Research On Wellbore Stability And Caving Characteristics In A Layered Ration Formation

With continuous development of shale gas resources,the rational formation layer of borehole wall stability problem increasingly prominent,the original homogeneous model based on isotropic hypothesis cannot reflect the rational formation of sidewall stability problem.To reveal the collapsed wall rock instability of stratified layers cause,explore the layered strata horizontal well casing collapse main control factors of indoor experiment mechanics test method,using the dragon stream set underground core direct shear and triaxial experiment.Based on experimental data,by considering the underground stress anisotropy and shale rock mass mechanics in the environment space bedding plane mechanical properties,and considering the seepage characteristics of the layered formation anisotropy,established the shale strength anisotropy well circumferential stress distribution model,calculated the layered strata formation caving pressure,combined with the actual drilling situation,analyzed the different bedding conditions on the sidewall rock collapse instability and borehole wall caving influence law.The main research work is:(1)We conducted laboratory experiments on rock mechanics.The experimental results showed that,the shale rock in Jiaoshiba area contains a large number of horizontal bedding fractures which display lower mechanical strength properties,and the shear strength of rocks along the bedding planes is much weaker than that perpendicular to bedding planes.The shale rocks have a strong anisotropy of mechanical parameters,and the variation of mechanical parameters is closely related with coring direction in shale rocks.The rock strength attains the highest value while the angle between coring direction and normal direction of bedding planes equal to 0°or 90°,the rock mechanical strengths reduce firstly and increase in the range of 30°to 90°,and the mechanical strength is lowest while the angle meets 60°.This paper successfully explain the main controlling factors for the wellbore instability of shale reservoirs drilled with horizontal well in Jiaoshiba area.(2)Studied the layered anisotropic seepage characteristics of strata,and reveal the bedding occurrence of horizontal Wells weeks the influence law of the seepage stress,without considering seepage are analyzed comprehensively,considering seepage isotropic seepage anisotropy,weeks seepage stress distribution law.(3)Layered stratigraphic principle is studied occurrence and relation between the Angle of principal stress,and the horizontal Wells weeks and transverse isotropic conditions is studied,and the elastic parameters,bedding,the influence law of occurrence of circumferential stress in the horizontal well,considering the maximum main stress is established and the intermediate principal stress of the horizontal well collapse pressure calculation model.The results show that the collapse pressure of the model is decreased with the increase of the well slope,and the calculation results are more consistent with the actual situation.(4)At the same time,the stability of borehole wall with anisotropic seepage is studied under the condition of both homogeneous and stratified strata.Based on the calculation model of the wall collapse pressure based on the mohr-coulomb criterion and the weak surface criterion,the distribution law of the collapse pressure of the borehole wall under different weak surface characteristics is analyzed.(5)Based on anisotropic formation caving pressure calculation model,further analysis when the instability of borehole wall collapsed,the direction of the borehole wall instability under the conditions of different bedding and borehole wall caving characteristics of borehole wall instability is calculated under the different conditions of hole enlargement ratio distribution,wall caving stratified layers model and predict the layered strata borehole wall caving characteristics.The research results of this paper lay a foundation for the structure of horizontal drilling well structure and optimization design of engineering parameters,and have high field application value.