Abstract:
A system and method is disclosed for treating a mixture of hydrocarbon and carbon dioxide gas produced from a hydrocarbon reservoir. The system includes a gas power turbine adapted to burn the produced gas mixture of hydrocarbon and carbon dioxide gas with oxygen as an oxidizing agent and a capture system to collect the exhaust gas from the power turbine. An inlet compressor receives exhaust gas from the capture system and compresses the exhaust gas for injection of the exhaust gas into a hydrocarbon reservoir and for recycle to the power turbine. The system may further include a membrane system that preferentially removes carbon dioxide and hydrogen sulfide from the produced gas stream before said stream is used as fuel gas in the power turbine. The carbon dioxide and hydrogen sulfide removed by the membrane system is combined with the exhaust gas, and the combined gas is injected into a hydrocarbon reservoir.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Patent Application No. 60/790,153, filed Apr. 7, 2006, which is incorporated by reference. 

   TECHNICAL FIELD 
   This invention relates to systems and methods used for enhanced oil recovery, and more particularly to a system and method for processing a mixture of hydrocarbon and carbon dioxide gas produced from a hydrocarbon reservoir. 
   BACKGROUND 
   Injection of carbon dioxide for tertiary enhanced recovery of oil from hydrocarbon reservoirs has been known and used worldwide since at least the 1980s. In particular, there are numerous carbon dioxide tertiary enhanced recovery projects in operation in the Permian Basin oil fields of west Texas. General literature about the conduct of such projects is well known and available from such sources as the Journal of Petroleum Technology and in papers published by the Society of Petroleum Engineers. Generally, in a carbon dioxide tertiary enhanced recovery project, the carbon dioxide is injected into a hydrocarbon reservoir via injection wells penetrating the producing formation. Oil, hydrocarbon gas, and water are produced through offsetting production wells. 
   Parrish, U.S. Pat. No. 4,344,486, incorporated by reference, teaches that the effectiveness of carbon dioxide as an aid to oil recovery is dependent on its miscibility pressure. As the carbon dioxide flows through a reservoir at an underground pressure above about 1,000 psi and a temperature of about 100 to 150° F., the carbon dioxide becomes partially miscible with the oil and helps push the oil toward the wellbore. The miscibility of the carbon dioxide with oil is dependent upon many factors including carbon dioxide purity, oil type, reservoir pressure, and reservoir temperature. The oil-carbon dioxide miscibility can be negatively affected by contaminants such as nitrogen, oxygen, oxides of nitrogen, carbon monoxide and methane. Parrish, U.S. Pat. No. 4,344,486, discloses that it is desirable for the carbon dioxide injection stream used in enhanced oil recovery to be substantially free from contaminants. 
   After carbon dioxide has been injected into the producing formation, the carbon dioxide will move through the producing formation driving a “flood front” of oil ahead of it toward the producing well. Ultimately, some of the carbon dioxide will reach the producing well and carbon dioxide will be produced in the production well together with the oil and hydrocarbon gases. The produced oil and gas mixture must be separated into its components. 
   At a primary field separation facility, oil is removed, treated, and sold. Free water, not entrained in the gas, is separated and disposed of or re-injected into the reservoir. The gaseous phase of the fluid stream is separated and sent from the primary field separation facilities to a central gas processing facility. In smaller fields, one gas processing facility may serve several fields. In a carbon dioxide enhanced recovery project, the produced gas will be a mixture of hydrocarbon gases and carbon dioxide. Additionally, some impurities such as hydrogen sulfide may also be present. 
   Combustion gas turbines capable of using low Btu gas as gas turbine fuel are well known in the art. Integrated gasification combined cycle (IGCC) systems have been used successfully to burn low caloric value (LCV) fuel. IGCC is a process in which a LCV fuel such as coal, petroleum coke, orimulsion, biomass or municipal waste may be converted to a low heating value synthetic fuel, which is used as the primary fuel in a gas turbine. Synthetic fuel has a heating value of about 125 Btu/scf to 350 Btu/scf. Typical natural gas has methane as its primary component and has a heating value of about 1,000 Btu/scf. The synthetic gas heating value and components may vary widely from one application to another and are highly dependent on the particular process producing the gas, the oxidant used, and the process feed stock. Further information on IGCC and flammability as a function of caloric value is discussed in a technical paper authored by R. D. Brdar and R. M. Jones, titled  GE IGCC Technology and Experience with Advanced Gas Turbines , and is incorporated by reference. 
   Zapadinski, U.S. Patent Publication 2004/0154793 A1, incorporated by reference, discloses a method and system of developing a hydrocarbon reservoir wherein hydrocarbon gas from the field is combusted with air as an oxidant in a gas engine and the exhaust gas resulting from the combustion is compressed and then injected into the hydrocarbon reservoir. The exhaust gas of the system taught by Zapadinski includes a high percentage of nitrogen and nitrogen oxides in addition to carbon dioxide. The nitrogen comes from using air as the oxidant, since air contains about 79% nitrogen. At a given injection pressure, injection of carbon dioxide containing nitrogen or nitrogen oxides into a hydrocarbon reservoir is less efficient in the enhanced recovery process than use of pure carbon dioxide due to the negative effects of nitrogen on the miscibility of the injected gas with the oil in the reservoir. Kovarik F S, “A Minimum Miscibility Pressure Study Using Impure CO 2  and West Texas Oil Systems: Data Base, Correlations, and Compositional Simulation,” Society of Petroleum Engineers Production Technology Symposium, November 1985, incorporated by reference. 
