Characterisation of Surface Electrical Charge and Wettability in Carbonates with Application to Controlled Salinity Waterflooding
M. D. Jackson and J. Vinogradov
Department of Earth Science and Engineering, Imperial College London, UK
It is well known that the wetting behavior of carbonate mineral surfaces is affected by their surface electrical charge [e.g. 1,2] and that the surface charge in saturated porous media can be characterized using measurements of streaming potential [e.g. 3,4]. Streaming potentials in porous media arise from the charge separation that occurs in the electrical double layer at mineral-fluid interfaces [e.g. 5]. If the fluid is induced to flow by an external potential (pressure) gradient, then some of the excess charge within the double layer is transported with the flow, giving rise to a streaming current. Divergence of the streaming current density establishes an electrical potential, termed the streaming potential. The closest plane to the mineral surface at which flow occurs in the double layer is termed the shear plane; the electrical potential at this plane is termed the zeta potential.
The relationships between streaming potential and surface electrical charge, and between surface electrical charge and wettability, suggest that measurements of streaming potential may be used to probe the wetting state of mineral surfaces in intact core samples. Previous studies have measured streaming potential in core samples saturated with oil, and with oil and brine, but the samples had not been aged to alter their wetting behavior [6-8]. Consequently, the relationship between streaming potential and wettability has not been investigated. Yet if streaming potential measurements, and the electrical properties interpreted from these measurements such as zeta potential and surface electrical charge, are affected by wettability in a direct and quantifiable manner, then streaming potential measurements may be of great utility in characterizing wettability and processes that affect wettability. The aim of this paper is to determine whether wettability alteration in carbonate core samples leads to measurable changes in streaming potential.
Our results suggest that aging carbonate samples in crude oil leads to measurable changes in the streaming potential coupling coefficient, zeta potential and excess charge transported by the flow of brine. Consequently, measurements of streaming potential could be used to probe the surface electrical charge properties of carbonate (and sandstone) samples saturated with brine and crude oil. The results may be used to quantify the wetting state; they may also to determine if and how changes in surface charge and wetting state are responsible for improved oil recovery during controlled salinity waterflooding. In the experiments reported here, aging of sample 1 caused some areas of the mineral surfaces to become oil-wet. This disrupted the continuity of the double layers at the mineral-brine interface, thus significantly reducing the excess electrical charge transported by the flow of brine, yielding zero streaming potential coupling coefficient, zeta potential and excess charge transported by the flow, within experimental error.
AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012