Wellbore Instability Mechanism And Countermeasures In Tight Sandstone Formation

Wellbore instability is a common problem in the oil drilling industry.Nearly all oil and gas fields in the world will suffer from wellbore instability.During the drilling process,instability of the borehole wall will cause a series of complex accidents such as jamming,stuck drilling,and leaks.In serious cases,it may even cause the borehole to be scrapped.Each year the borehole instability will cause huge economic losses,which seriously restricts development of the oil and gas exploration industry.The gas reserves have been proved abundant in the Cretaceous Bashkirchik tight sandstone reservoir in a China northwestern oilfield block.With high formation pressure,and stable production capacity,this block is one of the important tight sandstone gas producing areas in China.However,the wellbore instability problem during the drilling of tight sandstone reservoirs severely restricts the process of tight sandstone gas exploration and development.The main problems are as follows.It is difficult to determine the abnormally high pore pressure coefficient of the formation,which is the key parameter for the wellbore stability analysis;Traditional wellbore stability theory based on static poroelasticity mechanics does not consider the inertia of solids and fluids,and cannot fundamentally explain the phenomenon of wellbore instability under dynamic wellbore loading conditions.There is a lack of research on the strength weakening mechanism of tight sandstone formations under oil-based drilling fluid conditions.The key parameters of oil-based drilling fluids related to wellbore rock failure in tight sandstone reservoirs are not clear.There is a lack of research on the performance optimization method of oil-based drilling fluids for tight sandstone reservoirs.In response to the problems above,the following main works have been completed based on the theory and laboratory tests:(1)Pore pressure prediction model in tight sandstone formations based on the P-wave velocity equation of rocksBased on Man's theory of linear elasticity with initial stress,combined with Biot's porosity theory,P-wave velocity equation for porous media under stress is derived.It is theoretically explained that the P-wave velocity of rocks is the most sensitive to the stress on the direction of P-wave propagation.Based on the P-wave velocity equation,a model of the relationship between the P-wave velocity and stress of the rock under triaxial compression is established,and relevant experimental research is carried out using equipment such as the Rock Mechanics Laboratory GCTS to verify the P-wave velocity model of the rock.The variation law of P-wave velocity of rock with axial pressure,confining pressure and pore pressure was tested,and the response characteristics of P-wave velocity of rock were analyzed.A pore pressure prediction model based on the P-wave velocity equation was established to predict the pore pressure coefficient of abnormally high-pressure formations in the block studied.(2)Poroelastodynamic mechanism of wellbore instability in tight sandstone formationBased on Biot's poroelastodynamic s theory,considering the coupling of solid,fluid compressibility,inertia and viscous effects,a poroelastodynamic model was established to characterize the dynamic response of the wellbore rock under dynamic wellbore loading in a non-uniform stress field.Laplace transform,displacement transformation and other methods were used to obtain the analytical solution of the non-axisymmetric stress around the wellbore.The fluctuation phenomenon of the stress field around the wellbore under dynamic wellbore loading was analyzed,and the poroelastodynamic response mechanism of the wellbore rock was revealed.The research focuses on the potential shear failure area of the surrounding wall rock.The effects of factors such as pore elastic parameters,in-situ stress,and cyclic load parameters on the shear failure response of the wellbore rock are studied.The research results provide a theoretical basis for the stability analysis of the wall under dynamic borehole loading conditions.(3)Optimization of oil-based drilling fluid performance to maintain rock strength in tight sandstone formationsBased on the theory of fracture mechanics,the propagation mechanism of rock micro-cracks was analyzed,and the key factors that caused rock strength to decrease under oil-based drilling fluid conditions were studied from the perspective of wettability.The mechanism of wellbore rock weakening under oil-based drilling fluid conditions was revealed.Through wetting experiments,the influence law of wettability under oil-based drilling fluid conditions was studied.The effects of oil-water ratio and demulsification voltage on oil-based drilling fluid parameters were studied,and a comprehensive characterization model of rock wettability was proposed.An optimization method for oil-based drilling fluid performance based on wettability to maintain the strength of the wellbore rock was established,and the performance of the oil-based drilling fluid was optimized and evaluated for the target formation in the block studied.