   In both an IGCC and a typical combined cycle system, the gas turbine compressor uses atmospheric air as the source of oxygen for combustion. In such a system, air is the working fluid in the system and the turbine exhaust gas is released to the atmosphere after heat capture in a heat exchanger or heat recovery steam generator. Alternatively, in a semi-closed combined cycle, the turbine exhaust gas is recirculated back to the inlet compressor. Although a semi-closed combined cycle using atmospheric air as the oxygen source will result in an exhaust gas and overall working fluid enriched in carbon dioxide, the working fluid will still contain a large nitrogen component, making the carbon dioxide containing slipstream much less than optimal composition for use in the enhanced recovery process. Finally, substantially pure oxygen may be used as the oxidant in a semi-closed combined cycle. By using substantially pure oxygen instead of air for combustion, the purity of the carbon dioxide in the exhaust gas stream and the overall working fluid is much higher. The literature contains many articles that discuss various aspects of the semi-closed combined cycle process and the change in working fluid from air to carbon dioxide, and its effect on the performance of the inlet compressor and gas turbine due to the difference in fluid properties. Roberts S K, Sjolander S A, 2002, “Semi-Closed Cycle O2/CO2 Combustion Gas Turbines: Influence of Fluid Properties on the Aerodynamic Performance of the Turbomachinery. ” ASME GT-2002-30410. Proceedings of ASME TURBO EXPO 2002, Amsterdam, The Netherlands, Jun. 3-6, 2002, incorporated by reference. According to Roberts and Sjolander, two fluid properties that should be considered when switching from air to a carbon dioxide working fluid include the ratio of specific heats (γ) and the gas specific constant (R). At any given temperature, the carbon dioxide working fluid has a lower ratio of specific heats, lower gas specific constant, and higher density as compared to the air working fluid. The ratio of specific heats for carbon dioxide is approximately 1.28 at 300 K, the ratio for air is approximately 1.40 at 300 K and the ratio for water vapor (a product of combustion) is 1.14 at 300 K. Similarly, the gas specific constant differs significantly between carbon dioxide (188.9 J/kg-K), air (288.2 J/kg-K) and water vapor (461.5 J/kg-K). 
   Another journal article describes how the ratio of specific heats (γ) and the gas specific constant (R) are used to calculate the turbo machinery non-dimensional mass flow (π M ) and non-dimensional speed (π N ) parameters, which also need to be considered when changing the working fluid from air to carbon dioxide. Jackson A J B, Neto A C, Whellens, M W. 2000. “Gas Turbine Performance Using Carbon Dioxide as Working Fluid in Closed Cycle Operation.” ASME 2000-GT-153. ASME TURBOEXPO 2000, Munich, Germany, May 8-11, 2000, incorporated by reference. The large difference between the ratio of specific heats (γ) and gas specific constant (R) for carbon dioxide and air affects the turbo machinery non-dimensional mass flow and non-dimensional speed parameters and this presents a challenge for using existing turbo machinery equipment for a carbon dioxide working fluid. 
   SUMMARY 
   The present disclosure comprises a system and method for purification and re-injection of the carbon dioxide component of a produced hydrocarbon gas/carbon dioxide mixture. Instead of separating the hydrocarbon component from the hydrocarbon/carbon dioxide produced gas mixture (as is typically done in prior art systems), in the present invention, the produced gas mixture of hydrocarbon and carbon dioxide is used as a low Btu fuel gas for one or more gas fired turbines that are used to produce rotary energy used to drive electric generators and compressors. Low Btu fuel gas is defined here as a fuel gas with a heating value less than pipeline quality fuel gas, ranging as low as 100 Btu/scf heating value. The electricity produced by the electric generators may be sold and/or used in oil field and processing plant operations to enhance the economics of the project. 
   In the present invention, the exhaust gas from the turbine is captured, cooled, compressed, and re-injected into the producing formation via the injection wells. The captured exhaust gas consists of the original carbon dioxide component of the produced gas that was used as fuel gas and additional carbon dioxide formed by the combustion of the hydrocarbon component of the fuel gas in the turbine. To minimize dilution of the carbon dioxide stream, substantially pure oxygen is used as an oxidant and it is mixed with the fuel gas in the combustion process. 
   The system for treating the mixture of hydrocarbon and carbon dioxide gas produced from a hydrocarbon reservoir includes a gas power turbine adapted to burn the produced gas mixture of hydrocarbon gas and carbon dioxide as fuel gas with oxygen as an oxidizing agent. A capture system collects the exhaust gas from the power turbine. The exhaust gas includes a carbon dioxide component from the fuel gas and carbon dioxide formed as a product of the combustion of the oxygen and hydrocarbon component of the fuel gas. An electric generator is driven by the power turbine and an inlet compressor is driven by the power turbine. The inlet compressor receives cooled exhaust gas from the capture system and compresses the exhaust gas for further recompression and injection of the exhaust gas into a hydrocarbon reservoir. 
