Macroscopic And Microscopic Research On Fracture Morphology And Extension Propagation Rules Of Shale Hydraulic Fracturing

Socialism with Chinese characteristics has entered a “new era”.The People have put forward higher requirements for a better ecological environment.As a clean energy shale gas,its consumption and demand continue to increase.To be better exploit shale gas,it provides certain technical support for hydraulic fracturing design and reservoir reconstruction of shale gas reservoirs.This paper takes the Longmaxi formation shale in Changning area as the research object.The self-developed multifunctional true triaxial test system of fluid solid coupling test system was used to the horizontal well hydraulic fracturing experiments.The CT technique was used to scan the samples before and after experiments and the CT scanning images of internal fractures were reconstructed and visualized by 3D reconstruction.The influence of ground stress and fracturing fluid displacement on fracture morphology and extension caused by hydraulic fracturing was discussed.At the same time,the results of hydraulic fracturing experiments are verified by numerical simulation.Research shows:(1)The key of volume upgrading of shale fracturing is the complexity and extension of fracturing.The difference between ground stress and horizontal stress is the main factor controlling the complexity of shale hydraulic fracturing network.(2)The displacement has a certain range of influence on fracture complexity,too low displacement may lead to poor fracturing effect,but excessive displacement will cause hydraulic cracks to expand along the edge of the fracture surface,form a single fracture,to a certain extent,reduce the complexity of the fractures.(3)The 3D model reconstructed based on the different CT values of shale matrix,minerals and fractures can truly characterize the shale internal structure and fracture distribution.The distribution pattern of the cracks is in good agreement with the practice.(4)Shale hydrofracturing fractures can be divided into 3 categories: single transverse fracture,arc main fracture and complex fractures.The "fluctuation phenomenon" of water pressure curve is related to the formation and extension of reservoir fractures,and it is an obvious feature of complex fractures formed by volume fracturing of shale.(5)According to the experimental results,the whole process of the extension of shale hydraulic fracture initiation is divided into 3 stages: pre-crack stage,micro-fissure stage and fracture stage.Shale hydraulic fracturing process is actually the process of micro-fissure initiation,extension and expansion until the final generation of macroscopic hydraulic fracture under the action of water pressure.(6)Due to the influence of individual differences in shale samples and other factors,the numerical simulation of the hydraulic fracture morphology and distribution will not be exactly the same as those in physical experiment.However,the laws obtained by numerical simulation are generally consistent with those obtained by physical experiments.It is best to combine the two in actual research and analysis.