温度对超深裂缝性地层井壁稳定性的影响

温度效应对钻井井壁围岩稳定性的影响不可忽略，特别是超深（>6000 m）裂缝性地层。传统考虑温度效应的坍塌压力预测模型主要适用于连续性地层，温度对裂缝性地层坍塌压力影响的文献研究较少。针对上述问题，首先通过杜哈梅原理确定温度变化产生的诱导应力场，然后利用坐标转换，考虑裂缝渗流场和温度场耦合，获得裂缝面上应力分布特征，最后，将裂缝面应力场代入岩石破坏准则，建立了考虑温度效应的裂缝性地层井壁失稳预测力学模型，研究了温度和裂缝特征对井壁稳定性的影响。研究表明，相同应力和裂缝产状条件下，钻井液循环引起的井壁温度降低增大了井壁垮塌的程度，这与传统模型认为循环引起的温度降低有助于井壁稳定的结论相反；井筒液柱压力一定的条件下，井壁稳定性随裂缝产状发生变化，存在裂缝产状敏感区。对于超深裂缝性地层，随着钻井液循环导致井壁围岩温度降低，增大了井壁失稳风险和程度，在防止井壁失稳的坍塌压力当量密度设计方面应考虑温度和裂缝特征的影响。

The Effect of Temperature on Stability of Borehole Wall in Ultra-Deep Fractured Formation

Formation temperature plays a significant role in stability of the surrounding rock around the borehole wall, especially for the ultra-deep (> 6000 m) fractured reservoirs. Traditional prediction model on collapse pressure that considers temperature effect mainly applies to intact formation, whereas few works have been done to investigate the effect of temperature on collapse pressure of fractured formation. To address this problem, in this study, we first determined the induced stress field generated by temperature variation through Duhame's principle. Second, stress distribution on fracture surface was obtained with consideration of coordinate transformation and the coupling of fracture-seepage field and temperature field. Third, we characterized the effect of temperature on wellbore stability by incorporating the established stress field on fracture surface into the rock failure criterion. The results show that under the same stress and fracture incidence conditions, increasing temperature aggrandizes the extent of formation collapse. This result is contrary to the traditional conclusion that the temperature-reduction-induced collapse pressure drop may strengthen wellbore stability. Moreover, wellbore stability is sensitive to the variation of fracture incidence when wellbore pressure is constant. Furthermore, for the ultra-deep fractured formation, the risk of instability may increase when surrounding rock temperature decreases as a result of mud circulation. Our works highlight the importance of temperature on well stability, and shed light on the design of appropriate drilling mud to prevent well from collapsing.