Roll a Tank

Key Takeaways

• Tank rolling for homogenization before custody transfer sampling is the most operationally critical application, because the financial accuracy of oil sales volumes and crude oil quality payments depends on the oil in the measurement tank being fully homogeneous at the time of sampling: crude oil in a static storage tank stratifies over time by density, with the lightest components (gas and light hydrocarbon vapors) in the vapor space, the crude oil in the main volume, and the heaviest components (water, BS&W, sand, and dense asphaltenic oil) settled at the bottom; if a sample is taken from a tank that has not been rolled, the sample composition at any given draw height reflects the local composition at that height, not the average composition of the entire tank volume; API MPMS (Manual of Petroleum Measurement Standards) Chapter 8 specifies procedures for tank sampling and mixing that require the tank to be adequately mixed before sampling, with the mixing method (recirculation pump, portable mixer, or API-approved manual mixing) and the mixing duration sufficient to achieve sample-to-sample repeatability within specified tolerances; erroneous sampling of a stratified tank understates or overstates the water content (BS&W) of the oil, which directly affects the net oil volume (total volume minus water and sediment) transferred to the buyer and the price adjustment for quality deviations from the contract specification (oil with more than 0.5% BS&W typically requires a price discount or rejection by the receiving pipeline).
• Mechanical tank mixing equipment used to roll production and sales tanks includes recirculation pump loops (a pump on the tank outlet returns fluid to a submerged nozzle that creates a circular flow pattern within the tank), portable mixers (rotating impeller devices temporarily installed in a tank hatch that create vertical and horizontal flow patterns), and gas sparging (injection of nitrogen or produced gas through a perforated pipe header at the tank bottom that creates upward bubble flow that drags liquid upward and creates a mixing current); each method has different effectiveness for different tank geometries and fluid viscosities: recirculation loops are effective for large tanks but may leave dead zones near the walls if the nozzle is positioned near the tank center rather than tangentially; portable mixers work well for small to medium tanks with low-viscosity fluids but may not provide sufficient energy to re-suspend dense bottom sediments in large tanks; gas sparging is simple and effective for tanks without explosion-hazard restrictions on gas injection (gas sparging is not permitted in tanks containing fluids above the lower explosive limit without appropriate pressure control and explosion-proof electrical systems); the mixing time required to homogenize a tank (to achieve sample-to-sample repeatability within API specifications) depends on the tank volume, the mixing energy input rate, the fluid viscosity and density difference between phases, and the depth of settled sediment, and ranges from 15 minutes for small tanks with recirculation to several hours for large crude oil storage tanks with thick bottom sediment layers.
• Rolling a frac tank (a portable storage tank used for water, completion fluid, or produced fluid during well operations) has both the intentional mixing meaning (agitating chemicals in the frac tank to ensure uniform distribution before pumping the fluid) and the accidental overturn meaning (a frac tank that rolls over due to filling imbalance, unstable ground, or mechanical failure of the support legs) that must be distinguished by context: frac tanks used for stimulation fluid storage on a completion location hold 500 to 21,000 barrels of fluid and must be rolled (mixed) before injection to ensure that acid concentrations, friction reducers, crosslinkers, and breakers are uniformly distributed at the specified concentration throughout the fluid volume; if the chemical concentrations are not uniform, the frac fluid at the start of the treatment may be over-concentrated in chemical additives (potentially damaging the formation or exceeding equipment pressure limits) while the fluid at the end of the treatment may be under-concentrated (failing to achieve the desired rheology or breaking characteristics); the accidental overturn of a frac tank (rolling in the accident sense) is a significant safety incident that typically results from overfilling one compartment of an unlevel tank, collapse of the stand structure under load, or vehicle collision, releasing the full tank contents to the wellsite and requiring emergency containment and remediation of the released fluid.
• Paraffin and asphaltene management through tank rolling addresses the specific challenge posed by waxy crudes that deposit solid paraffin on tank walls and on the settled bottom sediment layer when the tank temperature falls below the wax appearance temperature (WAT) of the crude: as crude cools in the storage tank, paraffin wax crystallizes out of solution and deposits on the cooler tank walls and bottom, eventually forming a thick, solid wax layer that reduces the effective tank volume, impedes fluid transfer from the tank, and accumulates a sludge layer of mixed wax and water at the tank bottom; rolling the tank when the crude is warm (above the WAT) by recirculation through a heated exchanger re-dissolves the wax into the bulk crude and homogenizes the tank volume, delaying or preventing the deposit buildup; adding paraffin dispersant or wax inhibitor chemicals to the tank and rolling to distribute them throughout the crude provides ongoing wax deposit prevention between planned tank cleaning operations; tanks with chronic paraffin accumulation that resists rolling (because the wax is too solid to re-dissolve at ambient temperature) require mechanical cleaning (high-pressure hot water jetting inside the tank after isolating and degassing) followed by a rolling program with hot crude to prevent re-accumulation.
• Regulatory and environmental requirements for tank rolling include the prevention of volatile organic compound (VOC) emissions during the mixing operation (which brings dissolved light hydrocarbons to the surface and accelerates their vaporization into the tank vapor space), the containment of any fluids released from tank vents or safety valves during pressurization from gas sparging, and the proper disposal of BS&W and sediment mobilized from the tank bottom during rolling: EPA regulations in the United States (Clean Air Act regulations for oil and gas production facilities) restrict the VOC emissions from crude oil storage tanks and require vapor recovery systems on tanks above certain throughput thresholds; rolling a crude oil tank generates additional VOC emissions compared to static storage because the mixing action increases the surface area and turbulence at the oil-vapor space interface, accelerating evaporation of light ends; facilities subject to VOC emission limits must account for tank rolling events in their emission calculations and in some cases must connect the tank vapor space to a vapor recovery unit during rolling to capture the displaced vapors rather than releasing them to the atmosphere; produced water tanks that are rolled to resuspend settled oil for skimming and treatment before water injection must comply with produced water disposal regulations that specify maximum oil-in-water concentrations for injection or discharge.