The goal of this work is to evaluate sweep efficiency of various miscible flooding processes in a laboratory model, develop numerical tools to estimate sweep efficiency at the field scale, and identify solvent composition, mobility control method, and well architecture that improve sweep efficiency.

University of Houston, Houston, TX

Miscible and near-miscible gas flooding has proven to be one of the few cost-effective enhanced oil recovery techniques in the past 20 years. However, the sweep efficiency of such processes often is not high because of the adverse viscosity ratio and density difference between the solvent gas (often carbon dioxide) and the oil, coupled with reservoir heterogeneity. WAG processes often are used to improve sweep efficiency. Foams and direct thickeners have been developed and tested but not used routinely in the field. The effect of new well architectures on sweep efficiency is poorly understood. As the scope of miscible flooding is being expanded to medium-viscosity oils in shallow sands in Alaska and shallower reservoirs in the Lower 48, questions about sweep efficiency in near-miscible regions remain unanswered.

Results 
A high-pressure quarter five-spot cell has been constructed to conduct multicontact miscible water-alternating-gas (WAG) displacements at reservoir conditions. Multicontact miscible solvents were identified by conducting slimtube experiments for a medium-viscosity oil (78 cp). Coreflood experiments were conducted to determine microscopic displacement efficiency as a function of WAG ratio. Quarter five-spot experiments were conducted to infer sweep efficiency in a 3-D geometry at the laboratory scale. Gravity and compressibility terms were added to a compositional 4-phase streamline simulator. A parallel version of the simulator was coded and tested. Miscible or partially miscible solvents can be injected by a horizontal well and oil can be produced by a parallel horizontal well in the vapex mode if the vertical permeability is high. This has been tested in the high pressure cell. Surfactant alternating gas injections have been performed in a 1D sand pack to characterize foam formation and transport.

WAG improves the microscopic displacement efficiency compared to a program of continuous gas injection followed by waterflood in corefloods. WAG improves the sweep efficiency in a quarter five-spot compared to continuous gas injection followed by waterflood. Gas breakthrough is delayed by WAG injection. A decrease in the slug size improves the sweep.

Above minimum miscibility pressure (MMP)—as the reservoir pressure decreases—the gasflood followed by waterflood recovery increases, possibly because of high oil viscosity. An inverted nine-spot improves gasflood oil recovery slightly vs. a five-spot. In WAG floods, as the volume of total solvent injected drops, so does oil recovery, but not very drastically. Use of a horizontal production well lowers the sweep over a vertical production well during both continuous gas injection and WAG floods.

Simulation shows that gravity override increases as the gravity-to-viscous ratio increases; breakthrough sweep efficiency is higher for MCM cases than for FCM cases. The streamline simulator gives almost linear speedup with the increase in processor for the streamline part of the computation. Overall speedup was constrained by the finite difference part of the simulation. Vapex mode injection of ethane shows that the oil recovery is higher and gas breakthrough is later than the five-spot mode injection. Effects of gas composition and vertical spacing (between the horizontal wells) on recovery in vapex mode experiments were determined. As the surfactant slug to gas ratio increased, the foam strength decreased in the parameter range studied.

Benefits 
The experimental data on sweep efficiency help evaluate multicontact miscible flooding processes at the laboratory scale. Reservoir simulators should be tuned to such experiments before being used for field-scale process optimization. The methodology developed in this project would help improve the design of miscible oil recovery projects, leading to higher recoveries of residual oil.

Summary 
This project is aimed at evaluating the sweep efficiency at the laboratory scale, developing numerical tools to estimate sweep efficiency at the field scale, and identifying parameters to improve sweep efficiency.

The major elements of this project are to:

• Construct a high-pressure quarter five-spot physical laboratory model,
• Evaluate sweep efficiency for WAG floods for one Alaskan oil and solvent combination,
• Streamline Model WAG floods using a streamline compositional simulator that accounts for three-hydrocarbon phase behavior, gravity, and compressibility, and
• Evaluate the effect of horizontal wells, slug size, amount of solvent, and well pattern on sweep efficiency.

(December 2008)
This project has been completed and the final report is listed below under "Additional Information".

Funding 
This project was selected in response to DOE’s Oil Exploration and Production solicitation DE-PS26-04NT15450, February 2004.

$618,017 

$157,634 (20% of total)

NETL - Chandra Nautiyal (chandra.nautiyal@netl.doe.gov or 918-699-2021)
University of Houston - Kishore K. Mohanty (mohanty@uh.edu or 713-743-4331)

Final Project Report  [PDF]

Publications
Bhambri, P. and Mohanty, K.K., Two- and three-hydrocarbon phase streamline-based compositional simulation of gas injections, submitted to J. of Petroleum Science & Engineering.

Bhambri, P., A Three-Dimensional Four-Phase Compositional Simulator with Parallel Implementation, PhD Thesis, University of Houston, 2007.

Lewis, E. J., Experimental Study of Sweep Efficiency in Miscible Gas Injection Processes, MS Thesis, University of Houston, 2006.

Lewis, E., Dao, E.K., and Mohanty, K.K., Sweep Efficiency of Miscible Floods in a High Pressure Quarter Five-Spot Model, SPE 102764, SPE Annual Technical Conference and Exhibition, San Antonio, TX, September 24-27, 2006.

Dao, E.K., Lewis, E., and Mohanty, K.K., Multicontact Miscible Flooding in a High-Pressure Quarter Five-Spot Model, SPE 97918, SPE Annual Technical Conference and Exhibition, Dallas, TX, October 9-12, 2005.

Bhambri, P. and Mohanty, K.K., Streamline Simulation of Four-Phase WAG Processes, SPE 96940, SPE Annual Technical Conference and Exhibition, Dallas, TX, October 9-12, 2005.

Kumar, K., Dao, E., and Mohanty, K.K., AFM Study of Mineral Wettability with Reservoir Oils, Journal of Colloid and Interface Science, 289, 2005, pp. 206-217.