Composition-Transient Analysis in Shale-Gas Reservoirs With Consideration of Multicomponent Adsorption

Abstract

Summary Rate-transient analysis (RTA) has been widely studied to evaluate the shale-gas-reservoir parameters. However, few attempts have been made on composition-transient analysis (CTA) to evaluate formation properties about shale-gas reservoirs. In this work, we proposed a new CTA method and provided a physical interpretation of CTA. We first established a compositional simulator incorporating slip flow and extended Langmuir (EL) isotherm with multiple components. Then, a fully implicit iterative numerical solution of the nonlinear model was developed on the basis of perpendicular-bisection (PEBI) gridding. Finally, we defined the composition change and derivative as functions of pressure change and derivative. One important finding is that the free-gas composition is obviously different from the desorped-gas composition because of the different desorption capacity of different gas components. This contributes to the changes of composition of the produced gas over time. Another finding is that ultimate adsorption capacity determined the value of the composition change and composition derivative, which made adsorption content be easily determined by CTA. If the flow regimes are defined by composition, several flow regimes could be identified from the composition change and the composition derivative, which may not exist on the curves of pressure change and pressure derivative. The findings in this study could provide a more-effective and practicable way to identify reservoir parameters than the traditional well-test method. Furthermore, gas compositions could be measured at the wellhead, which makes the CTA feasible in practice.