Identification of Enthalpy Injection Thresholds in Butane-Steam-Assisted Gravity Drainage

Summary n-Butane (C4) has been widely studied and benchmarked as a steam additive to enhance bitumen gravity drainage through a combination of heating and dilution. The main objective of this research was to present a systematic comparison of C4-steam-assisted gravity drainage (C4-SAGD) with varying levels of enthalpy injection at a given injection pressure, in which the steam injection rate was adjusted to a fixed solvent injection rate. The comparison, based on the isenthalpic specification, classified different regimes of C4-SAGD; for instance, there is a minimum enthalpy injection (MEI) above which C4-SAGD achieves at least a SAGD-like oil production rate and a limiting enthalpy injection (LEI) below which vapor-chamber expansion is unlikely to occur. C4-SAGD with an enthalpy injection rate between MEI and LEI results in a lower rate of oil production than SAGD with a substantially reduced enthalpy requirement. Numerical simulations were performed using a heterogeneous sector model representative of the Surmont project in Canada. The model had dimensions of 1475×198×60 m3 with a single well pair and spanned a total of 11 years, including a 3-month preheating period, a SAGD stage for 1 year and 9 months, and a C4-SAGD stage for 9 years. Six simulation cases were conducted, including one SAGD case and five C4-SAGD cases with C4 concentrations of 3.8 mol%, 5.9 mol%, 10.2 mol%, 28.2 mol%, and 64.5 mol%. A greater concentration of C4 yielded a lower enthalpy injection rate because the steam injection rate was reduced while the C4 injection rate was kept constant at 35 t/d (tonnes per day) at the injection pressure of 2.75 MPa for all cases. Simulation results indicated that MEI corresponded to 36.5% of the SAGD enthalpy injection rate by the coinjection of 89.8 mol% water (i.e., steam) and 10.2 mol% C4, and LEI corresponded to 12.4% of the SAGD enthalpy injection rate by the coinjection of 71.8 mol% water and 28.2 mol% C4. The MEI C4-SAGD case achieved a SAGD-like oil recovery, but the total enthalpy required for oil production was reduced by 58% after 11 years in comparison with SAGD. The fundamental novelty of this research lies in the solvent-aided (SA)-SAGD theory based on the isenthalpic specification, which enabled the systematic identification of MEI and LEI in C4-SAGD. The specification of enthalpy and solvent rates connects the SA-SAGD performance directly to the associated energy and solvent costs.