Mechanism of coal wettability on storage state of undersaturated CBM reservoirs

ZHU Suyang; MENG Shangzhi; PENG Xiaolong; LI Xiangchen; ZHANG Qiangui; ZHANG Si

doi:10.13809/j.cnki.cn32-1825/te.2022.04.005

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Mechanism of coal wettability on storage state of undersaturated CBM reservoirs
Issue 4, Pages: 580-588+595(2022)
Published：2022
DOI： 10.13809/j.cnki.cn32-1825/te.2022.04.005
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DOI：

ZHU Suyang, MENG Shangzhi, PENG Xiaolong, et al. Mechanism of coal wettability on storage state of undersaturated CBM reservoirs[J]. Petroleum Reservoir Evaluation and Development, 2022, (4): 580-588+595. DOI： 10.13809/j.cnki.cn32-1825/te.2022.04.005.

摘要

岩石对流体的润湿性决定了孔隙中流体的分布特征，多孔介质中的小孔隙优先被润湿相流体占据。基于这一原理，分析了目前煤岩润湿性实验与煤层气赋存状态的矛盾，从煤岩润湿性的角度，提出了两种可能的煤层气宏观赋存模式。如果煤岩亲气，甲烷在毛管压力的作用下，可以被割理中的水封闭在煤基质中。由于煤基质和割理界面处的毛管压力，煤层气的吸附是平衡的，但赋存可以出现欠饱和状态，这也是排采过程中出现临界解吸现象的原因。当煤岩水相润湿时，水相需要占据煤岩中的小孔隙（基质），甲烷可以通过液相吸附的形态储存在基质中。液相吸附中，排采过程的临界解吸源自欠饱和的煤层气溶解状态。通过煤层气液相吸附的验证实验，也表明甲烷可以通过液相吸附的形式大量赋存于煤层中。

Abstract

The wettability decides the fluid distribution in porous media

and the wet-phase fluid will primarily occupy the smaller pore. Based on this principle

the paradox between laboratory test of coal wettability and state of fluids distribution is analyzed. On the view of wettability

two possible coalbed methane（CBM） storage types are proposed. When the coal is gas-phase wettability

owing to capillary pressure

the free gas and adsorption gas can be trapped in matrix （smaller pore system） by the water in cleat （larger pore system）. In matrix system

the sorption of gas is in equilibrium state

but the storage of gas is in the undersaturated state. The critical desorption process results from the capability pressure of water and gas. When the coal is water-phase wettability

the matrix is saturated by water

and the coalbed methane can store in the matrix in liquid-phase sorption state. This study designs the experiment to validate the gas liquid-phase sorption. The results indicate that a mass of methane can be adsorbed in the matrix in the liquid-phase sorption mode.