Transient numerical simulation of heat transfer processes during drilling of geothermal wells
The transient thermal history of a well drilling system has been identified as one of the main problems that the geothermal well drilling industry needs to solve. In particular, the estimation of temperatures, in and around a geothermal well during drilling (circulation) and shut-in (thermal recovery) conditions, is required. To overcome this problem, a computer simulator (WELLTHER) has been developed which uses a direct solution method to solve the finite difference equations describing the transient heat transfer processes in a wellbore during drilling and shut-in operations in the presence of the lost circulation to the formation. The new computer simulator uses a numerical model to account for the transient convective heat transfer in the formation surrounding a well, due to lost circulation. This feature of the present simulator is important, since previous wellbore simulators consider the heat transfer process in the formation (rock) as a merely conductive problem. The WELLTHER simulator is capable of accounting for these losses at any point in the well and it has been applied to the study of several Mexican geothermal wells. The results show that the effect of lost circulation on the shut-in temperature profiles can be reproduced satisfactorily. Likewise, a parametric analysis, carried out using the simulator, indicates that a number of assumptions made in previous numerical models are invalid and that certain factors ignored in previous models have a significant effect on the dynamic wellbore temperature distribution. Finally, a coupling of the new simulator with another computer code (STATIC TEMP) can be used as a tool to infer more reliably the static formation temperatures in geothermal systems.