Study On The Chemical-Mechanical Modeling Of Wellbore Stability In Shale And Its Plugging Performance Of Drilling Fluid

Wellbore instability of shale is a complicate and worldwide problem in the area of petroleum drilling engineering.To solve this problem,a systematic research method has been developed.The whole research idea is from pure mechanics and drilling fluid chemistry,to chemical-mechanical coupling,to mechanics,chemistry and thermodynamics coupling.At present,the qualitative evaluation of chemical-chemical coupling model is difficult to solve the quantitative expression of hydration in the problem of wellbore instability.Therefore,this paper quantitatively evaluates the hydration problem of chemical-mechanical coupling of mud shale wellbore stability.Firstly,establish the relationship between bound water and total water of single clay minerals.Then,establish the relationship between bound water and total water in shale formatio.Subsequently,study the effects of bound water on the mechanical parameters of rock based on chemical-mechanical coupling method.Finally,considering the drilling conditions of shale formation,the effects of membrane efficiency on the mechanical parameters of rock were studied through the triaxial test after immersion in different water-based drilling fluid systems.Specific understandings are as follows:(1)Simplified the physical model of clay mineral hydration based on the microscopic water absorption characteristics of single clay mineral,and establish the relationship between the bound water and its density of clay mineral.Then,using the bound water density as an intermediate variable,combined with previous experimental data for Na montmorillonite,this paper establish a power function model between bound water and total water on Na montmorillo-nite Finally,the model was verified by theremogravimetric and XRD analysis method,and the relative error of the model was less than 10%.(2)Based on the extended power function model of Na montmorillonite bound water and total water,a model for the relationship between bound water and total water of the single clay minerals of illite,chlorite and kaolinite was established.Finally,establish a composite functional relationship model between bound water and total water on shale formations.(3)Through the triaxial test of cores after immersion in clear water at different times,the XRD whole rock mineral analysis and clay mineral analysis results of the same batch of cores were used to solve the total water of the bound water.Influence,based on the above-mentioned relationship model of bound water and total water of shale,and then study the effect of bound water on mechanical parameters.It was found that the compressive strength,elastic modulus,cohesion and internal friction angle of the rock all decreased with the increase of the bound water content,and the poisson's ratio and the collapse pressure equivalent density increased with the increase of the bound water content.(4)Considering the actual drilling conditions of shale formations synthetically,this paper study the effect of membrane efficiency on rock mechanics parameters.Firstly,select two kinds of Water-based drilling fluid system using evaluate the plugging performance such as the HTHP filter,the cake pressure,the microstructure before and after the plugging,the wellbore stabilizer's plugging effect on the pore and crack,the pressure transmission,and the membrane efficiency.Filtrate loss reducers of HTHP less than 20 ml,a mud cake pressure capacity of more than 8 MPa,a porosity reduction rate of more than 80%after plugging,a permeability reduction rate of more than 75%after plugging,and a membrane efficiency of greater than 0.09,which can effectively reduce pressure transmission and achieve the effect of wellbore stability.Subsequently,the triaxial test of cores after immersion in different drilling fluid systems was used to study the effect of membrane efficiency on rock mechanical parameters.It was found that the compressive strength,elastic modulus,cohesion and internal friction angle of the rock all increased with the increase of the membrane efficiency.The poisson's ratio and the collapse pressure decreased with the increase of the membrane efficiency.