|Fig. 1: Schematic of Oil Recovery Phases.(Source: S. Aldourasry - after Donaldson et al. )|
Oil production is divided into three distinvictve phases which are primary, secondary and tertiary (Enhanced Oil Recovery, EOR). During the primary stage, the natural reservoir energy drives the production and in some instances artificial lift mechanisms are employed. The subsequent secondary production stage is aided by the injection of external fluid, commonly water or gas, into the reservoir. The secondary recovery aims towards maintaining the reservoir pressure and displacing hydrocarbons to the wellbore. Waterflooding remains the most prominent oil recovery method. The secondary recovery stage is discontinued when the injected fluid is produced in considerable amounts, which is a sign of approaching the economical limit. The combination of the primary and secondary methods produces up to 50% of the oil in place. Hence, there is still remaining oil trapped in the reservoir "residual oil"; for which, the role of Enhanced Oil Recovery (EOR) method arises. EOR is oil recovery by the injection of materials not normally present in the reservoir.  Fig. 1 is a schematic demonstrating the various stages of oil recovery.
This paper focuses on one critical aspect/measure during waterflooding stage, which is the Voidage Replacement Ratio (VRR). VRR is defined as the volume of injected fluid to the volume of the produced fluid. VRR is fundamental for comprehending the injection and production balance as it profoundly affects the pressure distribution within the reservoir and evidently the wells production rate. For a long time, complete voidage replacement is assumed to be optimal for all types of reservoirs and a common reservoir management practice. Meaning, the maximum oil recovery is generated when the amount of fluid injected equals the amount produced. However, recent papers have indicated otherwise. There are ongoing research efforts that target understanding the mechanism and impact of employing a VRR < 1 for different types of oil reservoirs.
The concept of Under Water Injection refers to maintaining a VRR < 1, at which the fluid injection volume is less than that of the volume produced. More recovery mechanisms are activated at VRR < 1 compared to VRR=1. Implementation of under water injection practice results in a pressure decline, which leads to dropping below the bubble point pressure (BPP) if continued for a sufficient period. Operating below the BPP triggers gas evolvement out of the oil solution or else known in the literature as solution gas drive. The concept of complementing waterflooding with solution gas drive to improve the oil recovery can be traced back to a paper by Dyes published in 1954. Dyes demonstrated experimentally the presence of an optimum pressure below BPP, at which additional oil recovery of 7-12 % is administered. He suggested that the use of both gas and water drives in recovery operations is superior to the use of either of the phases alone. 
Some significant mechanisms are observed when the pressure drops below the BPP and gas evolves out of solution as suggested by Vittoratos and West for heavy oil including: 
Oil depletion from water unswept reservoir volumes. Conventional waterfloods typically sweep the backbone of the reservoir i.e. preferred channels or communication paths where some fraction of the volume is not produced. Solution gas drive at VRR < 1 pushes the oil from these unswept reservoir regions.
Flow interference between the three phases, which results in preferential oil flow. The presence of gas slows down water flow and increases oil flow.
Chemical Changes which lead to the formation of emulsion flow or foamy oil, which are conditional to the oil properties.
Some of the negative attributes of this mechanism includes oil viscosity increase due to gas evolving out of solution, fast reservoir pressure decline i.e. losing reservoir energy if gas flows easily out of the reservoir, and subsidence might also occur in the case where rock compressibility is relatively large. [3,4]
There is still uncertainty and lack of understanding of the mechanisms involved in the waterflooding with VRR less than 1. Comprehensive studies at both micro and macro scales are required, simulation assisted studies will be of great benefit. Experiments conducted on micromodels allows visualization of fluid flow at the pore scale level and thus provides a better understanding and opportunity to investigate the parameters that improve the oil recovery. All factors impacting the fluid flow under such conditions shall be considered including oil chemistry and reservoir heterogeneity. Some reservoir management practices relevant to Voidage Replacement Ratio should be further examined and adapted accordingly.
Typical waterflood operations target injection at VRR equals to 1 or slightly higher. Simulation and laboratory experiments suggest that water flooding of heavy oil with periods of VRR in the range of 0.5 to 0.9 is optimal for oil recovery compared to continuous VRR=1 (Vittoratos and Delgado). Further work is required to validate the recent theoretical and experimental explanation. Most of the literature, when it comes to water injection below BPP at VRR <1, involves work pertinent to heavy oil; similar detailed research should also be extended to light oil too. Furthermore, Gas retention i.e. lower gas mobility within the reservoir after dropping below the BPP is the key towards achieving higher oil recovery in a combination of waterflood and solution gas drive mechanism with optimal VRR.
© Salem Aldousary. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 L. W. Lake., Enhanced Oil Recovery (Prentice Hall, 1996).
 A. B. Dyes, SPE 417-G, "Production of Water-Driven Reservoirs below their Bubble Point," One Petro SPE 417-G, J. Petrol Technol. 6, 31 (1954).
 E. S. Vittorato and C.C. West, SPE 129545, "Optimal Heavy Oil Waterflood Management May Differ from that of Light Oils," One Petro SPE 129545, 11 Apr 10
 D. E. Delgado, S. E. Vittorato and A. R. Kovscek, "Optimal Voidage Replacement Ratio for Viscous and Heavy Oil Walterfloods," One Petro SPE 165349S, 19 Apr 13.
 E.C. Donaldson, G.V. Chilingarian, T.F. Yen, Enhanced Oil Recovery, II: Processes and Operations (Elsevier, 1989).