Water, the third phase in petroleum production, represents a massive liquid volume that operators have to handle and dispose of cost-effectively and in an environmentally sound manner. Although data on water production often are unreliable or unavailable, one estimate places worldwide annual water production associated with oil and gas at 77 billion bbl.
In US operations alone, water production amounts to 14-18 billion bbl/year.
Various new technologies, as discussed in the special report starting on p. 41, help handle and determine the source of water downhole. But water production is a complex issue.
One can gain some understanding of the difficulty associated with water production from a January 2004 white paper prepared by Argonne National Laboratory for the US Department of Energy National Energy Technology Laboratory under Contract W-31-109-Eng-38. The study overviews many issues involved with water produced with crude oil, natural gas, and coalbed methane.
The study covers such aspects as the variability of its constituents, where it is generated, how it is managed and regulated in different settings, and the related costs associated with its production.
Produced water
Produced water characteristics and physical properties vary considerably and can change throughout a reservoir's life. Primary concerns include the oil and grease content, as well as the salt content.
Water also may contain organic and inorganic compounds that influence toxicity. Some of these occur naturally, while others are added as chemicals during operations. These chemicals in high concentrations may present a threat to crops and aquatic life, but discharges to small streams likely will have more of an environmental impact than discharges in open ocean where greater dilution occurs.
Because of these potential impacts on the environment, US regulatory agencies prohibit discharges in most onshore or nearshore locations.
Natural gas wells typically produce less water than oil wells, with the exception of wells producing from horizons such as coalbeds and Devonian-Antrim shales.
Coalbeds usually produce gas only after large amounts of water are removed.
For instance, the Argonne study says that within Wyoming's Powder River basin, water production from coalbed methane wells increased more than sevenfold during 1998-2001, with about 1.4 million bw/d being produced in 2001.
US regulations
In 1988, the US Environmental Protection Agency exempted oil and gas exploration and production wastes, including produced water, from the Resource Conservation and Recovery Act's hazardous waste provisions.
The Argonne study says most US onshore produced water is injected into Class II wells for either enhanced recovery or disposal.
The Underground Injection Control (UIC) program regulates this injection, and EPA has delegated authority to various state agencies to regulate the UIC program for protecting drinking water sources.
Most offshore produced water discharges require an EPA permit, under the National Pollutant Discharge Elimination System (NPDES). The permit limits the content of oil and grease, toxicity, and other constituents in the discharged water.
Regulations do allow some onshore operations to discharge water. This water is relative clean water from stripper oil wells, CBM wells, or other low-producing wells that can be used to irrigate croplands or for wildlife and livestock.
Handling water
One option for handling water production is to minimize its production by using downhole mechanical blocking devices or chemicals to prevent water from entering the wellbore.
Industry also has tested various devices that eliminate having to produce water to surface facilities.
These devices collect and separate the water either downhole or at the seafloor and include downhole oil-water or gas-water separators, dual-completion wells, and subsea separators. But to date these devices have not gained widespread acceptance.
Produced water also serves purposes such as dust control, vehicle washing, power plant makeup water, and fire control, but most enters disposal wells or is discharged offshore.
Because of the large volumes involved, water handling is a major expense that affects the economical limit of many fields.
The Argonne study estimates that handling costs range from as low as $0.01/bbl to hundreds of times that amount.
As the world's oil fields age, water production is bound to increase, and the cost and technology for handling it will influence ultimate oil and gas recovery.
