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[1] We report experimental measurements of the streaming potential coupling coefficient in sandstones saturated with NaCl-dominated artificial and natural brines up to 5.5 M (321.4 g L −1 of NaCl; electrical conductivity of 23 S m −1 ). We find that the magnitude of the coupling coefficient decreases with increasing brine salinity, as observed in previous experimental studies and predicted by models of the electrical double layer. However, the magnitude of the coupling coefficient remains greater than zero up to the saturated brine salinity. The magnitude of the zeta potential we interpret from our measurements also decreases with increasing brine salinity in the low-salinity domain (<0.4 M; 23.4 g L −1 of NaCl and 3.4 S m −1 ) but reaches a constant value at higher salinity (>0.4 M). We hypothesize that the constant value of zeta potential observed at high salinity reflects the maximum packing of counterions in the diffuse part of the electrical double layer. Our hypothesis predicts that the zeta potential becomes independent of brine salinity when the diffuse layer thickness is similar to the diameter of the hydrated counterion. This prediction is confirmed by our experimental data and also by published measurements on alumina in KCl brine. At high salinity (>0.4 M), values of the streaming potential coupling coefficient and the corresponding zeta potential are the same within experimental error regardless of sample mineralogy and texture and the composition of the brine.

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 Ca, Mg and CO. 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 CO partial pressure (pCO). 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 SO is also a PDI for the calcite surface, but new data presented here indicate that the value of pSO 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 ...

The formation, storage and chemical differentiation of magma in the Earth's crust is of fundamental importance in igneous geology and volcanology. Recent data are challenging the high melt fraction 'magma chamber' paradigm that has underpinned models of crustal magmatism for over a century, suggesting instead that magma is normally stored in low melt fraction 'mush reservoirs'. 1-9 A mush reservoir comprises a porous and permeable framework of closely packed crystals with melt present in the pore space. 1,10 However, many common features of crustal magmatism have not yet been explained by either the 'chamber' or 'mush' reservoir concepts. 1,11 Here we show that reactive melt flow is a critical, but hitherto neglected, process in crustal mush reservoirs, occurring because buoyant melt percolates upwards through, and reacts with, the crystals. 10 Reactive melt flow in mush reservoirs produces the low crystallinity, chemically differentiated (silicic) magmas which ascend to form shallower intrusions or erupt to the surface. 11-13 The magmas can host much older crystals, stored at low and even sub-solidus temperatures, consistent with crystal chemistry data. 6-9 Changes in local bulk composition caused by reactive melt flow, rather than significant increases in temperature, produce the rapid increase in melt fraction that remobilizes these cool-or cold-stored crystals. Reactive flow can also produce bimodality in magma compositions sourced from mid-to lower-crustal reservoirs. 14,15 Trace element profiles generated by reactive flow are similar to those observed in a well-studied reservoir now exposed at the surface. 16 We propose that magma storage and differentiation primarily occurs by reactive melt flow in long-lived mush reservoirs, rather than by the commonly invoked process of fractional crystallisation in magma chambers. 14 Magma reservoirs occur at several depths within the crust and typically grow incrementally through the intrusion of dykes or sills. 1,11,13,16,17 High melt fractions must sometimes be present in these reservoirs to produce eruptible, low-crystallinity magmas. 1,7,8,9,13 However, geophysical data suggest that reservoirs have low melt fraction even beneath active volcanoes 2-5 and crystal chemistry data indicate that long-term magma storage occurs at low or even sub-solidus temperature. 6-9 High melt fractions are therefore ephemeral, yet geochemical models typically assume differentiation occurs by crystal fractionation from low-crystallinity magmas; 11,14 moreover, geochronological data demonstrate that crustal magma reservoirs can be long-lived, spanning hundreds of thousands to millions of years. 17-21 Existing models of crustal magma storage and differentiation cannot reconcile these conflicting observations. We use numerical modelling to investigate the storage and chemical differentiation of magma in crustal reservoirs. The model describes repeated intrusion of mafic to intermediate sills into the mid-to lower crust, 12,13,16,21-23 the associated transport of heat via conduction and ad...

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.

The emerging concept of a magma reservoir is one in which regions containing melt extend from the source of magma generation to the surface. The reservoir may contain regions of very low fraction intergranular melt, partially molten rock (mush) and melt lenses (or magma chambers) containing high melt fraction eruptible magma, as well as pockets of exsolved magmatic fluids. The various parts of the system may be separated by a sub-solidus rock or be connected and continuous. Magma reservoirs and their wall rocks span a vast array of rheological properties, covering as much as 25 orders of magnitude from high viscosity, sub-solidus crustal rocks to magmatic fluids. Time scales of processes within magma reservoirs range from very slow melt and fluid segregation within mush and magma chambers and deformation of surrounding host rocks to very rapid development of magma and fluid instability, transport and eruption. Developing a comprehensive model of these systems is a grand challenge that will require close collaboration between modellers, geophysicists, geochemists, geologists, volcanologists and petrologists. This article is part of the Theo Murphy meeting issue ‘Magma reservoir architecture and dynamics’.

[1] We present measurements of the streaming potential coupling coefficient in intact sandstone samples saturated with NaCl brines at concentrations up to 5.5 molL À1. The values we record at low salinity are consistent with those reported previously. As brine salinity increases, the coupling coefficient decreases in magnitude, but is still measureable up to the saturated concentration limit. The magnitude of the zeta potential also decreases with increasing salinity, but approaches a constant value at high salinity. This behaviour is not captured by current models of the electrical double layer. Our results suggest that streaming potential measurements may be used to monitor flow in saline subsurface environments such as deep saline aquifers and hydrocarbon reservoirs. However, they were obtained at laboratory temperature. Future work will focus on the effect of elevated temperatures at high salinity. Citation: Jaafar, M. Z., J. Vinogradov, and M. D.Jackson (2009), Measurement of streaming potential coupling coefficient in sandstones saturated with high salinity NaCl brine,

We report measurements of the zeta potential on intact limestone samples obtained using the streaming potential method (SPM), supplemented by the more ubiquitous electrophoretic mobility method (EPM). The effect of the potential-determining ions (PDI) Ca, Mg and SO4, and the total ionic strength controlled by NaCl concentration, is investigated over the range typical of natural brines. We find that the zeta potential varies identically and linearly with calcium and magnesium concentration expressed as pCa or pMg. The zeta potential also varies linearly with pSO4. The sensitivity of the zeta potential to PDI concentration, and the IEP expressed as pCa or pMg, both decrease with increasing NaCl concentration. We report considerably lower values of IEP than most previous studies, and the first observed IEP expressed as pMg. The sensitivity of the zeta potential to PDI concentration is lower when measured using the SPM compared to the EPM, owing to the differing location of the shear 2 plane at which the zeta potential is defined. SPM measurements are more appropriate in natural porous samples because they reflect the mineral surfaces that predominantly interact with the adjacent fluids. We demonstrate that special cleaning procedures are required to return samples to a pristine zeta potential after exposure to PDIs. We apply our results to an engineering process: the use of modified injection brine composition to increase oil recovery from carbonate reservoirs. We find a correlation between an increasingly negative zeta potential and increased oil recovery.