Method and apparatus for generating steam for the recovery of hydrocarbon

Abstract

The instant disclosure provides a method of generating steam for the recovery of hydrocarbon from a hydrocarbon producing system including (i) generating supercritical steam from water; (ii) converting the supercritical steam to a subcritical steam; and (iii) injecting the subcritical steam into the system.

Claims

1. A method of generating steam for the recovery of hydrocarbon from a hydrocarbon producing system comprising: (i) generating supercritical steam from water; (ii) converting said supercritical steam to a subcritical steam, wherein said subcritial steam comprises a steam and water mixture; (iii) separating said subcritical steam into steam and water; and (iv) injecting said subcritical steam into said system.

2. A method as claimed in claim 1, wherein the water used to generate supercritical steam comprises water obtained from hydrocarbon recovery operations.

3. A method as claimed in claim 1, wherein the water used to generate supercritical steam comprises water obtained from hydrocarbon recovery operations and fresh water.

4. A method as claimed in claim 1, wherein the water used to generate supercritical steam comprises less than 10% by volume fresh water.

5. A method as claimed in claim 1, wherein the water used to generate supercritical steam comprises at least one of: silica 50 ppm wt or higher total dissolved solids 8500 ppm wt or higher hydrocarbon 5 ppm wt or higher CaCO.sub.3 1 ppm wt or higher.

6. A method as claimed in claim 1, wherein said supercritical steam is generated in a once-through steam generator.

7. A method as claimed in claim 1, wherein the pressure of said supercritical steam is in the range 22.2 MPa-50 MPa.

8. A method as claimed in claim 1, wherein the temperature of said supercritical steam is in the range 380-800 C.

9. A method as claimed in claim 1, wherein the conversion of supercritical steam to subcritical steam is carried out in an expansion unit.

10. A method as claimed in claim 9, wherein said unit is a valve or a turbine.

11. A method as claimed in claim 1, wherein impurities are removed from said subcritical steam prior to its injection into said system.

12. A method as claimed in claim 11, wherein said impurities are removed by separation of water from the steam.

13. A method as claimed in claim 1, wherein the pressure of said subcritical steam is in the range 0.5-22.0 MPa.

14. A method as claimed in claim 1, wherein the temperature of said subcritical steam is in the range 150-500 C.

15. A method as claimed in claim 1, wherein the quality of the subcritical steam injected into the formation is at least 80% by volume.

16. A method as claimed in claim 1, wherein the subcritical steam is used in recovery of hydrocarbon by steam assisted gravity drainage.

17. A method of producing hydrocarbon from a hydrocarbon producing system comprising: (a) generating steam by a method as claimed in claim 1; and (b) using said steam in a method of producing hydrocarbon from said formation.

18. A method as claimed in claim 17, wherein said steam is used to produce hydrocarbon by steam assisted gravity drainage.

19. A method for generating steam for the recovery of hydrocarbon from a hydrocarbon producing system comprising: (i) generating subcritical steam from water; (ii) separating said subcritical steam into water and steam; (iii) generating supercritical steam from said separated water; (iv) converting said supercritical steam to a subcritical steam; (v) optionally combining the steam produced in steps (ii) and (iv); and (vi) injecting said subcritical steam into said system.

20. A method as claimed in claim 19, wherein said subcritical steam is generated in step (i) in a once-through steam generator.

21. A method as claimed in claim 19, wherein said subcritical steam is separated in step (ii) in a separator.

22. A method as claimed in claim 19, wherein the water used to generate said subcritical steam in step (i) comprises: water obtained from hydrocarbon recovery operations, water obtained from hydrocarbon recovery operations and fresh water, less than 10% by volume fresh water, or at least one of: silica 50 ppm wt or higher total dissolved solids 8500 ppm wt or higher hydrocarbon 5 ppm wt or higher CaCO.sub.3 1 ppm wt or higher.

23. A method as claimed in claim 19, wherein the separated water used to generate supercritical steam in step (iii) is not purified.

24. A method as claimed in claim 19, wherein water obtained from hydrocarbon recovery operations and/or fresh water is added to the separated water and the mixture is used to generate supercritical steam in step (iii).

25. A method as claimed in claim 19, wherein the water used to generate supercrical steam in step (iii) comprises at least one of: silica 250 ppm wt or higher total dissolved solids 10,000 ppm wt or higher hydrocarbon 50 ppm wt or higher CaCO.sub.3 10 ppm wt or higher.

