EVALUATION OF C-14 AS A NATURAL TRACER FOR INJECTED FLUIDS AT THE AIDLIN SECTOR OF THE GEYSERS GEOTHERMAL SYSTEM THROUGH MODELING OF MINERAL-WATER-GAS REACTIONS

A reactive-transport model for 14C was developed to test its applicability to the Aidlin geothermal system. Using TOUGHREACT, we developed a 1-D grid to evaluate the effects of water injection and subsequent water-rock-gas interaction on the compositions of the produced fluids. A dual-permeability model of the fracture-matrix system was used to describe reaction-transport processes in which the permeability of the fractures is many orders of magnitude higher than that of the rock matrix. The geochemical system included the principal minerals (K-feldspar, plagioclase, calcite, silica polymorphs) of the metagraywackes that comprise the geothermal reservoir rocks. Initial simulation results predict that the gas-phase CO2 in the reservoir will become more enriched in 14C as air-equilibrated injectate water (with a modern carbon signature) is incorporated into the system, and that these changes will precede accompanying decreases in reservoir temperature. The effects of injection on 14C in the rock matrix will be lessened somewhat because of the dissolution of matrix calcite with “dead ” carbon