Stability of Aqueous Wetting Films in Athabasca Tar Sands

The existence of thin films of water that completely wet the sand grains has long been regarded as an important feature of the Athabasca oil sands deposit. Direct microscopic evidence, however, cannot be relied on to establish whether such films are present. The existence and stability of such films, therefore, must be inferred from the relevant surface chemical forces for the oil/brine/rock system. A detailed analysis of these forces shows that the stability of these thin wetting films is critically dependent on whether the zeta potentials (and charge densities) for the two electrical double layers bounding the film are of like sign. The zeta potential and charge density for the rock/brine interface will in almost all cases be negative in sign. Therefore, a requirement for the stability of a wetting film will be that these quantities are also negative at the brine/oil interface. New measurements of the electrophoretic mobility of small particles of Athabasca bitumen suspended in an aqueous phase are reported. These data show that the zeta potential at the bitumen/water interface is strongly negative. Consequently, these results suppose the hypothesis that wetting films will be stable in this instance. Introduction The great economic potential and geological significance of the Athabasca oil sands, as well as ready accessibility of outcrop specimens, have motivated extensive investigations of their chemical and physical properties for the past several decades. Although many details remain still unresolved, there is broad agreement regarding the gross physical nature of the quartz/bitumen/water mixture that constitutes the bulk of the resource. In particular, it is usually postulated that, even in the particular, it is usually postulated that, even in the bitumen-rich deposits where water content is very low, the aqueous phase is distributed in the form of continuous films that surround the quartz grains. In other words, the grains themselves are separated from the bitumen phase by envelopes of water. These envelopes are presumed to be much thicker than a simple monolayer or bilayer of water molecules (0.3 to 0.6 nm). The first published suggestion of such an arrangement was appended as a reader's comment to a general review of Athabasca oil-sands geology. Since then, others have reaffirmed this idea, occasionally pointing out also that, while present in the Athabasca material, such aqueous envelopes, separating oil from sand, are not an essential feature of all oil and tar sands. It is of some interest, however, so far as the Athabasca oil sands are concerned, that no direct observation of water films of greater-than-molecular thickness seems ever to have been made: thus, the evidence is, to date, indirect and equivocal. There is no doubt that the question of whether such wetting films are present is of more than academic importance. Rapid, complete separation of the Athabasca bitumen from sand is a key requirement for both current methods of mining and future in-situ technology. The modeling and optimization of such processes clearly will depend on a correct interpretation of the physical mechanisms involved, and this in turn requires a valid assessment of the initial physical state of the system. SPEJ P. 249