In recent years, Japan’s deregulation of geothermal development has allowed wells to be drilled near young volcanoes in national parks where geothermal surveys had not previously been conducted, and acidic fluids from wells have been found in some areas. It is necessary to increase the geothermal power output by maximizing the use of the available acidic fluids. For this purpose, it is important to understand the characteristics and distribution of acidic fluids in advance, as their production can cause corrosion problems that can significantly affect the design of wells and plant facilities. In this paper, the Onikobe geothermal field, which has been producing acidic fluids (pH 2.6 – 3.3) for many years, was selected as a model field. A conceptual model of the acidic fluid system was constructed. Based on the model, neutralization of acidic fluids was demonstrated by chemical equilibrium calculation and THC (coupled thermal-hydrological-chemical) simulation was performed to estimate the distribution of acidic fluids and to evaluate the validity of the model. Many iterative simulations between TH and THC models were performed until an acceptable level of geochemical results was obtained. The THC simulations resulted in an estimated acidic fluid distribution due to pH buffering, which was in good agreement with the chemical compositions and alteration mineral data measured in the existing wells. The model was evaluated using three statistical analyses; RMSLE (Root Mean Square Log Error), MALE (Mean Absolute Percent Error) and MAPE (Mean Absolute Percent Error) of pH and species concentrations. Sensitivity analyses were also performed on different sets of specific reactive surface areas of minerals to validate the model as this is one of the most uncertain parameters in THC simulations. The results suggest that the range of uncertain specific surface areas may be acceptable to some extent (about one or two orders of magnitude) if the pH buffer is established and geochemically stable over a long period of time. Such an estimation of chemical characteristics and distribution of acidic fluids is one of the few detailed field-scale THC simulations in the world and would contribute to the effective use of acidic fluids for power generation in the future.

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