Wettability characterization using streaming potential measurements

Abstract

The surface charge of carbonate minerals, which is also expressed in terms of the zeta potential, plays a key control on reservoir wettability, and changes in the zeta potential have been invoked to explain wettability alteration and the release of previously trapped oil during controlled salinity waterflooding (CSW). We report a method to characterize the zeta potential of carbonates, based on measurements of streaming potential, which can be used to determine the zeta potential of mineral-brine and oil-brine interfaces within the porous medium. The aim of this project was to determine the effect of total salinity, potential determining ion (PDI) contribution, and wetting state on the zeta potential of limestone. In the first part, we use the streaming potential method to obtain measurements of zeta potential on intact core samples at typical reservoir brine salinity and composition. We determine the impact on zeta potential of varying the total salinity, and the concentration of the PDIs calcium, magnesium and sulfate. The impact of each PDI was determined over a wide range of concentrations naturally found in sea water, formation brines, and typical compositions used in CSW. 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 (iso-electric point) 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 plane at which the zeta potential is defined. In the second part, we use the streaming potential method to investigate how the zeta potential changes when an oil phase is introduced in the rock sample. We establish a relationship between wettability and the zeta potential. This is done for samples that were aged in the presence and absence of a brine phase, in order to represent mixed-wet and oil-wet cases. In addition, measurements on non-aged samples were conducted in order to represent the water-wet case. We find that the more oil-wet the system is, the more negative the zeta potential gets with the oil-wet case being the most negatively charged. For the crude oil samples, we find that there is a strong correlation between the Amott Index and the zeta potential. Our findings suggest that the streaming potential method can be used to assess the impact of water chemistry and wetting state on the surface charge of limestone. The results are directly applicable to wettability characterization and understanding of wettability alteration that may take place during CSW.