   The system may include a heat recovery steam generator adapted to receive heat from the captured exhaust gas and convert said heat to steam. A steam turbine is driven by steam from the heat recovery generator, and an electric generator is driven by the steam turbine. In another embodiment of the system, steam produced in the heat recovery steam generator may be re-injected into the power turbine in order to increase the energy output of the power turbine, while at the same time eliminating the need for a separate steam turbine installation. Another embodiment of the system may include a recuperator wherein high-pressure air or nitrogen is cross-exchanged with the hot exhaust gas from the power turbine to recover the exhaust gas heat. After cross exchange, the heated high-pressure air or nitrogen can be let down through a turbo expander which is then used to drive an electric generator. The system may further include a membrane system upstream of the inlet to the power turbine, wherein the membrane preferentially removes carbon dioxide from the produced gas stream before the stream is used for fuel gas in the power turbine. The carbon dioxide and hydrogen sulfide removed by the membrane system is combined with the exhaust gas for injection of the combination into a hydrocarbon reservoir. 
   A method for conducting a carbon dioxide enhanced recovery process using the above equipment is also disclosed. The method includes providing produced gas from a hydrocarbon reservoir wherein the produced gas contains a hydrocarbon gas component and at least 50 mol % of a carbon dioxide component; providing substantially pure oxygen of at least 90 mol %; combusting the produced gas as fuel with a slight stoichiometric excess of oxygen to achieve substantially complete combustion; sending the combustion products to a power turbine; collecting the exhaust gas from the power turbine; removing heat and water from the collected exhaust gas; compressing the collected exhaust gas; and providing the compressed exhaust gas for injection into a hydrocarbon reservoir. 
   The method may also include driving an electric generator with the power turbine and driving a compressor with the power turbine. Alternatively, one or more compressors may be driven with electricity generated by one of the system&#39;s generators or from a power grid into which the generated electricity is fed. 
   The method may also include removing heat from the collected exhaust gas and using said heat to create steam; driving a steam turbine with said steam; and driving an electric generator with said steam turbine. 
   The method may also include removing heat from the collected exhaust gas and using said heat to create steam; then injecting such steam into the power turbine in order to increase the power output of the power turbine and eliminate the need for a separate steam turbine. 
   The method may also include removing heat from the collected exhaust gas in an exchanger and using air or nitrogen heated in the exchanger to drive a turbo expander, which is used to drive an electric generator. 
   The method may also include processing the produced gas through a membrane system to remove a portion of the carbon dioxide component and mixing the removed carbon dioxide with the compressed collected exhaust gas for reinjection into the reservoir. 
   The project may include further compressing the compressed gas using one or more stages of additional compression and driving the additional stages of compression with electricity produced by the system. 
   The project may include powering an air separation plant, which provides the oxygen for combustion of the fuel gas, with electric power from one or more of the system generators. 
   Among other advantages, the present invention allows carbon dioxide tertiary recovery projects to be implemented in smaller hydrocarbon fields where the initial investment and operating expenses of conventional prior art carbon dioxide/hydrocarbon processing plants are not economically justified. The present invention provides a source of carbon dioxide from the combustion of fuel in the power turbine for use in carbon dioxide tertiary recovery projects where there is no other economically viable source of carbon dioxide. 
   The present invention reduces air pollution by capturing the exhaust gases from the power turbine. The captive pollutants include nitrogen oxides and sulfur oxides. Additionally, capture and injection of the carbon dioxide combustion product prevents the carbon dioxide from being discharged into the atmosphere and reduces carbon dioxide that contributes to the “greenhouse” effect. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic of a prior art system and method for processing of a produced gas mixture of carbon dioxide and hydrocarbon gas without the removal of the hydrocarbon component; 
       FIG. 2  is a schematic of a prior art system and method for processing of a produced gas mixture of carbon dioxide and hydrocarbon gas with removal of the hydrocarbon component; 
       FIG. 3  is a schematic of a system and method of an embodiment of the present invention for processing of a produced gas mixture of carbon dioxide and hydrocarbon gas and re-injection of the carbon dioxide component into a hydrocarbon reservoir; and 
       FIG. 4  is a schematic of a system and method for processing of a produced gas mixture of carbon dioxide and hydrocarbon gas wherein the present invention further includes a membrane carbon dioxide removal system and re-injection of the carbon dioxide component into a hydrocarbon reservoir. 
   

