Characterization of Fractures in Geothermal Reservoirs Using Resistivity

The optimal design of production in fractured geothermal reservoirs requires knowledge of the resource’s connectivity, therefore making fracture characterization highly important. This study aims to develop methodologies to use resistivity measurements to infer fracture properties in geothermal fields. The resistivity distribution in the field can be estimated by measuring potential differences between various points and the data can then be used to infer fracture properties due to the contrast in resistivity between water and rock. In this project, a two-dimensional model has been developed to calculate a potential field due to point sources of excitation. The model takes into account heterogeneity by solving the potential field for inhomogeneous resistivity, therefore enabling fractures to be modeled with different resistivity from the rock. In order to enhance the difference in resistivity between fractures and rock, flow simulations have been performed of a conductive fluid injected into a reservoir and the potential difference has been calculated between two wells at different times as the fluid flows through the fracture network. These results, i.e. the graphs of voltage differences vs. time, correspond to the fracture networks and therefore have shown promising possibilities in indicating fracture locations. Future work will include further study of the relationship between fracture networks and the change in potential differences as conductive tracer is injected into the reservoir. Another future goal is to study the possibility of using the potential differences with inverse modeling to characterize fractures patterns.