Jan Vinogradov

Measurement of streaming potential coupling coefficient in sandstones saturated with natural and artificial brines at high salinity

Measurements of streaming potential can be used to monitor subsurface flow using electrodes installed along boreholes. However, the interpretation of the measurements requires an understanding of the streaming potential coupling coefficient, which dictates the magnitude of the streaming potential for a given pressure difference. Previous laboratory measurements of the coupling coefficient in earth materials focussed on crushed and intact rock samples saturated with artificial NaCl and KCl brines of relatively low salinity: salt concentrations were typically lower than average seawater. However, many subsurface brines are significantly more saline. We have measured the streaming potential coupling coefficient in four different intact sandstone samples saturated with seawater and artificial NaCl brines at concentrations up to 5.3 M/L. We obtain consistent results using three different sets of experimental apparatus. The values we record at low salinity are consistent with those reported previously. As brine salinity increases, the coupling coefficient decreases in magnitude, falling to a value of c. 1.5 x 10-10 V/Pa at 5.3M/L, and remains negative over the entire salinity range. The coupling coefficient measured for seawater is similar to that obtained for NaCl brine of the same ionic strength. The magnitude of the zeta potential also decreases with increasing salinity, but approaches a constant value of c. 17mV at salinities greater than c. 0.4M/L. This behaviour is not captured by current models of the electrical double layer. We hypothesize that ion interactions cause the reduction in thickness of the diffuse layer at high salinity to be less than predicted by the Poisson-Boltzmann equation, in which it is assumed that the ions are point charges. Moreover, the counter-charge required to balance the mineral surface charge is not accommodated entirely within the Stern layer. Consequently, the diffuse layer does not collapse to zero; rather, some of the counter-charge remains mobile within the diffuse layer, at a maximum concentration which is limited by the size of the hydrated counter-ions. Our hypothesis is supported by the observation that the Debye length is c. 0.5 nm at a salinity of 0.4 M/L, which is comparable with the diameter of a hydrated sodium ion. Our results suggest that streaming potential measurements may be used to monitor flow in more saline subsurface environments, such as deep saline aquifers and hydrocarbon reservoirs, than previously thought.

Zeta potential in oil-water-carbonate systems and its impact on oil recovery during controlled salinity water-flooding

Laboratory experiments and field trials have shown that oil recovery from carbonate reservoirs can be increased by modifying the brine composition injected during recovery in a process termed controlled salinity water-flooding (CSW). However, CSW remains poorly understood and there is no method to predict the optimum CSW composition. This work demonstrates for the first time that improved oil recovery (IOR) during CSW is strongly correlated to changes in zeta potential at both the mineral-water and oil-water interfaces. We report experiments in which IOR during CSW occurs only when the change in brine composition induces a repulsive electrostatic force between the oil-brine and mineral-brine interfaces. The polarity of the zeta potential at both interfaces must be determined when designing the optimum CSW composition. A new experimental method is presented that allows this. Results also show for the first time that the zeta potential at the oil-water interface may be positive at conditions relevant to carbonate reservoirs. A key challenge for any model of CSW is to explain why IOR is not always observed. Here we suggest that failures using the conventional (dilution) approach to CSW may have been caused by a positively charged oil-water interface that had not been identified.

