Electrokinetic properties of reservoir rock particles

Abstract

The surface charge properties of Berea sandstone (and its major components), Indiana limestone and Baker dolomite in concentrated electrolyte solutions were investigated. This was accomplished by crushing the rocks into fine particles which were then suspended in aqueous solutions to permit electrokinetic measurements. The electrophoretic mobilities (zeta potentials) of the particles were determined as functions of pH, ionic strength and ionic composition in ranges that are of interest in enhanced oil recovery (EOR) processes. To achieve this, it was necessary to investigate electrokinetic behaviour at much higher ionic strengths than are normally reported in the literature. In general, the Berea sandstone and its constituent quartz and clay fractions adopted a negative charge in the aqueous solutions, the magnitude of which was significantly influenced by pH, the ionic strength and the degree of hardness in the electrolyte solution. Under some conditions severe charge reduction and charge reversal were observed. The limestone and dolomite were more dramatically influenced by both pH and the presence of multivalent cations. The magnitude of the surface charges varied considerably and could adopt positive or negative surface charges depending on the solution properties. The sensitivity of these reservoir minerals to small changes in solution properties carries potentially serious implications for any EOR process whose efficiency depends indirectly on the rock surface charge. Examples would include the adsorption of surfactants, particularly ionic and amphoteric surfactants, and the chemical mobilization of fines causing permeability damage.