This paper describes a comparative study of the thermal behaviour of drilling fluids and the surrounding rock when air-water mixtures and conventional muds are used as drilling fluids in geothermal wells. The computations were performed with two numerical simulators: TEMLOPI/V.2, which is used to compute the transient temperature disturbance when mud is employed, and GEOMIST, which is used when air-water mixtures are employed as drilling fluid. Data from a well from the Las Tres Virgenes Mexican geothermal field are used in this analysis. The results of simulation include temperatures computed during circulation and shut-in, and the latter results are compared with temperatures logged during drilling stoppages. It was found that the thermal disturbance caused by circulation of an air-water mixture has a smaller thermal effect than the thermal disturbance caused by drilling muds. (C) 2003 Elsevier Ltd. All rights reserved.