A novel steam-CO2 combination flooding process for oil recovery hasbeen investigated systematically using a specially designed experimentalsystem.
Numerous experiments have been performed using both non-fractured and fracturedsandstone cores. The experimental results from non-fractured and fracturedsandstone cores are very encouraging. The oil recovery increased as much as 20%for non-fractured cores and 18% for fractured cores of original oil in place bysteam-CO2 process compared to steam-alone process.
In addition to the effects of injection temperature and injection rates on oilrecovery and irreducible oil saturation, the ratio of steam overCO2, a dimensionless number, was found to dominate the oil recoveryprocess. In non-fractured sandstone cores, there exists an optimum ratio ofsteam over CO2 which yields a maximum oil recovery with the lowestirreducible oil saturation. The ratio does not depend on injection temperatureand injection rates. In fractured cores, there still exits an optimumsteam-CO2 ratio which yields a maximum oil recovery with lowestirreducible oil saturation. However, it depends on injection temperature.
The theory to explain this novel steam-CO2 flooding process stillneeds further study, some fundamental mechanisms which contribute to theprocess have been investigated in our research.
Naturally fractured carbonate reservoirs represent a unique target for theapplication of enhanced oil recovery (EOR) technology. High divalent ionconcentrations in reservoir water and extensive fracture networks discouragethe use of chemical and miscible processes with possible exception of miscible CO2. Because of channeling of injected air, in-situ combustion wouldbe difficult to sustain1. For non-fractured eservoirs steam floodingis a proven and commercially process for oil recovery (especially heavy oil).Its recovery mechanisms are so well identified that design and operation ofthese methods are successfully implemented. The state of understanding andtechnology is quite controversial in case of using the process for fracturedreservoirs.
Recently, at the University of Wyoming laboratory studies of this littleexplored research area of steam flooding in fractured reservoirs with thepurpose of understanding the basic mechanisms that control the recoveryprocesses were performed2, 3, 4, 5.The results obtained from these studies have been very encouraging n terms ofanswering the questions about the important process mechanisms controlling therecovery oil from fractured sandstone cores.
For homogeneous (non-fractured) reservoirs, laboratory studies and fieldreports6, 7, 8, 9, 10,11, 12 indicated that the use of certain additives(carbon dioxide nitrogen, flue gases, caustic, etc.) with steam could resultin improvement in oil recovery. These observations lead us to believe that theeffects of additives to steam flooding of fractured reservoirs would be veryuseful investigations for improving the success of steam flooding processes infractured reservoirs.
The existing steam flooding system was combined with facilities of injectingadditives (CO2Nitrogen, etc.), By using this new experimentalapparatus, numerous experiments were performed both in the non-fractured andfractured sandstone cores. This paper summarizes the related research and fieldtest results. The experimental details as well as the results ofsteam-CO2 flooding in fractured