Well Casing and Annulus Cement Volume

Square inches of area over a foot of length is barrels divided by 1029.4, and once that is in hand the whole job is arithmetic: annulus capacity times the column height gives the slurry, casing capacity to the float collar gives the displacement. The annulus is a difference of SQUARES, which means it is far more sensitive to hole size than to casing size and is why an oversized or washed-out hole eats cement fast. Excess is where the honesty lives. A gauge hole needs little; a washed-out shale section can need double, and the spread between a reasonable low and high assumption is reported here in barrels and in feet of column, because that spread is the actual uncertainty in where the top of cement lands. A caliper log turns excess from a guess into a measurement, and running one before a critical job is the difference between hitting the planned top of cement and finding out later from a bond log. Getting the DISPLACEMENT wrong is the more dangerous error and it fails in both directions: over-displacing pumps cement past the float and back up the annulus from the wrong end, and under-displacing leaves cement inside the casing to drill out. Neither is recoverable cheaply, which is why the displacement is computed from the casing's own inside diameter and the measured depth to the float rather than estimated. Capacities assume a clean concentric annulus and a gauge hole apart from the entered excess; a real hole is neither. This does not design the slurry, its density, thickening time or free water, evaluate centralisation and mud removal -- which decide whether the cement actually bonds -- account for compressibility or losses to the formation, or address stage tools and multi-stage jobs. The cementing program, the service company's job design, and the well engineer govern.

annular capacity bbl/ft = (D_hole^2 - D_casing^2) / 1029.4 and pipe capacity bbl/ft = ID^2 / 1029.4; slurry = annular capacity x column length x (1 + excess); sacks = slurry in cubic feet / the yield per sack; displacement = casing capacity x depth to the float collar.

The oilfield capacity relations as every well-control manual writes them, with the 1029.4 constant converting square inches over a foot into barrels, and barrels at 42 gallons of 231 cubic inches, exact by definition. Excess and slurry yield are ENTERED. It does not design the slurry, its density, thickening time or free water, evaluate centralisation and mud removal, account for compressibility or losses to the formation, or address stage tools.

Two capacities and a volume; no service company data is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: hole_dia_in, casing_od_in, casing_id_in, cement_column_ft, excess_pct, float_collar_ft, slurry_yield_ft3_per_sack, low_excess_pct, high_excess_pct, annular_capacity_bbl_ft, casing_capacity_bbl_ft, annular_volume_bbl, slurry_volume_bbl, displacement_bbl, excess_spread_bbl

• A gauge hole apart from the entered excess a caliper log turns excess from a guess into a measurementthe caliper log
• A clean concentric annulus centralisation and mud removal decide whether the cement bondsthe cementing program
• No losses to the formation compressibility and losses are not modelledthe service company's job design