Pressure Analysis and Balanced Drilling in Yangyi Geothermal Field of Tibet by Coupling with Temperature

Abstract

Balanced or managed pressure drilling is of great benefits for geothermal wells, not only preventing from lost circulation/blowout and fracturing rock by controlling formation pressure but also improving productivity by mitigating reservoir damage. Geothermal pressures in Tibet are somehow complicated that both high and low pressures may be encountered within shallow formations, which leaves narrow window for drilling fluids with appropriate densities and makes it more necessary to analyze the pressures of the geothermal field and to model the drilling fluid hydraulics under high temperature. Physical, rheological and thermal properties of drilling fluid vary with temperature of rock mass, which influence annular pressure and pressure balance. This paper established mathematical models for coupling pressure-temperature and hydraulics-temperature under high temperature, which focused on wellbore temperature distribution and the variation of equivalent circulating density(ECD). Formation pressures and ECDs with annular friction loss, swab and surge pressure of the first medium-deep geothermal well ZK212 Yangyi Geothermal Field in Tibet were determined, with which the appropriate drilling fluid system was selceted and monitored to avoid lost circulation/blowout and fracturing formation.