Effect Of Borehole Stress Concentration On The Elastic Properties Of Near Wellbore Shale Formations

In oilfield exploration,sonic logging is commonly used in measuring the elastic properties of formations and extracting reservoir porosity.However,drilling induced stress concentration can change the stress distribution around a wellbore,which in turn changes the elastic properties of the formations near the borehole.This can affect the results of sonic logging,making it deviate from the results of the virgin formations.To study the effect of borehole stress concentration,a shale rock physics model with good generality is constructed first according to the microscopic grain structure of shale,which can properly simulate the stress dependency of shale elastic properties.The improved method is not limited to hydrostatic stress loading,it is applicable to arbitrary stress conditions as the influence of a general stress tensor is considered in the calculation of the elastic properties of shale.This method is developed based on the micro-crack model of Sayers-Kachanov and the rock physics modeling workflow of Pervukhina.The model uses four parameters that have clear physical meaning to describe the stress sensitivity of the elastic constants of a TI medium.These four parameters are specific crack tangential compliance,ratio of crack normal to tangential compliances,characteristic closure pressure of crack,and crack orientation anisotropy parameter.These model parameters can be inverted from the laboratory ultrasonic data of shale core samples.By combining the shale rock physics model with the stress distribution around a borehole,the spatial variations of the formation stiffness matrix can be obtained.At last,the velocities of P-wave,radial S-wave and tangential S-wave could be obtained by solving the Christoffel equation.Based on the established numerical simulation workflow,the influence of different model parameters,such as well deviation,in-situ stress and the stress sensitivity of shale,on the elastic properties of near wellbore shale formations is studied in detail.