| The vast reserves of shale gas have a good development prospect.But when drilling,because of the bedding plane,it is easy to cause shear wall collapse.In the past,the single weak plane model is commonly used as the mechanical evaluation method of borehole stability.While there has a big difference between the test data and the criterion,so it is necessary to research the criterion more deeply.Therefore,this paper takes Sichuan Longmaxi group as the research object,intending to provide the anisotropic mechanical evaluation method for of borehole stability for shale.This paper will provide an effective mechanics analysis method in order to get the accurate safety window of drilling fluid density for the effective prevention of well instability,through the experimental analysis and numerical calculation methods.The main contents and argues of this paper as follows:(1)We select clearly the mechanics analysis model of wellbore rocks,rock anisotropy characteristics and the present research of strength theory,and finally determine the research content and the technical route by researching the literature at home and abroad.(2)Through the analysis of shale macrostructure and microstructure,we initially learned the layered structure of shale significant characteristic.Using tri-axial compression test,we test the strength and deformation characteristics of rock under different confining pressure,and analyze the shale stress-strain curve characteristic.Draw the shale elastic modulus,Poisson's ratio,and the compressive strength changing with the angle.Observe the damage form of sample,and analyze its failure mode changing with the different confining pressures and the angle of characteristic.So when considering the borehole stability analysis problem,we can't ignore the anisotropic features.(3)Based on the test data of different parts of shale,through the analysis of the strength fitting degree of the isotropic and anisotropic characteristics criterion,we establish the anisotropic strength criterion is better than isotropic criterion to characterize shale strength characteristics.Compared with the commonly used,single weak plane criterion and other weak anisotropic criteria,Ramamurthy criterion can well fit different parts of the shale strength characteristics.The rule is not only considered anisotropic characteristics of stratification cause,also analyze the influence of confining pressure on anisotropic characteristics.It shows that in deep formation,the influence of confining pressure on rock anisotropy cannot be ignored.(4)By analyzing the formation characteristics of shale,based on the generalized Hoek's law,we establish the well circumferential stress distribution model on the basis of the transverse isotropic formation.In the model,it involves the borehole tilt angle,azimuth angle,anisotropic stress,bedding angle and direction,providing basis for anisotropic shale formation stability analysis.By comparing the different influencing factors,the axial stress and radial stress,along with the change of the well round angle,we analyze the influence law.(5)Combining circumferential stress and strength criterion,we establish formation caving pressure calculation model,and analysis the in-situ stress,elastic modulus and Poisson's ratio anisotropy degree,bedding angle and direction,which has the impact on the formation caving pressure.The results show that in the anisotropic formation,the collapse pressure of the wall will be influenced by many factors,and these factors in the final analysis are the representation form of anisotropy,thus the mechanics research method for predicting shale borehole wall collapse should fully consider the effect of anisotropy.(6)Through the analysis of engineering practice,with shale gas wells in Sichuan basin as an example,we get the formation caving pressure calculation.Comparing the calculation results with the collapse under other standards,it verifies the correctness of the mechanical evaluation method established in this paper. |
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