Modelling of chemical transport in geothermal reservoirs

Abstract

An original approach is presented for solving the problem of chemical (silica) transport and deposition in geothermal reservoirs by the method of characteristics. This analytical technique is used to analyse isothermal and non-isothermal transport of silica either in a porous medium or a single fracture. Analytic models of silica deposition are derived for the problem of constant rate injection into a one-dimensional channel or into a well in a uniform layer producing a radially symmetric flow. The changes in porosity and permeability resulting from deposition are included in the models. Various mathematical models describing the rate of deposition of silica from geothermal fluids are investigated. These deposition models, namely a first order rate of reaction, second order rate of reaction, and a combination of the first order and third order rates of reaction are combined with the silica transport equation and used to predict the variation of silica deposition in experimental packed columns and near reinjection wells in geothermal reservoirs. Modelling studies on the temperature effects of reinjection into a hot or cold reservoir is undertaken. The strong dependence of the rate of silica deposition on temperature is confirmed by the model. The radial flow model derived is also applied to some field data in Otake geothermal field Japan. Mathematical models of isothermal and non-isothermal silica deposition in a single fracture are successfully tested against some reported numerical results. The problem of variable rate isothermal injection in either one-dimensional or radial flow is also studied. Quasi-analytic solutions for silica deposition are derived using the method of characteristics. The models are validated with some reported experimental results and also applied in simulating the changes in injectivity of some of the reinjection wells in Tongonan geothermal field in the Philippines. The problem of chemical transport in a production-reinjection system is also examined. A high permeability fracture zone which connects the production area to the reinjection area is employed in the model. The mathematical models of silica deposition derived in the first part of the study are also incorporated in the sUica mass balance model. Simple time- dependent lumped parameter models of chloride, silica and temperature changes are developed. Several analytic models for the thermal effects of fluid flowing in the fractured zone are also investigated. The modelling procedures developed are applied to some of the production wells in Palinpinon geothermal field in the Philippines. The model matched the production silica, chloride and temperature changes observed in the wells very well.