GB2379685A

GB2379685A - Enhanced oil recovery with asynchronous cyclic variation of injection rates

Info

Publication number: GB2379685A
Application number: GB0225019A
Authority: GB
Inventors: Eliana Milena Arenas; Norbert Dolle; Tsuyoshi Matsuura; Anthony Robert Thompson
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 2002-10-28
Filing date: 2002-10-28
Publication date: 2003-03-19
Also published as: GB0225019D0

Abstract

A method for enhanced oil recovery (EOR) from an oil-bearing formation (8) involves cyclically varying fluid injection from different longitudinally spaced segments of a fluid injection well (1) into different regions (2, 3) of said formation. The fluid injection rates are cyclically and asynchronously varied. The pattern of injection may be selected such that a fluid injection peak coincides with a temporary interruption of fluid injection in another segment. Water may be used as the injection fluid.

Description

ENHANCED OIL RECOVERY PROCESS WITH CYCLIC VARIATION OF FLUID INJECTION RATE BACKGROUND OF THE INVENTION The invention relates to an enhanced oil recovery ("EOR") process wherein fluid is injected into an oil bearing formation via one or more fluid injection wells in order to enhance the production of crude oil from the formation.

The injected fluid may comprise water, steam, a steam foam or froth, nitrogen, and/or carbon dioxide. Water is the most commonly used stimulating fluid in EOR processes.

As a drive process'waterflooding'is sensitive to mobility ratios and reservoir heterogeneities. It is not uncommon to find'mature'waterflood projects in fractured and un-fractured carbonates in the Middle East and in the onshore of the US (Permian and Williston Basins) where water cuts approach uneconomic levels in many patterns while recovery factors are low.

Most water injection projects are not taking place under matrix conditions, but through induced fractures.

This is due to: insufficient water quality (particularly true when produced water is used), thermal effects, high reservoir oil viscosity, significant depletion, low permeability, and unfavourable rock properties. Regarding water quality, achieving the required treatment level to allow matrix injection is a costly and technically difficult operation that can turn many waterflood projects uneconomical.

Induced fractures should be seen as an integral part of most waterflood projects, which require proper management of their propagation and of their potential impact. If unsuccessful, consequences could range from short-circuiting the oil bank (poor sweep efficiency) to breaking of the cap-rock.

At the core of a successful waterflood is sweep efficiency. Natural or injection-induced fractures play different roles at injectors and producers, limiting our ability to manage fluid fronts and sweep.

Mitigation of early water breakthrough is currently done by injecting water blocking chemicals into the formation surrounding an oil production well and/or by inhibiting influx of water into selected regions of the oil production well by inserting expandable seals into the region (s) where water flows into the well. They can provide, if successful, an effective solution.

However, in tight reservoirs, where injection into the matrix is either only possible at extremely low rates or creates new fractures, fracture shut-off might not be the preferred solution. Employing the fractures to transfer water into the matrix, basically as an extension of the injector well, could be beneficial as long as the water injected into the fractures can be controlled.

One approach for controlling the water injection from a water injection well in highly fractured reservoirs is by a method known as Pressure Pulsing, which makes use of controlling the entire injector well. The well is injecting water into the reservoir, i. e. the fractures, until the watercut in the producer reaches an unacceptable level. At that moment the injector is shutoff, while the producer remains active, until the watercut has dropped.

In theoretical studies, this method has proved to result in additional oil recovery. However, in particular for heterogeneous fracture patterns and/or matrix properties, the performance of the known Pressure Pulsing EOR method is limited and there is a need for a more effective method of injection control that is able to enhance the sweep efficiency even more.

SUMMARY OF THE INVENTION In accordance with the invention there is provided a method for enhancing recovery of crude oil from an oil bearing formation by injecting a fluid from a fluid injection well through said formation towards an oil production well, wherein the fluid injection well comprises a plurality of longitudinally spaced segments through which fluid is injected into different regions of the formation, such that the fluid injection rates in said segments are cyclically varied in asynchronous cyclic patterns.

The injection well may comprise a first and a second segment, and fluid injection may be cyclically altered between said first and second segment.

As a first step of the method according to the invention, the fluid may be initially injected simultaneously via said segments into the oil bearing formation until at least some injected fluid reaches the oil production well, whereupon at least one segment of the fluid injection well is substantially closed and asynchronous cyclic variation of the fluid injection rate via various segments of the fluid injection well is initiated.

The cyclically injected fluid may be water or an aqueous fluid, such as steam, steam foam or a froth.

The method according to the invention may be applied in a tight oil bearing formation, such as a carbonate formation, which comprises natural and/or induced fractures.

The fluid injection well may comprise a substantially horizontal or tilted region through which fluid is injected into the oil-bearing formation.

The invention also relates to a fluid injection well which is configured for use in the method according to the invention, which well comprises various segments for injecting fluid into various regions of an oil bearing formation and means for cyclically varying the fluid injection rate in said segments in an asynchronous pattern.

