Release and Deposition of Clays in Sandstones

Abstract

ABSTRACT An attempt is made to prescribe conditions under which deposition or release of physically adhering clay particles occurs on pore walls in sandstones. Quantitative models for the physicochemical interactions between clay particles and pore walls are used to estimate rates of deposition and release in a single pore. It is found that the rates change rapidly from zero to a finite value governed only by the local Peclet number. This makes it possible to delineate regions in the ψ1 – ψ2 phase plane where deposition or release dominates. The onset of clay release or deposition is determined by these surface chemical parameters. However in the deposition or release dominated regimes, the actual rates are shown to be only functions of fluid pore velocity. Since the surface electrostatic potentials ψ1 and ψ2 are crucial in determining the onset of deposition or release, they were experimentally measured. Ottawa sand, kaolin, and fired and unfired Berea sandstone samples were studied. The sandstone samples were disaggregated and separated into the oxide fraction and the clay fraction. Surface charges were measured on each as a function of pH and ionic strength. These experimental results in conjunction with the regions of deposition and release in the ψ1 – ψ2 phase plane were used to construct stability diagrams. The stability diagrams delineate regions in the pH – ionic strength phase plane where clays will be released from pore walls. These regions are referred to as regions of instability for the sandstone. The results obtained for Berea sandstone agree well with those reported earlier. Similar stability diagrams can be constructed when other species such as polyvalent cations or anions are present in the system. Some experimental results are provided here towards this end.