Calibrating a Semi-Analytic SAGD Forecasting Model to 3D Heterogeneous Reservoir Simulations

Abstract The steam-assisted gravity drainage (SAGD) method is an efficient way of producing oil from many of Canada's Oil Sands reservoirs. Predicting oil production and steam injection rates is required for planning and managing a SAGD operation. While this can be done by simulating the fluid flow using commercially available thermal simulators, drainage area simulations can take days to run. This also involves significant expense for a business in the form of simulator license fees. For this reason, a proxy that reasonably predicts oil production and steam injection rates with low computational effort would be valuable. In this paper, a reliable proxy for predicting SAGD performance is developed. This model can handle different operating pressure strategies and uses a distinctive approach to capturing the impact of reservoir heterogeneity. The approach is an approximate solution using a semi analytical model based on relevant theories including Butler's SAGD theory. The model is orders of magnitude faster than full simulation and provides performance forecasts which have a level of accuracy suitable for many practical applications within an operating oil sands business. To capture the impact of near wellbore heterogeneity a novel parameter which accounts for the degree of near-well reservoir connectivity was developed. This parameter is calculated based on properties such as permeability, porosity and oil saturation inside an assumed 90-degree steam chamber above the producer. This parameter can take into account the distance of shale barriers above the producer which can act as a baffle/barrier for steam chamber development. First, this paper outlines the core theory underlying the model and then, by showing different examples will demonstrate its accuracy and applicability within an operating SAGD business. Results show strong correlation between simulated key production parameters such as peak oil rate and the parameter developed to account for near wellbore heterogeneity. By modifying the proxy to consider this parameter, the authors demonstrate a strong correlation between complex 3D thermal simulator results and this model in terms of oil production, cumulative steam oil ratio (cSOR) and instantaneous steam oil ratio (iSOR) predictions.