Interpretation of Geochemical Production Monitoring Data from Well SJ4-1, San Jacinto, to Indicate Reservoir Response to Production

Abstract

The chemistry of geothermal fluids discharged by wells gives insights on three main processes that may take place in the reservoir as a response to utilization: boiling, injection returns and groundwater incursion. The latest two reservoir processes are likely to cause cooling and, consequently, reduced energy available for electricity production. Enhanced boiling in producing aquifers can result in increased mineral precipitation potential and, hence, reduced permeability and well output. Chemical composition of San Jacinto reservoir fluid was computed using the WATCH program (Bjarnason, 1994) based on component concentrations measured in water and steam samples collected from surface pipelines. In the calculations, it is assumed that no transfer of heat nor mass occur on the way of fluids from the reservoir to surface (Arnórsson et al., 2007). The selected reference temperatures are based on downhole measurements and quartz geothermometry. In this study, geochemical production data of well SJ4-1 at San Jacinto, Nicaragua, collected since 2005 is reviewed as an attempt to identify physical and chemical processes possibly taking place in the producing aquifer. Well SJ4-1 is one of the wells that have the longest production history in San Jacinto with relatively stable output since initial production. It is considered to be fed by a shallow liquid aquifer, located at around -500 masl with an initial temperature of 275 °C. Reservoir tracer testing conducted in 2012 - 2013 has indicated this aquifer to be influenced by injection return from the system southern part. For the abovementioned features, study of well SJ4-1 is of special interest. The present analysis focuses on revising data on Chlorides (Cl), Boron (B), Silica (SiO2), Sodium (Na), Potassium (K), Non-Condensable Gases (NCG), Geothermometry, and Total Discharge Enthalpy as a function of time. Different periods in the production history of San Jacinto reservoir are marked with a different dominant reservoir process, including boiling, injection return and shallow boiled water inflow, as a result of the production/injection strategy adopted.