A Normalized Analytical Model for Instantaneous Steam/Oil Ratio of the Steam-Assisted Gravity Drainage Process and Its Applications in Athabasca Oil Sands

Summary Steam-assisted gravity drainage (SAGD), introduced by Butler et al. (1981), has demonstrated the commercial viability of oil sands development in Western Canada since the late 1980s. Nowadays, SAGD is widely applied as the primary thermal recovery method for heavy oil and oil sands resources in many commercial projects. Many initial development areas from these projects have accumulated over 10 years of production and are approaching their ultimate SAGD recovery factors. Based on the production data from these projects, Wang (2024) developed a simple normalized analytical model for the oil rate of the SAGD process. The model considers the normalized oil rate as a function of the normalized recovery factor. To forecast the steam injection rate of the SAGD process, a normalized analytical model for instantaneous steam/oil ratio (iSOR) is developed in this paper by extending the model of Edmunds and Peterson (2007) to the chamber rising and the chamber falling stages. In the new model, the normalized iSOR is also expressed as a function of the normalized recovery factor for all three stages of the SAGD process, including chamber rising, chamber spreading, and chamber falling. The new model demonstrates reliable results by validating it using field data and comparing it with existing analytical models. The new normalized analytical model for iSOR can be combined with the normalized oil rate model to forecast both the oil rate and iSOR for the SAGD process. By coupling these models with the five-component recovery factor method (Society of Petroleum Evaluation Engineers 2018), the new analytical models can be used for high, best, and low reserve estimations with corresponding steam rate forecasts and iSOR as the economic cutoff. Moreover, the new models also provide solutions for converting different cutoffs for the economic limit of SAGD projects. By running a Monte Carlo simulation, the new analytical models demonstrate the capability to capture the uncertainty of the oil rate and steam/oil ratio (SOR) forecast for the project at different stages of the SAGD process. The new model extends the existing models and provides a practical tool for reservoir engineers and reserve evaluation engineers to do quick production forecasts and reserve estimation with reasonable simplifications when limited data or time is available.