Geothermal Injection Monitoring with D.C. Resistivity Methods

Abstract

We consider injection into an idealized geothermal reservoir, assuming that the injected water differs in temperature and salinity from in place fluids. Changes information resistivity resulting from temperature and salinity variations are evaluated, and numerical simulation methods are used to predict effects which would be observed by means of dc resistivity arrays, a dipole-dipole array and a downhole surface array. Our calculations show that the dipole-dipole method is relatively insensitive to changes due to injection, but downhole surface measurements are very sensitive. From the simulated downhole surface measurements a bell shaped curve for resistivity change is obtained, from which the position of the chemical front may be approximately determined. Resistivity changes from temperature variations are rather small and probably cannot be detected in field measurements. Resistivity measurements are more than twice as sensitive when injected water is more saline than the in situ reservoir fluids. This suggests that it may be easier to monitor the location of injection water if geothermal brine is reinjected rather than fresher water.