Reservoir diffusion properties of the Longmaxi shale in Shizhu area, Southern Sichuan basin, China

Diffusion ability is an important indicator of shale gas reservoir quality. In this paper, the diffusion coefficient of the Longmaxi Formation is measured via the free hydrocarbon concentration method, and the diffusion ability, influencing factors, and seepage flow are discussed. Results show that the diffusion coefficient of the Longmaxi Formation is between 1.23x10(-5) and 2.98x10(-5) cm(2) s(-1) with an average value of 2.19x10(-5) cm(2) s(-1) (confining pressure 3.0 MPa). The diffusion coefficient is calculated for various pressures using an empirical formula (D=0.339K(0.67)/M-0.5) and experimentally measured data. The estimated, temperature-corrected diffusion coefficient of the Longmaxi Formation is 3.94x10(-6) -7.24x10(-6) cm(2) s(-1) with an average value of 5.28x10(-6) cm(2) s(-1) for depths from 1000 to 3000 m (confining pressure 16.7-39.7 MPa). The diffusion coefficient increases with increasing depth of the reservoir due to the changes in pressure and temperature. Fitting parameters show that the porosity of the reservoir and clay minerals is positively correlated with the diffusion coefficient, and the diffusion coefficient is also related to factors such as total organic carbon and the maximum reflectance of vitrinite (Ro). The diffusion flow rate is 0.177-0.204 m(3) d(-1) with an average of 0.182 m(3) d(-1). Linear seepage flow is 4.95x10(-4) -14.29x10(-4) m(3) d(-1) with an average of 8.87x10(-4) m(3) d(-1), calculated from the diffusion coefficient and permeability per unit flow. These results indicate that the migration of shale gas in the deep region of the reservoir is mainly by diffusion. Therefore, diffusion is an important shale gas flow mechanism.