Formation Collapse Pressure Calculation Method Based On Mohr Circular Envelope

Wellbore stability is the key to ensure the safety of drilling process,the determination of three-pressure profiles plays an important role in avoiding wellbore instability.The pore pressure and fracture pressure in the three-pressure profiles are relatively clear and easy to measure,while the acquisition of collapse pressure is more complicated.It is of great practical significance to accurately get the formation collapse pressure and determine the effective three-pressure profiles to find the optimal drilling fluid density.This dissertation took the tight sandstone of Xujiahe formation as an example,this dissertation presents a new method to determine the collapse pressure through a series of rock mechanics experiments,to ensure the wellbore stability in the drilling process.The main research contents and results of this dissertation are as follows:(1)The mathematical model of well circumferential stress field was established.The mechanical model of wellbore stability is established and derived completely,and the stress distribution around vertical and deviated wells was obtained.The mathematical model was calculated by MathCAD program,the stress distribution and failure zone of surrounding rock in different conditions were obtained and the stability of the wellbore was judged.(2)The basic physical properties and rock mechanics experiments of sandstone rock samples were carried out.The physical properties of rock samples were analyzed by rock mineral and clay mineral analysis,rock sample microstructure analysis,longitudinal and horizontal wave velocity testing,stereomicroscope analysis,rock sample density and porosity tests,which can ensure the homogeneity of rock samples.Sandstone rock mechanics experiments were carried out.Based on the results of uniaxial compression test,uniaxial tensile test,direct shear test and triaxial compression test of sandstone,the variation law of rock mechanical parameters,such as elastic modulus,compressive strength and longitudinal and horizontal wave velocity,with confining pressure was defined.(3)The parameters of the continuous tangent envelope line was determined.Based on the triaxial experimental data of Xujiahe formation sandstone,the common tangent of two adjacent circles was made,and the relevant parameters of the function are determined by data fitting,and the mathematical expression of the change of cohesion and internal friction Angle with confining pressure is obtained.(4)By using MathCAD program,the influence of rock mechanics parameters determined by traditional method and continuous tangent envelope method on borehole stability was analyzed.The influence of in-situ stress and borehole trajectory on wellbore stability was also analyzed.Several different methods for determining rock mechanical parameters was calculated with Xujiahe formation sandstone samples experimental data,and the collapse pressure was obtained.The study shows that the rock mechanics parameters determined by the continuous tangent envelope line have significant guiding significance for accurately obtaining the collapse pressure of strata.