煤层气储层中的流体压裂裂隙

FLUID PRESSURE-DEPENDENT FRACTURES IN COAL-BED GAS RESERVOIRS

煤中裂隙是煤层气运移产出的通道,对其成因进行探讨是煤层气地质学的重要内容。文章根据宏观 和微观观测,发现煤中存在一种流体压力致裂的特殊裂隙,这些裂隙的成因无法用诸如应力场的作用来解释。此 类裂隙的形成与煤中流体(液体和气体)密切相关,流体的来源有两类:煤化作用过程中形成的以甲烷为主的流体 和补给来的地下水,且以前者为主。当孔隙或裂隙中流体压力大于垂直裂隙(或基质孔隙)壁的正应力时,孔隙或 裂隙将沿最大主应力方向延伸、沿最小主应力方向张开;当流体压力降低时扩展终止。流体压力导致孔隙向割理 的转化、割理向继承性裂隙以及外生裂隙的进一步扩展。流体压力的集中是周期性的,即流体压力增加—裂隙扩 展—压力降低—裂隙闭合,这与烃源岩微裂缝排烃机理类似。

The fractures in coal formations are the migration and discharge channels of coal-bed gas. It is the important content of the coal-bed gas geology to investigate the origin of the fractures. According to the macro and micro observation, it is found there are special fractures depending on the fluid pressure in coal formations. The origin of the fractures can't be explained by the tectonic stress field. The origin of the fractures is correlated with the fluid (liquid and gas) closely. The fluid comes mainly from the fluid, most of which is methane, formed during the process of coalification and the recharged underground water, and the former governs. When the fluid pressure in pores or fractures exceeds the normal stresses that act vertically on the fracture (or matrix pores) walls, the pores or fractures will extend along the direction of the max. main stress and open along the direction of the min. main stress. When the fluid pressure depletes, the extension of the pores or fractures will terminate. The fluid pressure leads the conversion of pores into cleats. The cleats further develop to inherited fractures and exogenous fractures. The fluid pressure experiences cycle convergence, which causes the fracture extension while the fluid pressure increases, and the fracture closing while the fluid pressure decreases. It is similar to the mechanism of source rocks releasing hydrocarbon through micro cracks.