Summary
Evaluation and improvement of sweep efficiency are important for miscible displacement of medium-viscosity oils. A high-pressure quarter-five-spot cell was used to conduct multicontact miscible (MCM) water-alternating-gas (WAG) displacements at reservoir conditions. A dead reservoir oil (78 cp) was displaced by ethane. The minimum miscibility pressure (MMP) for ethane with the reservoir oil is approximately 4.14 MPa (600 psi). Gasflood followed by waterflood improves the oil recovery over waterflood alone in the quarter five-spot. As the pressure decreases, the gasflood oil recovery increases slightly in the pressure range of 4.550-9.514 MPa (660-1,380 psi) for this undersaturated viscous oil. WAG improves the sweep efficiency and oil recovery in the quarter five-spot over the continuous gas injection. WAG injection slows down gas breakthrough. A decrease in the solvent amount lowers the oil recovery in WAG floods, but significantly more oil can be recovered with just 0.1 pore volume (PV) solvent (and water) injection than with waterflood alone. Use of a horizontal production well lowers the sweep efficiency over the vertical production well during WAG injection. Sweep efficiency is higher for the nine-spot pattern than for the five-spot pattern during gas injection. Sweep efficiency during WAG injection increases with the WAG ratio in the five-spot model.
Introduction
As the light-oil reservoirs get depleted, there is increasing interest in producing more-viscous-oil reservoirs. Thermal techniques are appropriate for heavy-oil reservoirs. But gasflooding can play an important role in medium-viscosity-oil (30-300 cp) reservoirs and is the subject of this paper. Roughly 20 billion to 25 billion bbl of medium-weight- to heavy-weight-oil deposits are estimated in the North Slope of Alaska. Approximately 10 billion to 12 billion bbl exist in West Sak/Schrader Bluff formation alone (McGuire et al. 2005).
Miscible gasflooding has been proved to be a cost-effective enhanced oil recovery technique. There are approximately 80 gasflooding projects (CO2, flue gas, and hydrocarbon gas) in the US and approximately 300,000 B/D is produced from gasflooding, mostly from light-oil reservoirs (Moritis 2004). The recovery efficiency [10-20% of the original oil in place (OOIP)] and solvent use (3-12 Mcf/bbl) need to be improved. The application of miscible and immiscible gasflooding needs to be extended to medium-viscosity-oil reservoirs.
McGuire et al. (2005) have proposed an immiscible WAG flooding process, called viscosity-reduction WAG, for North Slope medium-visocisty oils. Many of these oils are depleted in their light-end hydrocarbons C7-C13. When a mixture of methane and natural gas liquid is injected, the ethane and components condense into the oil and decrease the viscosity of oil, making it easier for the water to displace the oil. From reservoir simulation, this process is estimated to enhance oil recovery compared to waterflood from 19 to 22% of the OOIP, which still leaves nearly 78% of the OOIP. Thus, further research should be directed at improving the recovery efficiency of these processes for viscous-oil reservoirs.
Recovery efficiency depends on microscopic displacement efficiency and sweep efficiency. Microscopic displacement efficiency depends on pressure, (Dindoruk et al. 1992; Wang and Peck 2000) composition of the solvent and oil (Stalkup 1983; Zick 1986), and small-core-scale heterogeneity (Campbell and Orr 1985; Mohanty and Johnson 1993). Sweep efficiency of a miscible flood depends on mobility ratio (Habermann 1960; Mahaffey et al. 1966; Cinar et al. 2006), viscous-to-gravity ratio (Craig et al. 1957; Spivak 1974; Withjack and Akervoll 1988), transverse Peclet number (Pozzi and Blackwell 1963), well configuration, and reservoir heterogeneity, (Koval 1963; Fayers et al. 1992) in general. The effect of reservoir heterogeneity is difficult to study at the laboratory scale and is addressed mostly by simulation (Haajizadeh et al. 2000; Jackson et al. 1985). Most of the laboratory sweep-efficiency studies (Habermann 1960; Mahaffey et al. 1966; Jackson et al. 1985; Vives et al. 1999) have been conducted with first-contact fluids or immiscible fluids at ambient pressure/temperature and may not be able to respresent the displacement physics of multicontact fluids at reservoir conditions.
In fact, four methods are proposed for sweep improvement in gasflooding: WAG (Lin and Poole 1991), foams (Shan and Rossen 2002), direct thickeners (Xu et al. 2003), and dynamic-profile control in wells (McGuire et al. 1998). To evaluate any sweep-improvement methods, one needs controlled field testing. Field tests generally are expensive and not very controlled; two different tests cannot be performed starting with identical initial states, and, thus, results are often inconclusive. Field-scale modeling of compositionally complex processes can be unreliable because of inadequate representation of heterogeneity and process complexity in existing numerical simulators. There is a need to conduct laboratory sweep-efficiency studies with the MCM fluids at reservoir conditions to evaluate various sweep-improvement techniques. Reservoir-conditions laboratory tests can be used to calibrate numerical simulators and evaluate qualitative changes in sweep efficiency. We have built a high-pressure quarter-five-spot model where reservoir-conditions multicontact WAG floods can be conducted and evaluated (Dao et al. 2005). The goal of this paper is to evaluate various WAG strategies for a model oil/multicontact solvent in this high-pressure laboratory cell. In the next section, we outline our experimental techniques. The results are summarized in the following section.
Characteristics and quantitative study on gas breakthrough in developing Yaha-2 condensate gas reservoir in Tarim Basin, China
