Some Effects of Contaminants on Real Gas Flow

Abstract

In gas well testing and deliverability forecasting, failure to make corrections for contaminants can lead to intolerable errors. In general, where impurities exceed ten percent by volume, corrected values of the real gas pseudopressure function should be used. Introduction Isothermal gas flow through porous media has been the object of considerable study - the work, in general, deals with the flow of ideal gases and is therefore not generally applicable to gas reservoirs having low permeabilities or high reservoir pressures. This is because ideal gas flow treatment implicity assumes that pressure gradients in the system are small. Furthermore, the pressure dependence of gas viscosity and the gas law deviation factor are usually neglected. Recently, Al-Hussainy et al. presented a theory for real gas flow. Unlike previous work, theirs demonstrated that the pressure dependence of viscosity and the gas law deviation factor could be taken into account by expressing the gas flow equation in terms of a pseudopressure function. A nonlinear flow equation similar to the diffusivity equation was developed that did not require the assumption of small pressure gradients. In addition, a table of the real gas pseudopressure function was presented for gases containing pseudopressure function was presented for gases containing small amounts of contaminants. In a companion paper Al-Hussainy and Ramey showed how the real paper Al-Hussainy and Ramey showed how the real gas flow theory could be applied to standard gas well tests. Again, this study was limited to gases containing small amounts of contaminants. The purpose of our paper is to investigate some of the effects of gas contamination on real gas flow. Particular attention is devoted to constructing tables of Particular attention is devoted to constructing tables of the real gas pseudopressure function corrected for various amounts of nitrogen, carbon dioxide and hydrogen sulfide. Comparisons are made with the uncorrected results of Al-Hussainy et al. Finally, several example calculations are presented for some typical well tests, showing the errors that can occur in failing to make adequate corrections for gas contaminants. Real Gas Flow Equations For completeness, some of the development of Al-Hussainy and Ramey is included here. Principal assumptions are:Flow is isothermal and laminar.Gas composition is constant.The gas law is pV = znRT.The reservoir is homogeneous and isotropic.The formation has constant thickness and no dip. The equation for radial flow of a real gas is (1) Eq. 1 can be expressed in terms of the real gas pseudopressure function defined by pseudopressure function defined by (2) which yields (3) where cg(p)is the isothermal compressibility of gas given by (4) The pressure Pm is an arbitrary base pressure. JPT P. 1157