Simulation o f Geothermal Reservoirs including Changes i n Porosity and Permeability due to Silica-Water Reactions

Abstract

Changes in porosity and permeability due to water rock reactions may affect a geothermal reservoir in two important ways. One is the possibility of c1oe;p;inp. of the pores pace in the vicinity of a reinjection well. The second concerns the long term evolution of a geothermal system, for example, the development of a self- sealing caprock. Two major types of reactions that may affect the porosity and permeability in the reservoir are dissolution - precipitation reactions and alteration reactions. Silica precipitation is the major self-sealing process in hot-water geothermal resevoirs ( Vhite,1973 ). The purpose of this work is to develop a mathematical model which can simulate the changes in porosity and permeability resulting from water-rock reactions in a geothermal reservoir.

The model being developed describes the flow of hot water in an axially symmetric porous medium. The model is a vertical cross-section in r- z coordinates. For simplicity, only a single dissolved chemicals pecies is being modeled. The fluid is single-phase water and only dissolution and precipitation of quartz a reconsidered. The kinetics for silica are those developed hy Pimstidt and Rarnes (1979) and Rlmstidt (1979).