Occurrence law of pore water in shale matrix: A case study of the Silurian Longmaxi Formation in southern Sichuan Basin

The existing practices reveal that the occurrence characteristics of pore water in shale reservoirs are of great significance to shale gas exploration and development. However, the occurrence characteristics and main controlling factors of pore water in shale reservoirs have not been clearly understood. With the shale of Silurian Longmaxi Formation in the Fushun area, southern Sichuan Basin, as an example, this paper discusses the occurrence characteristics of pore water in shale reservoirs through molecular simulation, combined with the centrifuge-NMR experiment. Then, the mineral compositions and pore structures in shale are analyzed by means of X-ray diffraction, large-area high-resolution backscatter imaging, SEM quantitative mineral evaluation and other technologies. The following research results are obtained. First, the pore water content in the Longmaxi Formation shale reservoir in the Fushun area ranges from 8.05 mg/g to 18.43 mg/g, with an average of 10.97 mg/g. It occurs in two phase states, i.e., adsorbed state and free state. The dominant pore diameter for the occurrence of adsorbed water is about 2–5 nm, while that of free water is about 5–10 nm. Pores with diameter greater than 20 nm are almost devoid of water. Second, under the action of hydrocarbon generation and water drainage in the evolution process of shale organic matter, almost all pore water in organic pores is drained out, with only a small amount of pore water adhering to the surface of some oxygen-containing functional groups in kerogen. The inorganic pores developed in the clay mineral (mainly illite) particles have strong hydrophilicity on the surface, which is the main occurrence site of pore water in shale reservoirs. Third, excessive illite content leads to higher content of pore water in the shale reservoirs, which occupies the gas occurrence space and is not conducive to gas enrichment. In the process of gas production, when the gas flows to the pores with diameter less than 1.4 nm where adsorbed water occurs, it can be hardly produced for its flow needs greater breakthrough pressure due to the action of capillary force. In conclusion, the researches on the microscopic occurrence characteristics of pore water in shale reservoirs and the controlling effect of shale compositions on the occurrence of pore water are conducive to the efficient development of shale gas in the Sichuan Basin, and lay a solid theoretical foundation for the high-quality development of shale gas in China. © 2023 Natural Gas Industry Journal Agency. All rights reserved.