Asphaltene Precipitation During Bitumen Extraction With Expanding-Solvent Steam-Assisted Gravity Drainage: Effects on Pore-Scale Displacement
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SummarySteam-assisted gravity drainage (SAGD) is a proved enhanced-oil-recovery technique for oil-sand extraction. However, the environmental and the economic challenges associated with steam generation limit the application of this technology. To address these issues, we have investigated the effectiveness of expanding-solvent-SAGD (ES-SAGD) over base SAGD on a bitumen sample (8.8 API). Experimental studies are conducted with a 2D physical model. Different strategies for solvent injection are tested (coinjection and cyclic injection) to examine the impact of the deposition of the asphaltene fraction of the bitumen on porous media and the behavior of the asphaltene fraction in produced oil. Toluene is used as asphaltene-soluble solvent, and n-hexane is selected as asphaltene-insoluble. Steam-chamber development is monitored with temperature profiles from 47 separate positions. The oil rate, recovery factor, and the produced-oil quality are evaluated together. The effectiveness of SAGD and ES-SAGD is discussed by considering the role of asphaltenes and their interactions with clays in both produced- and residual-oil samples. This study reveals that coinjection of hydrocarbon solvents with steam enhances the steam-chamber development with higher oil-production rate. Moreover, ES-SAGD results in recovery of more-upgraded oil and has a lesser environmental impact. We observe that the selections of solvent type and injection strategy are the most crucial parameters for the design of a hybrid SAGD process, and solvent cost and toxicity can be minimized with the recycling of solvent for continuous injection of solvents. High-energy consumption for steam generation during the SAGD process can be reduced by coinjection of proper solvent type with steam at a proper injection strategy. Our study reveals that the ES-SAGD process has environmental and economic benefits that are preferable to those of the base SAGD. However, some solvents can cause undesirable effects because of asphaltene destabilization and precipitation in production or transportation lines. The results of this work show that not only asphaltenes but also the other fractions of oil, along with the reservoir-clay type and the clay amount, affect the ES-SAGD performance.