   Like reference symbols in the various drawings indicate like elements. 
   DESCRIPTION OF PRIOR ART FIGURES 
   Referring now to  FIG. 1 , in one prior art scenario, a gaseous mixture  10  of carbon dioxide/hydrocarbon gas from the field is received in a gas processing facility  100  subsequent to the initial field separation from the oil and the water (not shown). The gas  10  is sent through several stages of compression  110 ,  130 ,  150  and  170  with inter-stage cooling  120 ,  140  and  160 . The pressurized carbon dioxide/hydrocarbon mixture  20  is then returned to the field for re-injection into an injection well. 
   Referring now to  FIG. 2 , in another prior art scenario, a gaseous mixture  10  carbon dioxide/hydrocarbon gas from the field is received in a gas processing facility  200  subsequent to the initial field separation from the oil and the water (not shown). The gas  10  is sent through several stages of compression  210 ,  230 ,  250  and  270  with inter-stage cooling  220 ,  240  and  260 . 
   Between the second and third stages of compression, the gas  20  is sent to a carbon dioxide/hydrocarbon removal unit  280 . Carbon dioxide  30  is removed from the gas  20  and the hydrocarbon gases  40  are sold. The hydrocarbon gas being sold may be further broken down into components and sold as methane, ethane, propane, butane, and heavier natural gas liquids. The pressurized carbon dioxide  50  is then returned to the field for re-injection into an injection well. While this separation of carbon dioxide from the hydrocarbon is illustrated simplistically as block  280  in  FIG. 2 , the process is complicated and requires a plant that has high initial construction costs and significant operating costs. 
   DETAILED DESCRIPTION 
   Referring to the attached  FIG. 3 , therein is illustrated a system  300  for processing and re-injection of the carbon dioxide component  30  of a hydrocarbon gas/carbon dioxide mixture  10  produced from a hydrocarbon reservoir. Instead of processing the hydrocarbon/carbon dioxide produced gas mixture  10  to separate the carbon dioxide and hydrocarbon (as illustrated in prior art  FIG. 2 ), the produced gas mixture  10  of hydrocarbon and carbon dioxide is used as a low Btu fuel gas  15  for the burners  500  of one or more gas fired power turbines  520  that are used to drive one or more electric generators  550 . The gas turbine may drive the generator directly via a coaxial shaft or through a gearbox. The electricity  90  is sold and/or used in oil field and processing plant operations to enhance the economics of the project. 
   The produced gas mixture  10  of carbon dioxide/hydrocarbon is generally received from the hydrocarbon field at 35 psia or less. The gas is compressed in one or more stages from about 35 psia to about 300 psia via first stage compressor  310  and second stage compressor  330 . The gas is cooled by intercooler  320 . The produced gas mixture  10  of hydrocarbon and carbon dioxide is used as a low Btu fuel gas  15  for the burners  500  of one or more gas fired power turbines  520  that are used to drive one or more electric generators  550 . All or part of the produced gas mixture It) may be used as fuel gas  15  for the burners  500 . After combustion of fuel gas  15  in burners  500 , combustion gas  16  passes through power turbine  520  and is collected as exhaust gas  17 . The exhaust gas  17  (comprised of carbon dioxide, water, and any residual oxygen from the power turbine  520 ) is collected at approximately atmospheric pressure, cooled in a heat exchanger or heat recovery steam generator  530 , further cooled and excess water vapor removed in condenser  380  and recompressed in inlet compressor  510  to about 250 psia. Compressed gas  18  exits from the high-pressure side of inlet compressor  510 . Substantially pure oxygen  52  from an oxygen plant  50  may be mixed with the fuel gas  15  to create a combustion mixture  25  for burner  500 . Substantially pure oxygen is defined as having 90 mol % or greater oxygen concentration. Alternatively, some burner arrangements are designed with the oxygen stream  52  injected directly into the burner  500  instead of. or in addition to, being mixed with the fuel gas  15  before the burner  500 . 
   Air separation plants suitable for producing substantially pure oxygen are well known in the art and are available from a number of vendors including Praxair and Air Products and Chemical. Such plants may use either cryogenic systems, vacuum pressure swing adsorption systems and other existing or future technology capable of producing substantially pure oxygen that contains less than 10% contaminants. In addition to these proven oxygen production technologies, there are new, emerging technologies such as the Ion Transport Membrane process (ITM) that also produce substantially pure oxygen and these processes would also be suitable for the present invention. 
   Referring also to  FIG. 3 , in an alternative embodiment of the system  300 , the produced gas mixture  10  is further compressed to approximately 700 psia (as opposed to 300 psia) via third stage compressors  350  before being combusted in the burners  500 . In this alternative embodiment, carbon dioxide return streams  23  would necessarily be returned at the inlet of intercooler  360  instead of at the inlet of intercooler  340 . Operating the system by supplying higher starting pressure gas stream  11  to burners  500  would improve the thermodynamic efficiency of the overall cycle. The number of stages of compression ( 310 ,  330 ,  350 , and  370 ) and stage pressures may vary in each installation of the system, but the concept of removing carbon dioxide/hydrocarbon fuel from one point and returning carbon dioxide to a downstream point remains the same. 
   In the system  300 , the carbon dioxide gas stream  17  from the heat exchanger  530  is further cooled in condenser  380  before being supplied as working fluid to the inlet compressor  510 . The additional cooling in condenser  380  can be used as a method not only to control the temperature, but also to control the water vapor content in the carbon dioxide gas stream, thus affecting the working fluid properties and providing additional control of the inlet compressor  510  and power turbine  520 . A portion  22  of the carbon dioxide gas stream  18  is drawn off from the outlet of inlet compressor  510  and sent to re-compressor  570  . A portion  19  of the carbon dioxide gas stream  18  is drawn off for use as working fluid  19  in burner  500  of the power turbine  520 . The carbon dioxide gas stream  23  from re-compressor  570  is then sent to compressors  350  and  370  and inter-stage coolers  340  and  360  for reinjection in the field. As illustrated in  FIG. 3 , in one embodiment of the invention, a portion  26  of the low pressure carbon dioxide gas stream  21  can be re-compressed in compressor  590  and sent to compressors  350  and  370  and intercoolers  340  and  360 , bypassing inlet compressor  510 . The compressed carbon dioxide stream  30  is returned to the field to be re-injected at about 2000 psi into the producing formation via the injection wells. 
