The complexity of thermal effect on rock failure in gas-drilling shale-gas wells

Gas drilling is a technique used to drill mining boreholes, oil and gas wells, and geothermal wells with air, natural gas, or nitrogen as the circulating fluid. Recently it has been employed to drill shale gas wells and proven to be effective. However, the performance of gas drilling is very unpredictable in many areas due to the lack of proper design of drilling parameters because of limited knowledge of rock failure mechanisms. This paper addresses the issue of thermal rock failure that has been controversial in the past few years. basis of analyses of thermal stresses induced by the frictional heating of drill bit and the cooling by gas expansion, this study reveals that the thermal effect on rock failure in gas drilling is significant. It is understood that both the frictional heating and the gas cooling can promote rock failure. However it not clear how the gas cooling will affect the frictional heating in the rock failure process. The thermal failure of rock is a complex process that requires further investigations by the means of experimental studies.