A New Method for Shale Gas Density Calculation Based on SRK Equation of State

Shale gas densities determined by using the conventional Soave-Redlich-Kwong (SRK) state equation are characterized by lower than actual values and significant relative errors. To enhance accuracy in shale gas density calculations under formation conditions, two parameters that are called contrast pressure and contrast temperature have been introduced to modify the conventional SRK method, and to establish an innovative method for the determination of shale gas density. At the same time, Newtonion iteration has been deployed to make computer programs, by which the shale gas densities of 210 samples were calculated under the pressures of 1-30 MPa and temperatures from 283.15 K to 428.15 K by means of SRK and modified SRK respectively, and then compared these data with that released from National Institute of Standards and Technology (NIST). It was found that the range of relative errors of shale gas densities calculated by SRK is from -11.10% to -0.05%, local relative errors are larger than 10%. On the other hand, the relative errors of shale gas densities calculated by improved SRK are from -2.39% to 2.11%, the maximum relative error is 2.39%. The research showed that the accuracy calculated by the new method based on SRK for calculating the density of shale gas was improved greatly, and may satisfy the demands for the evaluation of shale gas reservoirs and their application in development.