Suitably, said segments are separated by a number of open hole packers and each segment is connected to a fluid source via an interval control valve ("ICV"), which is configured to be cyclically opened and closed.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained in more detail and by way of example with reference to the accompanying drawings, in which: Fig. l schematically depicts a partially fractured reservoir into which fluid is cyclically injected via a first and second segment of fluid injection well in order to increase crude oil production from an adjacent oil production well; and Fig. 2 is a graph which shows a comparison on the basis of a numerical reservoir simulation technique of the calculated cumulative crude oil recovery of a conventional continuous waterflooding EOR process, of a waterflooding process with pressure pulsing the entire

injector well and of the cyclically moving waterflooding process according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS As depicted in Fig. l a fluid injection well according to the invention may be a horizontal open-hole injection well 1 which is divided into first and a second independently controlled fluid injection segments or zones 2,3 by employing one or more open hole packers 4 and open/close interval control valves (ICV's) 5,6. Water is injected in all intervals until the water-oil ratio of the effluents of the oil production well 7 reaches a high unacceptable threshold.

The injection interval (s) responsible for this behaviour is (are) shut-in while the production well 7 drains the injected water from the fracture network as well as the oil and water from the matrix 8 that flows into the fractures 9 when the pressure gradient is reversed.

Cycling of the fracture pressure level by opening and closing segments 2,3 of the injection well 1 is repeated over the life of the well. Water huff & puff results in an increased depletion of the oil in proximity to fracture interfaces 9. As with steam huff & puff, the oil/water ratio will decline cycle after cycle. The total incremental recovery will be a function of fracture density and fracture/matrix effective perm ratio, among other variables.

An advantage of the method according to the invention, compared to conventional waterflooding, is the use of the existing or induced fractures 9 to inject considerable amounts of water into the reservoir 8 while at the same time controlling the produced water.

The latter is important because the producer well 7, in

general, operates on a maximum liquid production constraint.

Simulations have been performed to test the application of pressure cycling in reservoirs with one distinct fractured zone. Fig. 2 illustrates the advantage of pressure cycling (curve 32) compared to permanent shut-off (curve 30). The advantage is dependent on the ratio of the matrix injectivity and the fractured zone injectivity. Relative improvement of pressure cycling compared to fracture shut-off versus this injectivity ratio is plotted in Fig 2. Typical improvements in the order of 10-30%, with peaks up to 60%, relatively to fracture shut-off are found.

The concept of pressure cycling has also been tested in densely fractured reservoir models. Fractures, again either natural or injection induced, are present along the entire length of the injector well. The injector 1 is divided into multiple intervals 2,3, each individually controllable with ICVs 5,6. Different permeabilities and lengths are assigned to each fracture.

Compared to the previously described prior-art of Pressure Pulsing, the Pressure Cycling technique according to the invention offers the flexibility to use and benefit from different zones with different flow characteristics to achieve an improved reservoir sweep and being able to counteract and control changing downhole conditions (e. g. new fractures).

Figure 2 illustrates that pressure cycling results in considerable additional recovery in heavily fractured reservoirs compared to conventional waterflooding and pressure pulsing. In Figure 2 the lowest curve 30 indicates that the cumulative crude oil recovery from a

conventional continuous waterflooding technique is about 18.1 % of total oil in place.

The intermediate curve 31 in Fig. 2 indicates that the cumulative crude oil recovery from the same reservoir with a conventional waterflooding process with pressure pulsing is about 20. 3% of total oil in place.

The upper curve 32 in Fig. 2 indicates that the cumulative crude oil recovery from the waterflooding process according to the invention, wherein fluid injection pressure is cyclically and alternatingly varied in the various segment (s) of the fluid injection well, is about 25.7% of total oil in place.

Thus, it will be appreciated that the cyclic variation of the fluid injection rate in various segments of the fluid injection well in accordance with the present invention generates a substantial improvement of the cumulative crude oil production from an oil-bearing formation.

Claims

C L A I M S 1. A method for enhancing recovery of crude oil from an oil bearing formation by injecting a fluid from a fluid injection well through said formation towards an oil production well, wherein the fluid injection well comprises a plurality of longitudinally spaced segments through which fluid is injected into different regions of the formation, such that the fluid injection rates in said segments are cyclically varied in asynchronous patterns.
2. The method of claim 1, wherein the injection well comprises a first and a second segment, and fluid injection is cyclically altered between said first and second segment.
3. The method of claim 1 or 2, wherein initially fluid is injected simultaneously via said segments into the oil bearing formation until at least some injected fluid reaches the oil production well, whereupon at least one segment of the fluid injection well is substantially closed and cyclic asynchronous variation of the fluid injection rate via various segments of the fluid injection well is initiated.
4. The method of any preceding claim, wherein the injected fluid is an aqueous fluid.
5. The method of claim 4, wherein the aqueous fluid is water.
6. The method of any preceding claim, wherein the oil bearing formation comprises natural and/or induced fractures.

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7. The method of any preceding claim, wherein the oil bearing formation is a carbonate formation.
8. The method of any preceding claim, wherein the fluid injection well comprises a substantially horizontal or tilted region through which fluid is injected into the oil bearing formation.
9. The method of any preceding claim, wherein the fluid injection well is divided into a plurality of hydraulically separated segments by a number of open hole packers.
10. The method of any preceding claim, wherein the fluid injection well comprises a plurality of control valves for cyclically varying fluid influx into the various segments.
11. A fluid injection well for use in the method of any preceding claim, the well comprising various segments configured for injecting fluid into various regions of an oil bearing formation and flow control means for cyclically varying the fluid injection rates in said segments.
12. The fluid injection well of claim 11, wherein said segments are separated by a number of open hole packers.
13. The fluid injection well of claim 11 or 12, wherein each segment is connectable to a fluid source via flow control means comprising an interval control valve, which is configured to be sequentially opened and closed.