   The captured exhaust gas  17  primarily comprises the original carbon dioxide component of the produced hydrocarbon/carbon dioxide gas  10  that was used as fuel gas  15 , and additional carbon dioxide formed by the combustion of the hydrocarbon component of the fuel gas in the burner  500 . To minimize dilution of the carbon dioxide stream for reinjection, substantially pure oxygen is mixed with the fuel gas in the combustion process. This minimizes the introduction of nitrogen into the combustion gas. If atmospheric air is used for combustion (instead of the substantially pure oxygen  52 ), the nitrogen will dilute the composite carbon dioxide gas stream being returned to the hydrocarbon reservoir for reinjection. As discussed in the background section of this application, the dilution of the carbon dioxide affects miscibility of the gas with the oil in the reservoir and therefore has unfavorable effects on the tertiary recovery process in the hydrocarbon reservoir. 
   The system  300  may include a matched set comprising an inlet compressor, combustor, and power turbine designed and manufactured by one company. It will be understood that various modifications may be made to the equipment design without departing from the spirit and scope of the invention. One such modification is to select individual manufacturers for the inlet compressor, combustor, and power turbine based on optimal equipment design. Another such set of alterations may include a change of the material of construction of the power turbine blades, modification of the power turbine cooling flow, and modification of controls for the inlet compressor, combustor, or power turbine. 
   Heat exchanger  530  may use excess heat from the power turbine exhaust gas  17  to form steam  60  that is used to drive steam turbine  540  and generator  560  to generate electricity  92 . Spent steam  61  from turbine  540  is cooled in condenser  390  and condensed to water  62 . The condensed water is then returned to heat exchanger  530  for creation of more steam  60 . In one embodiment a “once through steam generator” system formed of Inconel material may be used for the heat exchanger  530 . In another embodiment, the steam  60  may be injected into the power turbine  520  in order to increase the output power of the power turbine and possibly eliminate the need for a separate steam turbine  540 . In another embodiment, heat exchanger  530  is alternatively used to heat a high-pressure gas  80  consisting of air or nitrogen from air separation plant  50 . This high-pressure gas  80  may be used as power gas in a turbo expander engine  580 . The power gas  80  may be let down in pressure through a turbo expander  580  to drive a generator  582  and produce additional power  94 . 
   Referring now to  FIG. 4 , therein is disclosed the system and method  400 .  FIG. 4  includes a membrane separation system  700  used to separate a portion of the carbon dioxide contained in the low Btu fuel gas  15 . As the tertiary enhanced recovery project matures and more carbon dioxide has been injected into the reservoir, the mixture of carbon dioxide and hydrocarbon gas produced from the reservoir will increase in carbon dioxide content. A membrane separation system  700  may be installed to remove some of the carbon dioxide and other components of the fuel gas  15  including any hydrogen sulfide contained in the fuel gas  15 . Blizzard, Parro, and Homback discuss commercially available membrane technology in the article  Mallet Gas Processing Facility Uses Membranes to Efficiently Separate CO   2 , Blizzard G, Parro D, Homback K, Oil and Gas Journal, Apr. 11, 2005, incorporated by reference. 
   The removed carbon dioxide may be recycled as a carbon dioxide stream  14  to a lower pressure point in the system, such as the inlet of re-compressor  570  or the inlet of a separate recompressor (not shown). Removal of the carbon dioxide via the membrane system  700  has several advantages. First, partial removal of carbon dioxide from the fuel gas  15  reduces the total volume of gas  29  being sent to the fuel gas system, which can reduce the size and cost of certain fuel handling and combustion equipment. Second, by removing some of the carbon dioxide, the caloric value of the fuel gas  29  for the power turbine can be raised to a more typical range of 300 to 500 Btu/scf and the technology of the turbines may be more conventional. Third, hydrogen sulfide contaminants in the fuel gas  15  will be preferentially removed by the membrane, thereby significantly reducing any need for the equipment described in this invention to be constructed of expensive corrosion resistant materials when the produced gas  10  contains significant amounts of hydrogen sulfide contaminants. It should be noted that withdrawal of the fuel gas  15  from the produced gas  10  in this method  400  would occur downstream of intercooler  340  rather than upstream of intercooler  340  as in method  300  since membranes generally require lower inlet temperatures to operate properly. The carbon dioxide return stream  23  would therefore need to be cooled separately (not shown) before being returned to a downstream point. 
   The remaining system elements of  FIG. 4  are substantially the same as elements that were previously discussed with regard to  FIG. 3  and have been assigned the same reference numerals. 
   The produced carbon dioxide/hydrocarbon gas  10  is a low Btu gas that has many of the same properties of synthetic gas. Gas turbines developed for synthetic low heating value fuel gas are capable of use in the present invention. Table 1 lists a range of IGCC turbines manufactured by GE that may be suitable for use with the present invention. Information on these turbines is available at GEPower.com, the disclosure of which is incorporated by reference into this application. The invention is not limited to the use of only these specified gas turbine models. 
                               TABLE 1                       GE Gas               Turbine Model   Syngas Power Rating                           GE10    10 MW (50/60 Hz)           6B    40 MW (50/60 Hz)           7EA    90 MW (60 Hz)           9E   150 MW (50 Hz)           6FA    90 MW (50/60 Hz)           7FA   197 MW (60 Hz)           9FA   286 MW (50 Hz)                        
Because IGCC synthetic gas has a low heating value compared to natural gas, significantly more fuel must be injected in an IGCC turbine as compared to a standard gas turbine. The mass flow input is larger with an IGCC turbine and for this reason, the gas turbine has enhanced power output. Enhanced power output is also obtained when the low Btu produced gas  10  is used as fuel gas.
 
