Sweep Efficiency of Miscible Floods in a High-Pressure Quarter Five-Spot Model
Authors: Lewis Ed , Dao Eric , Mohanty Kishore Kumar 
Published: December 2007

Abstract
Evaluation and improvement of sweep efficiency are important for miscible displacement of medium viscosity oils. A high-pressure quarter 5-spot cell was used to conduct multicontact miscible water-alternating-gas (WAG) displacements at reservoir conditions. A dead reservoir oil (78 cp) was displaced by ethane. The minimum miscibility pressure for ethane with the reservoir oil is between 630 and 1000 psi. Gasflood followed by waterflood improves the oil recovery over just waterflood in the quarter 5-spot. As the pressure decreases, the gasflood oil recovery increases in the pressure range of 660–1380 psi for this undersaturated viscous oil. WAG improves the oil recovery in the quarter 5-spot over the continuous gas injection followed by waterflood. WAG injection slows down gas breakthrough. A decrease in the solvent amount lowers the oil recovery in WAG floods, but significantly higher amount of oil can be recovered with 0.1 PV solvent injection over only waterflood. Use of a horizontal production well lowers the oil recovery slightly over the vertical production well during WAG injection. Sweep efficiency is higher for lower reservoir pressure (in this undersaturated oil), for 9-spot pattern, and for 1:1 WAG injection compared to continuous gas injection followed by waterflood in the 5-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 gas flooding can play an important role in medium viscosity (30–300 cp) oil reservoirs, and is the subject of this paper. In the North Slope of Alaska alone, there is 10–12 billion barrels of oil in West Sak/Schrader Bluff formation alone.1 Similar reservoirs are also not developed in the Lower 48 states.
Miscible gas flooding has been proven to be a cost effective enhanced oil recovery technique. There are about 80 gasflooding projects (CO2, flue gas and hydrocarbon gas) in US and about 300,000 b/d is produced from gas flooding, mostly from light oil reservoirs.2 The recovery efficiency (10- 20% OOIP) and solvent utilization (3–12 MCF/bbl) need to be improved. The application of miscible and immiscible gas flooding needs to be extended to medium viscosity reservoirs.
McGuire et al.1 have proposed an immiscible wateralternating-gas flooding process, called VR-WAG (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 NGL (natural gas liquid) is injected, the C2+ 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 the oil recovery by 16% over that of just waterflood (22% vs. 19%). Still, 78% of the oil is expected to be left behind after this VR-WAG process.
Thus further research should be directed at improving the recovery efficiency of these processes for visous oil reservoirs. Recovery efficiency depends on microscopic displacement efficiency and sweep efficiency. Microscopic displacement efficiency depends on pressure,3,4 composition of the solvent and oil5,6 and small (core) scale heterogeneity.7,8 Sweep efficiency of a miscible flood depends on mobility ratio,9–12 viscous-to-gravity ratio,13–15 transverse Peclet number,16 well configuration, and reservoir heterogeneity,17–18 in general. The effect of reservoir heterogeneity is difficult to study at the laboratory-scale and is addressed mostly by simulation.19–20 Most of the laboratory sweep efficiency studies9,10,20,21 have been conducted with first-contact fluids or immiscible fluids at ambient pressure / temperature and may not be able to represent the displacement physics of multicontact fluids at reservoir conditions.