Reserve Pit
A reserve pit is an earthen excavation or lined surface containment structure constructed adjacent to a drilling location to store drilling fluids, drill cuttings, produced water, and other well-site waste generated during well construction operations, with post-drilling closure and reclamation requirements governed by state, provincial, and federal environmental regulations.
Key Takeaways
- Reserve pits serve as temporary containment for water-based and oil-based mud returns, cuttings, and pit fluids during drilling, preventing surface spills and providing a buffer volume for mud system management.
- Lined pits use high-density polyethylene (HDPE) geomembrane liners to prevent fluid migration into soil and groundwater, while unlined pits in low-permeability clay soils may be permitted in some jurisdictions but are increasingly restricted.
- Closure requirements typically mandate dewatering, solidification or off-site disposal of cuttings and pit contents, liner removal, backfilling with native soil, regrading, and revegetation to return the site to pre-disturbance land productivity.
- Closed-loop drilling systems eliminate the reserve pit entirely by processing all drilling fluid returns through mechanical solids control equipment and recycling clean fluid back to the active system, reducing environmental footprint.
- Environmental liability from reserve pits includes groundwater contamination from liner failure or unlined seepage, chloride and hydrocarbon leaching into soil, and improper disposal of oil-based mud waste, which is classified as hazardous in many jurisdictions.
Fast Facts
A typical onshore reserve pit for a 10,000-foot vertical well may hold 15,000 to 40,000 barrels of fluid and cuttings. HDPE liner thickness for reserve pits generally ranges from 40 to 60 mils (1.0 to 1.5 mm). In some US states including New Mexico and Colorado, unlined reserve pits have been banned or severely restricted since the 2010s. The average cost of pit closure and remediation in the US ranges from $30,000 to over $500,000 depending on pit size, fluid type, and contamination level.
Tip: When planning a well site in environmentally sensitive areas or near freshwater sources, design a closed-loop drilling system from the outset rather than retrofitting. The capital cost of closed-loop equipment is typically recovered through reduced pit closure liability, lower water disposal costs, and shortened reclamation timelines, particularly in jurisdictions where pit closures require third-party environmental certification.
What Is a Reserve Pit
The reserve pit, sometimes called the mud pit or waste pit, is one of the oldest features of a drilled well site. Before mechanical solids control technology existed, all drilling fluid circulated through the borehole was returned to large open earthen pits where cuttings settled by gravity and the clarified fluid was recycled to the active mud system. Even as modern facilities have dramatically improved fluid management, a reserve pit or equivalent containment structure remains a standard component of most onshore well sites because it provides emergency containment volume, a settling basin for fine-grained solids, and temporary storage during mud change-outs or well completion operations.
The physical size of the reserve pit depends on the planned well depth, hole diameters, expected cuttings volume, and the mud system design. In conventional vertical wells, the pit is typically a rectangular earthen berm construction with dimensions of 50 to 150 feet on a side and 6 to 12 feet deep. In horizontal multi-well pad drilling, the combined reserve pit may be substantially larger to accommodate simultaneous operations on multiple wellbores. Site surveys, geotechnical assessments, and regulatory permits are required before pit construction in most jurisdictions.
How Reserve Pits Work
During drilling, mud returns from the borehole flow across the shale shaker and other solids control equipment before entering the active mud tanks. Overflow, washdowns, and emergency discharges route to the reserve pit. The pit also receives drill cuttings that pass through or bypass the solids control equipment. At intervals, the pit is sampled and tested for chloride content, hydrocarbon concentration, and pH to assess fluid character and guide treatment or disposal decisions.
Upon well completion or abandonment, the pit closure process begins. Dewatering is the first step: residual fluid is pumped to tanker trucks for disposal at licensed facilities or treated on-site to meet discharge standards. If the cuttings and solidified waste meet burial criteria (typically low BTEX, low chlorides, acceptable pH), the pit may be backfilled in place after mechanical stabilization with kiln dust, lime, or fly ash. If the waste exceeds burial thresholds, it must be excavated and transported to a licensed solid waste disposal facility. Final reclamation involves contouring the backfilled area to the original drainage pattern, topsoil placement, and seeding with a regulator-approved native plant mix.
Liner integrity testing, including visual inspection and electrical resistivity surveys in some programs, verifies that the liner did not fail during operations. If liner failure or soil contamination is detected, additional remediation may be required, including soil excavation, bioventing, or pump-and-treat groundwater remediation. These contingencies can extend closure timelines by months to years and increase costs dramatically.
