Spontaneous potential (SP) is routinely measured using wireline tools during reservoir characterization. However, SP signals are also generated during hydrocarbon production, because of gradients in the water phase pressure (relative to hydrostatic), chemical composition and temperature. We suggest that measurements of SP during production, using electrodes permanently installed downhole, could be used to detect water encroaching on a well while it is several tens to hundreds of meters away. We simulate numerically the SP generated during production from a single vertical well, with pressure support provided by water injection. We vary the production rate, and the temperature and salinity of the injected water, to vary the contribution of the different components of the SP signal. We also vary the values of the so-called ‘coupling coefficients’ which relate gradients in fluid potential, salinity and temperature, to gradients in electrical potential. The values of these coupling coefficients at reservoir conditions are poorly constrained. We demonstrate that the SP signal peaks at the location of the moving waterfront, where there are steep gradients in water saturation and salinity. The signal decays with distance from the front, typically over several tens to hundreds of meters; hence the encroaching water can be detected before it arrives at the well. The SP signal at the well is dominated by the electrokinetic and electrochemical components arising from gradients in fluid potential and salinity. Larger signals will be obtained in low permeability reservoirs produced at high rate, saturated with formation brine of low salinity, or with brine of a very different salinity from that injected.

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