A Steam Injection Rate Allocation Method Considering the Dynamic Heterogeneity in Stratified Heavy Oil Reservoirs

Abstract The improvement of production profiles in stratified heavy oil reservoirs holds paramount significance within the domain of improved oil recovery. The separate-layer steam injection technique is an effective approach to minimize the recovery difference among layers. However, the accuracy of steam allocation is hampered by the dynamic heterogeneity after long-time steam flooding. This paper proposes an improved injection rate allocation model for stratified heavy oil reservoirs with a separate-layer steam injection process. Considering the time variable phenomena of temperature and water saturation in reservoirs during a steam flooding process, the classical Buckley-Leverett (BL) displacement theory is extended to establish an injection rate allocation optimization strategy for stratified heterogeneous heavy oil reservoirs. First, the water saturation at an oil-water front and the average water saturation (AWS) in a two-phase region are determined. Then, the layers occurring (hot) water breakthrough are identified. Furthermore, the water saturation at the outlets of the layers occurring water breakthrough is calculated, and the location of an oil-water front in pre-breakthrough layers is obtained. Finally, the injection rates of these two-type layers are programmed by assuming an equivalent water saturation at each outlet. The reservoir properties and injection parameters in an actual heavy oilfield are input into the optimization program. Field data shows that a permeability heterogeneity can highly affect the temperature and water saturation in layers. The results demonstrate that a high-permeability layer (HPL) occurs water breakthrough, in which the current water saturation exceeds the AWS in a two-phase region. Moreover, affected by a variation of temperature, a fractional flow curve in a layer with a low water-oil viscosity ratio (WOVR) is more convex, while a fractional flow curve in a layer with a high WOVR is more concave. As the water saturation in a layer before adopting the separate-layer steam injection technique increases, the optimized injection rate per unit reservoir thickness decreases. The injection rate allocation is found to be a strong function of separate-layer injection time. Furthermore, a decrease in the injection time results in a greater contrast of injection rate allocation. Based on the programmed optimization code, the contrasts of injection rate allocation among layers for 5 years and 10 years are 9.2 m3/(d·m) and 8.3 m3/(d·m), respectively. This research extends the application scope of the Buckley-Leverett displacement theory into a non-isothermal displacement process. It provides valuable insights for designing suitable injection rate allocation in stratified heterogeneous heavy oil reservoirs.