Effects of Lean Zones on SAGD Performance

As a large amount of mobile water exists in the lean zones of oil sands reservoirs, the analytical model considering mobile water can assist in the study of the mechanism on how lean zones affect SAGD performance. In 1981, Butler proposed a theory on the prediction of heavy oil and bitumen production [S1]. In 1991, Butler further obtained the transient expression for temperature distribution during SAGD production [S2]. In 2011, Sharma obtained the results for the expressions of conductive and convective heat fluxes based on a quasi-steady-state assumption and a simplified Carslaw and Jaeger’s equation [S3]. In 2013, Irani and Sahar modified Sharma’s result, obtained the velocity of the condensate, and updated the expression of heat flux [S4]. In 2013, Irani and Gates took the outflow convection into consideration and modified the heat flux equations [S5]. In 2015, Ji et al coupled the thermal expansion of connate water into the expression of heat flux [S6]. However, the role of the initial mobile water and time effect has not been studied in previous analytical models [S1–S6]. We also perform the analytical study as shown in the Supplementary Material. Owing to a large volume of mobile water in lean zones, the effect of mobile water on SAGD performance must be discussed first, to understand the effect of lean zones on such a performance. Analytical modelling is a good way to understand such an effect in homogeneous porous media. A previous understanding of the role of mobile water in SAGD performance is fully dependent on steady state flow. A transient solution is generated to explore the mechanisms of SAGD production in oil-sand reservoirs with mobile water changes in time. The analytical solutions presented in this section have prompted the current steady single-phase analytical method to become transient, with two-phase and free water saturation. In the analytical model, the outflow convection is omitted, but is considered in the numerical model [S7,S8].