An Improved GIP Method for Measurements of Shale Effective Porosity Through Pressure Decay

Shale gas reservoir samples exhibit low porosities, ultralow permeabilities, and high nanopore contents; thus, it is difficult to measure their porosities quickly and accurately. In this work, the decrease in the pressure inside a matrix cup is modeled via mass conservation and Darcy’s law, solving the problem of obtaining the equilibrium pressure via the gas intrusion porosimetry (GIP) method. The newly established mathematical model is used to fit the experimental data to obtain the equilibrium pressure before the experiment reaches actual equilibrium, enabling accurate and rapid measurements of the shale porosity. Compared with the GIP method, the improved GIP method reduces the saturation time required to obtain the equilibrium pressure and minimizes the influence of the saturation time on the porosity measurement. Moreover, the results obtained by the improved GIP method are consistent with the true porosities of the sample. However, to improve the measurement accuracy as much as possible, the saturation time of the new method should be 10~15 minutes. Repeated experiments show that the maximum error between the different tests is less than 5%, indicating good repeatability.