ON THERMOELASTICITY AND SILICA PRECIPITATION IN HYDROTHERMAL SYSTEMS - NUMERICAL MODELING OF LABORATORY EXPERIMENTS

We develop a numerical model to understand the evolution of fracture p ermeability in hydrothermal upflow zones resulting from the combined e ffects of thermoelastic stresses and precipitation of silica as high-t emperature, reactive fluid traverses temperature and pressure gradient s. Because we test the model by comparing the results with those from previously published laboratory experiments on cylindrical granite cor es, we solve the problem of radial flow under an applied pressure diff erence of a silica-saturated fluid through vertical, initially paralle l-walled cracks distributed evenly about the core sample. We emplace a steady state logarithmic temperature profile and assume that silica p recipitation occurs so that the silica concentration in the fluid rema ins at equilibrium with local temperature and pressure along the flow path. The model results show a rapid initial decrease in permeability resulting from thermoelastic stresses, followed by a further decrease resulting from silica precipitation. The greater the initial temperatu re gradient, initial permeability, and/or initial crack width (at a gi ven permeability), the greater the permeability decrease resulting fro m thermoelastic stresses. As a result of silica precipitation, the per meability eventually declines as t(-3/2). The model results agree with the general trends in the laboratory data, thus confirming that silic a precipitation is the main cause of the observed decrease in permeabi lity during the experiments. Disagreement between the model and labora tory data in detail suggests that complications such as reaction kinet ics,precipitation of other minerals and nonhomogeneous crack distribut ions need to be considered in the model. Thermoelastic stresses, thoug h not important at the laboratory scale, may be important at the field scale.