Quantitative Studies on the Characterization and Evaluation of Adsorbed Gas and Free Gas in Deep Shale Reservoirs

The quantitative characterization of adsorbed gas and free gas in shale reservoirs is a key issue in exploration and development of shale gas. Thus, the aforementioned topic is of great significance to the evaluation of reserves, the screening of favorable target areas, and the formulation of development plans. However, research on our current understanding of the quantities of adsorbed gas and free gas in deep shale gas reservoirs is still lacking. To address this problem, deep shales from the Longmaxi Formation in southern China were collected to conduct high-pressure isothermal adsorption experiments. The high-pressure isothermal adsorption model was used to describe the adsorption behavior of methane in deep shales, and the adsorbed gas and free gas in the deep shales were characterized quantitatively. The effects of temperature, pressure, and moisture on the adsorbed gas and the density of the free gas were analyzed. The results indicated that the excess adsorption isotherm curve for methane in deep shales increased and then decreased with the increase of pressure, and the modified Langmuir adsorption model may be used to describe the high-pressure adsorption behaviors. The adsorbed gas in shales decreases gradually with the increase of pressure, and the proportion of adsorbed gas and free gas is between 23 and 74% when the pressure reaches 50 MPa. The adsorbed gas in deep shales decreases with an increase of temperature, and the presence of water greatly reduces the adsorption capacity of the deep shale. The pore space occupied by the free gas in shale increased with the increase in the density of the free phase, and the ratio of the adsorbed gas to the free gas decreased. This research provides a useful reference for explaining how to best evaluate shale gas reservoirs, estimate the reserves in deep shales, and evaluate the adsorption and flow capacity of deep shale gas.