Geothermal energy is increasingly recognized as a potential of replenishing fossil fuels and meeting decarbonization obligations. HP/HT drilling is also increasingly recognized as a tool to provide an adequate supply of oil/gas. In these two types of activity, the high temperature is the common challenge for drilling. Under high temperature, rocks usually behave differently compared to the ambient condition. Whether the popular failure criteria can be used to evaluate wellbore stability under geothermal conditions is a question which has not been answered yet. In this paper, the triaxial compression tests under different temperature for six types of rock (Strathbogie granite, Tak granite, Tournemire shale, Slate, Carthage marble, and Crab Orchard sandstone) have been used to evaluate six different failure criteria. Two methods have been implemented to obtain rock cohesion and internal friction angle, which are grid search algorithm and Mohr failure line method. Both methods show that Modified Lade is the failure criterion that provides the best performance in predicting rock failure under geothermal drilling. The grid search algorithm guarantees the best performance of the failure criteria, providing with the best-fit rock parameters. The more practical method of Mohr failure line can still generate engineering reasonable predictions if a safety factor of 1.1 is considered, but only the Modified Lade should be used. The thermal influence on rock parameters is also analyzed and the cohesion is found to be decreased with the improvement of temperature, whereas the internal friction angle does not have a consistent relationship with temperature. Overall, considering the considerable variance of depth (several kilometers) and temperature (several hundred degree Celsius) of geothermal and deep oil/gas resources, the Modified Lade has proven to be the best choice to perform wellbore stability analysis.

When cutting a saturated rock under pressure, the PDC cutter is not only fragmenting the rock matrix, but also driving the pore fluid ahead of it. Because of the solid-fluid coupling in rock, different pore pressures induced by cutter will affect rock failure and lead to different MSE. The fact that cutting process is influenced by the pore pressure response in the rock is referred to as poroelastic effects in this paper. This paper continues the research in our previous work (Chen et al., 2018) and gives more insights into the poroelastic effects during rock cutting process. The influences of rock diffusivity coefficient and cutter speed are studied. The results show that the two parameters will affect pore pressure response in rock and further affect rock failure and MSE during cutting process. Based on the results, the cutting process can be identified as three conditions: undrained, drained and a transition zone between undrained and drained condition. In undrained and drained condition, MSE will be independent of cutter speed; while in transition condition, MSE decreases with increasing cutter speed. The transition boundaries for the three conditions are given. Cavitation in intact rock during cutting process is also studied. The results show that cavitation is easy to occur when cutting a hard rock with low original pore pressure. Cutting tests were conducted on Torrey Buff sandstone and Carthage marble to verify the poroelastic effects in cutting process. A good agreement between the model results and experiments is found. In general, the results in this paper can give a good understanding on the combined influence of formation permeability, depth of cut and RPM on cutting rock during drilling.

Cutting rock in pressurized conditions is essentially different as compared to the ambient conditions. Under pressurized condition, cutter is not only fragmenting the rock matrix, but also driving pore fluid to flow forward. Due to the coupling of stresses and pore pressure, the pore pressure change caused by cutter will affect effective stresses in the rock and eventually affect mechanical specific energy (MSE). The change in stresses and pore pressure caused by cutter movement are the poroelastic effects in cutting process. To study pressurized cutting process, we developed a cutting model based on the theory of linear poroelasticity. The model can predict coupled stresses and pore pressure in the rock and give a better understanding of the poroelastic effect in the cutting process. To obtain the desired solution, a Fourier Transform is employed. After the analytic solution in Fourier space is obtained, a Discrete Fourier Transform is employed to numerically invert the solution back to the real space. The Mohr-Coulomb failure criterion is also introduced to determine failure of the rock ahead of the cutter. Eventually, the model can predict cutting forces and MSE in pressurized cutting process. The model is also compared to the published experimental data and a good consistency between model and experiment results was found. The influence of hydrostatic pressure and the existence of cuttings on MSE are studied and poroelastic effect during cutting process is also discussed. The results in this paper can be applied to bit-rock interaction in well drilling.

Abstract The extended-reach well (ERW) and coiled tubing drilling (CTD) have been widely used in the development of shale gas. Although there are numerous advantages of CTD, the extension ability of CTD is restricted since the coiled tubing (CT) cannot rotate in the borehole during drilling process. Generally, the longer horizontal section of the horizontal ERW corresponds to the higher shale gas production. Therefore, we need to predict the ERW's maximum measured depth (MMD) while CTD and evaluate whether the predicted MMD can meet the requirements of shale gas development or not. In this study, a prediction model of MMD for horizontal ERW while CTD in shale formation is presented. The model is established based on the dynamic pressure balance of the bottom hole and the mud weight window of the drilled formation. Three processes of drilling, tripping in and tripping out are considered. The study shows that the buckling of the CT and the decreased pitch length result in a decrease in annular pressure losses, and all of them are conducive to the greater MMD of horizontal ERW while CTD. Moreover, drilling along the direction of minimum horizontal principal stress has a greater MMD in shale formation. In addition, the MMD decreases with the increase in tripping velocity. The annular size also has great effects on the extension ability of CTD. This work provides a practical tool for determining the ERW's MMD while CTD in order to meet the requirements of shale gas development and obtain good economic benefits. The study is significant to the development of shale gas.