Summary
Steam-assisted gravity drainage (SAGD) is the preferred thermal-recovery method used to produce bitumen from Athabasca deposits in Alberta, Canada. In SAGD, steam injected into a horizontal injection well is forced into the reservoir, losing its latent heat when it comes into contact with cold bitumen at the edge of a depletion chamber. Heat energy is transferred from steam to reservoir, resulting in reduced bitumen viscosity that enables the bitumen to flow toward the horizontal production well under gravity forces. Conduction is the main heat-transfer mechanism at the edge of the steam chamber in SAGD, and reservoir thermal conductivity is a key parameter in conductive-heat transfer. Conductive-heat transfer occurs at higher rates across reservoirs with higher thermal conductivity, which in turn affects the temperature profile ahead of the steam interface. Consequently, a reservoir with higher thermal conductivity will result in higher reservoir-heating rates, which lead to higher oil rates. However, when oil-sand reservoirs are heated from reservoir temperature to steam-chamber temperature, the thermal conductivity can decrease up to 25%, which affects the temperature profile and conductive heating at the edge of the steam-saturated zone known as the steam chamber. This study provides an analytical model that includes a temperature-dependent thermal-conductivity value. This novel approach is the first of its kind to incorporate a temperature-dependent thermal-conductivity value within an analytical SAGD model to predict temperature front, oil production, and steam/oil ratio (SOR).
Furthermore, if Butler's (1985) model is used, the results reveal that the arithmetic average thermal-conductivity values at reservoir and steam temperatures could be used for temperature-profile prediction, which would result in an error of less than 1% for the range of SAGD applications. The results of this study suggest that the minimum error for oil rates depends on the viscosity/temperature correlation. The optimum thermal conductivity should be calculated at the temperature that gives dimensionless temperatures [i.e., (T−Tr)/(Tst−Tr)] varying between 0.75 to 0.85 for m-values [Butler-suggested power constants (Butler 1985, 1991; Butler and Stephens 1981)] between 3 and 5.6. This study also investigates the effect of including temperature-dependent thermal conductivity on SOR variation and suggests that for both laterally expanding and angularly expanding reservoirs the SOR is independent of the thermal conductivity.
