Coupling effects of temperature, confining pressure, and pore pressure on permeability and average pore size of Longmaxi shale

The evolution due to temperature and pressure of shale reservoir permeability affects the productivity evaluation and development decision of shale gas reservoirs, which is very important for the exploration and development of unconventional gas reservoirs. This study analyzed the coupling effects of temperature (25, 50, and 75 degrees C), effective stress (15 and 30 MPa), and pore pressure (0.5, 2.0, 4.0, and 8.0 MPa) on the permeability of the shale sample in the Longmaxi Formation. As the temperature and pressure increased, the apparent permeability exhibited a downward trend, and the absolute permeability decreased with the rise of temperature or effective stress. An in-depth analysis of the gas slippage factors under the conditions of different temperature and pressure was conducted to evaluate the trend of the average pore width with temperature and pressure. The results were then verified by scanning electron microscopy (SEM). The results provide new insights into evaluating the permeability of the Longmaxi shale and can be used to enhance the gas recovery rate of deep shale gas reservoirs. This paper investigates the permeability characteristics and average pore size of Longmaxi shale under different temperature and pressure conditions. A customized high-temperature and high-pressure core permeability instrument was used to carry out seepage experiments on the Longmaxi shale. The influence of temperature, effective stress, and pore pressure on the apparent permeability were analyzed. The influence of the confining pressure at 30 MPa on the apparent permeability was greater than at 15 MPa. The absolute permeability and Klinkenberg parameters were calculated and analyzed. A good agreement was obtained between the calculated pore width derived from the Klinkenberg parameters and measured from the SEM images. The influence of temperature, pore pressure, and temperature on pore width was analyzed. image The effects of confining stress, pore pressure, and temperature on apparent permeability and absolute permeability were investigated. The Klinkenberg parameters decreased sharply with an increase in temperature under confining stress of 15 MPa but decreased slightly under confining stress of 30 MPa. The pore pressure had a negligible influence on the Klinkenberg parameters. The calculated pore width was highly consistent with the pore width derived from SEM image analysis.