Study On The Morphological Characteristics And Permeability Evaluation Of Hydraulic Fracturing Network In Anisotropic Shale

As a conventional and efficient rock mass modification method,hydraulic fracturing can significantly increase the permeability of shale in the process of shale gas production,thus improving the efficiency of shale gas production.However,due to the anisotropy caused by the typical bedding structure of shale,the process of hydraulic fracturing through the bedding becomes complex and diverse,which poses a great challenge to the actual hydraulic fracturing construction control.At the same time,the state of in-situ stress and injection rate will also affect the expansion and fracturing effect of hydraulic fractures.Therefore,it is of great practical significance to study the hydraulic fracture propagation mechanism and fracturing effect of anisotropic shale under different injection rates and in-situ stress states.In this paper,based on the Research Fund for talents of Guizhou University(Grant No.201901),the Special Research Funds of Guizhou University(Grant No.201903)and the National Natural Science Fundation of China(Grant No.51374257),the shale of wufeng-longmaxi formation is taken as the research object,and the anisotropic shale water is studied by means of theoretical analysis,hydraulic fracturing experiment,CT scanning technology and simulation analysis The characteristics of stress fracture propagation and the seepage effect of different pressure fracture network.The main conclusions are as follows:(1)Natural bedding shale is studied under different conditions of hydraulic fracturing mechanics effect,revealed that contains natural bedding shale middle angle,axial pressure and injection rate under the action of hydraulic fracture extension rule.(2)For the anisotropic shale samples after hydraulic fracturing,CT scanning technology was used to conduct CT tomography for the fractured shale samples,and then two-dimensional CT images were reconstructed using the visualization software AVIZO to establish a three-dimensional model of fracture network morphology of the fractured samples.Studied stratification Angle,injection rate and the axial stress(normal stress)under the effect of hydraulic fracture morphology change trend,Hydraulic fracturing indexes such as fracture volume ratio P33,tortuosity and fracture network growth index(FNGI)were used to evaluate and determine the fracturing effect of samples from the perspectives of fracture efficiency,crack growth pattern and potential of fracture network formation,achieve the best effect of fracturing fracture parameters configuration(axial pressure=25MPa,injection rate=6 ml/min,bedding Angle=60~o～90~o).(3)Based on the 3d reconstruction of the pressure fracture network,COMOSL,a multi-physics coupling software,was used to simulate the seepage of different fracture networks.The change law of hydraulic fracture permeability under different osmotic pressures and different permeable media was studied.By comparing the permeability of different fracture network,the hydraulic fracture network with the best water conductivity,the hydraulic fracture network in favor of methane extraction and the hydraulic fracture network most favorable for fracturing fluid backflow are obtained,and then the potential extraction effect of hydraulic fracture is quantitatively demonstrated.For the shale with the same bedding angle,when the injection rate increases,the tortuosity of the formed fracture network increases,and the equivalent permeability decreases;when the stress difference increases,the tortuosity of the formed fracture network increases,and the equivalent permeability decreases.The change of tortuosity of fracture network caused by different fracturing conditions will affect the conductivity of fracture network to the fluid,and the tortuosity and equivalent permeability are not linear,and the equivalent permeability does not decrease with the increase of tortuosity.When the tortuosity is about 3,the equivalent permeability is the largest and the permeability effect is the best.The research results can provide reference for the actual hydraulic fracturing construction design.