Prefracture Testing in Tight Gas Reservoirs

Abstract

Summary: A pressure transient test run before a fracture job in a tight gas reservoir is an invaluable part of the total testing program necessary to evaluate the efficacy of such a fracture job. This paper addresses the problem of treating the pressure-sensitive viscosity/compressibility (μct) product while analyzing the prefracture test data. A large variation of μct with pressure occurs in a typical low-flow-capacity (kh) well. These pressure-sensitive gas properties lead to a nonlinear gas flow equation. Use of the pseudopressure function partially linearizes the gas flow equation, while introduction of pseudotime and pseudopressure functions causes the complete linearization. This linearization permits a valid type-curve analysis. However, a semianalytical solution of the diffusivity equation that considers only the pseudopressure function suggests that the problem of nonlinearity is insignificant. We show that both the contentions presented in the literature are correct: complete linearization is required when the data are affected by wellbore storage, whereas partial linearization is quite adequate for a valid semilog analysis. Large variations of the μct product also cause the wellbore storage to last a long time, even in the presence of a downhole shut-in tool. We demonstrate that the convolution approach, for which the sandface rate measurement is required, shortens the testing time by orders of magnitude and yields the desired formation parameters of kh and skin.