A prediction model for borehole instability of ultra-deep fractured reservoir based on fracture and temperature effect

The traditional prediction method of wellbore instability in fractured formation cannot effectively solve the problem of surrounding rock collapse in ultra-deep fractured reservoir. This paper focuses on the coupling effect of complex geological conditions such as high stress, high temperature, fractured formation and drilling fluid. It establishes the induced stress field expression based on the heat exchange effect between drilling fluid and wellbore surrounding rock, and synthesizes the superposition effect of temperature and fracture. The strength failure criterion of wellbore surrounding rock in fractured formation is optimized, and the mechanical model of wellbore instability in fractured reservoir is established based on mechanical-thermal-chemical coupling model. The results show that as drilling fluid density increases, the stability of wellbore surrounding rock in ultra-deep fractured reservoir initially increases and then decreases. In other words, excessive drilling fluid density will aggravate wellbore collapse. In addition, the decrease in the temperature of the surrounding rock of the wellbore caused by the circulation of drilling fluid also leads to the increment in the degree of wellbore collapse. Based on these findings, reasonable drilling fluid density and properties can be optimized, and the instability problem of an ultra-deep fractured reservoir in Northwest China can be alleviated.