基于裂缝亚临界扩展理论的页岩吸水起裂微观力学机制  

Micro-mechanical mechanism of water imbibition inducing fracture initiation in shale based on subcritical fracture propagation theory

作　　者：杨斌[1] 张浩[1] 杨建 李越 曾港彬 刘国庆[1] 杨珊[1] YANG Bin;ZHANG Hao;YANG Jian;LI Yue;ZENG Gangbin;LIU Guoqing;YANG Shan(State Key Laboratory of Oil&Gas Reservoir Geology and Exploitation//Chengdu University of Technology,Chengdu,Sichuan 610059,China;Engineering and Technology Research Institute,PetroChina Southwest Oil&Gasfield Company,Deyang,Sichuan 618300,China)

机构地区：[1]油气藏地质及开发工程全国重点实验室·成都理工大学,四川省成都市610059 [2]中国石油西南油气田公司工程技术研究院

出　　处：《天然气工业》2025年第2期105-113,共9页Natural Gas Industry

基　　金：国家自然科学基金面上项目“深部破碎地层井壁力链结构承压演化机理与静/动力学失稳响应规律研究”(编号:52374005);国家自然科学基金青年基金项目“深层页岩裂缝亚临界扩展诱导井壁时滞性失稳规律研究”(编号:52104003);四川省自然科学基金青年基金项目“CO_(2)封存井筒安全完整纳米复合修复体系研究”(编号:22NSFSC4005)。

摘　　要：吸水起裂是页岩的典型特征,也是井壁失稳关键诱因和页岩气储层压裂后焖井制度的重要立足点,但现有理论尚难以解释页岩吸水慢速起裂扩展,而后趋于稳定或发展为快速失稳扩展的差异化现象。为揭示页岩吸水诱发裂缝扩展的微观力学机制,基于裂缝亚临界扩展理论,从Griffith能量平衡概念出发,测试计算了不考虑宏观地应力场时页岩吸水前后表面能大小,建立了毛细管力驱动裂缝亚临界扩展力学模型,并形成了页岩裂缝扩展分析图版。研究结果表明:①干燥页岩表面能平均为68.31 mJ/m^(2),吸水后页岩—水相界面能平均降低至4.62 mJ/m^(2),降幅与接触角呈显著的负相关性;②以0.1μm缝宽裂缝为例,毛细管力在裂缝尖端的机械能释放率为13.9 mJ/m^(2),能够克服吸水后裂缝的扩展阻力并诱发裂缝亚临界扩展;③毛细管力所产生的机械能释放率随缝宽增大而呈现先降低后快速升高的趋势,形成了类似“势阱”的区间,该区间的大小及缝宽分布范围决定了特定裂缝吸水后是否发生扩展,以及扩展后是趋于稳定还是演变为失稳破坏。结论认为,该认识有效揭示了裂缝发育特征、岩石—流体表/界面相互作用对页岩吸水起裂扩展的微观控制机理,对页岩地层井壁失稳预测、页岩气储层压裂后焖井致裂评估与排采制度优化等具有重要指导意义。Water imbibition inducing fracture initiation is one of the typical characteristics of shale,as well as a key cause of wellbore instability and an important foothold for the soaking system after the fracturing of shale gas reservoir.However,existing theories can hardly explain the differentiated phenomenon that fractures initiate slowly in shale after water imbibition and then tend to be stable or rapidly get unstable and propagate.In order to reveal the micro-mechanical mechanism of water imbibition inducing fracture propagation in shale,this paper measures and calculates the surface energy of shale before and after water imbibition without considering the macrogeostress field,based on the subcritical fracture propagation theory and the concept of Griffith energy balance.In addition,a mechanical model of subcritical fracture propagation driven by capillary force is established,and a chart for analyzing fracture propagation in shale is constructed.The following results are obtained.First,the average surface energy of dry shale is 68.31 mJ/m^(2),and the interface energy between shale and water phase after water imbibition decreases to about 4.62 mJ/m^(2),presenting an obvious negative correlation between the reduction and the contact angle.Second,taking the fracture of 0.1μm wide as an example,the mechanical energy release rate of capillary force at the fracture tip is 13.9 mJ/m^(2),which can overcome the fracture propagation resistance after water imbibition and induce subcritical fracture propagation.Third,with the increase of fracture width,the mechanical energy release rate generated by capillary force decreases first and then increases rapidly,forming a similar“potential trap”interval.The interval size and the fracture width distribution range determine whether a specific fracture propagates after water imbibition,and whether it tends to be stable or evolve into unstable failure after propagation.In conclusion,the research results effectively reveal the micro-controlling mechanisms of fracture developmen

关 键 词：页岩 吸水起裂 表面自由能 亚临界扩展 毛细管力 机械能释放率 

分 类 号：TE37[石油与天然气工程—油气田开发工程]