cellar

A cellar is the excavated pit or constructed below-grade enclosure at a drilling location that houses the wellhead, BOP stack, conductor casing, and associated wellhead equipment below the rig floor level, providing the vertical clearance required to install and operate the blowout preventer stack beneath the rotary table while maintaining the rig floor at a working height accessible to the drilling crew, and it is a fundamental structural element at every Western Canada Sedimentary Basin well site because the combined height of the BOP stack (typically 2.5 to 4.5 m for WCSB drilling rigs equipped with annular preventer plus two ram BOP preventers plus drilling spool), the wellhead tree, and the surface casing stub extending above grade would otherwise require an impractically high substructure to position the rig floor rotary table above the top of the BOP stack and still allow adequate clearance for the kelly, top drive, and traveling block to function within the derrick height envelope. The cellar is excavated during well site preparation before rig move-in, typically to a depth of 1.5 to 3.5 m below final grade depending on the planned BOP stack height, substructure design, and local frost depth requirements in WCSB cold-climate locations; the cellar dimensions are specified by the rig contractor and operating company based on the BOP stack configuration, the well control equipment test requirements (the pressure test mandrel and tree installation tools must fit within the cellar work envelope), and the need to accommodate personnel safely during BOP tests, wellhead installation, and tree running operations. In WCSB drilling operations, cellar construction is governed by the AER Directive 056 (Energy Development Applications and Schedules) site preparation requirements and the rig contractor's cellar design specifications, with cellar walls either left as open earth in competent formations (Cretaceous sandstone and carbonate-cemented formations in central and northern Alberta) or lined with timber cribbing, steel sheet piling, or pre-cast concrete panels in soft, wet, or frost-susceptible formations (organic-rich muskeg in the Peace River area, saturated silty soils in the NEBC Montney corridor) where unsupported earth walls would collapse into the cellar and bury the wellhead equipment. The cellar floor drainage system is critical for safe WCSB operations: the cellar accumulates drilling fluid, formation fluid, rain and snowmelt water, and test fluid during BOP pressure tests and wellhead connections, requiring a sump pump, gravity drain line, or both to keep the cellar floor dry and prevent personnel entering the cellar for BOP maintenance, wellhead inspection, or tree installation from working in flooded conditions; AER Directive 056 and Alberta OHS Code Part 36 require that cellars with accumulated liquids be pumped down before personnel entry and that cellar work be performed with a second person at the surface as a safety watch. Understanding cellar construction methods for different WCSB geological and climate conditions, the BOP stack clearance calculation that determines cellar depth, the drainage and safety requirements for cellar operations, the conductor casing and wellhead installation sequence relative to cellar construction, and the WCSB regulatory requirements governing cellar design and site restoration gives WCSB drilling engineers, rig contractors, well site construction supervisors, and HSE officers the civil and operational engineering framework to specify, build, and safely operate cellars throughout the drilling, completion, and abandonment lifecycle of WCSB wells.