26. An apparatus for generating steam for the recovery of hydrocarbon from a hydrocarbon producing system comprising: (i) an inlet for water; (ii) a supercritical steam boiler connected to said inlet for producing supercritical steam; (iii) an expansion unit connected to said supercritical steam boiler for converting said supercritical steam to subcritical steam, wherein said subcritical steam comprises a steam and water mixture; (iv) a separator connection to said expansion unit for separating said subcritical steam into steam and water; and (v) an outlet connected to said separator for delivery of the steam to said system.

27. An apparatus as claimed in claim 26, wherein said supercritical steam boiler is a once through steam generator.

28. An apparatus as claimed in claim 26, wherein said expansion unit is a valve or a turbine.

29. A system for producing hydrocarbon from a hydrocarbon producing formation comprising: (i) an apparatus as claimed in claim 26; (ii) a well arrangement connected to said outlet of the apparatus; and (iii) a means for the recovery of hydrocarbon.

30. A system as claimed in claim 29, wherein said well arrangement comprises: i) a first steam assisted gravity drainage (SAGD) well pair; and ii) a second steam assisted gravity drainage (SAGD) well pair.

31. A system as claimed in claim 30, further comprising an infill well, wherein said infill well is located in between said first and second SAGD well pairs.

32. An apparatus for generating steam for the recovery of hydrocarbon from a hydrocarbon producing system comprising: (i) an inlet for water; (ii) a steam boiler connected to said inlet for producing subcritical steam; (iii) a separator connected to said steam boiler for separating water and steam; (iv) a supercritical steam boiler connected to said separator for producing supercritical steam from separated water; (v) an expansion unit connected to said supercritical steam boiler for converting said supercritical steam to subcritical steam; (vi) an outlet connected to said separator for delivery of the steam to said system; and (vii) an outlet connected to said expansion unit for delivery of the steam to said system.

33. An apparatus as claimed in claim 32, wherein said steam boiler is a once-through steam generator.

34. An apparatus as claimed in claim 32, wherein said supercritical steam boiler is connected directly to said separator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1a is a schematic diagram showing the prior art method of steam generation for hydrocarbon recovery and FIG. 1b is a schematic diagram of a method of steam generation for hydrocarbon recovery according to the present invention;

(2) FIG. 2 is a schematic diagram of an apparatus of the present invention which also shows a method of cleaning an apparatus for generating steam;

(3) FIG. 3 is a schematic diagram of a further apparatus of the present invention; and

(4) FIG. 4 is a schematic diagram of a system of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(5) In FIGS. 1b, 2 and 3 features that are common are designated by the same reference numeral.

(6) Referring to FIG. 1a, it shows a schematic diagram of the generation of steam by a conventional technique. Thus water 1, which is generally purified to a significant level, is pumped to a OTSG 2 wherein subcritical steam 3 is generated. Typically the OTSG 2 is operated under conditions that produce a steam quality of about 80%. Under such conditions the steam generator is less likely to run dry which leads to the production of significant amounts of precipitate that sticks to the heat transfer surfaces of the OTSG. The subcritical steam 3 is usually fed to a steam separator 4 to remove the water 5. The resulting dry steam 6 is fed to the hydrocarbon producing system 7 wherein it mobilises hydrocarbon to enable its recovery. When the mobilised hydrocarbon 8 is produced at the surface, significant amounts of water 9 is also produced. This water 9 comprises impurities, e.g. hydrocarbon, silica from the formation and inorganic salts such as carbonates from the formation water. The produced water 9, and optionally the separated water 5, is pumped to a water purifier 10. In the purifier 10 the water is generally separated from organic compounds, e.g. hydrocarbon and then treated to remove impurities. Usually chemical treatments are carried out. The aim of the water purifier 10 is to purify the water to a level to enable it to be used as water 1 for the OTSG. In some cases, however, this is not possible and a proportion of water has to be disposed of as blow down 11. In this case fresh water 12 is used to supplement the water supply 1.

(7) This method has several disadvantages including: the quality of steam produced for recovery of hydrocarbon is limited to about 80% which means that 20% of the water supply is not used for steam generation; the produced water must be purified to a high level to enable it to be used as supply water; similarly the separated water must also be purified to a high level to enable it to be used as supply water or to be disposed of; the need to blow down water and concomitantly utilise fresh water has significant environmental impact.

