Research On The Water-based Drilling Fluid Invasion Into Shale Formation And Affect On Wellbore Stability

Oil-based drilling fluid has outstanding effect on stabilizing borehole wall,but its field application is limited due to its high cost,difficult cuttings treatment and great impact on the environment.Most of the sedimentary rocks encountered in the drilling process are mainly mud shale,which has the characteristics of low porosity,low permeability and easy hydration.When the water-based drilling fluid contacts mud shale,under the combined action of factors such as drilling pressure difference,capillary force and chemical potential difference,the drilling fluid will invade the mud shale formation,changing the pore pressure near the borehole wall and the stress state of the rock,thus causing borehole instability in the mud shale formation,resulting in complex downhole conditions.In the existing research,the drilling fluid invasion rule under single factor and its influence on wellbore stability are mainly studied.The drilling fluid invasion rule under multi-factor comprehensive action and wellbore instability study under hydration-seepage coupling action are rarely considered,and the instability rule of shale formation is not fully revealed.Therefore,it is of great significance to study the law of water-based drilling fluid invasion into shale formation under the action of multiple factors,which can provide theoretical basis for wellbore instability mechanism of shale formation and formulation of engineering and technical countermeasures for shale formation stability.With the support of the national natural science foundation general project(no 51574202),this paper,through theoretical research and numerical analysis,has carried out a study on the invasion law of mud shale formation drilling fluid under the comprehensive influence of multiple factors and its influence on borehole wall stability.the main work accomplished and the main results achieved are as follows:(1)According to the formation process of shale and the physical and chemical properties of shale,the mechanism of free water expansion after entering shale is clarified.(2)Combined with fluid dynamics principle,in low porosity and low permeability shale formation,the pressure difference required to form mud cake is large,which is difficult to reach in actual drilling.Based on this assumption and seepage mechanics,a drilling fluid invasion model of shale water layer under the action of differential pressure and chemical potential and shale oil layer under the action of differential pressure and capillary force is established.The analysis results show that when the solute concentration of drilling fluid is higher than that of formation solute,the solute concentration of formation near the well zone increases with the increase of drilling time,while the pore pressure of formation decreases first and then increases with the increase of radial distance.(3)A theoretical model for predicting the wellbore stress in shale formation considering the joint operation of seepage and hydration is established,and the model is solved by implicit difference method.The variation law of the wellbore stress under different conditions is obtained.The results show that the radial stress in the formation near the wellbore increases,while the circumferential stress decreases first and then increases after considering the hydration.With the increase of drilling time,the hydration area becomes larger.(4)Based on Mohr-Coulomb criterion,a prediction model of shale collapse pressure considering both seepage and hydration is established.The model analysis results show that when both hydration and seepage are considered,the shale collapse pressure has a collapse cycle.When drilling time is short and hydration is considered,borehole wall stability is good;When drilling for a long time,the collapse pressure with hydration is higher than that without hydration.Through the theoretical research in this paper,the invasion rule of water-based drilling fluid in shale water layer and oil layer and its principle of action on borehole wall stability are clarified.The research results can provide theoretical basis for safe and economic drilling of shale oil and gas.