Experimental Fluid-Rock Interaction Simulating Reinjection of Cooling Tower Condensate in Andesitic-hosted Reservoir of Southern Negros Geothermal Field, Philippines

Abstract

The fluid-rock interaction of geothermal steam condensate and andesitic rock was studied experimentally at 230oC and 20bars using a high temperature-pressure flow-through simulator. The experiment was designed to reproduce the reinjection of cooling tower condensate into the andesitic reservoir of Southern Negros Geothermal Field, Philippines. Effluent fluid samples were analyzed using IC and ICP-OES while rock samples were analyzed using XRD and SEM. Under geothermal conditions, the rapid equilibration of quartz increased SiO2 concentrations in reacted fluid up to 400mg/kg. Quartz crystals and feldspars are largely dissolved in the most reacted rock. The deposition of secondary mineral anhydrite, chlorite, and zeolites was observed in the run products. These occur mainly in the most reacted rock. The mineral deposition caused a drop in Ca, SO4, and Mg concentrations in the effluents and the buffering of Na and Al. Fluid pH rose slowly from 7 to 9 but did not reach a steady state indicating that andesite-fluid equilibrium was not achieved. The results of the fluid-rock interaction experiment afford understanding of the alteration processes occurring in the condensate fluid-injected reservoir and the permeability improvement of TWD with time.