Prediction of fracture distribution and evaluation of shale gas preservation conditions in Longmaxi Formation in Dongxi area

Commercial development of shale gas confirmed that mature shale has great potential for exploration, southeast Sichuan is the strategic pilot area of shale gas exploration and development in southern China, cracks are the key factor of shale gas preservation and development deployment, for the structural environment complex and special basin edge area, increasing the difficulty of shale crack research, but also provides a new direction for shale gas exploration and development. As a new field of oil and gas exploration in China, shale gas exploration has not yet developed a mature crack evaluation system. Taking Dongxi area of the southeast of Sichuan as an example, the stress field was simulated in the region by finite element numerical simulation. The model was constructed and calculated by rock mechanical parameters, the diagrams of maximum principal stress, minimum principal stress and differential stress distribution in Dongxi area were obtained through repeated debugging of the simulation results. Mohr-Coulomb criterion was used to calculate rock fracture coefficient and to predict the law of crack distribution in the study area, The prediction shows that the crack distribution of the study area is divided into four levels, GradeⅠcracks are distributed in bands near the east and west sides, grade cracks develop around gradeⅠcracks, the grade cracks are distributed in the northern dorsal nucleus and the western dorsal wing, respectively. At the same time, 10 preservation condition parameters, such as shale gas burial depth, distance from the ablative area, fracture distance from Qiyun Mountain, fracture action, differential stress and pressure coefficient, were clarified, and were used to determine the primary and secondary parameter weight by the combined weight method. Among them, the first stage parameters are buried depth(0.2), distance from the exfoliation area(0.1), fracture distance(0.1), fracture action(0.25), differential stress(0.15), and pressure coefficient(0.2). A relatively sound shale gas evaluation system and evaluation standards have been established. The two types of shale gas exploration targets are preferred, the favorable areas of the typeⅠand the typeⅡare located in the core and wings of the anticline core in the south of the study area and wings of the gentle folds in the middle of the study area, This study provides an important reference for shale gas exploration in southeast Sichuan.