A Newly Developed Flow-Reactor, with Ph Measurement System, for Laboratory Simulation of Water-Rock Interaction Processes

Abstract

A tube-type flow-reactor, with a pH measurement system, has been developed to simulate fluid-rock interaction processes over a wide range of temperatures and pressures. Our reactor system has an application in studying dissolution-deposition processes in natural flow systems, as well as fracture formation, for HDR (Hot Dry Rock) geothermal energy extraction. The flow system can accommodate the mixing of two preheated (hydrothermal) fluids, of differing input temperature and chemistry, to flow through a column of rock (either chip or fractured core) with a pre-set thermal gradient. Modifications to the flow reactor allow experiments are to be conducted at temperatures up to 500oC and pressures of 10- 50MPa. The pH of reacted hydrothermal fluid (i.e. fluid having undergone chemical water-rock interaction) in the flow system may be measured ëin-situí (under reaction conditions) or at room temperature. The pH of the hydrothermal fluid at ëreservoirí conditions was assessed using SOLVEQ92, based on; (i) the .E (potential difference) between an external pressure equilibrated, Ag/AgCl (0.1M KCl) reference electrode and a Pt-hydrogen electrode, for fluid which had interacted with granite at 20MPa and 100oC, 150oC, 200oC and 250oC, (ii) pH and .E of standard buffer solutions under the same experimental conditions; and (iii) major element chemistry of the reacted solutions. HDR experiments were undertaken at standard operating conditions of 20MPa and thermal gradient up to 425oC. We found that host rock characteristics, fluid chemistry, flow rate, temperature and temperature gradient are important factors that dictate the mechanisms of mineral dissolution and/or precipitation, and the condition of induced and natural fracture-pathways.