| Because of the characteristics of low porosity and low permeability,shale gas reservoir usually needs to be exploited by fracturing.In the process of hydraulic fracturing of shale reservoir,multiple physical fields such as stress field and seepage field are involved in the interaction.High stress and high pressure conditions will cause rock damage phenomenon,and the corresponding mechanical properties of rock will change accordingly.Therefore,it is of practical guiding significance to master the damage evolution law of shale under the action of multi-field coupling.In reference on the basis of predecessors’ research,this article 176 wellblock with bohai sea south sub-sag,righteousness as the study area,based on elastic mechanics,percolation theory and damage mechanics and finite element,and combined with the indoor experiment analysis of hydraulic fracturing rock mechanics experiments and seepage wall surrounding rock under the action of multi field coupling in the process of damage evolution process and the stability of wall rock,The main conclusions are as follows:(1)The rock mechanics parameters of the reservoir rocks in the study area were obtained by carrying out the rock triaxial compression experiment.The elastic modulus of the shale reservoir rocks ranged from 7.22 to 26.26 GPa,and the Poisson’s ratio ranged from 0.139 to0.226.Through the rapid pulse attenuation method,the permeability variation law of the core of Well Jiyacan 1 in the study area under different confining pressures was measured,which provided basic parameters for the establishment of numerical model in this paper.(2)Based on continuum mechanics and considering the change of effective stress,the equation of rock stress field including pore pressure variables is established.The seepage field equation of rock was established based on continuity equation and motion equation.Based on the maximum tensile stress criterion,Mulculum criterion and elastic damage theory of rock,the rock damage constitutive equation is established.Through the influence of rock damage on the related parameters of each physical field,the multi-field coupling relationship based on rock damage is established.(3)Using COMSOL Multiphysics,the numerical model of fractured shale reservoir was established to study the damage evolution of wellbore wall surrounding rock under different in-situ stress conditions and different seepage boundary pressure.The simulation results show that the larger the difference between vertical in-situ stress and horizontal in-situ stress is,the larger the volume ratio of the damage area is,and the damage of the wall rock expands more rapidly in the side with greater in-situ stress.A more ideal fracture zone can be obtained by increasing the seepage boundary pressure in a certain range.(4)The mechanical model of stress distribution of the surrounding rock of the horizontal well wall is established to analyze the stress state and plastic strain distribution of the surrounding rock of the borehole wall under different conditions(based on the background of the damage of the surrounding rock of the borehole wall),so as to explore the stability of the surrounding rock of the borehole wall.The larger the ratio of the maximum horizontal principal stress to the minimum horizontal principal stress is,the larger the plastic strain value of the surrounding rock is,and the maximum plastic strain value near the surrounding rock always appears at the horizontal side wall of the shaft wall.The higher the seepage boundary pressure is,the higher the effective stress of the surrounding rock will be.Reducing the seepage boundary pressure can ensure the stability of the wall rock. |
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