Optimal Waterflood Voidage Management Significantly Increases Oil Recovery with Minimal Incremental Cost

Abstract

Abstract Empirical data, laboratory experiments, and mathematical simulation methods indicate that for many heavy oil waterfloods the optimal voidage replacement ratio (VRR) is less than one. Many more recovery mechanisms are activated with VRR<1 than with VRR = 1. Some are readily understood with existing conventional reservoir engineering concepts, while others invoke emulsion & foamy oil multiphase flows that are activated under changing reservoir conditions. Not all the consequences of VRR<1 are positive for recovery. In aggregate, however, as displayed in empirical observations, VRR<1 can result in a significant increase in reservoir oil recovery, particularly for the heavier oils. This paper focuses on the quantification of the relative importance of the mechanisms activated by VRR<1 in saturated or nearly saturated reservoirs. The absolute value of the VRR<1 depends primarily on the oil quality and its associated properties, whereas the optimal time evolution of the VRR depends on the well spacing and the reservoir heterogeneity of the depositional environment. Numerical simulations have ranged from simple 1D models to a detailed fluvial depositional environment model. The role of relative permeabilities are demonstrated by the 1D model. The 3D field models of increasing geological complexity are useful for quantifying the incomplete waterflood sweep, and the role of VRR < 1 in activating solution gas drive to deplete the ‘cul-de-sacs’ of bypassed oil. We also propose here a numerical model to incorporate emulsion flow behavior into heavy oil water flooding, calibrated using data from a large scale (‘big can’) experimental waterflooding study of a heavy oil prone to emulsion formation. The methodologies developed in this study show that for heterogeneous heavy oil reservoirs the incremental recovery expected from an optimized VRR < 1 process is similar to that of other commercial IOR processes such as polymer flooding, but with very little incremental cost.