Producing hydrocarbons

Abstract

A method and apparatus for producing hydrocarbons from a subterranean formation. A first well is provided in the formation, the first well being separated by an isolating material into at least a first and second zone, the first zone being substantially isolated from the second zone. A second well is provided in the formation. The second well is separated by an isolating material into at least a first and second zone, the first zone being substantially isolated from the second zone. A first fracture is provided in the formation, the first fracture extending substantially between the first zones. A second fracture is provided in the formation, the second fracture extending substantially between the second zones of the first and second wells. A fluid is injected into the formation from the first zone in the first well. Hydrocarbons are produced at the second zone of the second well.

Claims

1. A method of producing hydrocarbons from a subterranean formation, the method comprising: providing a first well in the formation, the well separated by an isolating material into at least a first and a second zone, the first zone being substantially isolated from the second zone; providing a second well in the formation, the second well separated by an isolating material into at least a first and a second zone, the first zone being substantially isolated from the second zone; providing a first fracture in the formation, wherein the first fracture is a continuous, uninterrupted zone of fractures extending from the first zone of the first well to the first zone of the second well; providing a second fracture in the formation, wherein the second fracture is a continuous, uninterrupted zone of fractures extending from the second zone of the first well to the second zone of the second well; injecting a fluid into the formation from the first zone in the first well, to thereby put a portion of the formation between the first fracture and the second fracture under pressure; and producing hydrocarbons at the second zone of the second well.

2. The method according to claim 1, wherein each zone comprises openable openings providing a communicating path between the wells and the formation, the method comprising opening the openings in the first zone of the first well and the second zone of the second well, and closing the openings in the second zone of the first well and the first zone of the second well.

3. The method according to claim 2, the method further comprising closing the openings in the first zone of the first well and the second zone of the second well, and opening the openings in the second zone of the first well and the first zone of the second well, and injecting a fluid into the formation from the second zone of the first well.

4. The method according to claim 1, wherein the fluid is selected from any of carbon dioxide, hydrocarbons, methane, produced gas, nitrogen, hydrogen sulphide, water, surfactant, alkali, ketones, alcohols, aromatic hydrocarbons, hydrocarbons, solvents, and acid.

5. The method according to claim 1, wherein the fluid comprises any of a diluent, a solvent, a reactant and a surfactant.

6. The method according to claim 1, comprising providing the fractures by performing an operation selected from any of hydraulic fracturing, thermal fracturing, mechanical fracturing, and a combination thereof.

7. The method according to claim 1, wherein at least a portion of the first and second fractures are substantially perpendicular to a main axis of the first and second wells.

8. The method according to claim 1, wherein the first and second wells are disposed substantially horizontally in the subterranean formation.

9. The method according to claim 1, wherein the subterranean formation comprises a low permeability formation.

10. The method according to claim 1, wherein the first and second zone of each wellbore are hydraulically isolated from each other using any of a packer, a swell packer, a hydraulically set packer, and cement.

11. The method according to claim 1, further comprising subsequently changing the location of an interface between the first and second zones of either of the first and second wells.

12. A system for producing hydrocarbons from a subterranean formation, the system comprising: a first well in the formation, the well separated by an isolating material into at least a first and a second zone, the first zone being substantially isolated from the second zone; a second well in the formation, the second well separated by an isolating material into at least a first and a second zone, the first zone being substantially isolated from the second zone; a first fracture in the formation, wherein the first fracture is a continuous, uninterrupted zone of fractures extending from the first zone of the first well to the first zone of the second well; a second fracture in the formation, the second fracture is a continuous, uninterrupted zone of fractures extending from the second zone of the first well to the second zone of the second well; an injector for injecting a fluid into the formation from the first zone in the first well to thereby put a portion of the formation between the first fracture and the second fracture under pressure, wherein the injection of the fluid leads to production of the hydrocarbons at the second, zone of the second well.

13. The system according to claim 12, further comprising openable openings in each zone, the openings providing a communicating path between each well and the formation.

14. The system according to claim 13, wherein the openable openings are configured to be selectively opened or closed, to respectively determine that either the zone will be an injector zone or a production zone, or the zone will not be an injector zone or a production zone.

15. The system according to claim 12, wherein the injected fluid is selected from any of carbon dioxide, hydrocarbons, methane, produced gas, nitrogen, hydrogen sulphide, water, surfactant, alkali, ketones, alcohols, aromatic hydrocarbons, hydrocarbons, solvents, and acid.

16. The system according to claim 12, wherein the injected fluid comprises any of a diluent, a solvent, a reactant and a surfactant.

17. The system according to claim 12, wherein the first and second fractures are substantially perpendicular to a main axis of the first and second wells.

18. The system according to claim 12, wherein the first and second wells are disposed substantially horizontally in the subterranean formation.

