Reactive Geothermal Transport Simulations to Study Incomplete Neutralization of Acid Fluid Using Multiple Interacting Continua Method in Onikobe Geothermal Field, Japan

Abstract

Two types of fluids are encountered in the Onikobe geothermal reservoir, one is neutral and the other is acidic (pH = 3). It is assumed that acidic fluid is upwelling along a fault zone as incompletely neutralized volcanic fluid. In a previously developed porous medium model (Todaka et al., 2004), the acidic fluid was neutralized due to pH buffering by rock minerals, and acidity was suppressed for a very long time. Here we investigate a multiple interacting continua model (MINC) to examine the role of fracture-matrix interaction for incomplete neutralization in the reservoir. The multi-phase reactive geochemical transport code TOUGHREACT was used. Results indicate very extensive alteration of the primary mineral assemblage in the fracture zone, with significant decrease in porosity and permeability from extensive precipitation of secondary minerals. The secondary minerals may form coatings on the fracture walls, preventing geothermal fluid from further reacting with reactant minerals and maintaining low pH in the fracture zone (incomplete neutralization). It is demonstrated that the MINC model with rock matrix subgridding used in the present study is an important tool for representing fracture-matrix interactions in fractured geothermal systems.