煤层气井突然产气机理分析

Mechanism of gas production rate outburst in coalbed methane wells

煤层气井开采时一般先排水后采气,且见气时的产量不是缓慢而是突然升高。为了弄清煤层气井突然产气的机理,从煤储层的结构、产气过程等方面进行了深入分析和研究。结果表明:煤岩储层微观上为双重介质,由割理(裂缝)和基质岩块2个系统组成;割理和基质孔隙中均充满了地层水,煤层气为赋存在基质中的吸附气,需排水降压解吸后才能被采出;刚解吸出的少量气体饱和程度较小,多以气泡的形式分散在基质孔隙水中,由于受到基质毛管压力的限制,这些气体无法流动;随着解吸气量增多,气泡逐渐变成连续相,气体的饱和程度增加,压力升高,流动性也有所增强,但是煤岩基质孔隙一般较小,毛管压力较高,很多气体仍被限制在基质孔隙中,只有当气体压力升高到突破毛管压力之后,大量的解吸气才会倾泻到割理中,致使煤层气井产气量突然升高。煤层气井的产气压力低于解吸压力,而煤层气的解吸压力其实就是地层水的饱和压力或泡点压力。在煤层气开采过程中,可以采取相应节流措施来控制煤层气井的产量变化,以达到稳产及保护煤层和生产管柱的效果。

Coalbed methane （CBM） wells must drain water from coalbeds before producing gas. The gas production rate increases suddenly rather than slowly at breakthrough of gas at wells, which is a strange phenomenon so far people do not know its mechanism. In order to understand production characteristics of CBM, the mechanism of production rate outburst of CBM wells was analyzed. It is recognized that coal rocks are dual porosity media in microscope, consisting of cleats （fractures） and matrix blocks. Cleats and matrix pores of coals are saturated with formation water. CBM exists in the matrix with the form of adsorption, which can be exploited only after de- sorption from matrix under pressure drawdown through draining formation water. A little of desorbed gas cannot flow due to obstacle of capillary pressure, which are dispersed in water with a small saturation. As desorbed gas is increased and accumulated into continuous phase with a high saturation, the gas pressure and the flowing abili- ty are also increased simultaneously. Coal matrix pores are quite small with high capillary pressure, which can trap massive desorbed gas in coal matrix. When increasing gas pressure breaks the capillary pressure, the mas- sive desorbed gas flows suddenly into cleats of coal rocks, which results in the outburst of gas production rate in CBM wells. The production pressure of CBM is lower than its desorption pressure. The desorption pressure of CBM is actually the saturation or bubble pressure of formation water. Abrupt increase of production rate may damage coalbed or production tubes, so it is necessary to take a possible control of production rate in practice.