DIRECT STEAM GENERATION CO2 OUTPUT CONTROL

Abstract

Methods and systems generate steam and carbon dioxide mixtures suitable for injection to assist in recovering hydrocarbons from oil sands based on concentration of the carbon dioxide in the mixtures as influenced by temperature of water introduced into a direct steam generator. Increasing temperature of the water to above 200° C. before introduction into the direct steam generator may utilize heat from an electrical power generation unit. Enthalpy of this preheated water impacts amount of fuel needed to burn in the direct steam generator and hence the concentration of the carbon dioxide, which may be below 11% by mass percent of the steam.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application which claims benefit under 35 USC §119(e) to U.S. Provisional Application Ser. No. 61/697,026 filed Sep. 5, 2012, entitled “DIRECT STEAM GENERATION CO2 OUTPUT CONTROL,” which is incorporated herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None.

FIELD OF THE INVENTION

Embodiments of the invention relate to generating a stream containing steam and carbon dioxide for injection during steam assisted hydrocarbon recovery processes.

BACKGROUND OF THE INVENTION

Steam assisted gravity drainage (SAGD) provides an exemplary steam assisted recovery process for producing bitumen within oil sands. During SAGD operations, steam introduced into a reservoir through a horizontal injector well transfers heat to the bitumen upon condensation. The bitumen with reduced viscosity due to this heating drains together with steam condensate and is recovered via a producer well disposed parallel and beneath the injector well.

Steam generation costs limit economic returns of the SAGD. Relative to boiler or once through steam generation approaches, direct steam generation may facilitate lowering these costs due to improvements in efficiency, inherent makeup water replacement and reduced fouling issues. The direct steam generation operates by burning a fuel in a combustor and quenching resulting combustion products with water to produce a mixture of steam and the combustion products including carbon dioxide for injection.

The carbon dioxide may benefit the SAGD operations by lowering the steam to oil ratio. However, desired concentrations of the carbon dioxide within the steam to achieve such benefits for any particular SAGD application may not coincide with output from the direct steam generation. Dilution with pure steam can provide the desired concentrations but introduces expenses associated with boilers and steam transport.

Therefore, a need exists for methods and systems for generating steam and carbon dioxide mixtures for injection to assist in recovering hydrocarbons from oil sands.

BRIEF SUMMARY OF THE DISCLOSURE

In one embodiment, a method of generating a mixture of steam and carbon dioxide includes supplying fuel and oxidant into a direct steam generator. The method further includes heating water to above 200° C. for introducing in liquid phase into the direct steam generator. Combusting the fuel and oxidant in the direct steam generator as the water that is preheated is introduced produces the mixture that includes the steam and combustion products and that has a carbon dioxide level in mass percent of steam below 11 percent.

According to one embodiment, a system for generating a mixture of steam and carbon dioxide includes a device for heating water and a direct steam generator coupled in fluid communication with an output of the device for heating the water. The device for heating the water outputs the water in liquid phase at a temperature above 200° C. The direct steam generator combusts fuel and oxidant as the water from the output of the device is introduced into the direct steam generator to produce the mixture that includes the steam and combustion products and has a carbon dioxide level in mass percent of steam below 11 percent.

For one embodiment, a method of generating a mixture of steam and carbon dioxide includes combusting fuel and oxidant in a direct steam generator as water that is heated to above 200° C. and in liquid phase is introduced into the direct steam generator to produce the mixture that includes the steam and combustion products and has a carbon dioxide level in mass percent of steam below 11 percent. The method further includes introducing the mixture into a formation and recovering a hydrocarbon emulsion. The emulsion contains a condensate of the steam that is recycled for resupplying of the water heated before entering the direct steam generator.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings.

FIG. 1 is a schematic with a direct steam generator producing an injection mixture of steam and carbon dioxide at a concentration controlled by temperature of the water fed to the generator, according to one embodiment of the invention.

FIG. 2 is a graph of the temperature of the water fed to the generator versus the concentration of the carbon dioxide in the mixture produced by the generator, according to one embodiment of the invention.

DETAILED DESCRIPTION

Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow.

For some embodiments, methods and systems generate steam and carbon dioxide mixtures suitable for injection to assist in recovering hydrocarbons from oil sands based on concentration of the carbon dioxide in the mixtures as influenced by temperature of water introduced into a direct steam generator. Increasing temperature of the water to above 200° C. before introduction into the direct steam generator may utilize heat from an electrical power generation unit. Enthalpy of this preheated water impacts amount of fuel needed to burn in the direct steam generator and hence the concentration of the carbon dioxide, which may be below 11 percent by mass percent of the steam (i.e., mass of the carbon dioxide/mass of the carbon dioxide and steam expressed as a percentage).

