ON REACTIVE FLUID FLOW AND GEOCHEMICAL TRANSPORT MODELING: FROM NATURAL HYDROTHERMAL SYSTEMS TO GEOTHERMAL ENERGY DEVELOPMENT

Geochemical evolution in both natural hydrothermal systems and geothermal fields occurs through a complex interplay of multiphase fluid and heat flow and chemical transport processes. These complexities include (1) fracture-matrix interaction for fluid, heat, and chemical constituents, (2) gas-phase participation in multiphase fluid flow and geochemical reactions, (3) the kinetics of fluid-rock chemical interaction, and (4) heat effects on thermophysical and chemical properties and processes. Reactive fluid flow and geochemical transport modeling is a powerful tool to get insight into these processes, to test conceptual model, and to study the geochemical behavior, mineral alteration, changes in porosity and permeability. Three modeling examples are used to illustrate the usefulness of the reactive transport modeling. The first example deals with caprock alteration in a Long Valley Caldera (LVC) hydrothermal system in California. The observed sequence of argillic alteration in the LVC consists of an upper zone with smectite and kaolinite (in the lower-temperature region), a lower illite zone, and an intermediate mixed illite and smectite zone. The sequence is reasonably well reproduced in the numerical simulation. The second example investigates the formation mechanism of an impermeable barrier between acidic and neutral fluid zones in the Onikobe Geothermal Field, Japan. The third studies geothermal injection well silica scaling and loss of porosity and injectivity at Tiwi field, Philippines. Open challenges and future directions on the modeling will be discussed.