Drilling guides › Cement volume: slurry, sacks, mix water and displacement
Cement volume: slurry, sacks, mix water and displacement
A primary cement job has to put the right volume of slurry in the right place: enough to reach the planned top of cement, not so much that it is wasted or overloads a weak formation, and with the top plug landing on the float collar instead of being pumped past it. Every one of those outcomes comes down to a handful of capacity sums done before the job. This guide walks through them in the order a cementer or drilling engineer would, and finishes with a complete two-slurry example.
Where the slurry goes
Split the job into sections, because each one has a different capacity:
- Open-hole annulus: between the new casing and the drilled hole, from the shoe up to the previous casing shoe (or to the top of cement if that is deeper). The real hole is rarely gauge, so an excess is added here.
- Cased-hole overlap: between the new casing and the inside of the previous casing, from the previous shoe up to the top of cement. This diameter is known, so no excess is normally added.
- Shoe track: inside the casing between the float collar and the shoe, usually one to three joints. The top plug stops at the float collar, so this slurry stays inside the pipe and gives the best-quality cement around the shoe.
- Rat hole: any hole drilled below the shoe depth. It is full hole (no pipe in it) and is often given the same excess as the open hole.
Capacity formulas
Diameters are in inches. 1029.4 is the constant that turns inches squared into barrels per foot, so a 1 in ID holds 1 ÷ 1029.4 = 0.000971 bbl/ft. In metric units, capacity in m³/m is 0.7854 × (D² − d²) with diameters in metres.
Choosing the excess
Excess covers washouts, a hole drilled slightly over gauge, and slurry lost to the formation. Many operators use 20 to 50% over bit size in open hole when no caliper log is available, and less (often around 10 to 20%) over the caliper volume when one is. Soft or washed-out shale sections justify the higher end. Too much excess is not free: it raises the column height, the hydrostatic load on weak zones and the cost of the job.
From volume to sacks and water
The lab test for each slurry reports its density, its yield (the slurry volume one sack makes, in ft³/sk) and its mix water requirement (gal/sk). These depend on the cement class, additives and water ratio, so always take them from the lab sheet for your blend.
For reference, neat Class G cement mixed at 15.8 ppg is commonly quoted at about 1.15 ft³/sk yield and about 5.0 gal/sk of water. Lightweight lead slurries with extenders yield much more per sack and need much more water.
Displacement
Once the slurry is pumped, the top plug is dropped and pushed down with mud or water until it lands (bumps) on the float collar. The displacement volume is the capacity of the casing from surface to the float collar, plus any surface lines that were not already full:
Use the actual ID of each casing weight in the string, not a nominal figure, and apply a realistic pump efficiency. Watch for the plug to bump near the calculated strokes. If it has not bumped by the calculated volume, many programs allow over-displacing by no more than about half the shoe track before stopping, so that cement around the shoe is not replaced by mud.
Worked example 1: slurry volumes for a 9-5/8 in string
9-5/8 in casing (47 lb/ft, ID 8.681 in) is run in 12-1/4 in hole to 8,000 ft, with 10 ft of rat hole below the shoe. The previous 13-3/8 in casing (ID 12.415 in) is set at 3,000 ft. The shoe track is 80 ft. Plan: tail slurry from the shoe to 6,500 ft, lead slurry from 6,500 ft to a top of cement at 2,500 ft (500 ft inside the 13-3/8 in). Use 30% excess in open hole and rat hole.
In metric terms that is about 18.5 m³ of tail and 45.1 m³ of lead (1 bbl = 0.159 m³).
Worked example 2: sacks, mix water and displacement
The lab sheet gives: tail, Class G at 15.8 ppg, yield 1.15 ft³/sk, mix water 4.97 gal/sk; lead, an extended blend at 12.8 ppg, yield 1.85 ft³/sk, mix water 10.0 gal/sk. The pump is a triplex with 6 in liners and a 12 in stroke at 97% efficiency.
Half the shoe track is 40 ft × 0.0732 = 2.9 bbl, about 29 strokes, which is the most you would over-displace under the rule above if the plug has not bumped. Order bulk cement with a margin for dead volume in the silos and lines; the sack figures here are what ends up in the hole.
Practical checks
Before the job, compare the calculated slurry volume with a quick estimate from the annular capacity alone; an answer that is off by a factor of two usually means a wrong diameter or a dropped excess. Check the hydrostatic pressure of the full cement column plus spacer against the fracture pressure of the weakest zone, because a column that is too heavy causes losses and a low top of cement. During the job, track returns: if returns are lost while pumping slurry, the top of cement will be lower than planned and the excess was not enough.
Common mistakes
- Adding excess to the cased overlap, where the diameter is already known.
- Forgetting the shoe track or rat hole, which leaves the shoe short of cement.
- Using the casing OD instead of the previous casing's ID for the overlap.
- Calculating displacement to the shoe instead of the float collar, which pumps the shoe track cement out into the annulus.
- Using a yield or water figure from a different blend. Yield changes with every additive and water ratio.
- Ignoring the change in ID in a combination string when working out displacement.