Modeling pore size distribution of southern Sichuan shale gas reservoirs

•A numerical fitting model is proposed to quantitatively characterize the multimodal pore size distribution of shale.•The evolution of pore structure of marine shale subjected to four field conditions is thoroughly investigated.•A method to estimate pore volume of rocks under reservoir conditions is proposed.•A series of laboratory experiments is conducted to reveal the microstructure of marine shale from Sichuan Basin, China.

Understanding the pore structure of shale is important to investigating its fluid flow and hydrocarbon storage capacity and has gained prominence in the exploration for shale gas. Obtaining an accurate mathematical representation of the shale pore system, which can potentially aid in the numerical modeling of its hydraulic and mechanical behavior under a variety of field conditions, has been one of the most significant challenges in the studies of shale gas. In this study, a numerical fitting model was proposed to quantitatively characterize the multimodal pore size distribution (PSD) of the shale pore structure and produce fitting parameters with physical significance. Based on the fitting model and experimental data from mercury injection capillary pressure (MICP) and nitrogen gas (N2) adsorption tests, the pore structure evolution of marine Longmaxi shale was investigated. The sample was tested in various field conditions, such as with different extents of heterogeneity, quantities of clay and organic matter, cryogenic treatments and types of fracturing fluids. It was found that specimen milling decreased the macropore volume. Upon hydration and oven drying, the reorganization of clay and organic matter decreased the mesopore space and increased the micropore space, while the total pore volume remained relatively constant. In addition, cryogenic treatment of hydrated specimens gave rise to a remarkable expansion in either macro- or mesopore volume, especially when hydrated by a viscous fluid. Based on this analysis, a method for estimating the pore volume of shale under reservoir conditions is recommended. Supplemental field-emission scanning electron microscopy (FE-SEM) tests were also conducted to reveal the evolution of internal surface structures.