Evaluating the security of geological coalbed sequestration of supercritical CO2 reservoirs: The Haishiwan coalfield, China as a natural analogue

High concentrations of CO2 have been observed in the NO. 2 coal seam in the Haishiwan coalfield. The coal seam gas is almost pure CO2 introduced by dynamic metamorphism via faults that replaced pre-existing CH4. The CO2 concentration range in the coal mine is 34.1–98.64%, the measured gas pressure values range from 1 MPa to 7.5 MPa, and the temperature gradient of the eastern wells in the fourth formation of the Yaojie Group varies from 33 °C km−1 to 72 °C km−1, which could increase the coal seam gas pressure of the Haishiwan coalfields by 10.4–25.5% between depths of 700 and 1000 m; the gas pressure would reach 7.5 MPa at 700 m depth. Thus, supercritical CO2 storage potentially occurs in the Haishiwan coalfield, according to the critical point of the CH4–CO2 binary system.The pure CO2 storage capacity of the Haishiwan coal seam is 44.7 m3/t at a pressure of 7.5 MPa and a temperature of 313.15 K, and the volume of the gas sorbed increases with the decreasing proportion of CH4 in the gas. The high pore pressure and sorption swelling result in a lower permeability of the NO. 2 coal seam with 0.00039–0.00051 mD, which favour self-sealing processes. Supercritical CO2 could dissolve and mobilise hydrocarbons from coal liptinite.The precipitation and cementation of carbonates that plug rock pores results in a decrease in the fracture permeability of the cap rock and boundary fault of the NO. 2 coal seam. Natural analogue studies of supercritical CO2 occurring in coal may yield insights into the mechanisms of CO2 entrapment and the modes of CO2 storage.

► The supercritical conditions of CO2 have been observed in the Haishiwan coalfield.
► In suit interaction between coal and supercritical CO2 during geological time scales.
► The mechanism of natural analogue CO2 security storage in the coal seam.