鹿児島県白水越地熱地域における 酸性流体賦存域推定に係る熱－流体流動－化学反応モデリング

抄録

Acidic geothermal fluids (pH≤5) are estimated to constitute 30% of the total geothermal resources in Japan (NEDO, 1993). The objective of this project is to understand the chemical properties and distribution of acidic fluids for the power generation. Acidic fluids are classified into two major types, the Onikobe geothermal field as Cl-type and Shiramizugoe geothermal field as SO₄-type were selected for this project fields. In this paper, we describe the SO₄-type at the Shiramizugoe geothermal field. The geothermal conceptual model of the acidic fluid system was reasonably developed by various data. Based on the conceptual model, a coupled thermal, hydrological, and chemical (THC) simulation for the Shiramizugoe geothermal fields was performed using the TOUGHREACT code (v4) to understand the chemical properties and distribution of acidic fluids.

The geothermal conceptual model was reproduced by the THC simulation, and the distribution of acidic fluid was estimated by the THC model in good matching with measured data of chemical characteristics and alteration minerals as well as temperatures and pressures of the existing wells. At the Shiramizugoe geothermal field, the acidic fluid is neutralized during lateral flow from the east to the west, reproducing the conceptual model in which acidic and neutral fluids exist in the same reservoir. Such an estimation of the acidic fluid distribution will contribute to the effective utilization of acidic fluids in the future.