   Table 2 illustrates three different cases with molecular compositions of produced gas  10 . The heating values range from approximately 120 Btu/scf to 420 Btu/scf. Case I is the upper end of the Btu range for produced gas  10 . Case II is the lower end of the Btu range for produced gas  10 . The gas conditions listed in Case I and Case II are typical of the process illustrated in  FIG. 3 . Case III is typical of produced gas downstream  29  of a membrane separation system  700  as illustrated in  FIG. 4 . 
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
                 
               TABLE 2 
             
             
                 
                 
             
             
                 
               Case I (Mol %) 
               Case II (Mol %) 
               Case III (Mol %) 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               Nitrogen 
               2.75 
               1.90 
               1.35 
             
             
               Carbon Dioxide 
               80.00 
               90.70 
               79.00 
             
             
               Hydrogen Sulfide 
               1.65 
               1.50 
               0.10 
             
             
               Oxygen 
               -0- 
               -0- 
               -0- 
             
             
               Methane 
               8.50 
               3.00 
               6.50 
             
             
               Ethane 
               1.50 
               0.55 
               3.50 
             
             
               Propane 
               2.40 
               1.00 
               4.30 
             
             
               Iso-Butane 
               0.60 
               0.25 
               1.10 
             
             
               Normal-Butane 
               1.30 
               0.60 
               2.55 
             
             
               ISO-Pentane 
               0.45 
               0.20 
               0.65 
             
             
               Normal-Pentane 
               0.35 
               0.15 
               0.50 
             
             
               Hexanes 
               0.40 
               0.10 
               0.25 
             
             
               Heptanes+ 
               0.10 
               0.05 
               0.20 
             
             
                 
               100.00 
               100.00 
               100.00 
             
             
               Heating value 
               ~320 
               ~120 
               ~420 
             
             
               (Btu/CF HHV) 
             
             
                 
             
           
        
       
     
   
