井壁稳定性研究中考虑中间主应力强度准则的对比分析

Comparative analysis of intermediate principal stress strength criterion of wellbore stability

中间主应力σ_2对岩石强度具有显著影响,在井壁稳定性研究中需考虑σ_2效应。常用中间主应力强度准则有统一强度理论(UST)、Mogi-Coulomb(MG-C)准则、Drucker-Prager(D-P)准则、修正Lade准则、三维Hoek-Brown(H-B)准则和修正Wiebols-Cook(W-C)准则等6种。文中对比分析了上述6种强度准则在σ_1-σ_2平面内的曲线,并进行了井壁稳定分析。结果表明:在σ_1-σ_2平面内,除UST准则为双折线形式外,其他准则均为非线性形式;D-P准则所预测强度最高,其他准则预测结果相近。在当量坍塌压力计算中,UST准则(b≠0)、MG-C准则、修正Lade准则、三维H-B准则和修正W-C准则计算结果相接近,D-P准则所计算当量钻井液密度最低。对于安全钻井方位,UST准则下安全井壁稳定分布区域随着参数b值升高而逐渐减小;D-P准则下井壁稳定分布区域最高;MG-C准则、修正Lade准则、三维H-B准则和修正W-C准则所给出的井壁稳定分布区域相近。对比分析后,在井壁稳定性研究中推荐使用MG-C准则。该准则不仅反映了σ_2对岩石强度的影响,且形式简单,便于在实际工程中应用。

The intermediate principal stress σ2 has a significant influence on rock strength,which should be considered in the analysis of wellbore stability.The six commonly used intermediate principal stress strength criteria are Unified Strength Theory（UST）,MogiCoulomb（MG-C） criterion,Drucker-Prager（D-P） criterion,modified Lade criterion,3D Hoek-Brown（H-B） criterion and modified Wiebols-Cook（W-C） criterion.In this paper,the curves of these six strength criteria in σ1-σ2 plane are analyzed,which show that other criteria are nonlinear forms exception of UST which is the double broken line form.The D-P criterion predicts highest strength while the strength of others is similar.Then,the six strength criteria are used to calculate the equivalent collapse pressure to analyze borehole stability.The results show that the equivalent drilling fluid density calculated by UST criterion（b≠0）,MG-C criterion,modified Lade criterion,3D H-B criterion and modified W-C criterion are similar while the result calculated by D-P criterion is the lowest.The six different strength criteria indicate different safe drilling direction.The safe drilling direction range of UST decreases as the parameter b increases.The safe drilling directions given by MG-C criterion,modified Lade criterion,3D H-B criterion and modified W-C criterion are similar while the range of that of D-P criterion is the widest.From the above comparison,the Mogi-Coulomb（MG-C）criterion is recommended in wellbore stability analysis.The MG-C criterion not only considers the σ2 effect,but also has a simple formula,which is easy to be used in the practical engineering application.