Electromagnetic Telemetry Improves Drilling Efficiencies at The Geysers Geothermal Field

Abstract

The Geysers dry-steam field is interpreted as an ancient subduction complex formed as the Farallon plate was subducted by the North-American continent during the Mesozoic to early Tertiary periods. Commercial development of the geothermal resource has been ongoing since the early 1950s. The drilling environment is characterized by a complex, natural-fracture system subject to frequent circulation losses and low penetration rates. Wells are typically drilled with a mud-fluid system to a depth of 3,500 to 5,000 ft, and then switched over to an air-assisted drilling fluid as temperature increases and steam-producing fractures are penetrated. Frequent circulation losses while drilling upperhole sections with fluid have consistently challenged the effectiveness of negative- and positive-pulse-type motor/measure-while-drilling (MWD) systems for directional control. The pulse-type MWD systems’ inherent need for a continuous fluid column often requires costly cement-plugging procedures to maintain full fluid circulation. Another limiting factor for pulse-telemetry systems has been the inability to pump high concentrations of lost-circulation materials through the bottomhole assembly (BHA) without plugging tools, causing additional trip times. This paper presents case histories detailing the successful deployment of an electromagnetic telemetry system at The Geysers. In one case, fluid-circulation losses were cured with high volumes of lost-circulation material pumped through the system. Directional control was maintained despite frequent loss of a full-fluid column. In a second application, during the air-assisted drilling phase of the well, it became apparent that well trajectory was off target. In the past, this would have required costly plugback cementing and reorienting the well trajectory uphole with full-fluid circulation for a pulse-MWD system. Electromagnetic telemetry successfully enabled correcting the trajectory and maintaining directional control with an air-assisted fluid system to a depth of 4,840 ft.