Characteristics of the Zunil Geothermal Field (Western Guatemala)

Abstract

The Zunil geothermal field represents the marginal, shallow expression of a vast geothermal complex buried beneath active volcanic edifices (Cerro Quemadoo, Volcan Santa Maria, Western Guatemala) some kilometers to the west. The area lies at the edge of a tecto-volcanic depression where some1,000 m of Tertiary and Quaternary volcanic are underlain by a grandodioritic basement. High temperatures geothermal fluids reach the field from the west, upflowing along the inclined contact between the grandioritic and overlying volcanic, and along the fractures in the basement itself. A conglomeratic layer at the volcanic/basement contact, and the underlying weathered cap of the basement form the only permeable horizon of the succession; this horizon forms the local reservoir tapped by the productive wells. Its reduced thickness allows for a limited fluid storage such that field production relies on external recharge along the permeable horizon and underlying fractures in the grandiorite. Production testing and simulation models indicate a fairly rapid evolution of reservoir conditions from the liquid to the steam phase, due to pressure drawdown, in its turn due to a restricted inflow. The phenomena determines as upgrading of the fluid enthalpy, but a decline of mass output. Simulated reinjection into the reservoir proved to be an effective measure to slow down such an evolution and optimize the field exploitation.