Fact Sheet - Injection for Hydrological Purposes
Produced water has value for reuse in two ways. One value of produced water is as a source of raw water that can be treated to meet various end uses. It can be reused immediately or placed into storage or reserve for future use. Injection into an underground formation for later reuse (aquifer storage and recovery or ASR) is discussed in a separate fact sheet. A second value of produced water is as a fluid that can be used to occupy space or resist earth or fluid movement (i.e., for hydrological purposes). Several forms of hydrological reuse of produced water are discussed in this fact sheet. Produced water injection for enhanced oil recovery is an important example of a hydrological use of produced water to push oil toward a production well. However, because of its widespread use, it is discussed in a separate fact sheet.
The other potential hydrological uses of injected produced water include:
- Controlling surface subsidence in the wake of large withdrawals of ground water or oil and gas;
- Blocking salt water intrusions in aquifers in coastal environments; and
- Augmenting the regional ground water or local stream flows.
Subsidence control wells are injection wells designed to reduce or remediate the loss of land surface elevation due to the removal of ground water, oil, or gas. In their natural state in the formation, these fluids provide physical support to the soil and rock layers above them. When large quantities of the fluids are extracted, the upper formations can compress or collapse. Surface subsidence can cause damage to building foundations, roadways and railways, water wells, and pipelines. Land subsidence control is achieved by injecting water into an underground formation to maintain fluid pressure and avoid compaction.
The EPA (1999a) identified 58 subsidence control wells in the United States, but estimated that many more exist. The EPA considered only those subsidence control wells used to replace withdrawn ground water. These wells are classified as Class V injection wells. EPA also noted that injection wells used to control subsidence resulting from oil and gas production would be considered Class II wells. Because EPA (1999a) focuses on Class V wells, the report did not include a detailed discussion of Class II subsidence control wells.
None of the wells evaluated in EPA (1999a) injected into a drinking water aquifer. Therefore, the quality of the injected water did not necessarily match drinking water standards.
||Large subsidence basin in Wilmington oil field; Source: J. Veil, Argonne National Laboratory.
One of the most compelling examples of subsidence resulting from oil and gas extraction involves the Wilmington oil field in Long Beach, California. Since the 1930s, more than 1,000 wells withdrew about 2.5 billion bbl of oil. Between the1940s and the 1960s, this field experienced a total of 29 feet of subsidence, caused primarily by the withdrawal of hydrocarbons (Colazas et al., 1987). Subsidence in the Wilmington oil field caused extensive damage to Long Beach port industrial and naval facilities. A massive repressurization program, based on the injection of salt water into the oil reservoirs, reduced the subsidence area from approximately 50 km2 to 8 km2. Approximately 2.3 billion bbl of water were reinjected through 1969. The rate of subsidence at the historic center of the bowl has been reduced from a maximum of 28 inches per year in 1952 to zero in 1968. A small surface rebound has occurred in areas of heaviest water injection (Mayuga and Allen, 1969).
The California example is not unique. During an October 2008 visit to Venezuela, the author of this fact sheet was advised that the land surface on the eastern shore of Lake Maracaibo had subsided by up to 5 meters as a result of extensive oil production.
Salt Water Intrusion Barrier Wells
In some coastal regions, fresh water aquifers are connected hydraulically to salt water bodies, including estuaries and oceans. When pumped at modest rates, the natural hydrology allows the salt water/fresh water interface to remain at or near the coast, allowing onshore wells to produce fresh water. However, as coastal populations increase, and demand for fresh water likewise rises, additional fresh water may be withdrawn from these aquifers. This then changes the hydrological balance causing the salt water/fresh water interface to move inland and impact the onshore wells.
Some coastal communities have attempted to control salt water intrusion by injecting water into a low-water-quality or contaminated aquifer, with the intent to never withdraw the injected water. The injected water then produces a hydraulic barrier that physically blocks seawater intrusion (EPA 1999b). The hydraulic barrier is created by raising the piezometric head of the fresh water aquifer and preventing the salt water from moving inland.
