Real Gas Pseudopressures For CO2 Reservoirs

This paper presents a new method for correlating real gas pseudopressure values of gas reservoirs containing large amounts of CO2- Special attention is devoted to gas reservoirs in Colorado, New Mexico, and Utah. These reservoirs have 95 to 100% CO2 concentrations. The effects of CO2 on the skin factor also are analyzed. The main results of this study are that (1) the effects of CO2 on the conventional pressure analysis techniques are severe at higher mole fractions and at pseudoreduced pressures greater than one, (2) if real gas pseudopressure data are not properly corrected, the reservoir permeability calculated from pressure buildup and drawdown tests will be considerably less than the actual value, and (3) the proposed technique is simple, quick. and accurate enough to calculate pseudopressure values. This method is also useful in gas reservoirs with 0 to 100% CO2 concentrations. CO2 affects the skin zone both physically and chemically, in most cases favorably. The total skin factor is slightly dependent on time for very short transient flows. However, it ultimately will become constant as the CO2 gas sweeps the entire skin zone. Introduction The bulk of industry research and field testing of CO2 has been directed toward miscible displacement. This method of using CO2 appears to have greatest potential for oil recovery not possible by conventional producing methods. However, the potential for this process will he significant only if CO2 Can be found in enough quantity to treat many fields. The most plausible source of adequate volumes Of CO2 at a cost low enough for CO2 flooding appears to be either from existing known and undeveloped sources of naturally occurring CO2 or from future such discoveries. There are several areas in the U.S. where CO2 is known to occur naturally. Fig. 1 illustrates locations of wells that have produced significant concentrations of CO2. The pressure tests and correlation charts presented in this paper are from wells located in southern Colorado as shown in this figure. Actual CO2 reserves that might be contained in these various geographical areas ar-e unknown. Future large reserves of naturally occurring CO2 most likely would be located in the Four Corners area, the northeast New Mexico/southeast Colorado area, and central Mississippi, and would occur in reservoirs of high-purity CO2. This study analyzes pressure behavior of such reservoirs. Gas flow through porous media has been the object of considerable research. Up to the mid-1960's most published articles dealt with ideal gas. But most of these studies were- inadequate for gas reservoirs having high reservoir pressures, low permeabilities, and/or containing large amounts of contaminants such as CO2, nitrogen (N2), and hydrogen sulfide (H2S)- Several theories dealing with these problems were published. The most pertinent ones to this study are the papers by Al Hussainy et al., Al Hussainy and Ramey, and Zana and Thomas. Al Hussainy et al. introduced the concept of real gas potential, which eliminates the need to neglect the pressure dependence of gas viscosity and the gas deviation factor. The assumption of small pressure gradients was also eliminated. Al Hussainy and Ramey showed how the concept of real gas potential or the real gas pseudopressure could be used to analyze pressure transient tests. A few years later, Zana and Thomas investigated some of the effects of gas contaminants on real gas flow. They generated tables of the real gas pseudopressure function for various concentrations of N2, CO2, and H2S. Their study, however, did not consider the case of the high-purity CO2 reservoirs- Some of the other papers found useful to this study are by Carter, Dranchuk and Chwyl, Coats et al., Aziz et al., Robinson et al., Buxton, and Dewitt and Thodos. For instance, the study by Robinson et al. showed that there is a definite departure by the gas compressibility curve for CO2 from that of hydrocarbons, and that the value of this departure increases for higher amounts Of CO2. This departure is most significant at approximately 2,000 psia [13.8 MPa] and at low temperatures. Buxton determined the values of the gas compressibility factor at different concentrations Of CO2 in a mixture with hydrocarbon gases. Finally, Dewitt and Thodos experimentally demonstrated that the viscosities of various mixtures of gases increase with pressure as the CO2 content increases. This study investigates the pressure behavior of high- purity CO2 reservoirs-i.e., reservoirs with 60 to 100% CO2 concentrations. In particular, pseudopressure values of such reservoirs are generated and semiempirical relations are developed. Furthermore, a study by Keio Toi on diffusion of CO2 through glassy polymers and Ref. 12 provide the basis to investigate qualitatively the effects Of CO2 on the skin factor. Real Gas Pseudopressure Function As shown in Ref. 1, transient flow of real gas through porous media can be described by (1) SPEJ P. 180^