Experimental and numerical simulation research on hot dry rock wellbore stability under different cooling methods

Abstract

The exploitation of geothermal energy commonly occurs at depths ranging from 3000 to 10000 m. At these depths, the surrounding rocks experience substantial temperature and pressure, leading to alterations in their physical and mechanical properties. The infiltration of drilling fluid into the wellbore result in thermal exchange with the surrounding rocks, causing a decrease in their strength and stability. Consequently, the wellbore becomes more susceptible to instabilities. This objective of this study was to investigate the physical and mechanical properties of heated granite under three distinct cooling methods: liquid nitrogen cooling, water cooling, and natural cooling. Based on experimental findings, this study enhanced the failure criteria for high-temperature granite and established a thermal-solid coupling model to assess the stability of wellbores in high-temperature granite formations. To examine the impacts of temperature variations between the wellbore and drilling fluid, analyze the stress distribution around the wellbore at various depths, and evaluate the consequences of high-temperature fluid intrusion on wellbore stability. The research indicates that: (1) Under identical high-temperature conditions, it has been observed that liquid nitrogen cooling causes the most significant damage to granite, followed by water cooling. On the other hand, natural cooling has the least detrimental effect on the rock. (2) With increasing temperature, the physical and mechanical properties of the rock undergo two distinct stages of change. The first stage is a gradual variation that occurs between 25 and 400 °C, while the second stage is a rapid variation observed between 400 and 800 °C. Notably, the temperature of 400 °C serves as a critical threshold for the observed property changes in this granite. (3) The rock failure criteria have been modified based on the thermal damage characteristics of cold fluid intrusion into granite and a thermal-hydraulic-mechanical coupled wellbore model has been established. (4) The numerical simulation results revealed that liquid nitrogen cooling had the greatest impact on wellbore temperature, stress, and damage zone. At 500 s, the temperature on the wellbore surface decreased by approximately 700 °C due to liquid nitrogen cooling. The circumferential stress and radial stress decreased by 180 MPa and 35 MPa, respectively. (5) After the intrusion of cold fluid into high-temperature granite, the stress around the wellbore decreases and the rate of decrease in circumferential stress is significantly greater than that of radial stress. This will increase the likelihood of wellbore instability.