基于渗水试验的煤岩压裂液静态滤失特征分析

Analysis of static leak-off characteristics of fracturing fluid through coal rocks based on water seepage tests

压裂液滤失是制约煤储层压裂造缝效率、诱发储层物性伤害的关键,厘定压裂液静态滤失特征尤其是空间渗滤非均质性对于煤储层压裂液滤失机制深化、降滤失剂材料改进具有实际意义。为此采用大尺寸煤岩试样开展渗水试验研究,查明了清洁、冻胶压裂液在原生结构煤体、碎裂构造煤体中的渗水习性。研究结果表明,静态滤失过程中煤岩压裂液渗水系数和渗透速度受到压裂液体系、驱动压差及煤体结构的综合影响,整体上随着累计渗滤液量的增加均呈现出负幂指数形式的降低,清洁压裂液在原生结构煤体中初滤失现象明显。研究结果还表明,压裂液渗滤侵入原生结构煤岩导致裂缝两侧煤岩封闭性孔隙回弹响应,以及压裂液侵入构造煤岩开放孔隙内的甲烷气体置换效应,是导致煤岩压裂液静态滤失中渗滤驱动压差、渗滤速度降低的关键微观机制。

Leak-off of fracturing fluid was critical to restrict fracture efficiency and induce physical damage in coal reservoirs,determining the characteristics of static fracturing fluid leak-off,particularly the spatial heterogeneity of leak-off,was of practical significance for deepening the mechanism of fracturing fluid leakage and improving the materials of filtrate reducer in coal reservoirs.In the paper,water seepage tests were conducted with large-size coal samples,and the water seepage behaviors of clear and gelled fracturing fluid in primary structure coal and fractured tectonic coal were found out.The results showed that the seepage coefficient,seepage velocity in fracturing fluid static fluid-loss process were affected by the fracturing fluid system,driving pressure difference and coal structure,which decreased in negative power exponential form with the increase of accumulated leak-off volumes of fracturing fluid,the spurt loss of clear fracturing fluid in primary structure coal was obvious.Moreover,the stress response effect of closed pores at both sides of fractures in primary structure coal induced by fracturing fluid percolation,and the methane substitution effect of open pores of fractured tectonic coal,were the key micro mechanisms that led to the decrease of driving pressure difference and percolation velocity in the fracturing fluid static leak-off process of coal rock.