Hybrid steam generation with carbon dioxide recycle

DETAILED DESCRIPTION OF THE INVENTION

[0011] Embodiments of the invention relate to systems and methods of recovering hydrocarbons by injecting into a reservoir outputs from two different types of steam generators along with carbon dioxide. Synergistic results enable lower fuel consumption for such a hybrid based approach relative to using either type of steam generator alone. One steam generator vaporizes water by thermal transfer from combustion with exhaust from the combustion remaining separated from the steam. Since this type of steam generator outputs a limited carbon dioxide concentration with the steam, at least part of the carbon dioxide injected comes from recycling the carbon dioxide separated out of production fluids recovered from the reservoir. Another steam generator produces the steam by direct water contact with combustion products to produce a resulting fluid including the steam and additional carbon dioxide.

[0012] Figure 1 illustrates a system for recovering hydrocarbons that includes a processing unit 102, an exhausted steam generator 110 and a direct steam generator or DSG 116 that are all coupled to at least one production well 100 and at least one injection well 124. In an exemplary embodiment, the injection well 124 and the production well 100 provide a well pair for a steam assisted gravity drainage (SAGD) operation. Various other recovery operations including cyclic steam stimulation, solvent aided SAGD and steam drive may also employ processes described herein.

[0013] In operation, the processing unit 102 receives a mixture that is recovered from the production well 100 and includes hydrocarbons or oil, condensate from steam that is injected to heat and mobilize the oil and some carbon dioxide injected with the steam. The processing unit 102 may include liquid-gas separators, water-oil separators and treatment equipment for gas and water. The processing unit 102 separates the mixture into a sales stream 104 of the oil, a steam generator feed stream 106 of water and a gas recycle stream 108 of the carbon dioxide.

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[0014| Recovery of the carbon dioxide from produced fluids at the processing unit 102 costs less than attempting to recover the carbon dioxide in flue streams. Capturing dilute carbon dioxide within flue streams that are majority nitrogen requires expensive recovery procedures. Embodiments thus provide efficient carbon dioxide emission reduction options as exemplified further herein.

[0015] The exhausted steam generator 110 bums fuel in air to heat and vaporize the water input via the feed stream 106. Exhaust gases from combustion of the fuel and air exits a flue 112 of the exhausted steam generator 110 separate from a steam output 114 that conveys resulting vaporized water to the injection well 124. A boiler or once through steam generator (OTSG) may provide the exhausted steam generator 110.

[0016] In contrast to the exhausted steam generator 110, the direct steam generator 116 bums fuel, such as natural gas or methane from fuel inlet 118, in oxygen within a combustor where the water from the feed stream 106 is also introduced. An air separation unit (ASU) output 120 may supply the oxygen to the direct steam generator 116. A fluid output 122 from the direct steam generator 116 thus conveys carbon dioxide as a product of combustion along with steam from both water vaporization and the combustion to the injection well 124.

[0017] The gas recycle stream 108 also conveys the carbon dioxide that is recovered in the processing unit 102 back to the injection well 124 for introducing into the formation with at least the steam output 114 from the exhausted steam generator 110. In some embodiments, only some of the carbon dioxide recovered in the processing unit 102 mixes with the steam for injection to provide a desired carbon dioxide concentration for injection and an excess portion of the carbon dioxide is sent offsite for capture or sequestration. Carbon dioxide recycle rate via the recycle stream 108 and production rate split between the steam generators 110, 116 depends on retention rate of the carbon dioxide in the reservoir and desired carbon dioxide injection rate for a particular operation.

[0018] Increases in the retention rate of the carbon dioxide in the reservoir reduce the amount of the carbon dioxide produced and available for recycle, thereby requiring an increase in the production rate of the direct steam generator 116 relative to the exhausted steam generator 110 for a given desired carbon dioxide injection concentration. Similarly,

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PCT/US2014/016927 increasing the desired carbon dioxide injection concentration also raises the production rate of the direct steam generator 116 relative to the exhausted steam generator 110 and/or amount of the recycle stream 108 utilized for injection of the carbon dioxide. In some embodiments, level of the carbon dioxide may range between 1% and 25%, greater than 15% or less than 10% of the steam by mass.

[0019] Mixing of the steam output 114, the fluid output 122 of the direct steam generator 116 and the recycle stream 108 enables this concentration range of carbon dioxide being injected given that, for example, the fluid output 122 of the direct steam generator 116 may only provide carbon dioxide levels between 10% and 14% of the steam by mass that cannot otherwise be altered. Such control of the carbon dioxide concentration in the steam being injected provides flexibility. For example, a production profile may call for a lower carbon dioxide injection concentration in early production stages and more in later stages, which may be accomplished by increasing over time the amount of the carbon dioxide sent to the injection well 124 via the recycle stream 108.

[0020] In some embodiments, the direct steam generator 116 superheats the steam exiting through the fluid output 122. This superheating prevents condensation prior to introduction into the injection well 124. The condensation may otherwise occur since the steam from the exhausted steam generator 110 may cool as conveyed from a central processing facility to a wellpad or as a result of the carbon dioxide in the recycle stream 108 being introduced at a relatively cool temperature.

[0021] Locating the direct steam generator 116 at the wellpad and offsite from the exhausted steam generator 110 that may be at the central processing facility ensures that any such condensate is vaporized. The condensate may constitute at least five percent of the steam flowrate from the exhausted steam generator 110, represents thermal loss and would otherwise be returned for vaporization adding to system burdens. Compared with conveying steam, relative ease in transporting oxygen, fuel and water facilitates location of the direct steam generator 116 away (e.g., at least 10 kilometers) from the central processing facility and proximate (e.g., within 10, 5 or 1 kilometers) from a wellpad where the injection well 124 is located.