   As the tertiary carbon dioxide enhanced recovery project progresses, additional wells are placed on carbon dioxide injection and some injection wells are removed from carbon dioxide injection. Therefore, the carbon dioxide content in the produced gas will vary throughout the life of the project since it is a mixture of carbon dioxide gas from wells in various stages of their productive life. The above case examples in Table 2 are merely for illustrative purposes for produced gas from a typical Permian basin oil field in West Texas. Produced gas from other oil fields and from other areas will have different molar compositions. The present invention is not limited to the example compositions of produced gas or Permian basin produced gas. It will be understood that the system and method of the present invention may be used to process produced gas from one or more different hydrocarbon reservoirs and the carbon dioxide available for re-injection may be injected into one or more of the same or different reservoirs from which the gas was produced. 
   In addition to the economic advantages of the present invention that have been previously discussed, another advantage of the present invention is its unique control scheme. Several industry trade journal articles mentioned in the background section of this application note some of the differences between air and carbon dioxide working fluid and how existing power turbines and compressors designed for air service may work in carbon dioxide fluid service. These journal articles also discuss the difficulty and complexity of using standard turbo machinery equipment with carbon dioxide as the working fluid. The present invention includes a control method that enables the use of standard turbo machinery equipment. Use of standard turbo machinery as opposed to customized special order machinery can favorably affect the economics of the project via lower initial cost and lower replacement and operating costs. 
   As was briefly discussed earlier in the background section, the ratio of specific heats (γ) and the gas specific constant (R) are used to determine the non-dimensional mass flow (π M ) and non-dimensional speed (π N ) parameters of the turbo machinery. The present invention contains several process parameters that can be adjusted independently or simultaneously to obtain non-dimensional parameters for the carbon dioxide working fluid that sufficiently approximate the parameters for an air working fluid. The composition, pressure and temperature of the carbon dioxide stream feeding the inlet compressor  510 , the rotational speed of the inlet compressor  510 , and the rotational speed of the power turbine  520  are some of the adjustable parameters. 
   In the system  300  of  FIG. 3 , the condensing pressure and temperature of condenser  380  may be used to control the composition of the carbon dioxide exhaust gas that is fed to the inlet compressor  510 . A significant constituent in the carbon dioxide exhaust gas is water vapor, which is a product of combustion. At a higher condensing temperature, less water vapor will be condensed out of the carbon dioxide exhaust gas and more water vapor will remain in the carbon dioxide exhaust gas. For example, exhaust gas condensed at approximately 165° F. will contain approximately six times the water vapor content as exhaust gas that has been condensed at about 120° F. and approximately fifteen times the water vapor content as exhaust gas that has been condensed at 90° F. Due to the varied fluid properties of air, carbon dioxide, and water vapor, varying the exhaust gas composition by means of adjusting the condensing pressure and temperature will cause the weighted average of the ratio of specific heats (γ) and the gas specific constant (R) of the exhaust gas mixture to be significantly different, which will cause the non-dimensional mass flow and non-dimensional speed parameters to vary accordingly. Therefore, variation of the condensing pressure and temperature will allow control of the non-dimensional mass flow and non-dimensional speed parameters. This mechanism can be used as one way of controlling the overall equipment performance, but in addition, if the turbo machinery non-dimensional parameters with the carbon dioxide working fluid are made to sufficiently approximate the turbo machinery non-dimensional parameters with the air working fluid, then control of both the inlet compressor  510  and the power turbine  520  can be achieved, either independently or in combination with the other control parameters. Ultimately, changing the condensing pressure and temperature of the carbon dioxide gas can provide control of the cycle of the present invention. 
   The rotational speeds of the inlet compressor  510  and power turbine  520  can also be changed to affect the turbo machinery non-dimensional speed parameters. By reducing the rotational speed of the inlet compressor  510  or the power turbine  520 , the carbon dioxide working fluid mass flow is reduced and the turbo machinery non-dimensional mass flow parameters become more similar to the non-dimensional mass flow parameters for the turbo machinery with air as the working fluid. Additionally, steam injection into the power turbine, either at the turbine inlet or between turbine stages, may be used as a control method. The steam injection into the power turbine will further adjust the temperature and composition of the carbon dioxide working fluid, and will therefore change the non-dimensional parameters of the power turbine. Performed separately or in combination, modifying the inlet compressor  510  feed gas pressure, temperature and water vapor content, the inlet compressor  510  rotational speed, and the power turbine  520  pressure, temperature and water vapor content as well as the power turbine  520  rotational speed can be made to cause the turbo machinery non-dimensional speed parameters to sufficiently approximate the non-dimensional speed parameters of turbo machinery used in a conventional air cycle. 
