Reservoir assessment of Zunil I & II geothermal fields, Guatemala

Abstract

Conceptual models for Zunil I and II are presented from careful analysis of reservoir temperatures and initial pressures. The Zunil I reservoir is divided into an upper and lower reservoir by a lithological contact between the granitic formation and overlying volcanic rocks. Wells producing from the upper reservoir yield less total output than wells that produce from the deeper reservoir. There appears to be a main upflow in the western part of Zunil I at the intersection of northeast and northwest regional fault trends. A second upflow may be present near the eastern edge of Zunil I, possibly connected as a line source along a northeast trending fault to the main upflow in the western area. Pressure potentials are highest in the west, suggesting fluid upflow, with outflows to the northeast and east. The Zunil II area shows higher overall temperatures at shallower depths than Zunil I with a source upflow possibly located in the southeast area. Temperature and pressure distributions suggest that fluids flow to the northwest from the upflow zone. Wellbore simulations of shallow and deep producing wells show that cooling of fluids from re-injection of brine will not have an adverse effect on total steam output but will maintain pressures in the reservoir. A volumetric assessment of the Zunil II reserve shows that there is potential for 35 MWe for the next 25 years.