Fact Sheet - Underground Injection for Increasing Oil Recovery
Reinjection into an underground formation represents the most commonly used approach for onshore management of produced water. Some produced water is injected solely for disposal. Most produced water is injected to maintain reservoir pressure and hydraulically drive oil toward a producing well. This practice can be referred to as enhanced oil recovery (EOR), water flooding, or, if the water is heated to make steam, as steam flooding. In the context of improving oil recovery, produced water becomes a reusable resource rather than a waste product. The figure at right shows the wellhead of a small injection well.
||Injection well; Source: J. Veil, Argonne National Laboratory.
In the absence of produced water, operators would need to use other surface or groundwater supplies to conduct the water or steam flood. In typical water flooding cases, when sufficient produced water volumes are not available for injection, other sources of water must be used to supplement the water flooding operation. Historically, fresh water sources have been used for this purpose. However, due to the increasing scarcity of fresh water, other brines or water sources are now under consideration for replacing fresh water resources.
Examples of Onshore Produced Water Use for Increasing Recovery
Significant volumes of produced water are injected in the United States. In early 2003, Argonne interviewed staff from oil and gas agencies in three large oil- and gas-producing states (California, New Mexico, and Texas) to determine the number of injection wells in each state, and what percentage were used for EOR. The numbers of wells and water volumes injected are estimates. Nevertheless, they highlight the importance of injection and EOR as produced water management options.
California has nearly 25,000 produced water injection wells. The annual injected volume is approximately 1.8 billion bbl, distributed as follows: disposal wells — 360 million bbl; water flood — 900 million bbl; and steam flood — 560 million bbl.
New Mexico has 903 permitted disposal wells, 264 of which are active. The state has an additional 5,036 wells permitted for enhanced recovery; 4,330 of which are active. The approximate volume of produced water injected for disposal is 190 million bbl. The volume injected for enhanced recovery is about 350 million bbl.
Texas has 11,988 permitted disposal wells, 7,405 of which are active. It has an additional 38,540 wells permitted for enhanced recovery; 25,204 of those are active. The approximate volumes of produced water injected in 2000 were: 1.2 billion bbl disposed into nonproducing formations, 1 billion bbl disposed into producing formations, and 5.3 billion bbl injected for enhanced recovery.
In sum, operators in these three states inject more than 7 billion bbl of produced water per year for EOR. The ratio of produced water volume injected for water and steam flooding to the volume injected for disposal ranges from 1.8:1 to 4.0:1 for these three states.
Offshore Injection Produced Water Use for Increasing Recovery
Historically, produced water has rarely been injected for EOR at offshore platforms. Several factors account for this divergence from the onshore pattern. First, most U.S. platforms (as well as many international platforms) are authorized to discharge produced water to the ocean following treatment. Surface discharge represents in most cases the preferred option for operators. Second, at some point in the life of a field (when pressure maintenance is needed), the offshore wells at a platform do not generate sufficient produced water to meet the volumetric needs for water flooding. Third, platforms have ready access to virtually unlimited supplies of seawater. As discussed in the following section, operators must ensure that the water injected for EOR does not clog the pores of the producing formation. Seawater is nearly always cleaner than produced water, and it requires less pretreatment before injection. Therefore, seawater generally provides the preferred source of injection water for EOR.
Treatment before Injection
It is important to ensure that the water being injected is compatible with the formations receiving the water to prevent premature plugging of the formation or other damage to equipment. It may therefore be necessary to treat the water to control excessive solids, dissolved oil, corrosion, chemical reactions, or growth of microbes.
Solids are usually treated by gravity settling or filtration. Residual amounts of oil in the produced water not only represent lost profit for producers, but can also contribute to plugging of formations receiving the injectate. Various treatment chemicals are available to break emulsions or make dissolved oil more amenable to oil removal treatment.
Corrosion can be exacerbated by various dissolved gases, primarily oxygen, carbon dioxide, and hydrogen sulfide. Oxygen scavengers and other treatment chemicals are available to minimize levels of undesirable dissolved gases.
The water chemistry of a produced water sample is not necessarily the same as that of the formation that will receive the injected water. Various substances dissolved in produced water may react with the rock or other fluids in the receiving formation and have undesirable consequences. Before beginning a water flood operation, it is important to analyze the constituents of the produced water for the purpose of avoiding chemical reactions that form precipitates. If necessary, treatment chemicals can minimize undesirable reactions.
Bacteria, algae, and fungi can be present in produced water. They can also be introduced during water handling processes at the surface. Bacteria, algae, and fungi are generally controlled by adding biocides or by filtration.
In recent years, particularly in the North Sea region, where produced water discharge standards are becoming more stringent, offshore operators are beginning to evaluate injection of produced water combined with seawater or to replace seawater where it has been used previously. Studies have found some problems of reservoir souring (i.e., hydrogen sulfide production) when produced water is injected to a formation that had received seawater previously (Jenneman et al. 2004).
The reuse of produced water for EOR is an important aspect of reservoir and production management. Therefore, in addition to the following references, readers are directed to the extensive literature discussing reservoir management.
Hyne, N.J., 1995, "Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production," PennWell Publishing, Tulsa, OK, 536 pp.
Jenneman, G.E., R.H. Webb, A.J. Dinning, K. Voldum, and O. Bache, 2004, "Evaluation of Nitrate and Nitrite for Control of Biogenic Sulfides in Ekofisk Produced Water," 11th International Petroleum Environmental Conference, Albuquerque, NM, Oct. 11-15.
Veil, J.A., M.G. Puder, D. Elcock, and R.J. Redweik, Jr., 2004, "A White Paper Describing Produced Water from Production of Crude Oil, Natural Gas, and Coal Bed Methane," prepared by Argonne National Laboratory for the U.S. Department of Energy, National Energy Technology Laboratory, January. Available at http://www.evs.anl.gov/pub/dsp_detail.cfm?PubID=1715.