Viscous Oil Displacement via Aqueous Associative Polymers

Abstract

Abstract This study investigates the pore-level displacement of medium viscosity oil (200 cP) by brine and aqueous solutions of associative polymers. Associative polymers result in greater aqueous phase viscosities at the same concentration as conventional polymers. Studies are conducted in two-dimensional etched-silicon micro-models under a reflected light microscope. The pore network pattern of the micro-model replicates Berea sandstone. Results include the sweep pattern, oil recovery, and the pore-level distribution of residual oil. Generally, we find that brine and conventional polymer solutions at low concentrations result in severe fingering of the displacing fluid through the oil phase. Associative polymers lead to more stable displacement characteristics, apparently due to greater phase viscosity. Additionally, injection of associative polymers after breakthrough of brine mitigates fingering and improves viscous oil displacement. Experimental results show that associative polymers are a promising method to improve the displacement efficiency of viscous oils. Introduction Waterflooding accounts for about half of all oil recovered, but is generally limited to lighter oils with relatively low in-situ viscosity. A large number of fields holding viscous crude oil exist world-wide. These fields suffer from low recovery factors due to unfavorable mobility ratios in addition to low oil-phase mobility. Application of water injection for viscous oil recovery suffers from the high mobility of water leading to unstable displacement (Riaz et al., 2007). Heterogeneities in reservoir rock exacerbate unstable displacement. Nevertheless, for some situations such as Arctic and offshore reservoirs with viscous oils, there are perceived to be relatively few recovery process options except a water-based injectant. Addition of polymer to injection water reduces injected-phase mobility and provides a first-order solution to the problem of unstable displacement. Injection of viscous aqueous polymer solutions to improve volumetric sweep efficiency is a relatively mature concept. The extensive survey of Manning et al (1983) summarized field results of more than 250 polymer augmented water floods. Over the past decade, interest in polymer flooding has seen a resurgence and the oil volumes produced that are attributed to polymer flooding have grown, Principally, in the Daqing field (China), more than 250,000 bbl/d are produced by polymer injection and incremental oil recovery of up to 14 % is reported (Chang et al., 2006; Yupu and He, 2006). The mechanisms of polymer enhanced oil recovery have been studied with various methods and on various scales. Hele Shaw cells were used to visualise displacement of unfavorable mobility ratio floods (Benham and Olson, 1963; Allen and Boger, 1988). The processes involved in unstable flooding have been described theoretically (Sorbie et al., 1987; Araktingi and Orr, 1993) and examined experimentally (Tang and Kovscek, 2005; Riaz et al., 2007). The advantages of a stable displacement on volumetric sweep have been shown (for example) via streamline simulation (Wang et al, 1999) and field applications of polymer floods were simulated to improve interpretation of flood dynamics (Takaqi et al., 1992). A major cost for polymer injection projects is that of the polymer. In a typical application, 1 kg of polymer may be required to produced 1 m3 of incremental oil (i.e., 2.84 bbl oil / lb polymer) (Lake, 1989) Hence, an economical polymer should be injected resulting in the greatest oil recovery at the lowest polymer concentration.