Gas-based enhanced oil recovery (EOR) processes rely on the injection of gases such as carbon dioxide, nitrogen, and natural gas into heavy oil reservoirs to reduce inherent oil viscosity. Although these processes are very promising, they face the problem of limited and costly gas supply.
This study investigates the conditions, specifically temperature variation, under which freely available air at low temperatures, low pressures, and non-reactive environments for heavy oil recovery. To that end, preliminary experiments are carried out to demonstrate the possibility of beneficial effects of air temperature variation with time. Furthermore, this research aims to utilize the theory of optimal control to determine optimal air temperature versus time function to maximize the heavy oil recovery. For this purpose, the conditions necessary for optimal control are derived and utilized in a computational algorithm.
The preliminary experiments are executed by injecting air into a lab-scale heavy oil reservoir at different pressures (0.169-0.514 MPa absolute) and temperatures in the range of 25-90oC. Reservoirs of four different permeabilities (40-427 Darcy) are used in experiments. When air is injected with a periodic temperature variation between 90oC and 75oC that has an average of 78oC, the recovery is increased from 58.2% to 69.1% of the original-oil-in-place (OOIP) in comparison to that using constant temperature air injection at the maximum temperature of 90oC. That is a considerable improvement of oil recovery by 18.6%.
Furthermore, utilizing optimal control the optimal interfacial temperature versus time (control policy) is determined between 90oC and 82oC, which registers 20.66% increase in the oil recovery in comparison to that at the constant temperature of 90oC. The accuracy of optimal control is experimentally validated. The results show that the average relative difference between the predicted heavy oil recovery and the experimental value is a low value of 1.82%.