Synergistic Effects of Micronano Structures on Porosity and the Permeability of Shale Under Varying Effective Stresses and Temperatures: A Case Study of Fresh Outcrops from Lower Silurian Longmaxi Formation Shale in the Southern Sichuan Basin, China

To study the changes in porosity–permeability and the characteristics of the pore structure of shale under stress and high temperature, the Lower Silurian Longmaxi Formation shale in the southern Sichuan Basin, China, was investigated under conditions of continuous pressurization and heating. In addition, the pore compression coefficients and permeability stress sensitivity coefficients were analyzed and quantified. The mineral composition of these black shales was analyzed using scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR). Our results reveal that the porosity and permeability of the shales exponentially decreases with increasing stress, reflecting that microfracture development during increasing stress causes the pores in these shale samples to enlarge. However, the pore compressibility and stress sensitivity coefficient vary for each sample, and the quantitative results indicate an overall decrease with increasing stress, suggesting that the shale deformation is nonlinear during stress release. Based on the mineral composition analyses and SEM measurements, we conclude that the nonuniform changes during stress release are related to the complexity of the shale mineral compositions and the different intercrystalline/ interlaminar pores of the different minerals, which are affected in different ways by pressure. The NMR measurements reveal that the mesopores are most developed in the shale samples, the pore volumes of the micro- and macropores are small, and the nanoscale pores are mainly from 1–60 nm in diameter. The different types of pore sizes decrease with increasing stress, indicating that the porosities measured experimentally reflect the synergistic effects of the different pore sizes on porosity. As the temperature increases, the permeability of the shale decreases significantly, which is primarily caused by the thermal expansion effect. The high clay mineral content of the shales also causes their permeabilities to be sensitive to temperature.