Distribution Law of Occurrence State and Content Prediction of Deep CBM: A Case Study in the Ordos Basin, China

Abstract

The deep coalbed methane (CBM) resources in the Ordos Basin are enormous, and their exploration and development breakthrough are among the critical ways to improve CBM production in China. The occurrence state of deep CBM has unique characteristics caused directly by the change in methane density (ρ). By predicting key adsorption parameters and solving directly for adsorbed methane density (ρ_a), it is concluded that ρ_a decreases with increasing temperature and increases rapidly at first and then tends to stabilize with increasing pressure. Considering the characteristics of supercritical methane adsorption, a porosity (φ) prediction model for deep coal reservoirs was established based on these unique occurrence characteristics. A new equation for predicting gas content in deep coal seams was developed by combining the free gas content (V_fg) calculation method for unconventional oil and gas reservoirs and the adsorbed gas content (V_ad) method based on ρ_a. It was observed that the V_fg increased with pressure and φ but decreased with increasing water saturation and temperature. However, as temperature and pressure increased, the rate of increase in V_fg slowed down, probably because of the influence of φ decreasing with increasing temperature and pressure, which is similar to the change in ρ_a. Meanwhile, the V_ad increased with temperature and pressure, showing a trend of rapid increase followed by a decrease. These indicate that, as the depth and pressure increase and the temperature rises in deep coal seams, the negative effect of temperature gradually outweighs the positive effect of pressure. When φ increased to a specific value in low- to medium-rank coal, the V_fg can exceed the V_ad at depths between 2000 and 2500 m. Compared to high-rank coal, which has high V_ad, low- to medium-rank coals are more prone to experience the saturation phenomenon where the V_fg exceeds the V_ad.