压裂液处理对煤岩孔隙结构的影响

Effect of fracturing fluid treatment on pore structure of coal

关键词:  煤层气  压裂液  孔隙结构  氮气吸附  等温吸附

Key words:  coalbed methane  fracturing fluid  pore structure  nitrogen adsorption  isothermal adsorption

摘要:

煤层气压裂作业中必然发生压裂液与煤层的相互接触,煤岩微裂隙发育和毛管压力高等特点导致压裂液极易侵入煤层对煤岩孔隙结构造成严重损害,进而改变煤岩对煤层气的吸附能力。选取宁武盆地9号煤和现场用压裂液,采用氮气吸附和扫描电镜(SEM)表征压裂液处理前后煤样孔隙结构,探求不同体系压裂液处理对煤岩孔隙结构的影响,开展压裂液处理前后煤样的等温吸附实验。结果表明:压裂液处理后煤样孔径分布、比表面积及孔隙分形维数都将发生变化;压裂液体系对煤样孔隙结构的影响程度为瓜胶压裂液>活性水压裂液>清洁压裂液;压裂液处理后煤样比表面积与孔隙分形维数增量越大,其对甲烷气体吸附能力越强,煤岩孔隙结构的变化会改变甲烷气体的吸附能力。

Abstract:

Fracturing fluid and the coal seam will touch each other during fracturing operation of coal. The developed micro-fractures and high capillary pressure of coal make it easy for fracturing fluid intrude into coal seam and therefore cause serious damage to the pore structure, which affects further the adsorption capacity of coal bed methane. Using the NO.9 coal sample from Ningwu Basin and three fracturing fluid samples on site, the characterization of pore structure of coal, which was treated with fracturing fluid, was analyzed with nitrogen adsorption method. Combined with the analysis results of scanning electron microscopy (SEM), the influence of different fracturing fluid on pore structure of coal was investigated. Methane isothermal adsorption experiments were also carried out using the treated coal. The results show that pore size distribution, specific surface area and pore fractal dimension of coal change after treated by fracturing fluid. The results also indicate that gel fracturing fluid has the most impact on pore structure of coal, followed by activated water fracturing fluid, and clean fracturing fluid having the least. The larger the increase of specific surface area and pore fractal dimension of coal treated by fracturing fluid, the stronger its ability of methane adsorption. Therefore, the change of pore structure in coal will change its adsorption ability of methane.