煤层气吸附-解吸机理再认识

Reconsideration of Coalbed Methane Adsorption-Desorption Mechanism

煤层气是以吸附成藏的非常规气藏,其圈闭机理与以游离气成藏的常规气藏不同。本文分析了游离气与吸附气的圈闭机理,研究认为煤层气的吸附并不存在临界解吸现象,生产过程中的临界解吸是煤层气-水关系以及毛管压力导致的,并提出了一种水动力学封闭形式。研究认为煤层裂缝-割理系统被水饱和,溶解态煤层气会以浓度差扩散的方式慢速散失,但是游离气和吸附气可以被水动力学圈闭在煤层的基质中。根据水动力学封闭机理,煤层气的吸附是平衡的,不存在欠饱和状态,而排采过程中会出现临界解吸现象,表现出欠饱和赋存的状态,是毛管压力作用的结果。

Coalbed methane is an unconventional gas reservoir in a sorption state,of which the trapping mechanism is totally different from the conventional gas reservoir in a free state.The trapping mechanism of free gas and sorption gas is analyzed in this paper.It is investigated that the undersaturated state can hardly occur in coalbed methane sorption process and the critical description results from the gas-water relationship and the capillary pressure.Meanwhile,a new hydraulic trapping mechanism is reported.The research indicates that the cleat-fracture system of coal seam is saturated with water and the methane leakage occurs in the solution state.However,the free gas and sorption gas can be trapped in the coal matrix.With reference to the hydraulic trapping mechanism,gas sorption is an equilibrium process without any undersaturated state and the methane production experiences the critical desorption process,showing the undersaturated storage state.