Microscopic occurrence and movability mechanism of pore water in deep shale gas reservoirs: A typical case study of the Wufeng-Longmaxi Formation, Luzhou block, Sichuan Basin

Abstract

Water in pore system (i.e., pore water) is of great significance for shale gas exploration and exploitation. However, the microscopic occurrence and movability mechanism of pore water is still unclear. In this study, the pore water of deep shale gas reservoirs of the Wufeng-Longmaxi Formation in the Luzhou block was quantitatively evaluated by a new theoretical approach. The microscopic distribution and movability of pore water were investigated by combining NMR technology and theoretical models, and then the influencing factors were discussed. The results show that the signal amplitude of dry shale originates from clay minerals and organic matter. For clay minerals, illite has the greatest influence on the signal amplitude, followed by illite-smectite mixed layer and chlorite. Pore water mainly exists in an adsorbed phase, with an average adsorbed ratio of 69.26%, and an average adsorption thickness (H) of 0.5392 nm. Adsorbed and free water coexist in pores of 3–100 nm, and have equivalent content and saturation in pores of 10 nm and 6–7 nm, respectively. The specific surface area and pore volume control the occurrence of adsorbed and free water respectively. With the increase in average pore size, the water-holding capacity of pore system weakens. Adsorbed water mainly occurs in pores related to organic matter and quartz, while free water tends to occur in clay minerals pores. The adsorption affinity of spherical pores is the strongest, followed by cylindrical and slit pores. The effective pore size threshold available for fluid flow in spherical, cylindrical, and slit pores is 6H, 4H, and 2H respectively. With increasing centrifugal pressure difference, the water saturation decreases, the adsorbed ratio and mobility increase. Shale with higher pore volume has stronger mobile potential and mobility. Clay minerals are essential for pore water movability, while organic matter and brittle minerals inhibit pore water movability. This study offers a novel approach for quantitatively characterize the occurrence and transport of pore water and potentially has wide application prospects.