Offshore Disposal of Oil-Based Drilling-Fluid Waste: An Environmentally Acceptable Solution
- Ed Malachosky (Arco Oil and Gas Co.) | B.E. Shannon (Arco Oil and Gas Co.) | J.E. Jackson (Arco Oil and Gas Co.) | W.G. Aubert (Arco Oil and Gas Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 1993
- Document Type
- Journal Paper
- 283 - 287
- 1993. Society of Petroleum Engineers
- 1.10 Drilling Equipment, 1.7.7 Cuttings Transport, 2.4.3 Sand/Solids Control, 4.1.2 Separation and Treating, 6.5.3 Waste Management, 1.14 Casing and Cementing, 6.5.5 Oil and Chemical Spills, 1.6 Drilling Operations, 1.11 Drilling Fluids and Materials, 4.3.4 Scale, 5.6.5 Tracers
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Oily cuttings and waste fluid are byproducts of oil-based drilling muds. In such difficult drilling environments as the Gulf of Mexico, where oil-based fluids often are preferred, personnel safety, environmental, and economic concerns are exacerbated by the necessity to transport these cuttings and fluids to shore for disposal. This paper describes a process for on-site preparation and subsequent disposal of a slurry of cuttings by annular pumping. The disposal includes all cuttings and waste oil mud generated during drilling with oil-based fluids. Wastes are displaced down a casing annulus and into permeable zones below the surface casing setting depth. Descriptions of environmental and safety problems arising from onshore disposal, benefits of annular pumping, and equipment used for slurry preparation and pumping are described. This technique eliminates the need for platform cuttings storage, cuttings transportation to shore, and the environmental effects of onshore disposal.
U.S. governmental regulations currently prohibit the discharge of free oil into the Gulf of Mexico during drilling operations. This ban on oil discharges has resulted in the necessity for operators to transport the waste mud and cuttings generated during drilling with an oil-based mud to an appropriate onshore waste treatment or disposal facility or to switch to exotic and sometimes unproved water-based systems. Although these water-based mud systems have not always failed, they have not proved to be fully acceptable alternatives to the oil-based systems they have displaced.
Both economic and environmental considerations have dictated that more wells be drilled from fewer surface installations. As a result, more highly deviated and often horizontal wellbores have been drilled through the young, troublesome, water-sensitive shales that have plagued gulf drilling operations in the past.1 Successful drilling under these conditions in many areas of the gulf2 and other offshore environments3 requires the use of an oil-based fluid. Consequently, the implementation of an environmentally acceptable disposal method for oily cuttings and mud at the drillsite would reduce drilling costs and minimize the potential environmental exposure that would result from transport of this material to shore for disposal. Thus, a technique for converting the cuttings and mud into a slurry for slurry disposal by pumping it down the annulus of a well was developed. As this paper describes, annular pumping is the one-time displacement of drilling fluids and solids into a nonproductive, permeable zone (primarily sandstone) for disposal.
The U.S. Environmental Protection Agency (EPA) regulates discharges from offshore oil and gas production facilities under the General National Pollutant Discharge Elimination System (NPDES) Permit No. GMG280000.4 These discharges are defined in the Oil and Gas Point Source Category of Title 40 of the U.S. Code of Federal Regulations (40 CFR 435). The permit allows the discharge of water-based drilling fluids and cuttings to the gulf, as long as the discharge does not result in a sheen on the ocean's surface (the so-called "no-free-oil restriction"). Additionally, the suspended particulate phase of the drilling fluid must pass a 96-hour toxicity test (LC50>30,000 ppm). Permit No. GMG280000 expired at midnight, Eastern Standard Time, July 1, 1991; however, the EPA published proposed Permit No. GMG2900005 for the western Gulf of Mexico. This permit places comparable restrictions on oil-based mud use and will, when finalized, replace the expired permit.
As a result of these regulations, oil-based drilling fluids and the cuttings generated by drilling with these fluids cannot be discharged into the Gulf of Mexico. Consequently, the use of an oil-based mud system called for waste disposal in a commercial, onshore, nonhazardous oilfield waste (NOW)disposal location, where the waste is either buried or land-farmed. Historically, movement of drilling wastes to a NOW facility was not adequately tracked, nor was waste disposal well documented. Common industry practice included trucking mud and cuttings from offshore and onshore locations to the commercial facility. Often, the only records of these transactions were transportation and disposal invoices. Although these operations were conducted in accordance with applicable regulations, because non-NOW material was deposited at these sites, certain sites have been classified by the U.S. EPA as Comprehensive Environmental Response, Compensation, and Liability Act Superfund Sites. Remediation costs are shared by all who used the site for disposal regardless of the type of wastes taken to the site.
The prohibition on the discharge of oil-based waste has caused a re-evaluation of the economic, safety, and environmental consequences of a "no discharge"offshore drilling operation. The associated expense, employee exposure to transfer operations, potential oil-spill cleanup exposure, and possible future liability of commercial NOW sites all have combined to create a need for an onsite disposal option.
Because oil-based drilling-fluid systems have been the operational fluid of choice for drilling in the Gulf of Mexico, new procedures and techniques that would continue to allow the use of these fluids in the offshore environment have been sought. Oil-based-mud cuttings cleaning systems* have been developed, patented, and field tested. These systems, although operationally and environmentally sound, proved too slow to provide real-time treatment of generated waste and proved to be comparatively expensive for the offshore environment.
Several efforts have been reported6-11to develop technologies that would render oil-based-mud waste suitable for offshore discharge. Cuttings washers, incinerators, and solvent-extraction systems all failed to produce environmentally acceptable effluents, were too expensive to use, or were considered unsafe for offshore application. Consequently, when the need for oil-based muds was identified, many operators relied on the on-site collection of the oily cuttings and mud in boxes. These boxes then were transported to shore by dedicated work boats, and the contents were buried in commercial NOW disposal facilities.
Annular pumping for waste-water disposal has been used in several situations. In one application, drilled cuttings are recycled for use as road-building material and the water used to clean the cuttings is pumped into a permeable zone below the surface casing.12,13 In many areas, produced water routinely is pumped into production zones for EOR purposes.
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