Application Of Well Logging In Evaluating Wellbore Stability Of Shale Formation

Wellbore instability is a serious problem encountered during the drilling process.The downhole accidents caused by the collapse of the borehole wall will bring great economic losses to the exploration and development of oil fields.It is therefore of great significance to keep the stability of the wellbore.In the past research on the stability of wellbore,mostly through the indoor experiment method with downhole core.Although this method precision is higher,but due to poor continuity,high cost makes application is restricted.Logging data can provide large amounts of wellbore information with good continuity.This dissertation use the logging data with a small number of laboratory experiments to evaluation and analysis the wellbore stability of shale formation.The main research contents are follows:(1)Based on experiments,shale structure,mineral composition,CEC value and specific surface were tested.Utilized gamma-ray spectral logging data to comprehensively evaluate the clay mineral constituents and content in the shale formation,as well as the physic-chemical properties of shale.The results show that the clay mineral content is high in the study area,and the hydration swelling of the shale is strong.(2)Well stress field and rock mechanics properties are analyzed by using triaxial rock mechanics experimental instrument and log data.(3)Utilizing the migration rules of aqueous solutions in formations near borehole walls,combined with the basic equation of seepage flow in shale,this paper analyzes the influence of the seepage field on the formation pore pressure and borehole wall stability.Under the influence of seepage,the formation pore pressure increases with time and decreases as the distance increases.(4)The influence of different types of ions and different molar concentration of ions on the swelling clay mineral surface potential is evaluated with Zeta potential tester.This paper proposes a quantitative computation model for hydration stresses in an exploratory manner by utilizing the diffuse double-layer theory.And establish the shale formation hydration stress profile by using logging data,the hydration stress will have an important effect on the effective stress in the well,and affect the stability of borehole.(5)This dissertation discusses the influence of hydration stresses on pore pressures and rock mechanical parameters of shale formation,based on mechanical analysis of wellbore stability.Combined the seepage field with the hydration evaluation model to evaluate the stability of shale borehole walls,thereby obtaining the "safe density window" under the action of seepage effect and hydration stress.From above researches,the comprehensive analysis was conducted in the paper.The method is practical in study area,which can be reference for drilling fluid design in oilfield.