Reserve Pit Regulations Across International Jurisdictions
In Canada, Alberta's AER has progressively tightened reserve pit requirements under Directive 050 and Directive 055. Operators are required to submit a waste management plan identifying pit locations, liner specifications, waste classification, and reclamation timelines. Reclamation certificates under Alberta's Environmental Protection and Enhancement Act require third-party environmental site assessments confirming that pit sites meet regulatory soil and water quality standards before a certificate is issued. WCSB operators increasingly use closed-loop or closed-containment systems to avoid the cost and liability of pit reclamation, particularly in environmentally sensitive Peace River and foothill areas.
In the United States, reserve pit regulation is primarily a state responsibility, with federal overlay from the EPA's Clean Water Act and the Resource Conservation and Recovery Act (RCRA). Under RCRA's Bentsen Amendment, drilling fluids and produced water are specifically exempted from hazardous waste classification, but states may impose more stringent rules. New Mexico, Colorado, Wyoming, and Pennsylvania have enacted detailed pit construction, lining, and closure regulations. The EPA's effluent guidelines for the oil and gas extraction category (40 CFR Part 435) govern discharge from pits in certain situations. Federal lands managed by the Bureau of Land Management require a Drilling Plan that specifies pit design and reclamation procedures under Onshore Order No. 1.
In Norway, the Petroleum Safety Authority and the Norwegian Environment Agency jointly govern drilling waste management on the Norwegian Continental Shelf. Offshore drilling does not use reserve pits: all cuttings and mud returns are processed and either discharged (water-based cuttings), shipped to shore for disposal, or re-injected into disposal wells. Norwegian land operations are minimal, but where they exist, the Pollution Control Act regulations apply stringent containment and cleanup requirements consistent with Norway's broader environmental protection framework.
In the Middle East, Saudi Arabia's Meteorology and Environmental Protection Administration (MEPA) and Saudi Aramco's internal environmental standards govern drilling waste management. Aramco requires that all drilling waste be contained, with lined pits mandatory for water-sensitive desert environments where hydrocarbon contamination could migrate through aeolian sands toward shallow aquifers. The Gulf region's high ambient temperatures also accelerate evaporation from open pits, concentrating salts and hydrocarbons and increasing contamination risk. Closed-loop and cuttings-reinjection technologies are widely deployed on Aramco's high-volume drilling campaigns.
Synonyms and Related Terminology
Reserve pits are also known as mud pits, waste pits, slush pits, or sump pits. In Canadian terminology, "drilling waste containment area" is sometimes used in regulatory documents. Related concepts include drilling fluid, drill cuttings, closed-loop drilling, solids control, produced water, and well site reclamation.
Frequently Asked Questions
Q: What is the difference between a lined and an unlined reserve pit?
A: A lined pit uses a synthetic geomembrane, typically 40-60 mil HDPE, installed over a prepared soil sub-base to prevent fluid infiltration into native soil and groundwater. An unlined pit relies on the natural low permeability of clay soils as the containment barrier. Unlined pits are now prohibited or highly restricted in most jurisdictions because natural soil permeability is rarely sufficient to prevent chloride and hydrocarbon migration over a multi-month drilling program.
Q: What is a closed-loop drilling system and how does it eliminate the need for a reserve pit?
A: A closed-loop system processes all drilling fluid returns through a complete mechanical solids control train (shakers, centrifuges, hydrocyclones) and recycles clean fluid back to the active mud system without allowing any overflow to reach an open pit. All solids are captured in enclosed collection tanks or bins for off-site disposal. The system is sealed so no fluid contacts the ground surface, eliminating the reserve pit entirely and dramatically reducing site footprint and closure liability.
Why Reserve Pits Matter
Reserve pits represent one of the most significant environmental liability points in onshore drilling operations. Contamination from pit seepage or failure can affect surface and groundwater for decades, generating remediation costs that can exceed the value of the well itself. Regulatory requirements for pit closure and reclamation are increasing in stringency across North America and internationally, raising both the cost and timeline of post-drilling site restoration. The growing adoption of closed-loop systems reflects the industry's recognition that eliminating the reserve pit entirely is more cost-effective than managing its long-term liability. For operators, understanding pit regulation, liner specifications, waste classification, and reclamation certification requirements is essential to managing project economics and maintaining social license to operate.