Aquifer fluid compositions at the Berlín geothermal field, El Salvador in 2012

Abstract

The Berlín geothermal reservoir is liquid-dominated and sub-boiling at deep levels with a temperature of ~300°C. However, after 20 years of continuous power production and the increase in power generation from 8 to109 MW, there has been a decline in pressure of about 20 bar with the result that extensive boiling of the aquifer water now occurs in the feed zone of some wells. This change in conditions affects scale formation potential from the boiling geothermal water. Electric power generation from geothermal resources is important for El Salvador since it provides about 24% of the country’s demands. Therefore, maintenance of the productive capacity and adequate resource management at Berlín are important variables for the company operating the field, LaGeo S.A. de C.V. To quantify the scaling potential, it is necessary to understand the geochemical processes occurring in the reservoir, as well as changes in the chemical composition of the fluid as it boils in producing aquifers and wells, to lay out an appropriate exploitation and management plan and an adequate monitoring plan to help make timely and correct solutions concerning scaling problems that may occur in wells. In this study, an evaluation of the physical conditions of 11 production wells operating in the Berlín geothermal field was carried out. Ten wells with two-phase production were evaluated, supported by the WATCH program developed by Arnórsson et al. (1982), version 2.4 (Bjarnason, 1994), to calculate aquifer water compositions and aqueous species distribution for each well at selected aquifer temperatures. Temperature equations were used for equilibrium constants for selected mineral dissolution reactions, presented by Karingithi et al. (2010), to calculate the saturation indices for common hydrothermal minerals in geothermal reservoirs. These minerals include calcite, clinozoisite, epidote, grossular, magnetite, prehnite, pyrite, pyrrhotite and wollastonite.