Abstract 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 qualify capillary trapping effects.

中文翻译：

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量化毛细管捕集对煤层气采收率的影响

摘要 煤基质中水的存在会引起毛细管力，从而影响产气量。在这项工作中，采用综合实验和数值建模方法来量化毛细管捕集对煤层气（CSG）采收率的影响。实验工作是获得接触角随压力的变化规律。然后将实验数据输入一个完全耦合的现场比例数值模型，连同一个经典的相对渗透率模型来描述割理系统中的气水两相流。进行了一系列敏感性研究以评估影响产气性能的因素。结果表明，毛细管捕集能明显改变产气曲线，使产气量和累计产气量均降低，并且峰值生产率出现较晚。平均孔径的减小导致产率提前衰减，降低了采收率，平均孔径为1.0 μm的情况下，有3.85%的气体被截留。孔径分布 (PSD) 决定了能够发生气体突破的孔的百分比。结果表明，大孔隙率高的煤层可提前220天、高出911.0立方米/天的峰值瓦斯率。较高的朗缪尔压力常数提供较低且较晚的峰值气体速率，并且截留气体百分比也较大。较高的朗缪尔体积常数是确定原位气体含量的关键参数，会导致更高和更晚的峰值。还观察到接触角对产气量的显着影响。