New mathematical model and key parameters of capillary pressure curves of high-rank coal samples

Capillary pressure is an important parameter that affects the occurrence and development of coalbed methane. In order to establish an effective mathematical model to characterize capillary pressure curves of high-rank coalbed methane reservoirs, coal samples from No.3 coal seam in southern Qinshui Basin was taken as the research object, high-pressure mercury intrusion experiments were carried out, and the adaptability of the typical mathematical models of the capillary pressure curve currently in common use are systematically evaluated. Then a new mathematical model with higher fitting degree for high-rank coal is established. The results show that capillary pressure curves of 6 coal samples have no middle flat section, and the overall performance showed the protruding to upper left, which is obviously different from those of the conventional sandstone or low-rank coal reservoirs. BC model, He Chengzu’s model and Li’s model could not fit the capillary pressure curves of high-rank coal samples well. A new empirical model of capillary pressure curve, is established by processing capillary pressure data. The fitting degree of this model and capillary pressure curve data reaches over 97%, and the fitting effect is better than the existing models. In the log-log coordinates, the logarithmic difference between capillary pressure and the minimum capillary pressure is linear with the logarithmic difference between mercury saturation and the minimum capillary pressure, and the linear relationship can be used to directly obtain the slope a and power exponent b of the new capillary pressure model. The values of slope a and power exponent b are inversely proportional to the capillary pressure, when other conditions are equal.