Research On The Influences Of Stress State On The Crack Structure Of Carbonate Rocks

The crack is one of the main kinds of secondary pores in the carbonate reservoirs.The change of the stress state may make crack deformed or closed,which has an important influence on the oil and gas accumulation and percolation ability.In this paper,experimental and numerical simulations methods are conducted to study the effects of stress state on the deformation of crack structure in carbonate rocks.Physical models containing controllable cracks are set up based on substantial laboratory tests and the P-wave velocities are measured under different stress conditions.The characteristics of the effective pressure-compressional wave velocity response relationship have been analyzed.On the basis of that,a new calculation method is proposed to quantify the crack closure pressure according to the effective pressure-compressional wave velocity response curve.Besides,the influences of the initial crack aspect ratio and crack angle on the crack closure pressure are analyzed.Combined with the sensitivity of the rock elastic wave velocity on crack parameters,the inversion algorithm of the crack structure parameters is developed based on the P-wave velocities.Experimental tests indicate that the preparing method of physical models in this paper is feasible and effective and the crack closure pressure is higher when increasing the initial crack aspect ratio.The results also reveal that when the uniaxial pressure and crack surface is not vertical,the uniaxial pressure should be resolved,only when the angle reaches Byerlee critical angle,might the crack closes and the crack closure pressure is higher with higher angle.On the basis of theoretical analysis,a simulation method was developed to predict the pore structure deformation in two-dimensional porous medium under different states of stress using COMSOL Multiphysics software.The simulation results indicate that the smaller crack aspect ratio is,the stronger the stress sensitivity of the rock apparent elastic parameters,when the crack porosity remains the same.Only when the crack aspect ratio becomes bigger,will the crack porosity have a significant impact on the rock stress sensitivity.As a result,a prediction model of crack density variation with the effective stress is set up on the basis of the inversion of crack parameters according to the P-wave velocities and effective stress measured by laboratory tests and the results of numerical simulations.The reliability of this model is analyzed according to the statistical theory.This research provides a foundation for predicting the crack parameters of the formation based on the elastic wave velocities and it is also helpful to improve the exploration and production efficiency of cracked oil and gas reservoirs.