Investigating the Effects of Fluid Flow and Consequent Porosity and Permeability Changes through an Artificially Fractured Carbonate Geothermal System

In a geothermal system, understanding how the permeability of carbonate host rocks develops overtime is a key parameter in the efficiency of geothermal energy production. This study contributes to pre-existing research into the longevity of fluid flow and how this may change as a result of potential precipitation-dissolution reactions, concentrating on geothermal systems. Here, the results of laboratory experiments to study long-term fluid flow changes through artificially fractured carbonates in a simulated geothermal system, with a focus on potential dissolution-precipitation reactions, are presented. Three main variables were monitored, including: comparing limestone and dolomite samples, studying the effects of temperatures up to 80°C, and comparing the effects of closed-loop and open-loop systems. Overall findings generally showed a decrease in the permeability of the limestone and dolomite samples throughout the experiment. Despite this, the micro-structural, CT scan and water sample results for many limestone and dolomite samples suggested an increase in dissolution throughout the experiment, generally showing a positive correlation with temperature. The cause of the occurrence of both an increase in dissolution, yet decline in permeability is discussed, with evidence of small grains blocking the pore throats of larger grains in the SEM images. The implications of a decrease in permeability in the carbonate host rock, as suggested by the laboratory experiments, would imply a reduction in the long-term efficiency of a geothermal system in a natural setting.