A large pressure pulse decay method to simultaneously measure permeability and compressibility of tight rocks

Abstract

Permeability and pore volume compressibility are two key parameters of rocks for the evaluation and exploitation of oil/gas reservoirs. This study presents a large pressure pulse decay method (LPPD) for simultaneously estimating rock permeability and compressibility. A numerical solution of the LPPD model is first developed using the finite difference method. A globally convergent least-squares method is proposed to determine permeability and pore volume compressibility. Then, a series of large pressure pulse decay tests are conducted on a coal sample by taking water as the testing fluid. The proposed method is applied to processing the experimental data obtained from large pressure decay experiments under different confining pressures. The results indicate that the fitting coefficients are all larger than 0.99, which shows that the LPPD can well match the experimental data. The estimated permeabilities are compared and verified with Brace's method and modified Brace's method. Since the pore volume and the pore volume compressibility of the sample are not considered in Brace's method, the calculated permeabilities of the new method are smaller than the other two methods. The estimated values of pore volume compressibility are also verified by experimental results, and the maximum error between them is −6.93%, which shows that the proposed method is accurate and reliable for estimating the two parameters. At last, a new simpler method is proposed to calculate the pore volume compressibility, which shows acceptable approximation results. Compared with other transient methods of measuring permeability, the LPPD method can simultaneously measure rock permeability and pore volume compressibility. With the obtained parameters, the permeability of rock under different effective stresses can be obtained. The results in this study can be beneficial to the evaluation and exploitation of oil/gas reservoirs.