Simulation and resistivity modeling of a geothermal reservoir with waters of different salinity

Abstract

Apparent resistivities measured by means of repetitive dipole-dipole surveys show significant changes within the Cerro Prieto reservoir. The changes are attributed to production and natural recharge. To understand better the observed geophysical phenomena, we performed a simple reservoir simulation study combined with the appropriate DC resistivity calculations to determine the expected magnitude of apparent resistivity change. We consider production from a liquid dominated reservoir with dimensions and parameters of the Cerro Prieto ‘A’ reservoir and assume lateral and vertical recharge of colder and less saline waters. Based on rather schematic one- and two-dimensional reservoir simulations, we calculate changes in formation resistivity which we then transform into changes in apparent resistivity that would be observed at the surface. Simulated changes in apparent resistivities over the production zone show increases of 10 to 20% over a 3 year period at the current rate of fluid extraction. Changes of this magnitude are not only within our ability to discern using proper field techniques, but are consistent in magnitude with some of the observed effects. However, the patterns of apparent resistivity changes in the simulated dipole-dipole pseudosection only partially resemble the observed field data. This is explained by the fact that the actual fluid recharge into the ‘A’ reservoir is more complicated than assumed in our simple, schematic recharge models. DC resistivity monitoring appears capable of providing indirect information on fluid flow processes in a producing geothermal reservoir. Such information is extremely valuable for the development of quantitative predictions of future reservoir performance.