Self-potential anomalies induced by water injection into hydrocarbon reservoirs

The injection of cold water into a hydrocarbon reservoir containing relatively warmer, more saline formation brine may generate self-potential anomalies as a result of electrokinetic, thermoelectric, and/or electrochemical effects. We have numerically assessed the relative contributions of these effects to the overall self-potential signal generated during oil production in a simple hydrocarbon reservoir model. Our aim was to determine if measurements of self-potential at a production well can be used to detect the movement of water toward the well. The coupling coefficients for the electrochemical and thermoelectric potentials are uncertain, so we considered four different models for them. We also investigated the effect of altering the salinities of the formation and injected brines. We found that the electrokinetic potential peaked at the location of the saturation front (reaching values of 0.2 mV even for the most saline brine considered). Moreover, the value at the production well increased as the front approached the well, exceeding the noise level (similar to 0.1 mV). Thermoelectric effects gave rise to larger potentials in the reservoir (similar to 10 mV), but values at the well were negligible (less than or similar to 0.1 mV) until after water breakthrough because of the lag in the temperature front relative to the saturation front. Electrochemical potentials were smaller in magnitude than thermoelectric potentials in the reservoir but were measurable (> 0.1 mV) at the well because the salinity front was closely associated with the saturation front. When the formation brine was less saline (similar to 1 mol/liter), electrokinetic effects dominated; at higher salinities (similar to 5 mol/liter), electrochemical effects were significant. We concluded that the measurement of self-potential signals in a production well may be used to monitor the movement of water in hydrocarbon reservoirs during production, but further research is required to understand the thermoelectric and electrochemical coupling coefficients in partially saturated porous media.