19. The system according to claim 12, wherein the subterranean formation comprises a low permeability formation.

20. The system according to claim 12, wherein the first and second zone of the each wellbore are hydraulically isolated from each other using any of a packer, a swell packer, a hydraulically set packer and cement.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates schematically a cross section view of a formation having a first and a second well;

(2) FIG. 2 is a flow diagram showing exemplary steps; and

(3) FIG. 3 is a graph comparing productivity of primary oil depletion compared with oil depletion using the techniques described herein.

DETAILED DESCRIPTION

(4) Described herein is a method and system for enhanced oil recovery, which can be particularly useful for tight and ultra-tight formations such as but not restricted to shale oil formations or formations considered to be shale-rich formations. Reservoirs in low or ultra-low permeability formations are often termed shale reservoirs, but may also be other types of reservoir such as tight carbonate or sandstone.

(5) FIG. 1 shows schematically a first well 1 and a second well 2. In a typical tight formation, the wells are disposed substantially horizontally. It will be appreciated that the wells may be at any angle to best match the shape of the oil-bearing subterranean formation in which they are located. Furthermore, the first well 1 and the second well 2 are shown as being disposed parallel to one another. While this configuration may be optimum, it will be appreciated by the skilled person that the wells may deviate from being parallel to one another, again dependent on the formation in which they are located. The distance between the first well and the second well can be selected depending on many factors, such as the pressure in the reservoir, the permeability of the formation, the viscosity of the oil to be produced and so on. A typical distance may be around 400 m, but it will be appreciated that this can vary greatly.

(6) The first well 1 is divided into zones; in the example of FIG. 1, a first zone 3, a second zone 4 and a third zone 5 are shown. It will be appreciated that many more zones may be provided along the length of the first well 1. The zones are substantially hydraulically isolated from one another by isolating material 12, meaning that fluids cannot pass from one zone to another (or at least, the flow of fluid is severely restricted between zones depending on the type of isolation used).

(7) Similarly, the second well 2 is divided into zones; in the example of FIG. 1, a first zone 6, a second zone 7 and a third zone 8 are shown. It will be appreciated that many more zones may be provided along the length of the second well 2. Again, the zones are substantially hydraulically isolated from one another by isolating material 13, meaning that fluids cannot pass from one zone to another, or the flow of fluid is severely restricted between zones depending on the type of isolation used.

(8) The zones in the first well 1 and the second well 2 may be any suitable length, depending on factors such as the pressure in the reservoir, the permeability of the formation, the viscosity of the oil to be produced and so on. A typical length is around 25 m to 100 m but can vary greatly.

(9) There are various ways that zones can be hydraulically isolated from one another. For example, packers, swell packers, hydraulically set packers or cement may be used to ensure no or little fluid communication between zones.

(10) Fractures are induced between the zones of the two wells 1, 2. In the example of FIG. 1, a first fracture 9 is induced between the first zones 3, 6 of the first well 1 and the second well 2 respectively, a second fracture 10 is induced between the second zones 4, 7 of the first well 1 and the second well 2 respectively, and a third fracture 11 is induced between the third zones 5, 8 of the first well 1 and the second well 2 respectively. Note that in FIG. 1, the fractures are shown as clean lines extending between the first well and the second well. This is for illustrative purposes only. In reality, each fracture comprises a series of fractures of different lengths and sizes, and each fracture may be thought of as a zone of fractures rather than a single fracture. For the sake of simplicity, the term “fracture” is used herein to refer to a fractured region.

(11) The fracturing operation must be carefully controlled to ensure that each fracture extends substantially between corresponding zones of the first well 1 and the second well 2. The fractures in FIG. 1 are shown as being substantially perpendicular to the wells 1, 2. It will be appreciated that, again, factors such as the shape and permeability of regions of the formation between the wells 1, 2 may dictate that the fractures deviate from being perpendicular to the wells 1, 2.

(12) The fractures are induced by any suitable means. Examples of techniques for inducing fractures between the wells include hydraulic fracturing, thermal fracturing, mechanical fracturing, and a combination of those methods. Where hydraulic fracturing is used, a fracturing may include proppants to ensure that a portion of the fractures remain open after the fracturing operation is complete.

(13) In use, different zones are designated as injector zones or production zones. In the example of FIG. 1, the first and third zones 3, 5 of the first well 1 are designated as injector zones, and the second zone 7 of the second well 2 is designated as a production zone. The remaining zones are closed.

(14) An injection fluid is injected through the first 3 and third zone of the first well 1. The main fluid path for the injection fluid is from the injector zones towards the production zone (the second zone 7 of the second well 2). This ensures that the injection fluid is forced through the formation between the wells 1, 2 and carries hydrocarbons with it. By forcing injection fluid through the formation in this way, a greater volume of the oil-bearing formation is available for production of oil, and oil production yields are increased. The arrows in FIG. 1 show the direction of flow of both injection fluid and produced oil towards the production zone 7. This type of flooding is termed cross-flooding.

