古龙页岩力学特征与裂缝扩展机理

Mechanical characteristics and fracture propagation mechanisms of the Gulong shale

刘合 孟思炜 王素玲 董康兴 杨柳 陶嘉平 梁立豪

He LIU Siwei MENG Suling WANG Kangxing DONG Liu YANG Jiaping TAO Lihao LIANG

古龙页岩油是中国对纯页岩型页岩油规模化勘探开发的首次探索。明确古龙页岩岩石力学特征与裂缝扩展机制，对指导古龙页岩油工程靶体优选、压裂工艺设计与施工参数优化具有重要意义。在对古龙页岩开展矿物分布、薄片及岩石力学测试分析的基础上，明确了古龙页岩“千层小薄饼”力学特征，分析了地质-工程多因素控制下的裂缝扩展机理。结果显示，古龙页岩具有黏土矿物含量高（平均为46.6 %）、储集层塑性强、页理发育程度高及力学各向异性强的典型特征。与常规页岩脆性断裂存在显著不同，古龙页岩典型岩样力学性质表现出高频波动特征，抗压强度大于20 MPa的波动次数为3.33次/cm，断裂过程呈渐进式的稳定破坏过程，具有缓慢的峰后跌落段，断裂轨迹表现为锯齿状随机路径。同时，在高密度页理缝的控制下，古龙页岩压裂裂缝形态复杂，但缝高和缝长受到明显抑制，人工裂缝“扩不高、延不远”，成为古龙页岩油储集层有效改造的主要制约。建议古龙页岩油压裂改造应遵循“控近扩远”原则，加强压裂施工过程控制，抑制近井裂缝过度发育，促进主裂缝充分延伸，提高有效改造体积。

The Gulong shale oil represents China’s first attempt at large-scale exploration and exploitation of the oil contained in shale sequences without intercalations. Clarifying the rock mechanical characteristics and fracture propagation mechanisms of the Gulong shale is vital for guiding the selection of landing zones and fracturing design and engineering parameter optimization. In this study， the mineral distribution， thin section observation and rock mechanics tests are performed to clarify the Gulong shale as “fine layered” texture in mechanics and reveal the fracture propagation mechanisms under the control of multiple geological and engineering factors. It is shown that the Gulong shale is characterized by high clay mineral content （Avg. 46.6 %）， strong plasticity， a foliation intensity of up to 1 000～3 000 stripes per meter and strong mechanical anisotropy. Unlike the brittle fracturing of conventional shale， the typical rock samples from Gulong exhibit high-frequency fluctuation in mechanical property， with a fluctuation frequency of 3.33 times per cm for a compressive strength greater than 20 MPa. The fracturing process is observed as a steady gradual process with a slow post-peak stress decline， and along a random path in a zigzagged shape. Meanwhile， in the case of high-density foliation fractures， the hydraulic fractures in the Gulong shale are of complex morphology， with their height and length being significantly constrained. The limited vertical and horizontal extension of hydraulic fractures has been a major constraint for the effective stimulation of the Gulong shale oil reservoir. It is thereby suggested that the hydraulic stimulation of the Gulong shale oil reservoir should follow the principle of controlling near-wellbore fracture branching and further extending distal fracture networks， while placing the fracturing treatment under more effective control to suppress the development of near-wellbore fractures and boost the extension of main fractures to sufficiently expand the stimulated reservoir volume.

10.11743/ogg20230402