Dispersion of Tracers in Fractures - Experiments at Stanford University

Abstract

Although reinjection can help to maintain reservoir pressure and fluid volume, in some cases a decrease in reservoir productivity has been observed (Horne, 1982). This is caused by rapid flow of the reinjected water through fractures connecting the injector and producers. As a result, the water is not sufficiently heated by the reservoir rock, and a reduction in enthalpy of the produced fluids is seen. Tracer tests have proven to be valuable to reservoir engineers for the design of a successful reinjection program. By injecting a slug of tracer and studying the discharge of surrounding producing wells, an understanding of the fracture network within a reservoir can be provided. In order to quantify the results of a tracer test, a model that accurately describes the mechanisms of tracer transport is necessary. One such mechanism, dispersion, is like a smearing out of a tracer concentration due to the velocity gradients over the cross section of flow. If a dispersion coefficient can be determined from tracer test data, the fracture width can be estimated. The purpose of this project was to study the dispersion of a chemical tracer in flow through a fracture. To this end, an experimental apparatus was designed and constructed in 1984 (Gilardi, 1984), and early experiments showed that Taylor dispersion could accurately describe the mechanism of tracer transport through fractures. During 1985, the apparatus has been modified to measure tracer concentration at a greater number of points and with greater precision.