(15) Different zones can change their function. For example, once sufficient oil has been extracted using the first 3 and third 5 zones of the first well as injector zones, these zones can be closed off and the second zone 4 of the first well 1 can become an injector zone (along with, say, a fourth, sixth, eighth and so on zone). This allows more of the formation to be subjected to the injection fluid and increase yields. In this case, the second zone 7 of the second well 2 will be closed off, and the first 6 and third 8 zones of the second well 2 are opened for production.

(16) One way to change the injector and production zones is to provide openable openings in each zone. The openings provide a communicating path between the wells and the formation. The openings can be selectively opened or closed depending on which zone will be an injector zone and which zone will be a production zone.

(17) Similarly, different wells can change their function. In the example of FIG. 1, the first well 1 is used to inject fluid, and the second well 2 is used to produce hydrocarbons. This may be reversed so the second well becomes an injector well, and the first well becomes a production well.

(18) Any suitable injection fluid may be used. Examples include carbon dioxide, hydrocarbons, methane, produced gas, nitrogen, hydrogen sulphide, water, surfactant, alkali, ketones, alcohols, aromatic hydrocarbons, hydrocarbons, solvents, and acid.

(19) Injection fluids with different functions may also be used. For example, injection fluids may act as a diluent, a solvent, a reactant or a surfactant. Different combination of fluids can be used to optimize production. Furthermore, the type of injection fluid may be selected based on the type of hydrocarbon to be produced, the pressure and temperature in the formation, the viscosity of the hydrocarbon, the distance between wells and so on.

(20) Turning now to FIG. 2, a flow diagram shows exemplary steps of the cross-flooding technique described herein. The following numbering corresponds to that of FIG. 2:

(21) S1. A first well 1 is provided in the formation. The first well has at least a first 3 and a second 4 zone, the first and second zones being substantially hydraulically isolated from each other.

(22) S2. A second well 2 is provided in the formation. The second well has at least a first 6 and a second 7 zone, the first and second zones being substantially hydraulically isolated from each other. The second well 2 is optimally substantially parallel to the first well 1.

(23) S3. The formation is fractured so that a first fracture 9 extends substantially between the first zone 3 of the first well 1 and the first zone 6 of the second well 2. A second fracture 10 extends substantially between the second zone 4 of the first well 1 and the second zone 7 of the second well 2.

(24) S4. In this example, the first zone 3 of the first well 1 is used as an injection zone, and the second zone 7 of the second well 2 is used as a production zone. Injection fluid is injected from the first zone 3 of the first well.

(25) S5. The injected injection fluid is forced through the formation towards the second zone 7 of the second well 2, carrying hydrocarbons from the formation with it. Hydrocarbons are therefore produced at the second zone 7 of the second well 2.

(26) S6. As mentioned above, the designations of injection zones, production zones, injection wells and production wells may be changed at any point, and the method reverts to step S4. Furthermore, interfaces may be moved between different zones and the method reverts to step S4. Interfaces may be moved by, for example, changing the location of packers.

(27) The systems and methods described above allow the maximization of pressure gradients across the formation to provide improved oil recovery rates by reducing the distance that injected fluid must travel through the formation before production, while minimizing fluid channelling between connected fractures.

(28) The isolated zones in each well 1, 2 allow for injection of injection fluids to occur offset to production as shown in FIG. 1, requiring injected fluid to traverse the formation in a direction substantially parallel to a main axis of the wells, allowing hydrocarbons to be produced where the induced fracturing may be less substantial and less connected than in the direction orthogonal to the wellbore. Furthermore, the distance that injection fluid (and produced hydrocarbons) must traverse in the direction parallel to the wellbore through the formation is relatively small compared to the distance typically traversed between wells in a conventional flood, allowing for larger pressure gradients and more economic production rates.

(29) The completions configuration for these wells can be relatively simple. Several methods are available. One exemplary method consists of using several packers for zonal isolation in the wellbore along with a tubing string running a portion of the wellbore and penetrating at least one packer where the tubing string may have one or more sliding sleeves to control and or restrict the flow in each zone. This configuration requires much less complicated completions than in either the adjacent and proximal well configuration or the single well configurations discussed above, and is thus more reliable and less expensive.

(30) The system may be provided with monitoring systems to determine the efficiency of production at each production zone. Production zones can be changed as a result of this monitoring.

(31) FIG. 3 shows modelled recovery rates of oil from tight formations. The solid line represents primary depletion of oil without any injection fluid. The dashed line gives the example of a traditional CO.sub.2 flood from well to well. It can be seen that over time, cumulate recovery improves marginally. Using the cross-flooding techniques described herein (dotted line), secondary depletion is expected to improve and recovery is significantly improved over the lifetime of the well.

(32) The cross-flooding techniques described above can lead to cost-effectively allowing the production of significant oil reserves in formations that cannot be cost-effectively produced using existing techniques. The method maximizes pressure gradients and minimizes the distance that injection fluid and hydrocarbons must traverse through the formation while minimizing potential channelling effects and rapid breakthrough due to fracturing.

(33) The skilled person will appreciate that various modifications may be made to the above described embodiments without departing from the scope of the present invention.