Quantifying Capillary Trapping on Ultimate Coal Seam Gas Recovery: A Laboratory Study

This work presents the results for two sets of laboratory measurements: (i) the change of contact angle in response to different surface roughness, temperature, and water chemistry; and (ii) a series of gas desorption experiments on two coal samples in a pressure chamber to quantify the impact of water on gas desorption and its implications to ultimate gas recovery. Coal transient directional deformation was measured using strain gauges and the visual monitoring of coal desorption was achieved through one of the four transparent observation windows and was recorded using high resolution camera. The desorption data was collected and analyzed to determine the capillary trapping effect. The results show that the contact angle has a negative relation with surface roughness and temperature, while no evident relation with water salinity is observed. The existence of water in coal can reduce the gas recovery rate evidently, indicating that the capillary pressure in pores hinder the gas flow capacity and even cause trapping effect. Lower desorption and recovery rate was observed for the tighter coal sample. The gas emerging characteristics captured by the camera present that more bubbles are formed on the matrix surface under lower pressure, and the gas bubbles tend to be larger, demonstrating that the gas slug is easily generated under lower pressure conditions and could hinder the gas-water two-phase flow in cleats, which is unfavorable for gas production. The measured matrix strains reveal that the strain under the presence of water condition is smaller compared to dry coal, showing the capillary induced trapping effect also influences the matrix shrinkage. The results of the sorption-induced strain, bubble pattern in the cleats, and the existence of capillary trapping offers a new set of information for better understanding what may occur in the reservoir during gas depletion.