Effects of temperature on seepage capacity for a multi-type ultra-deep carbonate gas reservoir

Ultra-deep carbonate gas reservoirs are buried at great depths and have high temperatures, and the impact of high temperature on the seepage capacity of multi-type reservoirs is still unclear. The study selected cores from the fourth member of Dengying Formation in the Gaoshiti-Moxi area to measure the gas single-phase permeability of rock samples during the heating and cooling process, as well as the gas-water interfacial tension and gas-water two-phase relative permeability at different temperatures. This allowed researchers to obtain the effect of temperature on the seepage capacity of multi-type ultra-deep carbonate gas reservoir. The research results show that within the range of 20–120 °C, with the change of temperature, the gas single-phase seepage capacity of different types of reservoir rock samples changes as a power function. The decrease in gas-phase permeability during the heating process is jointly affected by the increase in gas viscosity, the expansion of dolomite crystals, and the migration of rock particles after embrittlement. After one heating and cooling process, fractured-cavity type rock samples had the highest irreversible degree of permeability at 82.52%, due to the development of micro-fractures, followed by 27.63% for pore type due to the development of small pores and throats, and the lowest was 9.46% for pore-cavity type. Fractured-cavity rock samples are temperature-sensitive, while pore-type and pore-cavity-type rock samples are temperature-resistant. The upper-temperature limit of the target multi-type gas reservoir is concentrated around 46–50 °C. The temperature increase mainly improves the gas-displacing water efficiency and gas-water two-phase seepage capacity by reducing the water-gas viscosity ratio, which is about 1/3 of the normal temperature at the formation temperature. The gas-water phase permeability curves of multi-type reservoirs under high-temperature conditions can better represent the two-phase seepage characteristics of actual formations. The effect of temperature on the seepage capacity of multi-type ultra-deep carbonate gas reservoirs can provide a theoretical basis for the efficient development of such gas reservoirs.