Water distribution in pore systems and its influences on gas-bearing property of deep shale: A case study of the Longmaxi Formation in the Luzhou area, southern Sichuan Basin

The deeply-buried (>3500 m) Longmaxi Formation (LMX) shale in the southern Sichuan Basin, China, has become an attractive target for shale gas exploration owing to its huge resource potential. Exploration shows that the deep shale has a wide range of gas-in-place (GIP) contents, with variable gas yields, but the reason remains unclear, especially the impact of pore water on gas-bearing property lacks systematic research. In the present study, a suite of deep LMX shale samples was collected from the well FB1 in the Luzhou area of the southern Sichuan Basin, and techniques such as the pore water content measurements, low-pressure gas (CO2 and N2) adsorption and high-pressure methane adsorption experiments of the moist and dry samples were employed to investigate the distribution of water in the nanopores and its effects on the gas-bearing property. The results show that the deep shale is characterized by a high water content, with a high water saturation (average up to 69.80%), and the water occurs in both the inorganic and organic pores. The water reduces the effective specific surface area and pore volume of the shale averagely by 79.01% and 22.56%. Consequently, the water results in the decrease of methane adsorption capacity averagely by 45.13%. The GIP content models of two typical shale samples indicate that their total gas content is < 3 m3/t under the actual conditions, without development potential, and it will increase significantly to >3 m3/t with decreasing water saturation to <40–50%, especially under overpressure conditions. The gas-bearing property of deep LMX shale reservoirs in the complex faulted zones or structural-complex zones would mainly depend on the pore water content except for the properties of shale itself (e.g., maturity, TOC content, mineral composition, porosity and pore structure).