Enhanced Heavy-Oil Recovery by Alkali-Surfactant Flooding

Abstract

This study presents the results of laboratory core studies investigating the recovery mechanisms of alkali-surfactant flooding in heavy oil reservoirs. Specifically, mixtures of water and alkali-surfactant systems have been injected into cores containing heavy oil (11 000 mPa×s and 15 000 mPa×s). Salinity is varied in order to generate oil-in-water vs. water-in-oil emulsion systems, and the effects of generating different emulsions are compared. The application of this work is for the many heavy oil reservoirs in countries such as Canada and Venezuela containing viscous oil that still has some limited mobility under reservoir conditions. Alkali-surfactant (AS) flooding has considerable potential for non-thermal oil recovery after primary production. Experiments were performed on cores with varying permeability, at different AS injection rates. All tests were performed on gas-free oil systems. The response from direct injection of AS systems is compared to AS injection after waterflooding. Pressure and oil recovery information is obtained from core floods, and these results are interpreted based on a semi-theoretical framework obtained from phase behavior and bulk liquid studies. It is demonstrated that both oil-in-water and water-in-oil emulsions can lead to the recovery of additional oil. Alkali-surfactant flooding is already an established technique in conventional oil reservoirs, whereby enhanced oil recovery is a result of reduced trapping of oil due to the lowered oil/water interfacial tension. In addition, the injection of these chemicals may lead to the formation of emulsions, as has been documented by previous researchers. In our work, we demonstrate that in heavy oil systems, emulsion formation is a necessary requirement for the production of heavy oil. When these emulsions form, AS injection can lead to considerable improvements in the flooding response, even without the addition of polymers to stabilize the flood. Introduction Several countries in the world, notably Canada and Venezuela, contain massive resources of unconventional heavy oil and and bitumen. With issues of resource stability and rising oil prices, international interest is now shifting rapidly towards Canada's oil sands. The oil sands are characterized as unconsolidated, high porosity and high permeability reservoirs. While ease of flow is therefore not a significant concern, the single biggest obstacle to successful recovery from the oil sands is the high oil viscosity. Heavy oil reservoirs are a special subset of the oil sands, whereby the oil viscosity at reservoir temperature and pressure varies on the order of 50 - 50 000 mPa×s (cP). While this oil is still very viscous, it does have some limited mobility at resevoir conditions. As much as 20% of the oil may be recovered by solution gas drive1, but in many cases the recovery is much lower. At the end of primary production, significant oil still remains in the reservoir, but the reservoir energy has now been depleted. This is the target for enhanced heavy oil recovery. In order to recover additional heavy oil after primary production, a fluid usually has to be injected in order to displace oil to the production wells. However, mobility ratio concerns dominate displacement of viscous oil, and most EOR processes focus on reduction of the oil viscosity or improvement in the mobility ratio. Unfortunately, many of the heavy oil reservoirs in Canada are relatively small and thin, making them poor candidates for expensive thermal processes. Ideally, the displacement mobility ratio should be improved in an inexpensive (i.e. non-thermal) fashion. This work investigates the potential for alkali-surfactant flooding to be used for enhanced heavy oil recovery. The injection of alkalis and/or surfactants into oil reservoirs is not a new technology. As early as in the 1920's, Nutting proposed the injection of alkaline solution into reservoirs for oil recovery2. The injection of a combination of alkali and surfactant was discussed in the 1950's by Reisberg and Doscher3. Since then, chemical injection (alkali and/or surfactant) has become an accepted enhanced oil recovery methodology in many conventional oil applications.

Stability Proxies for Water-in-Oil Emulsions and Implications in Aqueous-based Enhanced Oil Recovery

“…Recovery in chemical flooding of heavy-oil reservoirs have been shown to depend on flow rate, and consequently on the shear rate [5][6][7][8][9][10]. This is consistent with capillary number dependencies associated with emulsion flow [11].…”

“…The issue of mobility ratio or control arises because viscous water has much lower viscosity than heavy oil and consequently the displacement front develops fingers and oil recovery is generally poor. Alkali-surfactant (AS) flooding has been shown to be a potential viable technology for EOR in heavy-oil reservoirs [5][6][7][8][9][10]. However, in the absence of a thickening agent such as polymers, the increase in water saturation as water displaces oil only worsens the mobility control issue.…”

“…Bryan and Kantzas [6] state that emulsification, entrainment and entrapment of dispersed phase and wettability reversal mechanisms can contribute to oil recovery during the injection of alkaline agents and surfactants in heavy-oil reservoirs. Recovery in chemical flooding of heavy-oil reservoirs have been shown to depend on flow rate, and consequently on the shear rate [5][6][7][8][9][10].…”

Chemical Methods for Ugnu Viscous Oils

“…Generally speaking, addition of highly hydrophilic surfactants (high HLB), low salinity, and high WOR lead to O/W emulsions (Bryan, 2007). The goal of this work is to identify chemicals for forming oil-in-water emulsions with a heavy Alaskan oil and conduct displacement studies to understand the recovery of heavy oils.…”

Study of the influence of asphaltenes on modeling of viscosity of Nigerian crude mixtures

“…The high viscous nature of heavy oil, viscosity in the range of 1x10 4 cP, requires that the viscosity be reduced to approximately 400cP for its effective and efficient production and transportation. Researchers in this field have reported several techniques employed by industry to achieve reduction in heavy oil/bitumen viscosity such as the use of heating system in the formation and/or pipeline [1,2], addition of liquid diluents and injection of liquids or gaseous diluents into the formation, and the use of water and emulsion [3][4][5]. Although these methods, to an extent have been effective in reducing the oil viscosity the high costs of energy and diluents restrict their frequent applications.…”