陆相页岩CO2压裂裂缝起裂和扩展特征试验研究

富含黏土矿物的陆相页岩气储层在传统水基压裂过程中易出现基质黏土吸水膨胀现象,降低页岩双重孔隙介质渗透特性,使得页岩气开采遇到一定的阻碍。因此,针对高水敏性页岩气储层,采用CO₂等无水压裂技术的特性研究显得愈发重要。采用延长陆相页岩和致密砂岩试样开展CO₂与清水室内压裂对比试验,揭示了CO₂压裂起裂及扩展特征。实验结果表明:CO₂压裂起裂压力远低于清水压裂,平均值仅为后者的60%;页岩丰富软弱层理面削弱最大主应力对主裂缝扩展的影响,导致沿层理面产生剪切裂缝;低黏度CO₂的高渗滤导致孔隙压力升高,并降低基质缺陷及弱面处的有效应力,有利于裂缝尖端沿着试样内部缺陷薄弱区域扩展,从而形成粗糙裂隙表面。可见,CO₂无水压裂技术适用于超低渗透性页岩气储层改造,研究结果可为水敏性陆相页岩气的现场压裂施工与参数优化提供支撑。

Experimental Study on Fracture Initiation and Propagation Characteristics of Continental Shale by CO2 Fracturing

Continental shale gas reservoirs with abundant clay content are prone to absorb water and clay swelling occurs during water-based fracturing, which reduces the permeability of the dual-pore medium of shale and hamper the shale gas exploitation. Consequently, studying the feasibility of nonaqueous fracturing for water-sensitive shale gas reservoirs is necessary. Comparative tests between CO₂ fracturing and water fracturing were conducted using continental shale and tight sandstone samples to reveal the fracture initiation and propagation mechanism of CO₂ fracturing. Results show that the breakdown pressure of CO₂ fracturing is around 60% of that of water fracturing. Abundant soft bedding of shale weakens the influence of the maximum principal stress on the fracture propagation, leading to the presence of shear fractures along the soft bedding. High leak off of CO₂ with low viscosity leads to increased pore pressure around the drilling hole and the reduction of effective stress at the matrix defect and weak bedding, which is conducive to the expansion of crack tip, thus forming the rough fracture surface. It can be seen that CO₂ fracturing is suitable for the stimulation of shale gas reservoirs with ultralow permeability, and the research results are helpful for the field fracturing construction and parameter optimization of water-sensitive continental shale gas reservoirs.