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.