Zeta potential of artificial and natural calcite in aqueous solution

Despite the broad range of interest and applications, controls on calcite surface charge in aqueous solution, especially at conditions relevant to natural systems, remain poorly understood. The primary data source to understand calcite surface charge comprises measurements of zeta potential. Here we collate and review previous measurements of zeta potential on natural and artificial calcite and carbonate as a resource for future studies, compare and contrast the results of these studies to determine key controls on zeta potential and where uncertainties remain, and report new measurements of zeta potential relevant to natural subsurface systems. The results show that the potential determining ions (PDIs) for the carbonate mineral surface are the lattice ions Ca2+, Mg2+ and CO32-. The zeta potential is controlled by the concentration-dependent adsorption of these ions within the Stern layer, primarily at the Outer Helmholtz Plane (OHP). Given this, the Iso-Electric Point (IEP) at which the zeta potential is zero should be expressed as pCa (or pMg). It should not be reported as pH, similar to most metal oxides. The pH does not directly control the zeta potential. Varying the pH whilst holding pCa constant yields constant zeta potential. The pH affects the zeta potential only by moderating the equilibrium pCa for a given CO2 partial pressure (pCO2). Experimental studies that appear to yield a systematic relationship between pH and zeta potential are most likely observing the relationship between pCa and zeta potential, with pCa responding to the change in pH. New data presented here show a consistent linear relationship between equilibrium pH and equilibrium pCa or pMg irrespective of sample used or solution ionic strength. The surface charge of calcite is weakly dependent on pH, through protonation and deprotonation reactions that occur within a hydrolysis layer immediately adjacent to the mineral surface. The Point of Zero Charge (PZC) at which the surface charge is zero could be expressed as pH, but surface complexation models suggest the surface is negatively charged over the pH range 5.5-11. Several studies have suggested that SO42- is also a PDI for the calcite surface, but new data presented here indicate that the value of pSO4 may affect zeta potential only by moderating the equilibrium pCa. Natural carbonate typically yields a more negative zeta potential than synthetic calcite, most likely due to the presence of impurities including clays, organic matter, apatite, anhydrite or quartz, that yield a more negative zeta potential than pure calcite. New data presented here show that apparently identical natural carbonates display differing zeta potential behaviour, most likely due to the presence of small volumes of these impurities. It is important to ensure equilibrium, defined in terms of the concentration of PDIs, has been reached prior to taking measurements. Inconsistent values of zeta potential obtained in some studies may reflect a lack of equilibration. The data collated and reported here have broad application in engineering processes including the manufacture of paper and cement, the geologic storage of nuclear waste and CO2, and the production of oil and gas.

Zeta potential in intact natural sandstones at elevated temperatures

Supporting data are included in PDF and CSV files; any additional data may be obtained from the corresponding author (e-mail: j.vinogradov@imperial.ac.uk). TOTAL is thanked for partial support of Jacksons Chair in Geological Fluid Mechanics and for supporting the activities of the TOTAL Laboratory for Reservoir Physics at Imperial College London where these experiments were conducted. The Editor thanks Andre Revil and Paul Glover for their assistance in evaluating this paper.

Measurements of spontaneous potential in chalk with application to aquifer characterization in the southern UK

We report the first measured values of the streaming potential coupling coefficient in chalk samples saturated with natural groundwater, and preliminary field measurements of the spontaneous potential (SP), at both ambient and pumped conditions, at a test site in the Berkshire Chalk aquifer in the southern UK. The ultimate aim of the work is to use measurements of SP, in conjunction with borehole data, to characterize groundwater flow and aquifer properties. Laboratory measurements yield a value of the streaming potential coupling coefficient of −60 ± 4 mV MPa−1 and a corresponding zeta potential of −13 ± 1 mV. A negative zeta potential contrasts with previous published open-system measurements on artificial calcite, and may reflect the presence of organic material in the natural chalk samples or HCO3 and SO4 ions in the groundwater. Field measurements at ambient conditions show temporal variations in SP consistent with flow processes within the aquifer, but no coherent spatial variations. Measurements during water abstraction demonstrate that voltages at the ground surface and in monitoring boreholes become more positive during pressure drawdown and more negative during pressure build-up, consistent with the negative values of streaming potential coupling coefficient and zeta potential observed in the laboratory. Moreover, the magnitude of the change in voltage is similar to that estimated using the laboratory value of the coupling coefficient. Our results suggest that measurements of SP may make a valuable contribution to characterizing groundwater flow in the UK Chalk aquifer.

Temperature-dependence of the zeta potential in intact natural carbonates

The zeta potential is a measure of the electrical charge on mineral surfaces and is an important control on subsurface geophysical monitoring, adsorption of polar species in aquifers, and rock wettability. We report the first measurements of zeta potential in intact, water-saturated, natural carbonate samples at temperatures up to 120 °C. The zeta potential is negative and decreases in magnitude with increasing temperature at low ionic strength (0.01 M NaCl, comparable to potable water) but is independent of temperature at high ionic strength (0.5 M NaCl, comparable to seawater). The equilibrium calcium concentration resulting from carbonate dissolution also increases with increasing temperature at low ionic strength, but is independent of temperature at high ionic strength. The temperature dependence of the zeta potential is correlated with the temperature dependence of the equilibrium calcium concentration and shows a Nernstian linear relationship. Our findings are applicable to many subsurface carbonate rocks at elevated temperature.