   The unique control scheme of the present invention presented thus far can be used during normal, steady-state operations and for fine-tuning during transient operations. Additional control methods are also necessary to provide proper control during transient conditions. If not properly controlled, the use of carbon dioxide as the working fluid can result in rotating stall of the power turbine compressor due to the high density and other different fluid properties of carbon dioxide as compared to air. The amount of flow through the inlet compressor  510  can be controlled during startup, surge, or stall conditions by varying the amount of low-pressure carbon dioxide  26  that is taken off upstream of the inlet compressor  510  and sent to compressor  590  before being sent to compressors  350  and  370  and intercoolers  340  and  360 . 
   It should be noted that if the turbo machinery non-dimensional parameters of the present invention are similar to the non-dimensional parameters for turbo machinery used in an air cycle, it may be possible to use standard turbo machinery equipment or make relatively minor adaptations to standard turbo machinery equipment for use with the carbon dioxide working fluid at significant cost savings. In addition, control of the inlet compressor feed gas via the control scheme of the present invention is superior to prior art systems that recycle steam to adjust the fluid properties, as taught by Stahl, U.S. Pat. No. 4,434,613, incorporated by reference. 
   Because of the continuous operation of the tertiary enhanced oil recovery process, the produced gas  10  in the present invention is always available. Therefore, startup of the semi-closed combined cycle system  300  or  400  of the present invention can be performed using the carbon dioxide/hydrocarbon fuel source  15  or  29  and oxygen  52  instead of using a natural gas fuel source and air as the oxidant. This is a unique feature of the present invention. In prior art systems discussed in the literature, the fuel gas is often pipeline natural gas and the carbon dioxide stream is not available when the plant is ready to startup. In general, the prior art literature suggests that startup of a similar semi-closed combined cycle should be done using air and natural gas fuel in an open cycle arrangement until enough carbon dioxide is produced to enable the cycle to operate as a semi-closed cycle. Ulizar I, Pilidis P. 2000, “Handling of a Semi-Closed Cycle Gas Turbine with a Carbon Dioxide-Argon Working Fluid.” Journal of Engineering for Gas Turbines and Power, 122(3), pp. 437-441. The present system  300 ,  400  will not require operation as an open cycle during startup and this minimizes the complexity of the control design. 
   Another advantage of the present system  300 ,  400  is that no air pollution removal equipment is required in the systems illustrated in  FIG. 3  and  FIG. 4 . Any pollution (nitrogen oxides and sulfur oxides) generated by the combustion of fuel gas in the power turbine is gathered and injected with the carbon dioxide into the hydrocarbon reservoir. The system  300 ,  400  and method of the present invention produces electricity without adding nitrogen oxides, sulfur oxides and carbon dioxide to the environment. Reduction of carbon dioxide emissions in the course of producing electricity is considered beneficial in order to reduce “greenhouse” gas effects. Greenhouse gas allowances and greenhouse avoidance tax credits may be of significant economic benefit and further improve system  300  and  400  economics. 
   At least one of the differences between the present invention and the disclosure of Zapadinski, U.S. Patent Publication 2004/0154793 A1 is that the present invention uses substantially pure oxygen as the oxidation agent in the burners  500  of power turbine  520  instead of air. Use of oxygen ensures suitable carbon dioxide quality for reinjection in the hydrocarbon reservoir. Contaminants such as nitrogen or uncombusted hydrocarbons reduce the partial pressure of carbon dioxide so that an increased injection pressure is required to obtain the same carbon dioxide flood properties as substantially pure carbon dioxide. Kovarik F S, “A Minimum Miscibility Pressure Study Using Impure CO 2  and West Texas Oil Systems: Data Base, Correlations, and Compositional Simulation,” Society of Petroleum Engineers Production Technology Symposium, November 1985. The initial cost and operating costs for the enhanced oil recovery process increases as additional equipment and energy is required to produce the higher carbon dioxide injection pressure. Therefore, an important component of the present invention is the use of substantially pure oxygen for oxidating agent in the burners  500 , which thereby lowers the required carbon dioxide injection pressure, which results in the advantage of lower initial equipment costs for the system  300 ,  400  and lower operating costs for the system  300 ,  400 . 
   Zapadinski, U.S. Patent Publication 2004/0154793 A1 further differs from system  300 ,  400  in that Zapadinski also does not disclose the use of low Btu gas in combination with IGCC technology for the burners  500  and power turbine  520  in the invention. The use of low Btu gas in combination with IGCC technology offers a significant operational and cost advantage. 
   Parrish, U.S. Pat. No. 4,344,486 discloses a method and system of enhanced recovery of hydrocarbons from underground reservoirs using carbon dioxide obtained by combusting a mixture of oxygen and produced gas from the underground reservoir. In Parrish, the exhaust gas is compressed and injected into the hydrocarbon reservoir. Parrish discloses a method whereby all of the produced carbon dioxide is reinjected in the underground reservoir for oil recovery and the cycle taught is a once-through process with no recirculation of carbon dioxide. In contrast, the present invention discloses a semi-closed combined cycle that recirculates a portion of the carbon dioxide and injects the remaining carbon dioxide in underground reservoirs for oil recovery. An important advantage of the present invention is the recycle of carbon dioxide and its use as a working fluid in the semi-closed combined cycle. 
   A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.