(8) Referring to FIG. 1b, it shows a schematic diagram of the generation of steam by a method of the invention. The water 13, which is generally less pure than the water used in conventional steam generation, is pumped to a once-through supercritical steam generator 14. The supercritical steam 15 produced is generally fed to an expansion unit 16 such as a relaxation/expansion valve or turbine. In unit 16 the supercritical steam is converted to subcritical steam 17. The pressure and temperature of the conversion process can be controlled so that subcritical steam of different conditions can be produced. Generally the steam quality is at least 90% by weight. The subcritical steam 17 is fed to a separator 18 where impurities are removed. If the steam is dry, the impurities typically deposit in the separator 18 as solids. More typically, the subcritical steam 17 comprises a percentage of water (e.g. less than 5% by weight) and the impurities are concentrated in the water 19. In the separation the water is separated from the steam thereby purifying the steam. The resulting steam 20 is fed to a hydrocarbon producing system 21 as described above in relation to FIG. 1a. In the methods of the invention, however, a much greater volume of the water 13 fed to the steam generator 14 is converted to steam 20 that is injected into the hydrocarbon producing system 21. Moreover, as described above, the degree to which the produced water 23 must be purified to enable it to be utilised as water 13 for the generation of steam is decreased. As a result, little if any water is blown down and much less fresh water, if any, is required.

(9) Referring to FIG. 2, it shows a schematic diagram of an apparatus 25 of the present invention. The apparatus 25 comprises a water inlet 26, preferably fluidly connected to a water tank 27. The inlet 26 supplies water to a once-through supercritical steam generator 14. Any conventional supercritical steam boiler may be used, e.g. a Benson boiler. The supercritical steam 15 that results is fed to an expansion unit 16a and or 16b, preferably a turbine 16b, to convert it to subcritical steam 17. As shown expansion units may be provided in parallel. Alternatively a single expansion unit may be employed. The pressure and temperature of the unit 16 is preferably controlled by controller 28. Energy is released during the conversion of supercritical steam 15 to subcritical steam 17 and this is preferably captured. Optionally the energy is supplied to the once-through supercritical steam generator 14. The subcritical steam 17 is fed to a separator 18 where steam 20 and impurities are separated. Preferably the impurities are separated in water 23. The resulting steam is then pumped to a hydrocarbon producing system 21.

(10) FIG. 2 also shows how the apparatus of the present invention may be cleaned. The dotted line a indicates a pipe that may be used to convey supercritical steam to an inlet of the supercritical steam boiler 14. O.sub.2 may be injected into the supercritical steam as it flows through the pipe so that when the O.sub.2-containing supercritical steam enters boiler 14 it combusts any hydrocarbon present therein. The O.sub.2 containing supercritical steam is preferably routed out of the boiler 14 via the outlet for supercritical steam. Alternatively a separate outlet (not shown) may be used.

(11) The dotted line b indicates a pipe that may be used to convey O.sub.2 containing supercritical steam to the separator 18. The O.sub.2 containing supercritical steam preferably passes through both outlets of the separator. Preferably, however, the O.sub.2 containing supercritical steam is not injected into the formation. Thus preferably a waste pipe c is provided.

(12) Referring to FIG. 3, it shows a schematic diagram of an apparatus 25 of the present invention. The apparatus comprises a once-through steam generator 14, an expansion unit 16a or 16b and a separator 18 as shown in FIG. 2. The apparatus shown in FIG. 3, however, additionally comprises a conventional steam boiler 29 that generates subcritical steam. The water fed to this boiler may be water obtained from hydrocarbon recovery operations, fresh water or a mixture thereof. The subcritical steam produced is fed to a separator 30 where steam 31 and water 32 are separated. The water 32 is fed to the once-through steam generator 14. Since the generator 14 generates supercritical steam, there is no need to purify the water 32.

(13) Referring to FIG. 4, it shows a system 33 for the recovery of hydrocarbon from a hydrocarbon producing system 21. Each SAGD well pair comprises an injector well 34, 35 and a producer well 36, 37. The vertical separation (arrow a) between each well pair is about 5 m. The horizontal separation (arrow b) between each well pair is about 100 m. The injector wells 34, 35 are at the same depth in the reservoir and are parallel to each other. Similarly the producer wells 36, 37 are at the same depth in the reservoir and are parallel to each other. The producer wells 36, 37 are preferably provided with a liner (not shown) as is conventional in the art. The arrangement also preferably includes infill well 38.

(14) Subcritical steam 17 is injected into the hydrocarbon producing system via injection wells 34, 35. This mobilises hydrocarbon and it drains under gravity to producer wells 36, 37 as well as infill well 38. Hydrocarbon is pumped to the surface from these wells where hydrocarbon 22 is separated from produced water 23. The produced water undergoes treatment as hereinbefore described in purifier 24 and thereafter is used to generate steam in apparatus 25.