Matrix Diffusion and Its Effect on the Modeling of Tracer Returns from the Fractured Geothermal Reservoir at Wairakei, New Zealand

Abstract

Tracer tests performed at the geothermal reservoir at Wairakei, New Zealand have been analyzed, using a mathematical and physical model in which tracer flows through individual fractures with diffusion into the surrounding porous matrix. Model calculations matched well with the observed tracer return profiles. From the model, first tracer arrival times and the number of individual fractures ( the principal conduits of fluid flow in the reservoir ) joining the injector-producer wells can be determined. If the porosity, adsorption distribution coefficient, bulk density and effective diff u s i o n c o e f f i cent are known, fracture widths may be estimated. Hydrodynamic dispersion down the length of the fracture is a physical component not taken into account in this model. Future studies may be warranted in order to determine the necessity of including this factor. In addition to the tracer profile matching by the matrix diffusion model, comparisons with a simpler fractur e flow model by Fossum and Horne (1982) were made. The inclusion of the matrix diff u s i o n effects was seen to significantly improve the fit to the observed data.