The Establishment And Application Of Three Pressure Profiles In Changning Shale Gas Horizontal Wells

The shale formation is prone to wellbore collapse and fracture loss during horizontal drilling.In this paper,the complex conditions such as well collapse and lost circulation appearing in the horizontal well drilling of shale formation in Changning area,guiding by the theory and method of rock mechanics,drilling engineering and logging,starting from the study of rock mechanics and failure criteria.Make full use of well-resolution and continuous well logging curves,focus on calculating rock mechanics parameters,in-situ stress,three-pressure and safe drilling fluid density window of shale horizontal wells,and apply it to the evaluation of borehole stability in shale drilling.The following results and understanding.(1)When calculating the pore pressure of the horizontal well and the overburden pressure,if the measuring depth is regarded as the vertical depth,pore pressure and the overburden pressure of the horizontal well will be too large.To this end,correcting firstly the measuring depth of the shale horizontal well,then calculating the overburden pressure,and then calculating the pore pressure by the Eaton.(2)Based on the in-situ stress model of the Thiercelin,considering the influence of shale bedding angle and inclination on horizontal the in-situ stress,a new model for shale horizontal in-situ stress calculation is established.(3)Aiming at the existing problem of evaluating the horizontal well collapse and fracture position angle of the horizontal well,this paper proposes a relatively simple numerical analysis method for calculation of the collapse and fracture position angle of the horizontal well wall under three common in-situ stress conditions,Which provides important parameters for the calculation of collapse pressure and fracture pressure in shale horizontal wells.(4)Based on the stress distribution of the inclined shaft wall,combined with the three rock strength failure criteria of Mohr-Coulomb,Drucker-Prager and Mogi-Coulomb,the calculation model of collapse pressure and fracture pressure of horizontal well is given.Study on the influences of the inclination angle and the azimuth angle of the well on the collapse pressure equivalent density and the fracture pressure equivalent density value by programming calculation and mapping simulation under different failure criteria.At the same time,analyze the applicability of the three shear failure criteria,and propose a new method for predicting the density window of safe drilling fluid.(5)Based on the single weak surface of the rock strength and the stress distribution state of the borehole wall,the calculation model of the collapse pressure of the inclined well with weak surface is established.Through the programming,calculation and simulation,the effects of bedding plane occurrence on the stability of shale shaft wall under different angles and azimuths of wells were studied.(6)Based on the established collapse pressure and fracture pressure calculation model,The effects of horizontal ground stress ratio,Poisson's ratio,cohesion and internal friction angle and pore pressure on the shale horizontal well collapse pressure equivalent density,fracture pressure equivalent density and safe drilling fluid window under different in-situ stress conditions are simulated.Based on the above method and model,a set of logging evaluation software suitable for the rock mechanics parameters,in-situ stress and three pressure profile calculation and borehole stability analysis of shale in Changning area was developed.It is used in the calculation of rock mechanics parameters,ground stress and three pressure profile and safety drilling fluid density window of the horizontal wells in Changning area,and the wellbore stability analysis.The results are reasonable and the effect is good,which is worthy of popularization and application.