FIG. 1 illustrates a direct steam generator (DSG) 100 that produces a mixture 101 of steam and carbon dioxide. The steam generator 100 may integrate with a steam assisted production process used in connection with an injection well 102 and a production well 104. An output of the steam generator 100 couples to the injection well 102 to convey the mixture 101 into a formation.

In an exemplary steam assisted production operation, the injection well 102 and production well 104 each include horizontal lengths that pass through the formation and may be disposed parallel to one another with the horizontal length of the injection well 102 above the production well 104. This configuration of the injection well 102 and the production well 104 exemplifies a conventional steam assisted gravity drainage (SAGD) well pair. The steam in the mixture 101 condenses and transfers heat to hydrocarbons in the formation that then drain with condensate of the steam by gravity to the production well 104 for recovery.

An emulsion 106 of the hydrocarbons and the condensate recovered from the production well 104 upon separation provides products and part or all of feed water 108 to the steam generator 100. The water 108 pumped to the steam generator 100 may need additional treatment if being recycled depending on configuration of the steam generator 100. In some embodiments, separation of the emulsion 106 occurs without significant heat loss from the water 108 relative to when recovered from the production well 104.

For some embodiments, one or more heat exchangers, such as a first heat exchanger 110, transfers heat to the water 108 from any components of the emulsion 106 recovered through the production well 104. The water 108 exits the first heat exchanger 110 through a heater input conduit 112 coupled to a device, such as a second heat exchanger 114, for heating the water 108 to a temperature above 200° C. prior to being introduced into the steam generator 100.

Depending on how much heat is lost and/or recovered, initial temperature of the water 108 upon introduction into the second heat exchanger 114 thus may range from ambient up to a temperature, such as 200° C., corresponding to temperature of the emulsion coming from the production well 104. In some embodiments, the second heat exchanger 114 transfers heat from an electrical power generation unit 116 to the water 108 increasing the temperature of the water 108 to above 200° C. For example, the electrical power generation unit 116 may utilize a gas turbine burning natural gas with resulting exhaust used by the second heat exchanger 114 instead of, or in addition to, a second cycle to increase electricity production.

The second heat exchanger 114 may not rely only on waste heat from the electrical power generation unit 116 since the heat in common practice would otherwise raise steam production for the second cycle. Increasing size and firing rate of the gas turbine compensates for the heat removed by the second heat exchanger 114. However, resulting fuel savings in the steam generator 100 outweighs additional fuel burned in the electrical power generation unit 116, as shown in Table 1 herein.

Location of the electrical power generation unit 116 on-site enables employing the second heat exchanger 114 with the steam generator 100. With respect to being on-site, the electrical power generation unit 116 supplies power needs of a facility supporting the steam assisted production process. Demand for the power comes from associated equipment including an air separation unit, evaporator and/or carbon dioxide conditioning/compression system.

A heater output conduit 118 conveys the water 108 from the second heat exchanger 114 for introduction into the steam generator 100 under sufficient pressure to be in liquid phase. In operation, fuel 120, such as hydrocarbons including natural gas, and an oxidant 121, such as oxygen separated from air, supplied to the steam generator 100 combust inside the steam generator 100 as the water 108 is introduced. The water 108 makes direct quenching contact with resulting combustion products and is thereby vaporized into steam. This steam in combination with the combustion products produces the mixture 101 output from the steam generator 100.

In some embodiments, a portion of the water 108 (e.g., from the heater input conduit 112 as shown) enters into the steam generator 100 at a temperature below 200° C. in an area of the steam generator 100 upstream from where the water 108 above 200° C. is introduced. The water 108 that is below 200° C. when entering the steam generator 100 may ensure sufficient cooling in a head of the steam generator 100 where temperatures may be highest in the steam generator 100. The head also includes injectors of the fuel 120 and the oxidant 121 and is most susceptible to thermal damage.

For some embodiments, the second heat exchanger 114 increases temperature of the water 108 such that the water 108 is above 250° C., between 250° C. and 300° C., or between 250° C. and 280° C. and at a pressure above 6000 kilopascals when output from the second heat exchanger 114 and/or introduced into the steam generator 100. Further, the water 108 may enter the steam generator 100 at more than 30° C. below a temperature of the mixture 101 output. For example, the mixture 101 may exit from the steam generator 100 above 280° C. and at least 6000 kilopascals for introduction into the formation through the injection well 102.

FIG. 2 shows a graph with a line plotting the temperature of the water 108 fed to the steam generator 100 versus the concentration of the carbon dioxide in the mixture 101 produced by the steam generator 100. Depending on the temperature of the water 108, the mixture may thus contain a carbon dioxide level in mass percent of the steam below 11 percent or below 10 percent. In some embodiments, controlling temperature of the water 108 fed to the steam generator 100 adjusts the carbon dioxide level to a selected value to achieve a threshold steam to oil ratio.

