PREDICTING WATERFLOOD RECOVERY PERFORMANCE

Based on a workshop sponsored by PTTC’s Midwest Region on February 17-21, 1997, in Evansville, IN.

BOTTOM LINE

For waterfloods, an ongoing prediction and surveillance program can determine ultimate recovery and improve timing of events, such as initial water breakthrough and peak production. This helps maximize oil recovery at the lowest water-oil ratio and operating cost level.

PROBLEM ADDRESSED

Waterflooding is the most successful and widely used enhanced oil recovery process. This is because water is widely available and inexpensive relative to other fluids, easy to inject, and highly efficient in displacing oil. There are several reservoir engineering aspects to consider in successfully calculating expected waterflood recovery performance.

KEY WORDS:

Waterflood, Water Injection, Waterflood Surveillance, Displacement, Breakthrough

SPEAKERS

James T. Smith, James T. Smith & Associates

William M. Cobb, William M. Cobb & Associates

TECHNOLOGY OVERVIEW

Oil recovery can be determined at any time in the life of a waterflood if four factors are known:

• First, the amount of oil-in place (OIP) at the start of a waterflood must be identified. This is a function of the oil saturation and floodable pore volume, which is highly dependent on net pay discriminators such as permeability and porosity cutoffs.
• Second, the areal sweep efficiency (the percentage of the reservoir area that the water will contact) must be known. It primarily depends on the relative flow properties of oil and water, the pattern and pressure distribution between injection and production wells, as well as directional permeability.
• Third, the vertical sweep efficiency (the fraction of a formation in a vertical plane that the water will contact) must be identified. Vertical sweep efficiency depends primarily on the degree of reservoir stratification.
• Fourth, the displacement sweep efficiency must be known. This represents the fraction of oil which water will displace in the invaded zone. 

LESSONS LEARNED

1. Waterflood recovery (Np) can be computed at any time in the life of a project by using the formula: Np = N * EA * EV * ED .
   
   N represents the oil in place in the floodable pore volume at start of water injection
   
   EA (areal sweep efficiency) represents the fraction of floodable pore volume area swept by the injected water
   
   EV (vertical sweep efficiency) represents the fraction of the floodable pore volume in the vertical plane swept by the injected water
   
   ED (displacement efficiency) represents the fraction of oil saturation at the start of water injection which is displaced by water in the invaded zone.
   
   
2. In addition, waterflood recovery is dependent on a number of other technical variables. Some of the more common include: oil saturation at the start of waterflooding, residual oil saturation, connate water saturation, and free gas saturation at the start of water injection. Also affecting recovery rates are the water-floodable pore volume, oil and water viscosity, and effective permeability to oil measured at the immobile connate water saturation, as well as relative permeability to water and oil. Other determining factors involve reservoir stratification, waterflood pattern, pressure distribution between injection and producing wells, injection rate, and oil formation volume factor.
   
3. Monitoring performance is particularly important, since the engineering aspects of waterflooding are continuous. There are several monitoring methods and procedures.

Forecasts may differ from actual production due to inaccurate fluid saturations, rock properties, or geological descriptions (stratification, permeability distribution, or continuity). Operational considerations, such as the timing of well conversions, also can affect forecasts even if the reservoir database is accurate. Production curves are valuable for monitoring and detecting changes in well or reservoir performance. The extrapolation of oil cut, the water-to-oil ratio (WOR), and X-value curves can help estimate future production when water cut exceeds 65 to 70 percent. Pressure transient testing can be used to monitor changes in formation damage, reservoir pressure, and parting pressure gradients. There are several common pressure tests used in surveillance, including buildup, falloff, step rate, and the Hall plot. Pulse tests or interference tests occasionally are run in large projects to determine the pressure communication between wells and to estimate interwell rock properties.

Significant improvements in efficiencies in large, multi-well patterns can be achieved through the careful management of flood injection rates in each waterflood pattern. Balancing injection and production rates within and between patterns can substantially reduce produced water, improve long-term production rates, and enhance ultimate recovery.

Stratification in reservoirs can create major problems, leading to low vertical sweep efficiencies. Waterflood prediction techniques assume all injection water enters the various layers in proportion to the flow capacity of each layer. Although all layers flood simultaneously, injection water seeks zones of highest permeability. Therefore, water must be injected into each layer based on the displaceable hydrocarbon pore vol ume for ideal injection. Thin, high-permeability channels in stratified reservoirs prevent efficient flooding of other zones. Injection profiles identify perforations that are not taking injected water, possibly because the perforations are plugged. These profiles also may reveal injectivity contrasts within a perforation set. A failure to correct for high concentration of injection over small intervals (thief zones) can result in poor vertical sweep, lower oil recovery, and increased water production. An injection profile should be conducted every six months during the first two years of a well’s injection life, annually thereafter, but within 30 to 60 days of any remedial work.

To ensure that injection into each layer is in proportion to its displaceable hydrocarbon pore volume, profile modification may be required. Some techniques that have been successfully used include: selective perforating, low-pressure squeeze cementing, acidizing, fine-sand injection, and polymer injection. The benefits of profile modification may not be observed for several months at the producing well.

CONNECTIONS:

James T. Smith
James T. Smith & Associates
PO Box 1990, Cody, WY 82414
Phone 307-527-6494, Fax 307-527-6688

William M. Cobb
William M. Cobb & Associates
12770 Coit Rd., Suite 907, Dallas, TX 75251
Phone 972-385-0353, Fax 972-788-5165, E-mail wcobbassoc@aol.com

For information on PTTC’s Midwest Region and its activities contact:
David G. Morse, Petroleum Geologist, Oil and Gas Section,
Illinois State Geological Survey Natural Resources Bldg.,
615 E. Peabody Dr., Champaign, IL 61820
Phone 217-244-5527, Fax 217-333-2830, E-mail morse@geoserv.isgs.uiuc.edu

Disclaimer: No specific application of products or services is endorsed by PTTC. Reasonable steps are taken to ensure the reliability of sources for information that PTTC disseminates; individuals and institutions are solely responsible for the consequences of its use.

The not-for-profit Petroleum Technology Transfer Council is funded primarily by the US Department of Energy’s Office of Fossil Energy, with additional funding from universities, state geological surveys, several state governments, and industry donations.

Petroleum Technology Transfer Council, 2916 West T. C. Jester, Suite 103, Houston, TX 77018
Toll-free 1-888-THE-PTTC; Fax 713-688-0935; E-mail hq@pttc.org; web www.pttc.org