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[00221 In some embodiments, the fluid output 122 from the direct steam generator 116 supplies more remote wellpads relative to the central processing facility than those supplied by the exhausted steam generator 110. The steam output 114 thus mixes with the carbon dioxide from the recycle stream 108 but may not mix with the fluid output 122 of the direct steam generator 116. All injection into the reservoir however may still contain the carbon dioxide with the steam as desired.

[00231 As shown in the following table, process modeling compared results associated with two comparative cases and two exemplary cases all for a 90,000 barrel per day SAGD facility having all electrical power generated by a natural gas-fired combined cycle (NGCC) plant. These four cases include generating all required steam with an OTSG, generating all required steam with a DSG, and DSG-OTSG hybrid applications (e.g., as shown in Figure 1 ) with either 40 or 20 percent carbon dioxide retention in the reservoir. Reservoir modeling shows that while such fractions of the injected carbon dioxide may be retained in the reservoir significant remaining amounts may return to the surface in the produced gas making recycling possible.

[0024) For this analysis, an ideal carbon dioxide injection rate selected corresponds to the DSG that has carbon dioxide at 11.4% of the steam by mass. This carbon dioxide concentration results in lowering the steam to oil ratio (SOR) from 2.5 to 2.125. Such reduction in the SOR derives from benefits associated with injecting the carbon dioxide with the steam, which benefits may include viscosity reduction of the hydrocarbons from dissolution with the carbon dioxide, insulating effects of the carbon dioxide or carbon dioxide pressure support.

<td rowspan="2"></td><td rowspan="2"> OTSG</td><td rowspan="2"> DSG</td><td colspan="2"> DSG-OTSG Hybrid</td>

<td> 40% CO2 Retention</td><td> 20% CO2 Retention</td>

<td> SOR</td><td> 2.5</td><td> 2.125</td><td> 2.125</td><td> 2.125</td>

<td> Steam (tons per</td><td colspan="4"> tour)</td>

<td> OTSG</td><td> 1566</td><td> 0</td><td> 824</td><td> 1082</td>

<td> DSG</td><td> 0</td><td> 1264</td><td> 482</td><td> 234</td>

<td> Total</td><td> 1488</td><td> 1264</td><td> 1264</td><td> 1264</td>

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<td rowspan="2"></td><td rowspan="2"> OTSG</td><td rowspan="2"> DSG</td><td colspan="2"> DSG-OTSG Hybrid</td>

<td> 40% CO2 Retention</td><td> 20% CO2 Retention</td>

<td colspan="5"> CO2 Flowrates (tons per hour)</td>

<td> DSG</td><td> 0</td><td> 144</td><td> 58</td><td> 29</td>

<td> Recycle</td><td> 0</td><td> 0</td><td> 87</td><td> 115</td>

<td> Total</td><td> 0</td><td> 144</td><td> 144</td><td> 144</td>

<td colspan="5"> Electrical Loads (megawatts)</td>

<td> Facility base</td><td> 92</td><td> 92</td><td> 92</td><td> 92</td>

<td> ASU</td><td> 0</td><td> 94</td><td> 38</td><td> 19</td>

<td> Evaporator</td><td> 0</td><td> 18</td><td> 6</td><td> 3</td>

<td> CO2 compress</td><td> 0</td><td> 12</td><td> 7</td><td> 10</td>

<td> Total</td><td> 92</td><td> 217</td><td> 143</td><td> 123</td>

<td colspan="2"> Fuel Flowrates (tons per</td><td colspan="3"> lour)</td>

<td> OTSG</td><td> 78</td><td> 0</td><td> 41</td><td> 54</td>

<td> DSG</td><td> 0</td><td> 54</td><td> 21</td><td> 11</td>

<td> NGCC</td><td> 15</td><td> 36</td><td> 24</td><td> 20</td>

<td> Total</td><td> 93</td><td> 90</td><td> 86</td><td> 85</td>

<td> Relative usage</td><td> 1</td><td> .96</td><td> .92</td><td> .91</td>

<td colspan="5"> Green House Gas Footprint (tons per hour)</td>

<td> CO2 emissions</td><td> 251</td><td> 103</td><td> 176</td><td> 200</td>

[00251 The DSG in the hybrid applications produces about 38 and 19 percent of the total steam generated for respective 40 and 20 percent carbon dioxide retention cases. Relative fuel usage shown in the table compares total fuel used by being normalized to the OTSG case. The relative fuel use in the hybrid cases range between 0.91-0.92, which represents reduction in fuel operating expense relative to not only the OTSG case but also the DSG case that is 0.96.

[00261 While capital expense is not quantified in results shown in the table, the hybrid application provides lower capital costs than the DSG case due to ability to use smaller air separation units given that not all steam is generated by the DSG. The hybrid application

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WO 2014/130455 PCT/US2014/016927 also still enables capturing some of the carbon dioxide produced with extent of the capture depending on steam production rate split between the OTSG and the DSG. Carbon dioxide emissions thus drop 20-30% in the hybrid application relative to the OTSG case.

[0027] The preferred embodiments of the invention have been disclosed and illustrated. However, the invention is intended to be as broad as defined in the claims below. 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 arc within the scope of the claims below and the description, abstract and drawings are not to be used to limit the scope of the invention.