The oil saturation inside the steam chamber during steam-assisted gravity drainage (SAGD) has an important impact on economics and conservation. However, the SAGD residual oil saturation is difficult to determine and model because it involves long-term thermal effects and three-phase flow. SAGD has been widely studied and piloted, but improved understanding of long-term drainage effects represents a fundamental issue that requires improved understanding.
In the first part of the paper, we present a sensitivity test done on the shapes and the endpoints of the two-phase relative permeability curves. We find that the water relative permeability and oil relative permeability in the gas-oil system are the main factors that determine the magnitude and shape of the oil saturation curve as a function of time.
Secondly, we demonstrate how to adjust the krog relative permeability curve to match a theoretically determined residual oil saturation, which is supported by laboratory data. We propose that, during SAGD, the flow of oil can be split into two regimes. In the first regime, close to the edge of the steam chamber, oil drains quickly in a short period of time. In the second regime, oil drains slowly within the steam chamber for a longer period of time as it is produced by "film drainage". To capture these flow regimes, the oil relative permeability curve in the gas-oil system, krog, is split into two. At higher liquid saturations, the first flow regime is represented and at lower liquid saturations, the second regime.
Thirdly, the krog curve was adjusted so that the decrease in oil saturation with respect to time closely matched the theoretical curve while maintaining oil production rates expected for SAGD. Using this new curve at different pressures, we show that the residual oil saturation increases at lower SAGD operating pressures.
In a steam-assisted gravity drainage (SAGD) process, bitumen drainage occurs mainly along the transition zone, which is the mobile liquid region at the boundary of the steam chamber. Bitumen drainage also continues to occur within the body of the steam chamber over a long period of time such that the residual oil saturation gradually falls.
In this paper, the term residual oil saturation refers to the average oil saturation within the steam chamber where the steam chamber includes all grid blocks in a numerical model containing any amount of steam. It is difficult to obtain residual oil saturation data as a function of time in the field so, in this study, we relied on theoretical and laboratory data. We calibrated reservoir simulation results to the theoretical and laboratory data and then used the simulator to investigate the effects of SAGD operating pressures on the residual oil saturation.
Multiphase flow in a porous medium can be described using Darcy's Law as follows: Equation (1) (Available in full paper)
where kr denotes the relative permeability of a phase. Relative permeability is a function of saturation only.