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. Inversion of the measured signals in conjunction with normally available reservoir data could be used to determine the water saturation in the vicinity of the well, and to regulate flow into the well using control valves in order to maintain or increase oil production and delay or prevent water production.
Downhole monitoring of streaming potential, using electrodes mounted on the outside of insulated casing, is a promising new technology for monitoring water encroachment towards an intelligent well. However, there are still significant uncertainties associated with the interpretation of the measurements, particularly concerning the streaming potential coupling coefficient. This is a key petrophysical property which dictates the magnitude of the streaming potential for a given fluid potential. The coupling coefficient can be measured experimentally, but previous studies have obtained data for sandstone cores saturated with relatively low salinity brine (less than seawater). Formation and injected brine in hydrocarbon reservoirs is typically more saline than this. Extrapolating data obtained at low salinity into the high salinity domain suggests that the coupling coefficient falls to zero at approximately seawater salinity. If this is the case, then streaming potential signals will be very small in most hydrocarbon reservoirs.
We present the first measured values of streaming potential coupling coefficient in sandstone cores saturated with brine at higher than seawater salinity. We find that the coupling coefficient is small, but still measurable, even when the brine salinity approaches the saturated concentration limit. Consistent results are obtained from two independent experimental set-ups, using specially designed electrodes and paired pumping experiments to eliminate spurious electrical potentials. We apply the new experimental data in a numerical model to predict the streaming potential signal which would be measured at a well during production. The results suggest that measured signals should be resolvable above background noise in most hydrocarbon reservoirs, and that water encroaching on a well could be monitored while it is several tens to hundreds of metres away.
Copyright 2005, held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors. Annual Logging Symposium held in New Orleans, Louisiana, United States, June 26-29, 2005. ABSTRACT measured electrokinetic and pressure signals which confirm experimentally that the normalized electric field is a function of permeability. The signals also corresponded qualitatively with those predicted by the model. INTRODUCTION We have studied the electrokinetic signals generated by acoustic sources centered in a borehole using a finite element model, and by making borehole and laboratory measurements. Electrokinetic signals are generated when acoustic sources move fluid in the pore space of a rock. These signals therefore contain information on the dynamic as well as the static properties of a reservoir. We have found that a reliable permeability log can be derived from electrokinetic measurements. The finite element model combines Biot's equations for acoustic propagation in a porous medium with a simplified form of Maxwell's equations. The model handles the effect of mudcake as well as the presence of horizontal and radial layers. Of the various types of electrokinetic signal generated, the normalized electric field (the ratio of the electric field to the pressure) of the Stoneley wave has the most significant dependence on permeability independent of porosity. We have also constructed a simple logging tool with acoustic and electrical transducers, designed to give minimum noise or spurious electrical signals. The tool has been used to test the model results in three well-characterized water wells as well as the Callisto nuclear-tool calibration pits. In one case we were able to invert the field data with the help of the model to obtain permeability.