Analysis of Bottomhole Rock Stress in Deep-Well Drilling Considering Thermal-Hydro-Mechanical Coupling

Drilling is a key step in the exploitation of deep oil and gas resources. In order to clarify the stress state of the rocks and the mechanism of rock breakage in deep-well drilling, a thermal-hydro-mechanical coupling model for deep-well drilling was established, and the effects of drilling on the temperature, pressure, and stress in the formation were studied. Furthermore, the effects of the formation parameters and wellbore parameters on the bottomhole stress were analyzed. The results revealed that after the formation was drilled, the temperatures in different horizontal in situ stress directions were not significantly different, but the difference in the pore pressure between the maximum and minimum horizontal stress directions was large. The average effective stress at the bottom of the hole was the smallest, and in some areas, it was tensile stress. For deep-well drilling, as the formation pressure increased, the in situ stress increased, and the permeability decreased, leading to greater average effective stress of the bottomhole rock. As a result, it was harder to break the rock, and the drilling efficiency decreased. Reducing the wellbore pressure and wellbore temperature is conducive to forming tensile stress near the borehole axis in the bottomhole, causing tensile damage. The average effective stress of the formation near the shoulder of the drill bit was compressive stress, and it is advisable to take advantage of the rock shear failure characteristics to improve the drilling efficiency in this area. The results of this study can help us to understand the stress state of the bottomhole rocks and the mechanism of rock breakage and can provide a reference for the optimization of drilling tools and drilling parameters in deep-well drilling.