Abstract

This paper examines the wettability change during Steam Assisted Gravity Drainage (SAGD) and Expanding Solvent - SAGD (ES-SAGD). The qualitative and the quantitative analyzes of residual oil for steam and steam-solvent coinjection cases are achieved to investigate the impact of clay migration and asphaltene precipitation on wettability alteration. The solvent selection in ES-SAGD is made according to their solubility in asphaltenes; insoluble (n-hexane), soluble (toluene), and intermediate soluble (cyclohexane). Five experiments (one SAGD and four ES-SAGD) are conducted on a Canadian bitumen. Different solvent injection strategies are followed: coinjection and cyclic injection. Wettability is determined through contact angle measurements on the spent rock samples for both inside and outside steam chamber zones. Residual oil saturation is defined via solvent extraction and with a thermal method; Thermogravimetric Analysis (TGA). Two solvents are used for the extraction: toluene and mixture of 90%dichloromethane+10%methanol. The asphaltene fractions of the residual oil samples are further characterized by determining clay content, Solubility Profile; and carbon, hydrogen, sulfur, nickel, and vanadium contents. Both the thermal and the two solvent extraction methods yield more or less the same residual oil saturations. The asphaltene content of the residual oils (22 to 27 wt%) is found lower than the asphaltene content of original bitumen (34 wt%). However, higher metal content is detected on the residual oil asphaltenes. Analysis of residual oil asphaltenes shows a significant presence of clays in the inside steam chamber region for SAGD, which inhibits effective steam chamber propagation by reducing permeability. This asphaltene-clay interaction increases the oil- wetness of the rock surface and impacts the oil production adversely. However, this effect is minimized by the addition of solvents. The wettability measurements on spent rock samples also support these findings. The elemental analysis of asphaltenes reveals that with the increase in precipitation of asphaltenes (for ES-SAGD with n-hexane), there is an increase in vanadium and nickel concentrations. In terms of asphaltene Solubility Profiles, higher polarity was found for asphaltenes originated from inside the steam chamber zone for ES-SAGD with n-hexane, where the effect of n-hexane in the vapor phase is dominant. This work shows that the effectiveness of ES-SAGD may be in part caused by contributions from wettability changes, clay migration, and asphaltene precipitation in addition to by oil viscosity reduction alone. This study provides information on the interaction of clay, asphaltenes, solvent, and steam during SAGD and ES-SAGD. It explains the behavior of clay and asphaltenes during SAGD and ES-SAGD when different solvents are used.

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