Tank: Drilling, Production, and Storage Vessels, API 650 Design, and Secondary Containment in WCSB Batteries
A tank is a metal or plastic vessel used to store or measure a liquid, and it is among the most ubiquitous pieces of surface equipment in the oil and gas industry, appearing at every stage from the drilling rig to the export terminal. In an oil field the tanks fall into three broad functional types: drilling tanks, production tanks, and storage tanks, and each is engineered for a different fluid, pressure, and service life. Drilling tanks are the mud pits and active circulating tanks that hold drilling fluid as it cycles down the drillpipe and back up the annulus, including the suction, settling, reserve, and trip tanks that let the crew manage mud volume and monitor for the small gains or losses that signal a kick or lost circulation. Production tanks receive fluids after the wellhead and separation train, and they include treating tanks, wash tanks, gun barrels for oil-water separation, and the measuring or dump tanks that quantify produced volumes for accounting. Storage tanks hold conditioned crude oil, condensate, produced water, or chemicals until they are trucked or pipelined away, and they are the most visible tanks on any lease, the upright cylindrical vessels that define the skyline of a battery. Construction varies with duty. Low-pressure atmospheric storage tanks in the Western Canadian Sedimentary Basin are commonly built or rated to API Standard 650 for welded steel tanks, while smaller shop-fabricated vessels and bolted or fiberglass tanks serve produced-water and chemical service. A standard 400 barrel field tank holds roughly 63.6 cubic metres, and tank sizes are quoted in both barrels and cubic metres because Canadian regulatory reporting uses cubic metres while North American partner accounting often uses barrels, with one cubic metre equal to about 6.29 barrels. Tanks are never just vessels in isolation. Provincial regulation requires secondary containment, typically a lined dike or remote impoundment sized to hold the largest tank's volume plus freeboard, so a leak or rupture cannot reach soil or surface water. The Alberta Energy Regulator addresses tank and battery requirements through Directive 055 for storage of oilfield waste and fluids and Directive 058 and related rules for facility design, while measurement-related tank duties fall under Directive 017. Tanks also concentrate hazards: hydrocarbon vapours create fire and explosion risk, hydrogen sulphide can accumulate in vapour space, and gauging or thief-hatch work has caused fatalities, which is why tank entry, venting, and vapour-recovery design carry strict safety controls across the basin.
Key Takeaways
- Three functional types in the field: Oilfield tanks divide into drilling tanks, the mud pits and trip tanks that circulate and monitor drilling fluid; production tanks, the treating, wash, and dump tanks that handle and measure produced fluids; and storage tanks, the upright vessels that hold conditioned crude, water, or chemicals before trucking or pipeline shipment. Each is built for a distinct fluid and service life.
- API 650 governs welded storage tanks: Atmospheric crude and water storage tanks in the WCSB are typically designed to API Standard 650 for field-welded steel, while smaller bolted, shop-built, or fiberglass tanks serve chemical and produced-water duty. The standard sets shell thickness, weld quality, foundation, and venting so the vessel safely holds liquid at near-atmospheric pressure.
- Secondary containment is mandatory: Regulation requires a lined dike or remote impoundment around tanks, sized to hold the largest tank plus freeboard, so a rupture cannot reach soil or surface water. AER Directive 055 sets these storage and containment requirements, and inadequate containment is a common compliance finding during facility inspections.
- Dual-unit volume reporting: A standard 400 barrel field tank holds about 63.6 cubic metres. Canadian regulatory and Petrinex reporting uses cubic metres, while partner and North American market accounting often uses barrels, with one cubic metre equal to roughly 6.29 barrels, so operators track and reconcile both units on every battery.
- Tanks concentrate serious hazards: Hydrocarbon vapour creates fire and explosion risk, hydrogen sulphide can collect in vapour space, and opening a thief hatch to gauge can release a lethal cloud. Vapour-recovery units, proper venting, gas detection, and strict tank-entry procedures are required because routine gauging and sampling have caused fatalities across the basin.
Drilling Tanks and the Active Mud System
On a drilling rig the tanks form the surface circulating system that the entire well-control scheme depends on. Drilling fluid is drawn from the suction tank by the mud pumps, sent down the drillpipe, and returns up the annulus through the shale shakers into the settling and active tanks. Crews monitor total active volume closely, because a steady gain can mean an influx and a steady loss can mean fractured formation. The trip tank is a small calibrated tank used while pulling or running pipe, letting the crew verify that the hole takes or gives back exactly the volume of steel removed or inserted, an early and sensitive indicator of a kick or losses before they become serious.
Production and Storage Tanks on a Battery
Downstream of the wellhead, production tanks finish the job the separators began. A wash tank or gun barrel gives oil and water more residence time to separate by gravity, treating tanks apply heat, and dump tanks measure the clean oil for reporting. Conditioned crude then flows to storage tanks that buffer production against trucking or pipeline schedules. These atmospheric tanks need pressure and vacuum venting, emergency relief, and increasingly vapour-recovery units that capture flash gas instead of venting it, both for emissions compliance under provincial methane rules and to recover saleable product. Tank levels, temperatures, and water cuts are logged for the volumetric accounting that drives royalty and partner settlements.
Fast Facts
Tank gauging has a deadly history that reshaped oilfield safety. Across North America, workers opening thief hatches to manually gauge or sample crude have been overcome and killed by hydrocarbon vapours and hydrogen sulphide venting from the tank, sometimes in seconds and with no warning. Investigations of these flash-gas exposure fatalities drove the shift toward automated level measurement, closed sampling, and vapour-recovery systems, so that a routine task once done with a steel tape and an open hatch is now engineered to keep workers out of the vapour entirely.
Related Terms
The tank connects to much of the surface production vocabulary. A Measuring Tank is the specialized calibrated dump tank that quantifies produced liquid for accounting, while a Separator conditions the well stream upstream by removing gas before fluids reach the tanks. Custody Transfer is the accounting event that storage and sales tanks ultimately serve, and a Battery is the grouped surface facility where wells, separators, treaters, and tanks are assembled into one reporting and measurement unit.
WCSB Field Scenario: A Containment Upgrade at a Central Alberta Oil Battery
An operator running a multi-well group battery in the Cardium fairway near Pembina, Alberta, faced an AER inspection finding that its three 400 barrel crude storage tanks shared an earthen dike with insufficient capacity and a degraded liner. Under Directive 055, the containment had to hold the largest tank's volume plus freeboard, roughly 70 cubic metres, on an impermeable base. The operator budgeted about CAD 240,000 to install a new synthetic-lined containment, upgrade thief hatches with vapour-recovery tie-ins, and add automated level gauges.
The upgrade eliminated the open-hatch gauging exposure, captured flash gas that had been venting, and brought the site into compliance ahead of the regulator's deadline. The recovered vapour added a small saleable gas stream, and the closed-gauging retrofit removed the single most dangerous routine task the field operators had been performing at the battery.