Thermal―hydrodynamic-chemical (THC) modeling based on geothermal field data

摘要

Data on fluid chemistry and rock mineralogy are evaluated for a number of geothermal fields located in the volcanic arc of Japan and Kamchatka, Russia, Common chemical characteristics are identified and used to define scenarios for detailed numerical modeling of coupled thermal-hydrodynarnic-chemical (THC) processes. The following scenarios of parental geothermal fluid upflow were studied: (1) single-phase conditions, 260℃ at the bottom ('Ogiri' type); (2) two-phase conditions, 300 ℃ at the bottom ('Hatchobaru' type); and (3) heat pipe conditions, 260℃ at the bottom ('Matsukawa' type). THC modeling for the single-phase upflow scenario shows wairakite, quartz, K-feld spar and chlorite formed as the principal secondary minerals in the production zone, and illite-smectite formed below 230℃. THC modeling of the two-phase upflow shows that quartz, K-feldspar (microcline), wairakite and calcite precipitate in the model as principal secondary minerals in the production zone. THC modeling of heat pipe conditions shows no significant secondary deposition of minerals (quartz, K-feldspar, zeolites) in the production zone. The influence of thermo-dynamic and kinetic parameters of chemical interaction, and of mass fluxes on mineral phase changes, was found to be significant, depending on the upflow regime. It was found that no parental geothermal fluid inflow is needed for zeolite precipitation, which occurs above 140℃ in saturated andesite, provided that the porosity is greater than 0.001. In contrast, quartz and K-feldspar precipitation may result in a significant porosity reduction over a hundred-year time scale under mass flux conditions, and complete fracture sealing will occur given sufficient time under either single-phase or two-phase upflow scenarios. A heat pipe scenario shows no significant porosity reduction due to lack of secondary mineral phase deposition.