Resistivity Monitoring of Fluid Migration at the Cerro Prieto Geothermal Field

摘要

Apparent resistivities, measured by means of repetitive, dipole-dipole dc resistivity surveys since 1979, show significant and somewhat systematic changes over the area of the Cerro Prieto alpha reservoir. These changes are attributed to temperature and salinity changes, consequences of production, and natural recharge. To better understand the observed geophysical phenomena, we performed a series of simple reservoir simulation studies combined with the appropriate dc resistivity calculations to determine the magnitude and form of resistivity change. We considered production from a liquid-dominated reservoir with dimensions and parameters of the Cerro Prieto alpha reservoir and assumed lateral and vertical recharge of colder, less saline waters. The associated apparent esistivities increase 10 to 20% over the production area during a 3 year period at the current rate of production. These calculated changes agree in magnitude with the observed changes for the same production period. However, displayed in conventional pseudosection form, the patterns of calculated resistivity change only partially resemble the observed data. This is explained by the fact that natural recharge into the alpha reservoir is more complicated than our simple two-dimensional schematic recharge model. It is concluded that if properly conducted, dc resistivity monitoring appears capable of providing indirect information on fluid flow processes, including reinjection in a producing reservoir. (ERA citation 08:025910)