Quantifying the impact of capillary trapping on coal seam gas recovery

The presence of water in coal matrices could cause capillary force and consequently affect gas production. In this work, an integrated experimental and numerical modelling approach was adopted to quantify the impact of capillary trapping on coal seam gas (CSG) recovery. The experimental work was to gain the change rule of contact angle with pressure. The experimental data was then fed into a fully coupled field scale numerical model, together with a classic relative permeability model to describe gas-water two-phase flow in the cleats system. A series of sensitivity studies were conducted to assess the factors affecting gas production performance. The results show that the capillary trapping can alter the gas production curve evidently, with both the production rate and the cumulative production being reduced, and the peak production rate appears later. The decrease of the mean pore size leads to earlier attenuation of production rate and reduces the recovery rate, and 3.85% of gas is trapped for the average pore radius equals to 1.0 μm case. Pore size distribution (PSD) determines the percentage of pores, in which gas breakthrough is able to happen. The results illustrate that coal seams with a high portion of large pores can have a 220 day earlier and 911.0 m³/day higher peak gas rate. Higher Langmuir pressure constant delivers a lower and later peak gas rate, and the trapped gas percentage is also larger. Higher Langmuir volume constant, a critical parameter in determining in-situ gas content, leads to higher and later peak value. A noticeable impact of contact angle on gas production has also been observed. The developed framework and findings from this work are expected to help better understand the capillary trapping effect on unconventional gas reservoirs and proposes a way to quantify capillary trapping effects.