This paper presents the results of a comprehensive study to improve our understanding of high-mobility-ratio waterflood (HMRWF) and to improve performance prediction. Published data on heavy-oil water-injection field projects are limited. Several successful HMRWF projects have been reported, and they show significant oil recovery at high watercut. However, the range of reported recovery is large—waterflood recoveries of approximately 1 or 2% to 20% of original oil in place (OOIP) have been reported for similar reservoirs. Higher viscosities result in lower recovery.
Mechanistic studies using fine-scale simulations show that the viscosity (or mobility) ratio primarily controls oil recovery response, and that the recovery is lower at higher viscosity ratios. Further, viscous fingers dominate high-viscosity-ratio floods, and mobile water can significantly reduce recovery. Field-scale simulation results indicate that heterogeneity plays a more important role for a HMRWF than conventional waterfloods. The amount of primary production before the start of the waterflood has a larger effect on incremental oil recovery for high-mobility-ratio floods. Further, highly-correlated, thin, thief zones reduce recovery of HMRWF more severely, and rock wettability (relative permeabilties) strongly influences oil recovery. These results indicate that, in addition to reservoir geology, accurate viscosity and relative permeability measurements are essential for a reliable performance prediction.
Waterflood has been conducted in many high viscosity reservoirs in the past, and several water injection projects in high viscosity reservoirs are ongoing and planned around the world (Oefelein and Walker 1964; Jennings 1966; Roark 1960; Nelson 1976; Woodling et al. 1993; Jenkins et al. 2004; Adams 1982; Edgson and Czyzewski 1985; Kasraie et al. 1993; Smith 1992; Lim et al. 1993; Ko et al. 1995; Etebar 1995; Pallant et al. 1995; Foerster et al. 1997; Yang et al. 1998; Forth et al. 1996; Yao 1999; Hanafy 1999; Cook et al. 2000; Jayasekera and Goodyear 2000; Capeleiro Pinto et al. 2001). However, published data on HMRWF performance is limited and results are sometimes conflicting. Further, it has been postulated that some of the recovery mechanisms might be different (Cook et al. 2000). It is apparent from the literature that our understanding of HMRWF performance is inadequate.
This paper presents the results of a comprehensive study to provide improved insight into mechanisms governing HMRWF and to help to improve performance prediction. The specific objectives were to: (1) evaluate published field data, (2) determine key parameters that govern the process using analytical methods and fine-scale mechanistic numerical models, and (3) quantify the effects of reservoir heterogeneity on HMRWF field performance. Accordingly, the paper is divided into four parts. The first presents a review of published field data. Next, definitions of mobility ratio are examined and a preferred definition is proposed. This is followed by a fine-scaled mechanistic modeling of HMRWF to identify key parameters. Finally, field-scale simulations are conducted to delineate key differences between HMRWF and conventional waterflood.