• Cellar depth calculation for WCSB BOP stack clearance requirements: The minimum cellar depth is determined by: (rig floor height above grade) minus (substructure height) minus (BOP stack height) minus (wellhead spool and tree height above surface casing stub). For a WCSB rig with a 6.7 m substructure, rig floor at 7.0 m above grade, BOP stack of 3.8 m (annular preventer 1.2 m + double ram BOP 1.8 m + drilling spool 0.8 m), and wellhead elevation requirement of 0.5 m above cellar floor, the cellar depth = 7.0 minus 6.7 minus 3.8 minus 0.5 = minus 4.0 m (4.0 m below grade). Standard WCSB drilling rig cellars are 1.8 to 4.0 m deep; workover rig cellars may be shallower (0.9 to 1.8 m) where smaller BOP stacks are used on production wellheads already partially installed.
• Cellar lining methods in WCSB soft-ground and muskeg locations: WCSB Peace River lowlands, NEBC Liard Basin, and Grande Prairie area well sites often encounter organic muskeg, saturated silty clay, and frost-sensitive alluvial soils that require cellar lining to prevent wall collapse. Timber crib cellars (horizontal 150 mm x 300 mm timber planks in a log-cabin pattern, anchored to driven corner posts) are the traditional WCSB approach for competent soil conditions; steel sheet pile cellars (interlocking Z-pile driven to 1 m below cellar floor, then excavated inside) are used in saturated or very soft soil conditions; pre-cast concrete cellar boxes (factory-manufactured 3-piece concrete structures lowered into an oversized excavation and backfilled) are used on multi-well WCSB pad sites where the cellar investment is amortized over multiple wells. Frost protection insulation (25 to 75 mm extruded polystyrene on cellar walls and floor) is specified for WCSB cellars in areas with design frost depth below 1.5 m to prevent heave damage to the wellhead and surface casing.
• Conductor casing and surface casing installation relative to WCSB cellar construction: The WCSB cellar construction sequence typically proceeds as: (1) excavate cellar pit to design depth; (2) install conductor casing (typically 508 mm or 762 mm OD) by jetting, driving, or drilling to 15 to 30 m depth through the cellar floor center; (3) line cellar walls as required; (4) install cellar drainage system (perforated pipe at cellar floor level to sump, or gravity drain to adjacent pit); (5) move in rig and nipple up BOP stack on conductor after drilling surface hole and cementing surface casing. The conductor casing weld or connection at the cellar floor level must be designed to carry the combined weight of the BOP stack, wellhead, and any suspended casing loads without exceeding the conductor load rating.
• Cellar safety requirements for personnel entry during WCSB BOP and wellhead operations: Alberta OHS Code Part 36 classifies the cellar as a confined or restricted-access work area when accumulated liquid is present or when gas detection indicates H2S or combustible gas in the cellar atmosphere. Required controls for cellar entry during WCSB operations include: confined space entry permit when cellar meets the Alberta OHS Code Part 40 confined space definition (enclosed, limited entry/exit, not designed for continuous occupancy, potential for hazardous atmosphere); continuous gas monitoring with a 4-gas detector calibrated for H2S (STEL 15 ppm, TWA 1 ppm), LEL, O2, and CO2; safety watch at surface with rescue equipment; and communication between cellar worker and driller. WCSB wellhead trees, Christmas trees, and BOP test equipment are designed to minimize cellar entry time by allowing valve actuation, pressure gauging, and test connection from above grade where possible.
• Cellar abandonment and site reclamation requirements under WCSB regulations: At WCSB well abandonment, the cellar is required to be reclaimed under AER Directive 020 and AER Directive 079 (Alberta) and BCOGC Drilling and Production Regulation (British Columbia) to restore the ground surface to its pre-disturbance condition. Reclamation requires: removal of all wellhead equipment to below grade; cement fill of the cellar void space to within 0.5 m of ground surface; placement of clean native soil or gravel fill to grade; vegetation re-establishment on non-cultivated sites; and submission of a reclamation certificate confirming no contamination was left in the cellar. Steel or concrete cellar liners are either removed and recycled or grouted in place if removal is impractical; conductor casing is cut and capped at the cement fill level. Cellar reclamation cost on a WCSB abandoned well ranges from $8,000 to $25,000 depending on liner type, depth, and site contamination.

Cellar Flooding Delaying BOP Test on a WCSB Montney Horizontal Well

A northeast British Columbia Montney horizontal well site in the Dawson Creek area experienced a cellar flooding event during spring breakup when the cellar sump pump failed due to a burned-out motor while the rig was rigging up. Snowmelt and rainwater accumulated to 1.2 m depth in the 2.5 m deep cellar over 18 hours, completely submerging the wellhead and lower BOP spool. The rig crew could not perform the mandatory AER Directive 036 BOP pressure test required before drilling out the surface casing shoe because the test manifold connections were underwater and cellar entry under flooded conditions was prohibited by the NEBC site safety plan without confined space entry procedures not yet in place for the new location. The crew pumped the cellar with a trailer-mounted 3-inch diesel pump for 4 hours to dewater the cellar, inspected all submerged connections for integrity, completed the confined space entry permit documentation, and performed the BOP test 6 hours after the originally planned test time. The incident was reported to the BCOGC as a procedural delay; investigation found the sump pump had been installed without an overload-protection circuit breaker. The company added automatic cellar level alarms and dual-pump redundancy (one electric, one pneumatic backup) to its WCSB cellar design standard for all new well sites.

Related Terms

Blowout preventer is the primary well control equipment installed in the cellar on the wellhead, with the cellar depth sized to provide vertical clearance for the full BOP stack height below the rig floor rotary table; WCSB BOP stack configurations of annular preventer plus double ram BOP plus drilling spool typically require 2.5 to 4.5 m of cellar clearance below the substructure base. Wellhead equipment including the casinghead, casing spools, and tubing head is installed in the cellar on the surface casing stub, with the cellar providing the working space required for flange-up operations, pressure testing, and tree installation by WCSB wellhead service crews during casing and completion operations. Conductor casing is the largest-diameter casing string (typically 508 to 762 mm OD in WCSB operations) that penetrates the cellar floor at its center, providing structural support for the BOP stack and wellhead loads and isolating the shallow soil zone from the wellbore during surface hole drilling before surface casing is run. Rig floor is positioned above the cellar at the top of the substructure, with its elevation above grade determined by the substructure height; the vertical distance from the rig floor rotary table to the top of the BOP stack in the cellar defines the maximum connection length available for kelly-bushing to BOP-top operations during drilling and completion activities. Site reclamation at WCSB well abandonment requires cellar decommissioning as one of the final steps before the reclamation certificate is issued, involving removal of wellhead equipment, cement fill of the cellar void, and restoration of the ground surface to pre-disturbance conditions in compliance with AER Directive 079 and the Alberta Environmental Protection and Enhancement Act requirements for upstream oil and gas site restoration.