Study On Rock Mechanics Mechanism Of Mesh - Like Induced Joints In Shale Gas Wells

Shale gas as unconventional natural gas resource has attracted the attention of shale gas resource countries, horizontal well drilling and volume stimulation are the two core technologies. However, due to the lack of recognize in the formation and propagation regularity of fracture in shale reservoir, field fracturing isn’t satisfying, shale gas exploration and development are lagged seriously. Based on the heterogeneity of shale formation and the mechanical properties, with the help of the numerical simulation software, development regularity of induced fracture around borehole in shale gas reservoir in the process of borehole pressurization were studied, thus, the primary control factors of network fracture system were analysized. The main results are listed below:(1) The natural fracture on borehole can change the direction of fracture initiation, whether the induced fracture initiates along the natural fracture or not is related to the angle, length, mechanical property of fracture and in-situ stress. Under given condition in the paper, the angle of natural fracture between maximum principal stress less than and equal to the criticle angle(70°), is the precondition that induced fracture initiates along natural fracture, the length of natural fracture makes no difference. Meanwhile, the weaker the mechanical property is, open natural fracture in the same angle is more possible. Influence degree of fracture property on the initiation of induced fracture is not the same when the ratio of in-situ stress is diffenrent. As a whole, the smaller ratio of in-situ stress is, the more possible induced fracture initiation along the natural fracture on the borehole wall is.(2) Propagation of induced fracture is mainly effected by the morphology of natural fracture after initiation, under certain distribution of natural fracture, the induced fracture can branch and deviate along natural fracture. When the angle of natural fracture is less than and equal to the criticle angle(77°), natural fracture around borehole distributes in a certain area, the length of fracture is long, it is possible for induced fracture to develop along natural fracture. Besides, through the research on initiation along the natural fracture in different angle, find out that initiation direction deviation maximum principal stress can promote the chance of connecting induced fracture. The in-situ stress has impact on fracture extension. In the smaller stress ratio condition, induced fracture can connect natural fracture easily. However, the distribution of simple fracture in formation only changes local fracture morphology, and forming effective complex fracture is impossible.(3) In the formation with bedding, the induced fracture can become complex fracture when it develops along bedding plane. The formation with smaller bedding angle, smaller space distance, weaker mechanical property is good for the induced fracture to extend long bedding plane. However, the formation with bedding is unable to form network fracture. Reserches show that though the combined effect of bedding and natural fracture, the induced fracture can be more complex and the natural fracture plays a leading role. Besides, a specific angle between bedding and maximum stress, moderate space distance and mechanical property can complicate the induced fracture. Moreover, the smaller difference between in-situ stress is, the more complex induced fracture is.(4) The keys that determine the formation of complex induced fracture are complex natural fracture system, appropriate angle, space distance and mechanical property of bedding plane, and small diffenrance between in-situ stress. As for well track design in shale, formation characteristic should be considered, and changing well track can adjust stress distribution around borehole, and the relationship of borehole and bedding. Manage the mutual relation of horizontal trajectory, in-situ stress and weak structural plane reasonably, is beneficial to achieve the complex induced fracture system.The topic is from the project of national natural science foundation of China:rock mechanical fundermental research on regulation and control of fracture pattern around borehole in hard brittle shale formation (code:51274172).