Laboratory Measurements and Numerical Modeling of Streaming Potential for Downhole Monitoring in Intelligent Wells

Downhole monitoring of streaming potential, using electrodes mounted on the outside of insulated casing, is a promising new technology for monitoring water encroachment toward 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 that dictates the magnitude of the streaming potential for a given fluid potential. We present the first measured values of streaming potential coupling coefficient in sandstones saturated with natural and artificial brines relevant to oilfield conditions at higher-than-seawater salinity. We find that the coupling coefficient in quartz-rich sandstones is independent of sample type and brine composition as long as surface electrical conductivity is small. The coupling coefficient is small in magnitude, but still measurable, even when the brine salinity approaches the saturated concentration limit. Consistent results are obtained from two independent experimental setups, 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 that would be measured at a well during production. The results suggest that measured signals should be resolvable above background noise in most hydrocarbon reservoirs. Furthermore, water encroaching on a well could be monitored while it is several tens to hundreds of meters away. This contrasts with most other downhole monitoring techniques, which sample only the region immediately adjacent to the wellbore. Our results raise the novel prospect of an oil field in which the wells can detect the approach of water and can respond appropriately.

Tidal influence on self-potential measurements

Long-term surface and borehole self-potential (SP) monitoring was conducted in the UK Chalk aquifer at two sites. The coastal site is c. 1.7 km from the coast and the inland site is c. 80 km from the coast. At both sites, power spectral density analysis revealed that SP data contain the main ocean tidal periodic components. However, the principal lunar component (M2), the dominant ocean tidal component, was most significant at the coastal site. The M2 signal in surface-referenced SP data at the inland site was partly due to telluric currents caused by the geomagnetic ocean dynamo. Earth and/or atmospheric tides also contributed, as the SP power spectrum was not typical of a telluric electric field. The M2 component in borehole-referenced data at the inland site was below the significance level of the analysis method and was two orders of magnitude smaller than the M2 signal in borehole- referenced SP data at the coastal site. The tidal response of the SP data in the coastal borehole is, therefore, primarily driven by ocean tides. These cause changes in fluid pressure and chemical concentration gradients within the coastal aquifer, leading to time varying electrokinetic and exclusion-diffusion potentials. Borehole-referenced SP measurements could be used to characterize and monitor tidal processes in coastal aquifers such as the intrusion of seawater.

Self‐Potential as a Predictor of Seawater Intrusion in Coastal Groundwater Boreholes

Abstract Monitoring of self-potentials (SPs) in the Chalk of England has shown that a consistent electrical potential gradient exists within a coastal groundwater borehole previously affected by seawater intrusion (SI) and that this gradient is absent in boreholes further inland. Furthermore, a small but characteristic reduction in this gradient was observed several days prior to SI occurring. We present results from a combined hydrodynamic and electrodynamic model, which matches the observed phenomena for the first time and sheds light on the source mechanisms for the spatial and temporal distribution of SP. The model predictions are highly sensitive to the relative contribution of electrochemical exclusion and diffusion potentials, the exclusion efficiency, in different rock strata. Geoelectric heterogeneity, largely due to marls and hardgrounds with a relatively high exclusion efficiency, was the key factor in controlling the magnitude of the modeled SP gradient ahead of the saline front and its evolution prior to breakthrough. The model results suggest that, where sufficient geoelectric heterogeneity exists, borehole SP may be used as an early warning mechanism for SI.

Measurements of Zeta Potential in Intact Carbonate Samples with Application to Controlled Salinity Waterflooding

Summary Laboratory experiments and field trials have shown that oil recovery from carbonate reservoirs can be increased by modifying the brine composition injected during recovery in a process termed controlled salinity water-flooding (CSW). However, CSW remains poorly understood and there is no method to predict the optimum CSW composition. This work demonstrates that improved oil recovery (IOR) during CSW is strongly correlated to changes in zeta potential at both the mineral-brine and oil-brine interfaces. We report experiments in which IOR during CSW occurs only when the change in brine composition induces a repulsive electrostatic force between these interfaces. The polarity of the zeta potential at both interfaces must be determined when designing the optimum CSW composition. Results show that the zeta potential at the oil-water interface may be positive at conditions relevant to carbonate reservoirs and this has a significant impact on the choice of optimum bring composition for CSW. We report new measurements of zeta potential in intact carbonates at reservoir conditions of temperature and brine composition to understand how the zeta potential changes during CSW.