压裂液对煤储层渗透性伤害的微观作用机理研究

Hydraulic fracturing is the main well stimulation in coalbed methane (CBM) industry. However, coal reservoir is very easily damaged by fracturing fluid and results in impacting stimulation effects. The study of microcosmic damage mechanism of fracturing fluid for permeability in coal reservoirs has important theoretical and realistic significance for coal reservoir protection research and fracturing fluid optimization in CBM wells. The project will aim at different rank coals, combine with subject theories and testing means of coal petrology, coal chemistry, reservoir physics and seepage mechanics, and carry out following contents. (i) Simulation the retention effect of fracturing fluid in coal reservoir during flowback process. (ii) Finding out damage features of fracturing fluid for permeable properties of coals with different ranks. (iii) Understanding differences in pore structure and surface physical and chemical properties of coal reservoirs before and after fracturing fluid treatment. (iv) Revealing the interaction mechanism of fracturing fluid composition and components and surface physical and chemical properties in coal, i.e. the microcosmic damage mechanism of fracturing fluid for permeability in different rank coal reservoirs. (v) Ultimately providing theoretical and technological basis for the development of new fracturing fluids system suitable for different rank CBM development.

水力压裂是煤层气井的主要增产措施，但煤储层极易受压裂液的伤害而影响增产效果。研究压裂液对煤储层渗透性伤害的微观作用机理，对煤储层保护与煤层气井压裂液优化研究具有重要的理论和现实意义。本项目拟以不同煤阶煤样为研究对象，结合煤岩学、煤化学、油层物理学、渗流力学等学科理论与测试手段，模拟压裂液返排过程中在煤储层中的滞留效应；查明压裂液对不同煤阶煤储层渗透性的伤害特征；认识压裂液高压作用前后不同煤阶煤孔隙结构与表面理化性质特征差异；揭示压裂液成分与煤岩组分、表面理化性质的相互作用机理，即压裂液对不同煤阶储层渗透性伤害的微观作用机理；最终为适合不同煤阶煤层气开发的新型压裂液体系的研制提供理论依据和技术基础。

水力压裂过程中的煤储层渗透性伤害问题是制约煤层气井增产效果的重要因素之一。本项目以山西河曲长焰煤、柳林焦煤和晋城无烟煤样品为研究对象，结合高压岩心流动驱替实验、核磁共振驱替实验、CT扫描驱替成像实验研究了在压裂液滞留效应的静态和动态特征；通过工元分析和低温液氮吸附实验研究了压裂液对煤的组分和微小孔结构的影响；通过XRD、FTIR、13C-NMR实验，揭示压裂液对煤表面物理化学结构特征的影响；通过润湿性测试分析了压裂液对煤亲水性的影响；最终总结了压裂液对煤渗透性伤害的微观作用机理。结果表明：（1）高压作用下压裂液会产生滞留效应，且在微孔中的压裂液滞留效应最强，其次是大孔及裂隙；高煤级煤样滞留率高于中、低煤级，与孔隙度大小和孔喉结构复杂程度有关；压裂液主要滞留在煤的孔隙中，样品尾端的滞留效应最明显。（2）压裂液高压处理后，沙曲煤和永红煤中C-O、羰基、羧酸等含量增多，且产生了醛基和酮基，使煤的接触角变小，亲水性增强，使液体更容易在煤中滞留。（3）随着煤级的升高，芳香环层片堆砌高度和直径增大，层间距减小，压裂液滞留效应增强。高压压裂液剪切作用会使煤体发生糜棱化，导致煤芳香环层片堆砌高度和直径增大，层间距减小，煤储层更趋于有序性，更容易产生滞留现象。（4）挤压应力效应、水锁效应、表面改性效应和滞留效应是煤储层压裂液渗透性伤害的主要作用机制。研究成果能够为煤层气井低伤害性压裂液研制和储层保护策略提供理论依据。