双井筒试验下页岩水力裂缝扩展与机制研究

水力压裂是页岩气开采的主要技术,研究水力裂缝的扩展规律有利于促进复杂缝网的形成。为了研究裂缝扩展规律和破裂机制,对页岩进行了真三轴双井筒水力压裂模型试验。结果表明:(1)竖向应力越大,裂缝形态越单一,多条水力裂缝间越不易沟通,页岩破裂压力越大;(2)水平应力差越大,双井筒水力裂缝与天然层理沟通越明显,越易形成垂直于水平最小主应力的裂缝;(3)双井筒出口在不同层理面时,两条水力裂缝形态相似,各自会沟通层理和天然裂缝;(4)双井筒出口在同一层理面时,两条主裂缝容易相交,与天然裂缝的沟通较少,不易产生缝网;(5)通过对上升斜率RA和频度FA值综合分析得到,页岩水力压裂以剪切破坏为主,在不同层理压裂更易产生复杂缝网。

Study on Hydraulic Fracture Propagation and Mechanism of Double Wellbore Experiment in Shale

Hydraulic fracturing is the main technology of shale gas exploitation. The study of hydraulic fracture propagation is helpful to promote the formation of complex fracture network. In order to study fracture propagation and fracture mechanism, true triaxial double wellbore hydraulic fracturing model experiments was carried out on shale. The results show that: (1) With higher vertical stress, the shape of fracture is single, it is more difficult for the fractures to contact each other, and the fracture pressure of shale is higher. (2) With greater horizontal stress difference, the more obvious the contact between double wellbore fractures and natural fractures is, and the fractures perpendicular to the horizontal minimum principal stress are easy to form. (3) When the two wellbore outlets are on different bedding planes, the two hydraulic fractures are similar in shape, and each will contact the bedding and natural fractures. (4)When the double wellbore outlets are on the same bedding plane, the two main fractures are likely to intersect and communicate less with natural fractures, and it is difficult to produce seam mesh. (5) Through the analysis of RA and FA values, it is found that the hydraulic fracturing damage of shale is mainly shear damage, and fracturing in different layers is easier to produce complex fracture networks.