EPA (1999b) identified 315 salt water intrusion barrier wells in the United States. However, EPA suspects that nearly 700 salt water intrusion barrier wells exist in the nation. The wells identified by EPA inject water that typically meets drinking water standards promulgated by EPA (for the list in alphabetical order, see http://www.epa.gov/safewater/consumer/pdf/mcl.pdf). It is paramount that, prior to injection, the produced water has been treated to the extent necessary for the purpose of meeting these standards.
Liske (2005) and Ouellette et al. (2005) report on a project to evaluate possible new uses for produced water generated in the San Ardo field in central California. Among the options being considered is control of salt water intrusion in the Salinas River valley. This area has overdrawn ground water for domestic and agricultural uses, resulting in the salt water/fresh water interface moving six miles upstream. Unlike the other examples in this fact sheet, the treated produced water is not expected to be injected. Instead, it would be discharged to the Salinas River or used locally for irrigation, thereby avoiding ground water withdrawal and reducing the driving force of the salt water intrusion.
Stream Flow Augmentation
Although not an injection activity, stream flow augmentation is briefly discussed here. Many oil- and gas-producing regions are located in arid or semi-arid climates. Fresh water resources frequently are in short supply, and competing users strain the available resources. Streams may dry up seasonally. Produced water can potentially be used to augment stream flows. Where discharges are permitted, treated produced water meeting applicable discharge standards could be directly discharged to surface water bodies. Produced water could also be injected into formations exhibiting hydrologic interconnection with surface water bodies, or allowed to infiltrate to the water table through holding ponds.
When used for stream augmentation, the quality of produced water must be controlled to avoid impairment of the surface water quality pursuant to the criteria adopted by the host state. Moreover, the quantity of the added produced water must not increase erosion or damage to stream channels.
The State of New Mexico began a tax credit program in 2002, encouraging oil and gas operators to treat produced water and discharge it to the Pecos River rather than inject it underground. (For the tax credit form, see http://www.state.nm.us/tax/forms/year03/rpd41221.pdf [PDF]). New Mexico had not been upholding its river flow commitments of 34,000 acre-feet to Texas under the Pecos River Compact. The state hoped to achieve some flow augmentation through the addition of the treated produced water covered by the tax credit program. The tax credit was set at $1,000 per acre foot of cleaned water, not to exceed $400,000 per year per company.
Colazas, X.C., R.W. Strehle, and S.H. Bailey, 1987, "Subsidence Control Wells, Wilmington Oil Field, Long Beach, California," Proceedings of the International Symposium on Class V Injection Well Technology, Sept. 22-24, Washington, D.C., Underground Injection Practices Council Research Foundation.
EPA, 1999a, "The Class V Underground Injection Control Study, Volume 20, Salt Water Intrusion Barrier Wells," EPA/816-R-99-014t, U.S. Environmental Protection Agency, Sept. Available at http://www.epa.gov/ogwdw/uic/class5/pdf/study_uic-class5_classvstudy_volume20-salineintrusionbarrier.pdf [PDF].
EPA, 1999b, "The Class V Underground Injection Control Study, Volume 23, Subsidence Control Wells," EPA/816-R-99-014w, U.S. Environmental Protection Agency, Sept. Available at http://www.epa.gov/safewater/uic/class5/pdf/study_uic-class5_classvstudy_volume23-subsidencecontrol.pdf [PDF].
Liske, B., 2005, "Recovery of More Oil-in-Place at Lower Production Costs While Creating a Beneficial Water Resource," presented at the DOE/PERF Water Program Review, Annapolis, MD, Nov. 1-4. Available at http://www.perf.org/pdf/liske.pdf [PDF].
Mayuga, M.N. and D.R. Allen, 1969, "Subsidence in the Wilmington Oil Field, Long Beach, California, USA," IAHS-AISH Publication 88.
Ouellette, R., R. Ganesh, and L.Y.C. Leong, "Overview of Regulations for Potential Beneficial Use of Oilfield Produced Water in California," presented at the 12th International Petroleum Environmental Conference, Houston, TX, Nov. 8-11. Available at http://ipec.utulsa.edu/Conf2005/Papers/Ouellette_Overview.pdf [PDF].