Study On Propagation Law And Seepage Law Of Shale Hydraulic Fracture Network Under High Stress Conditions

The permeability of shale reservoirs is extremely low.It is necessary to perform hydraulic fracturing on shale reservoirs to form complex fracture networks,which are helpful for large-scale production of shale gas.The formation mechanism of hydraulic fracture network and the permeability law of fracture network in shale reservoir under the condition of high in-situ stress are of great significance to guide the hydraulic fracturing production of shale reservoir.X-ray diffraction experiments,field emission scanning electron microscopy experiments and low-field nuclear magnetic resonance experiments are carried out on the shale of the Longmaxi Formation.The mineral composition of shale and the distribution law of shale pore size are analyzed.The anisotropic characteristics of shale mechanical properties are studied by carrying out uni-triaxial compression test and Brazilian splitting test.A variety of brittleness evaluation methods are used to analyze the brittleness characteristics of shale.Confining pressure and bedding affect the evaluation results of the brittleness degree of shale.The brittleness index(BI_new)method based on stress-strain curve is proposed to evaluate the brittleness characteristics of shale.It has independently developed a high-stress rock triaxial hydraulic fracturing test system,which can simulate the full-diameter shale hydraulic fracturing test under a stress environment of 60MPa and a high displacement of 2.4L/min.The experiment system is used to carry out shale hydraulic fracturing tests under different confining pressure,different stress difference coefficient conditions and different bedding angles.The hydraulic fracturing pressure curve under high stress conditions presents two forms of single fracture and secondary fracture during the shale fracture stage.There are 10 main modes of interactive expansion between hydraulic fractures and closed or open bedding planes.Due to the effects of high confining pressure,pump injection displacement,fracture pressure and reduced shale brittleness.The fracture morphology formed by shale fracturing is more complicated.Acoustic emission experiments show that the acoustic emission b value in the whole hydraulic fracturing process is different due to the different propagation trends of hydraulic fractures.The general rule is that the smaller the peak acoustic emission b value is,the more complex the shale hydraulic fracture network is.Three-dimensional reconstruction of fracture networks in shale samples after hydraulic fracturing using a micro-X-ray computed tomography scanner.Hydraulic fractures are mainly initiated in the open hole section of the bottom hole or the middle section of the wellbore.The crack width is mainly in the range of 100-3200μm.The connectivity is higher than 94%,and the fracture network connectivity is good.The relationship between characteristic parameters such as fracture opening,fracture volume ratio,fracture morphology distribution,fracture connectivity,fracture fractal dimension and fracture network morphology is studied.The smaller the hydraulic fracture network volume,the better the pore connectivity.The depressurization seepage test and numerical simulation research on the shale samples after CT scanning show that the apparent permeability of the shale samples increased gradually with the decrease of the average pore pressure.The magnitude of the increase is more pronounced at low pore pressures.During the whole test process,there is a slippage effect.Fracture connectivity,fracture opening and fracture area ratio are positively correlated with equivalent permeability.The analytic hierarchy process is used to evaluate the influence of the geometric characteristics of hydraulic fracture network on the gas seepage law.The greater the fracture connectivity and fracture opening of the hydraulic fracture network,the greater the flow conductivity of the hydraulic fracture network.In this paper,the research on the expansion of full-diameter shale hydraulic pressure fracture network and the permeability law of shale with fracture network under high stress conditions is carried out.Simulation of the formation law and transportation capacity of hydraulic fracturing shale fracture network under the condition of real high in-situ stress.It can provide a theoretical basis for understanding the shale fracture law,reservoir hydraulic fracturing stimulation and hydraulic fracturing effect evaluation under the condition of high in-situ stress.