Reactive Flow in Supercritical Geothermal Settings and Its Impact on Electrical Fluid and Rock Properties

Abstract

In recent years, the investigation of superheated/ supercritical hydrothermal systems linked to magmatic intrusions in volcanic environments became a more and more prominent target of geoscientific and engineering studies as the exploitation of reservoirs with fluid temperatures above 350 °C is expected to increase the energy output by up to a factor of ten. Deep resistivity surveys are regarded to provide a convenient means for detecting supercritical roots of geothermal reservoirs, as electrolytes, dissolved in aqueous solutions, have the tendency to associate at near- and supercritical conditions. The removal of charge carriers from the solution causes the reduction of fluid conductivity by an order of magnitude. However, mass transfer and diffusion-controlled chemical reactivity of aqueous solutions are enhanced at near- and supercritical conditions. In consequence, resistivity contrasts between sub- and supercritical systems may be blurred by fluid-rock interactions, what could reduce the spatial resolution of electrical measurements at field scale. Due to a lack in experimental data for the high-temperature range, in the framework of the European-Mexican GEMex project we have studied the impact of hydrothermal alteration processes on the electrical properties in reactive flow experiments under pressure and temperature conditions of unconventional high-enthalpy geothermal reservoirs (T more than 350 °C, pfluid = 25 MPa). We have tested water-rock systems of various lithologies (volcanic rocks, limestones) with different permeabilities. The measurements were supplemented by a number of additional tests, comprising microstructural investigations as well as the chemical analysis of fluid samples, after they percolated through the rocks at various temperatures.