Study On Stability Of Shale Well Walls Based On Fluid-Structure Interaction Theory

In the development of shale reservoirs,wellbore instability often occurs.It is generally believed that the instability of the borehole wall in shale reservoirs is closely related to the loss of drilling fluid.The changes in shale mechanical strength parameters caused by shale hydration make it easier for the reservoir wellbore to enter a damaged state,leading to instability of the wellbore.In this paper,through relevant experimental research,combined with the theory of fluid-solid coupling,the fluid-solid coupling process is realized by ABAQUS secondary development,and the borehole instability model is established,and the impact of drilling fluid intrusion on borehole stability during drilling is analyzed.X-ray diffraction,scanning electron microscopy and CT scanning experiments are used to conduct fabric analysis and physical property analysis of Longmaxi Formation/Longtan Formation shale reservoir cores.The results show that the main primary minerals in the shale are quartz,pyrite and dolomite,which exhibit obvious hard and brittleness,and the clay minerals are mainly illite and Imonite mixed layers.Different degrees of micro-nano pores and micro-cracks are developed inside the core.In-situ stress core test,rock mechanical strength test,and permeability stress sensitivity test were carried out on Longmaxi/Longtan shale reservoir cores,and the elastic modulus,Poisson’s ratio and permeability of Longmaxi and Longtan formations were obtained.Fitting model with changes in effective stress.Through the strength weakening experiment,it is obtained that the elastic modulus and Poisson’s ratio of the Longmaxi rock sample change rapidly during the initial immersion(soaking time 0-2 days),and tend to be flat after the immersion time 2-10 days,indicating that the rock sample is in In the early stage of immersion,the drilling fluid is easy to enter the rock sample along the micro-fractures,resulting in more fractures and the strength of the rock decreases.Combining the fluid-solid coupling principle and the stress-seepage fitting model established by experiments,the FORTRAN language is used to compile the ABAQUS subroutine to realize the fluid-solid coupling process,and a wellbore reservoir model is established on this basis.The analysis results show that with the continuous drilling activities,the plastic damage near the well circumference will continue to increase,and the damage will lead to the continuous development of micro-fractures,which in turn causes the area of penetration damage to continue to increase.For this reservoir,the increase in drilling fluid equivalent density has less impact on the damage around the borehole wall,and hydration is more likely to cause borehole wall instability.Therefore,in engineering practice,solving the instability of the borehole wall should strengthen the plugging properties of the drilling fluid and improve the inhibition of the drilling fluid,and it cannot be achieved by only increasing the density of the drilling fluid.