Drilling guides › Mud tank volume: rectangular and cylindrical pits
Mud tank volume: rectangular and cylindrical pits
Knowing the volume per inch of every pit turns a level reading into a pit gain or loss. It is also how the active system volume on the kill sheet is checked.
Pit volume is the first line of kick detection. Flow sensors and gas detectors help, but the plain fact that more mud is coming out of the well than is going in shows up first as a level rising in the active pits. That only means something if each inch of level can be turned into barrels quickly and correctly.
Rectangular tanks
5.615 is the number of cubic feet in one barrel; 67.38 is 5.615 × 12.
Length times width is the floor area of the pit, and each foot of mud on that area is that many cubic feet. Dividing by 5.615 turns cubic feet into barrels, and dividing by 12 more gives the volume of a one-inch layer, which is what a level float or a dipstick reads.
Worked example
A pit 30 ft long, 10 ft wide and 8 ft deep:
A 2 in rise in this pit is a 9 bbl gain.
Why trip tanks are small
A trip tank exists to measure small volumes accurately. Compare a 4 ft × 4 ft trip tank with the pit above:
Worked example: resolution
One stand of 5 in drill pipe needs about 0.70 bbl of fill. In the trip tank that is almost 3 in of level change, easy to read. In the active pit it is about a sixth of an inch, which is lost in the ripple from the agitators. This is why hole fill on trips is always measured on the trip tank.
Vertical cylindrical tanks
A 10 ft diameter tank holds 13.99 bbl per foot of height, or about 1.17 bbl per inch.
The constant is π ÷ 4 ÷ 5.615: the area of a circle in square feet turned straight into barrels per foot of height.
Horizontal cylindrical tanks
Some storage tanks and older trip tanks are cylinders lying on their side. Here the volume per inch changes with the level: small near the bottom and top, largest at the middle. A straight proportion of height gives the wrong answer.
R is the radius, h the mud level and L the length, all in feet, with the arc cosine in radians.
Worked example: horizontal tank
A tank 8 ft in diameter and 30 ft long has mud 3 ft deep (R = 4 ft, h = 3 ft).
A straight 3/8 of the full volume would say 100.7 bbl, almost 9 bbl too much. For tanks like this, a strapping table worked out once and taped to the tank is the practical answer.
Metric
Worked example: metric pit
A pit 9 m long, 3 m wide and 2.4 m deep holds 9 × 3 × 2.4 = 64.8 m³, or about 408 bbl. Each centimetre of level is 9 × 3 × 0.01 = 0.27 m³, about 1.7 bbl.
Usable and dead volume
Not all the mud in a pit can be pumped. The suction sits some distance above the floor, and below that level the mud is dead volume. Sloped or V-shaped bottoms, internal stiffeners, agitator shafts and gun lines all change the real volume per inch, usually near the bottom. The level the sensor reads is also not always the true average level: agitators, mud guns and, on floating rigs, the roll and heave of the hull all move the surface.
From a level change to a kick size
When two or more active pits are connected, they rise together, so their barrels per inch add up. Two pits like the 30 ft × 10 ft example give about 8.9 bbl per inch between them.
Worked example: pit gain
With both pits on line, the level rises 1.2 in during a connection with the pumps off and no transfers.
That is the pit gain to write on the kill sheet after shutting in. The volume also feeds the kick tolerance check and helps the crew judge how high the influx reaches in the annulus.
The more pits that are in the active system, the less the level moves for the same gain, and the harder a small kick is to see. Keeping the active system as small as the operation allows is one of the simplest ways to catch kicks early.
Common mistakes
- Using outside dimensions from a drawing instead of measured inside dimensions.
- Counting dead volume as usable when planning a kill mud or cement mix.
- Leaving a pit in or out of the active total without telling the driller and mud logger, so the alarm compares the wrong numbers.
- Transferring between pits during drilling without recording it, which looks like a gain in one pit and a loss in another.
Good practice
- Measure the inside dimensions, and note sloped bottoms or internal frames that take volume.
- Keep a strapping table for each pit, especially odd-shaped ones.
- Agree with the mud logger which pits are in the active system before drilling ahead, so pit gain alarms are set on the right total.
- Check level sensors against a manual reading every tour, and log every transfer, dilution and addition of mud from the reserve.
- Set the pit gain alarm as tight as the surface noise allows. A few barrels is common on land; floating rigs often need compensation for motion.