In addition to providing control of the carbon dioxide level in the mixture 101, approaches described herein may reduce operating and capital expenses compared to similar approaches that lack the second heat exchanger 114 used to increase the temperature of the water 108 above 200° C. The Table 1 shows this comparison made with process models for a 90,000 barrel per day facility. As shown in the Table 1, firing rate of the steam generator 100 decreases by 16 percent when the second heat exchanger 114 is employed as described herein. This reduction enables using fewer steam generators along with smaller air separation units and carbon dioxide processing systems per given amount of steam output. Total fuel usage also drops with use of the second heat exchanger 114.

	TABLE 1
	without 2nd	with 2nd
	heat exchanger	heat exchanger
DSG head H20 temperature	170° C	170° C
DSG non-head H20 temperature	170° C	280° C
CO2 mass % of steam	11.4%	9.5%
H2O flowrate (tonne/hr)	1148	1166
O2 flowrate (tonne/hr)	210.7	175.6
Total power (MWe)	216.8	199.3
Fuel- DSG(tonne/hr)	53.7	44.8
Fuel- power generation (tonne/hr)	35.8	38.8
Total fuel (tonne/hr)	89.5	83.6

In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as an additional embodiment of the present invention.

Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims, while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents.

Claims

1. A method of generating a mixture of steam and carbon dioxide, comprising:

supplying fuel and oxidant into a direct steam generator;

heating water to above 200° C. for introducing in liquid phase into the direct steam generator; and

combusting the fuel and oxidant in the direct steam generator as the water that is preheated is introduced to produce the mixture that includes the steam and combustion products and that has a carbon dioxide level in mass percent of the steam below 11 percent.

2. The method according to claim 1, wherein the heating of the water includes heat exchange with heat from an electrical power generation unit.

3. The method according to claim 1, wherein the heating of the water includes heat exchange with fluids recovered from a hydrocarbon production well and then with heat from an electrical power generation unit.

4. The method according to claim 1, wherein a portion of the water is introduced into the direct steam generator at a temperature below 200° C. in an area of the direct steam generator upstream from where the water above 200° C. is introduced.

5. The method according to claim 1, wherein the water is above 250° C. and at a pressure above 6000 kilopascals when introduced into the direct steam generator.

6. The method according to claim 1, wherein the water is supplied by recycling condensate of the steam recovered following injection of the mixture into a formation.

7. The method according to claim 1, further comprising injecting the mixture into a formation to assist recovery of hydrocarbons.

8. The method according to claim 1, wherein the fuel includes hydrocarbons and the carbon dioxide level in mass percent of steam is below 10 percent.

9. The method according to claim 1, wherein the fuel is natural gas and the oxidant is oxygen separated from air.

10. The method according to claim 1, wherein the water is introduced into the direct steam generator at more than 30° C. below a temperature of the mixture output.

11. The method according to claim 1, wherein the water is between 250° C. and 280° C. when introduced into the direct steam generator and the mixture is output from the direct steam generator above 280° C. and at least 6000 kilopascals.

12. A system for generating a mixture of steam and carbon dioxide, comprising:

a device for heating water and configured for outputting the water in liquid phase at a temperature above 200° C.; and

a direct steam generator coupled in fluid communication with an output of the device for heating the water and configured to combust fuel and oxidant as the water from the output of the device is introduced into the direct steam generator to produce the mixture that includes the steam and combustion products and has a carbon dioxide level in mass percent of steam below 11 percent.

13. The system according to claim 12, wherein the device for heating the water includes a heat exchanger to transfer heat from an electrical power generation unit.

14. The system according to claim 12, wherein the device for heating the water includes a first heat exchanger for transferring heat from fluids recovered from a hydrocarbon production well and a second heat exchanger for transferring heat from an electrical power generation unit.

15. The system according to claim 12, further comprising an injection well in fluid communication with the mixture output from the direct steam generator.

16. The system according to claim 12, wherein the device for heating the water is configured to heat the water to between 250° C. and 300° C.

17. The system according to claim 12, wherein the direct steam generator is operable to provide the carbon dioxide level in mass percent of steam below 10 percent and combust the fuel that includes hydrocarbons.

18. The system according to claim 12, wherein the direct steam generator is coupled to receive a portion of the water at a temperature below 200° C. in an area of the direct steam generator upstream from where the water above 200° C. is introduced.

19. A method of generating a mixture of steam and carbon dioxide, comprising:

combusting fuel and oxidant in a direct steam generator as water that is heated to above 200° C. and in liquid phase is introduced into the direct steam generator to produce the mixture that includes the steam and combustion products and has a carbon dioxide level in mass percent of steam below 11 percent;

introducing the mixture into a formation; and

recovering a hydrocarbon emulsion containing a condensate of the steam that is recycled for resupplying of the water heated before entering the direct steam generator.

20. The method according to claim 19, wherein the introducing of the mixture into the formation is through a horizontal injection well above a horizontal production well through which the emulsion is recovered in a steam assisted gravity drainage process.