Fact Sheet - Onsite Burial (Pits, Landfills)
Burial is the placement of waste in man-made or natural excavations, such as pits or landfills. Burial is the most common onshore disposal technique used for disposing of drilling wastes (mud and cuttings). Generally, the solids are buried in the same pit (the reserve pit) used for collection and temporary storage of the waste mud and cuttings after the liquid is allowed to evaporate. Pit burial is a low-cost, low-tech method that does not require wastes to be transported away from the well site, and, therefore, is very attractive to many operators.
Burial may be the most misunderstood or misapplied disposal technique. Simply pushing the walls of the reserve pit over the drilled cuttings is generally not acceptable. The depth or placement of the burial cell is important. A moisture content limit should be established on the buried cuttings, and the chemical composition should be determined. Onsite pit burial may not be a good choice for wastes that contain high concentrations of oil, salt, biologically available metals, industrial chemicals, and other materials with harmful components that could migrate from the pit and contaminate usable water resources.
In some oil field areas, large landfills are operated to dispose of oil field wastes from multiple wells. Burial usually results in anaerobic conditions, which limits any further degradation when compared with wastes that are land-farmed or land-spread, where aerobic conditions predominate.
The use of earthen or lined pits is integral to drilling waste management. During most U.S. onshore drilling operations, the cuttings separated by the shale shaker are sent to a pit called the reserve pit located near the drill rig. The pit is generally open to the atmosphere, so it also accumulates stormwater and washwater from the rig. The strategic location of small pits near drilling sites can also help minimize spillage of waste materials. Unless site characteristics are such that no significant threat to water resources can occur, liners are generally required. Where pits must be constructed adjacent to water bodies or on sloping terrain, engineering precautions incorporated into the design will help to ensure pit integrity. Precautions should be taken to prevent disposal of chemicals, refuse, debris, or other materials not intended for pit disposal.
At the end of the drilling job, any hydrocarbon products floating on top of the pits are recovered and any free water or other liquids are collected and disposed of, usually in an injection well. The remaining cuttings are covered in place using native soils, the surface is graded to prevent water accumulation, and the area is revegetated with native species to reduce the potential for erosion and promote full recovery of the area's ecosystem. Reserve pits should be closed as soon as possible following the generally accepted closure procedures in the region.
For wastes with constituent concentrations only slightly higher than those allowed for traditional pit burial, the wastes may be blended with clean, local soil to dilute and reduce the high concentrations to acceptable levels before the waste/soil mix is buried. The objective of this approach is to incorporate wastes that meet required criteria into the soil at a level below the major rooting zone for plants but above the water table (Bansal and Sugiarto 1999). Costs for such burial depend on the volume of soil required to be mixed to bring concentrations of oil and grease to within appropriate oil and grease limits (e.g., <3% by weight), pit excavation costs, and loading/hauling costs of soils and cuttings. The average drilling waste burial costs are estimated at $7 to $8 per barrel (Bansal and Sugiarto 1999).
Landfills are used throughout the world for disposing of large volumes of municipal, industrial, and hazardous wastes. In landfills, wastes are placed in an engineered impoundment in the ground. At the end of each day or on some other cycle, the waste is covered with a layer of clean soil or some other inert cover material. Modern design standards require clay or synthetic liners, although, in some areas, unlined landfills continue to operate.
Landfills can be used for disposing of drilling wastes and other oil field wastes. In some circumstances, these are offsite commercial operations established to receive wastes from multiple operators in an oil field (e.g., the West Texas region). In other cases, oil companies with a large amount of drilling activity in an area may construct and operate private landfills. For example, TotalFinaElf designed and built a controlled landfill to dispose of inert wastes at a remote site in Libya, where other management alternatives were not readily available. At this landfill, a bottom liner overlaid by a geological barrier was developed to prevent contamination of the soil. A top liner, which is drawn over the waste during non-active periods, will be installed permanently after the landfill is closed. Two collection pits collect rainwater and subsequent leachate (Morillon et al. 2002).
Wastes suitable for burial are generally limited to solid or semi-solid, low-salt, low-hydrocarbon content inert materials, such as water-based drill cuttings. Costs for disposing of cuttings that have been stabilized prior to dilution and burial are estimated at $9-10 per barrel of waste (Bansal and Sugiarto 1999).
Factors to consider for burying drilling wastes include the following:
- Depth above and below pit. Areas with shallow groundwater are not appropriate; a pit location of at least five feet above any groundwater is recommended to prevent migration to the groundwater. The top of the burial cell should be below the rooting zone of any plants likely to grow in that area in the future (normally about three feet).
- Type of soil surrounding the pit. Low-permeability soils such as clays are preferable to high-permeability soils such as sands.
- For offsite commercial landfills, any protocols required by the facility accepting the waste (not all facilities have the same acceptance criteria).
- Prevention of runoff and leaching. Appropriate types and degree of controls to prevent runoff and leaching should be implemented. Natural barriers or manufactured liners placed between the waste material and the groundwater help control leaching.
- Appropriate monitoring requirements and limits.
- Time required to complete the burial.
- Chemical composition of the buried cuttings.
- Moisture content or condition of buried cuttings.
The advantages of onsite burial of drilling wastes include the following:
- Simple, low-cost technology for uncontaminated solid wastes.
- Limited surface area requirements.
Concerns include the following:
- Potential for groundwater contamination if burial is not done correctly or contaminated wastes are buried, and the resulting liability costs.
- Requirements for QA/QC, stabilization, and monitoring.
Bansal, K.M., and Sugiarto, 1999, "Exploration and Production Operations – Waste Management A Comparative Overview: US and Indonesia Cases," SPE54345, SPE Asia Pacific Oil and Gas Conference, Jakarta, Indonesia, April 20-22.
E&P Forum, "Exploration and Production (E&P) Waste Management Guidelines," 1993, Report No. 2.58/196, London, U.K., September.
Morillon, A., J.F. Vidalie, U.S. Hamzah, S. Suripno, and E.K. Hadinoto, 2002, "Drilling and Waste Management," SPE 73931, presented at the SPE International Conference on Health, Safety, and the Environment in Oil and Gas Exploration and Production, March 20-22.