A Study On Formation Shear Slip Mechanism Considering Mud Shale Water-Absorbent

In this century,due to the deterioration of the quality of oil and gas resources,the exploration and development of shale oil has gradually become an important part of the original stable production in China.Due to the low permeability and low porosity characteristics of shale reservoirs,horizontal wells and volume fracturing construction methods are required when shale oil is exploited.However,with the progress of hydraulic fracturing in various shale oil and gas fields,stratum slippage deforms the casing,resulting in frequent failure of bridge plugs to run in.The problem of stratum shear slippage during shale reservoir volume fracturing is seriously affected.The construction and production on-site have caused certain economic losses.Therefore,this paper first analyzes the mechanical conditions for the formation of water inlet channels on weak structural planes in fracturing.On this basis,through laboratory rock mechanics experiments,studies the change rule of rock mechanical properties of mud shale after water absorption.Secondly,the stress analysis of the weak structural plane of the reservoir is carried out.Based on the Mohr-Coulomb criterion,the mechanical conditions of formation slip of shale reservoir are defined.By simplifying the discrete fracture network model,the influence of fracture-induced stress on formation slip is analyzed,and the formation slip mechanism under volume fracturing is obtained.Finally,based on the data obtained from the laboratory mechanics experiment,combining the stratum slip criterion and the stratum slip mechanism,a mudstone water-absorbing stratum slip model under volume fracturing conditions was established,and the fluid-solid coupling finite element method was used to solve the problem.The changes in pore pressure,longitudinal displacement of formation,shear stress of lithological interface and shear failure area of lithological interface during fracturing are clarified.In order to verify and apply the finite element model,use on-site construction data to establish W202H75-1 The formation slip model of the well is compared with the actual casing damage position.According to the laboratory rock mechanics experiment results of mud shale under different water content conditions,When mud shale absorbs water,mechanical parameters such as elastic modulus,Poisson's ratio,cohesion and internal friction Angle of rock samples will be greatly reduced,thus increasing the risk of formation slip.The finite element numerical calculation results show that with the increase of fracturing stages and construction pressure,the injection of fracturing construction fluid in the formation increases,and the formation pore pressure has an overall upward trend.The change of pore pressure leads to the irregular deformation of the lithologic interface,increases the shear stress of the weak structural surface,and enlarges the damaged area of the lithologic interface.In the process of fracturing,the water absorption phenomenon of shale leads to the weakening of rock mechanics properties,which aggravates the shear failure of the weak structural plane.