is supplied to the lower branch 7A.

Table 1: List of variables used in the equations.

Variable	Description	Dimension	Unit
C	constant	-	-
D	Constant	-	-
hl	thickness of the lower section of the layered oil reservoir	L	M
hu	thickness of the upper section of the layered oil reservoir	L	m
krg	relative permeability of the non-aqueous fluid	-	-
krw	relative permeability of the aqueous liquid	-	-
Kl	permeability of the lower section of the layered reservoir	L2	Darcy
Ku	permeability of the upper section of the layered reservoir	L2	Darcy
qg	injection rate of the non-aqueous fluid	L3T-1	m3/s
qw	injection rate of the aqueous fluid	L3T-1	m3/s
vg	rate of advance of the non-aqueous liquid	LT-1	m/s
vw	rate of advance of the aqueous liquid	LT-1	m/s
W	width reservoir	L	m
µg	viscosity of the non-aqueous fluid	ML-1T-1	cP
µw	viscosity of the aqueous fluid	ML-1T-1	cP
ϕl	porosity of the lower section	-	-
ϕu	porosity of the upper section	-	-

Claims

1. A method of improving the oil-productivity and recovery of a layered oil reservoir having an upper layer that is more permeable than a lower layer of the oil reservoir, wherein:

- a viscosified aqueous liquid is injected through an injection well into the upper layer;

- a non-aqueous fluid that is miscible with the oil is injected into the lower layer, and

- oil is recovered from the layered oil reservoir through a production well,

- the viscosity of the aqueous liquid is so selected that the pressure gradient in the injected aqueous liquid in the upper layer is larger than or equal to the pressure gradient in the injected non-aqueous fluid in the lower layer, or
that the pressure gradient in the upper layer allows limited cross flow of the gaseous fluid from the lower into the upper layer; and

- at least one of the injection and production wells comprises a permeable fluid transfer section with a substantially horizontal orientation.

2. The method of claim 1, wherein the viscosified aqueous liquid is injected through a first injection well and non-aqueous fluid is injected through a second injection well.

3. The method of claim 2, wherein the second injection well has a substantially horizontal permeable fluid transfer section through which the non-aqueous fluid is injected into the lower layer.

4. The method of claim 3, wherein the production well has a substantially horizontal fluid transfer section which is oriented substantially parallel to the substantially horizontal fluid transfer section of the second injection well.

5. The method according to claim 1, wherein the ratio of the respective injection rates is so selected that the rate of advance of the aqueous liquid in the upper layer is larger than or substantially equal to the rate of advance of the non-aqueous fluid in the lower layer.

6. The method of any preceding claim, wherein the aqueous liquid comprises a polymer, a surfactant polymer and/or an alkaline surfactant polymer.

7. The method of any preceding claim, wherein the viscosity of the aqueous liquid is a factor of 2 or higher than the injected water.

8. The method of any preceding claim, wherein the non-aqueous fluid comprises Carbon Dioxide and/or Hydrogen Sulphide.

9. The method of any preceding claim where the non-aqueous fluid is first contact miscible or multi-contact miscible with the reservoir oil.

10. The method of any preceding claim, wherein the average permeability of the upper layer is higher than the average permeability of the lower layer.

11. The method of claim 3, wherein the first and second injection wells are substantially parallel horizontal branches of a multilateral well.

12. The method of any preceding claim, wherein the crude oil produced from the layered oil reservoir is converted into a transportation fuel and/or other marketable products.

Drawing