Understanding how SAGD works is important as most of the in-situ production of bitumen in Alberta is using SAGD technology. A key parameter in simulation-based SAGD performance prediction is the residual oil saturation, which usually in SAGD simulations is a fixed number based on typical two-phase end-point relative permeability curves, combined with Stone's model to yield oil's relative permeability. Therefore, in simulations residual oil saturation (Sor) never goes below a certain number (typically 0.15-0.2). In reality, based on retrieved cores, there is evidence of Sor continuously decreasing to as low as 0.03 and below. This paper explains the reason for this discrepancy and suggests modifications to the relative permeability model for more realistic simulations.
Observations from retrieved cores suggest the residual oil saturation is dependent on the length of SAGD operation. This is also supported by Cardwell-Parsons correlation albeit for a two-phase system. The discrepancy between the current simulation results and actual observations point to the inadequacy of the relative permeability models containing the end points where krog and krog vanish at abscissas Sw, Sg less than 1. In this work effect of extending mobility of the oil phase all the way to Sw, Sg = 1 and krw, krg all the way to Sw, Sg = 0 is examined. First, a column drainage is numerically simulated, and relevant curve parameters are retrieved by comparing results against field results. These curves are then extended to simulate SAGD.
The results show that the modified relative permeability curves mimic the observed behavior better with residual oil phase saturation progressively decreasing with time, rather than remaining constant after a certain point. Improved correlation with observed saturations obtained using modified curves suggest that the fixed residual saturations resulting from current models are a myth. When extended to solvent aided processes, the model reinforces the benefit of solvent additives even further.
To the best of the authors’ knowledge most current SAGD simulations are done in a manner resulting in a fixed residual oil saturation. The proposed method presents an opportunity for better prediction of oil saturation with time and location and the corresponding performance of the SAGD process.