Sun Weiji, Hou Mengru, Liang Bing, Liu Qi, Zhao Hang, Hao Jianfeng
School of Mechanics and Engineering, Liaoning Technical University, Fuxin, Liaoning 123000, China.
School of Architecture and Civil Engineering, Anhui Polytechnic University, Wuhu, Anhui 241000, China.
ACS Omega. 2024 Jul 20;9(30):32525-32535. doi: 10.1021/acsomega.4c00826. eCollection 2024 Jul 30.
This study aimed to investigate the effect of the microstructure of shale on fracture initiation and extension during hydraulic fracturing. The Longmaxi Formation shale reservoir in the Sichuan Basin was considered as the research object; its structure was modeled from a microscopic perspective, and a zero-thickness cohesive unit was embedded within the solid unit. Numerical simulations were performed to study the effect of mineral content on the microextension of the hydraulic fracture, extension behavior, and evolution law of shale. The results showed that changes in the mineral content resulted in changes in the forces between molecules within the minerals, which, in turn, affected the shale's brittleness. The percentages of brittle mineral content in the Long I, II, and III reservoir sections are 60.37, 47.60, and 53.56%, respectively. The fracture initiation pressures of the three reservoirs were 29.22, 31.42, and 30.22 MPa, respectively, and a linear correlation was found between the fracture initiation pressures and the brittle mineral contents of the reservoir sections. An increase in the reservoirs' percentage of brittle mineral content facilitated the fracture initiation, with a corresponding gradual decrease in the resistance to fracture initiation. The pore pressures of the fractures in the three reservoirs after fracture initiation were 0.90, 1.18, and 1.00 MPa, respectively. The larger the percentage of brittle minerals was, the lower was the fracture pore pressure. The greater the length, number, area, and width of the cracks were, the more likely they were to form longer and wider cracks. Hence, reservoirs with a high percentage of brittle minerals should be prioritized as the target formations for hydraulic fracturing operations. The results of this study reveal how the mineral content affects the extension of microscopic hydraulic fractures in shale reservoirs. As such, this work can provide a theoretical basis for rationally selecting a hydraulic fracturing operation layer in shale gas reservoirs.
本研究旨在探究页岩微观结构对水力压裂过程中裂缝起裂与扩展的影响。以四川盆地龙马溪组页岩气藏为研究对象，从微观角度对其结构进行建模，并在固体单元中嵌入零厚度粘结单元。通过数值模拟研究矿物含量对水力裂缝微观扩展、页岩扩展行为及演化规律的影响。结果表明，矿物含量的变化导致矿物内部分子间作用力的改变，进而影响页岩的脆性。龙一段、龙二段和龙三段储层脆性矿物含量分别为60.37%、47.60%和53.56%。三个储层的裂缝起裂压力分别为29.22MPa、31.42MPa和30.22MPa，且发现裂缝起裂压力与储层脆性矿物含量之间存在线性关系。储层脆性矿物含量百分比增加有利于裂缝起裂，同时裂缝起裂阻力相应逐渐降低。三个储层裂缝起裂后的裂缝孔隙压力分别为0.90MPa、1.18MPa和1.00MPa。脆性矿物百分比越大，裂缝孔隙压力越低。裂缝的长度、数量、面积和宽度越大，越容易形成更长、更宽的裂缝。因此，应优先将脆性矿物含量高的储层作为水力压裂作业的目标层位。本研究结果揭示了矿物含量如何影响页岩气藏中微观水力裂缝的扩展。据此，这项工作可为合理选择页岩气藏水力压裂作业层位提供理论依据。
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