The Myth of Residual Oil Saturation in SAGD-Simulations Against Reality

摘要

Understanding how SAGD works is important as most of the in-situ production of bitumen in Alberta isusing SAGD technology.A key parameter in simulation-based SAGD performance prediction is the residualoil saturation,which usually in SAGD simulations is a fixed number based on typical two-phase end-pointrelative permeability curves,combined with Stone's model to yield oil's relative permeability.Therefore,insimulations residual oil saturation(S_(or))never goes below a certain number(typically 0.15-0.2).In reality,based on retrieved cores,there is evidence of S_(or)continuously decreasing to as low as 0.03 and below.Thispaper explains the reason for this discrepancy and suggests modifications to the relative permeability modelfor more realistic simulations.Observations from retrieved cores suggest the residual oil saturation is dependent on the length ofSAGD operation.This is also supported by Cardwell-Parsons correlation albeit for a two-phase system.Thediscrepancy between the current simulation results and actual observations point to the inadequacy of therelative permeability models containing the end points where kr_(og)and kr_(og)vanish at abscissas Sw,Sg less than1.In this work effect of extending mobility of the oil phase all the way to Sw,Sg = 1 and k_(rw),k_(rg)all the wayto Sw,Sg = 0 is examined.First,a column drainage is numerically simulated,and relevant curve parametersare 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 withresidual oil phase saturation progressively decreasing with time,rather than remaining constant after acertain point.Improved correlation with observed saturations obtained using modified curves suggest thatthe fixed residual saturations resulting from current models are a myth.When extended to solvent aidedprocesses,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 resultingin a fixed residual oil saturation.The proposed method presents an opportunity for better prediction of oilsaturation with time and location and the corresponding performance of the SAGD process.