A Mechanism For Initiation of Bitumen Displacement From Oil Sand

Abstract

Abstract According to an oil sand structure recently proposed in this laboratory, connate water in high-grade oil sand appears in at least two types of configurations: as pendular rings at grain-to- grain contact paints, and as a roughly 10 nm thick film which covers the sand surface. The aqueous film exists due to the double layer repulsive forces between the bitumen and sand surfaces. The study has now been extended to include a theoretical model regarding the separation of bitumen from oil sand aggregates immersed ill aqueous solutions. The model describes the displacement mechanism in terms of the capillary forces at the bitumen/aqueous interface, the viscous drag of the bitumen and the stability of the aqueous film on the sand surfaces. The model successfully explains the observed results of the crumble test;a high rate of crumbling on an oil sand pellet was observed in a NaOH solution, but the presence of Ca++ caused a dramatic decrease in the effect of NaOH. The theory was further confirmed by a microscopic disintegration experiment. Here the bitumen displacement from individual sand grains was observed and video-recorded under a high resolution optical microscope. Introduction The previous paper(l) dealt with an accurate description of the microscopic structure of in-situ Athabasca oil Sand; especially the manner In which connate water and bitumen are distributed in the void space between the sand grains. In-situ, the sand grains occupy roughly 65% of the total volume. In the case of high-grade oil sand, the pore space is filled with bitumen and water. The water occupies between 10 and 15% of the pore volume and forms pendular rings at the contact points between the sand grains. These pendular rings (bulk aqueous phase) cover roughly 30% of the sand surface and the remaining 70% of the surface is covered by a thin film of water (film aqueous phase), which connects the bulk aqueous phase. The thickness of this aqueous film is of the order of 10 nm. In lower-grade oil sand, clusters of fine particles exist within the framework formed by the coarse sand grains. These clusters of fine particles are saturated with water. Thus the amount of connate water in oil sand increases linearly with increasing fines content. The main features of the structural model of the Athabasca oil sand described above are schematically shown in Figure 1. Theoretical considerations demonstrated that the aqueous film has been held in place throughout geological times due to the double layer repulsive forces between the negatively charged sand and bitumen surfaces. Recently, similar analysis was also reported by A.C. Hall et al(2,3). It has long been recognized that the presence of the aqueous film is the single most characteristic and fortunate feature of the Athabasca oil sands(4,7). Without this aqueous film or the hydrophilic nature of the sand, the hot water extraction process of bitumen would not work. The above model provides good grounds for the existence of this aqueous film.