Influence Of Confining Pressure On Pore Structure And Permeability Of Sandstone

The problem of rock seepage involves all aspects of social life,including water conservancy engineering,mineral exploitation,oil and gas development and other practical engineering fields.In mining engineering,due to stress unloading,cracks in surrounding rock of excavation face will be generated and expanded,which will change the stress field and seepage field,and finally induce engineering disasters.In oil and gas development,rock seepage is affected by the change of confining pressure.With the recovery of underground fluid,the change of pore fluid seepage and pore pressure will affect oil and gas production.Therefore,it is of great significance to study the influence of confining pressure on porosity and permeability during rock seepage.In this paper,the influence of confining pressure on the variation of porosity and permeability is analyzed by studying the petrophysical and mechanical properties,microscopic pore structure and seepage characteristics of sandstone reservoir in a block of Shengli Oilfield.The main research contents of this paper include:（1）Based on the experimental study of core seepage,the composite exponential function relationship between confining pressure,porosity and permeability is fitted from the experimental results,and the parameter values and variation rules under different confining pressures can be predicted by knowing the initial porosity and permeability.（2）The microscopic pore structure inside the rock sample was studied by mercury injection experiment.It was found that the average pore throat radius had a good correlation with the permeability,which could be expressed asK（28）1.7868r^{1.23 34}.The larger the average pore throat radius was,the better the permeability of the rock sample was.The pore throat distribution of different cores is fitted by Origin,which conforms to the normal distribution.The heterogeneous distribution of core pores in the numerical simulation in the following paper is described by the normal probability density function.（3）Conventional triaxial compression experiments were carried out on different cores,and mechanical parameters such as elastic modulus and Poisson’s ratio of rock under different confining pressures were obtained,which provided the parameters for subsequent modeling.Based on the finite element software COMSOL,the numerical model of sandstone under different confining pressures is established to simulate the changes of porosity and permeability of sandstone under normal laboratory confining pressures and beyond the range of experimental confining pressures.The numerical simulation results under normal experimental confining pressures are compared with the experimental results.The results show that:With the increase of confining pressure,the porosity and permeability of rock decrease to a certain extent,and the change of confining pressure has a more obvious effect on core permeability,which is basically consistent with the experimental conclusion.By comparison,it is found that both porosity change rate and permeability change rate increase in a logarithmic function relationship with the increase of confining pressure,while the evolution trend of change rate slows down gradually.Finally the numerical simulation results compared with other experimental results,verifies the confining pressure and the function relation between the core porosity and permeability of composite index of rationality and the validity of the d/a,the predicted values and the real value of error control in a reasonable range,so as long as the core can be measured in practical engineering of initial porosity and initial permeability,The porosity and permeability under different confining pressures can be directly predicted.