Slurry Yield: Sack Volume, Water Ratio, and Cement Job Design in WCSB Wells

Slurry yield is the volume of cement slurry produced when one sack of dry cement is mixed with its design quantity of mixing water and any additives, and it is the parameter that converts a sack count into the cubic metres or cubic feet of slurry available to fill an annulus. It is reported in cubic feet per sack (ft3/sk) in field units, where one sack of cement is taken as 94 pounds, and in cubic metres per kilogram or cubic metres per tonne in metric. The yield is computed from the absolute volumes of every component: the dry cement contributes its mass divided by its absolute density of roughly 3.14 grams per cubic centimetre, the mixing water contributes its full volume at one gram per cubic centimetre, and each additive contributes its mass divided by its own absolute density. Summing those absolute volumes and dividing by the constant 7.4805 gallons per cubic foot gives the yield directly. For neat API Class G cement at a water-to-cement ratio near 44 percent by weight, the slurry weighs about 15.8 pounds per gallon and yields close to 1.15 ft3/sk, a number cementers in the Western Canadian Sedimentary Basin commit to memory. Yield is inseparable from slurry density, water requirement, and mix water, because raising the water ratio raises both the yield and the volume but lowers the density, while adding extenders such as bentonite or pozzolan raises yield dramatically and lightens the column. A bentonite-extended lead slurry can yield well above 2 ft3/sk, whereas a densified slurry weighted with hematite and run at low water ratio can fall below 1.0 ft3/sk. Accurate yield is the foundation of every primary cementing job: the engineer first calculates the annular and shoe-track volume to be filled from the caliper or open-hole gauge, applies an excess factor for washouts, then divides by the yield to arrive at the number of sacks to order and blend. Underestimating yield leaves the top of cement short of the design depth and can fail to isolate a gas zone; overestimating it wastes product and can lift cement into unwanted intervals. In the WCSB, primary jobs on surface, intermediate, and production casing across plays such as the Montney, Cardium, and Viking all begin from a yield figure, and AER Directive 009 and Directive 008 set the cementing and zonal-isolation expectations that those volumes must satisfy. Yield also governs the economics of large multi-stage horizontal completions, where small per-sack volume errors multiply across hundreds of tonnes of blend and translate into real CAD swings on the cement bill.

Calculating Yield for a Class G Lead and Tail System

A two-slurry production job is the common WCSB pattern: a lighter lead slurry fills most of the annulus and a denser tail slurry covers the pay and shoe. Suppose the lead is Class G with 8 percent bentonite at a 12.5 ppg target, yielding about 1.8 ft3/sk, and the tail is neat Class G at 15.8 ppg, yielding 1.15 ft3/sk. The cementer calculates each interval volume separately, divides by the matching yield, and orders two distinct blends. Because the lead yields far more slurry per sack, fewer sacks fill the long upper annulus, while the tail's tighter yield delivers the strength and density needed across the producing formation.

Why Lab-Confirmed Yield Beats the Textbook Number

Published yields assume clean water and nominal additive densities, but field mix water in the WCSB often carries dissolved solids, and bulk cement can vary batch to batch. A pilot test on the actual rig water and blend run in the service company lab returns the true yield, density, thickening time, and fluid loss before pumping. A yield that comes back 5 percent low on a 200 tonne intermediate job means the planned sacks fall short of design top of cement, so the lab check directly protects against a remedial squeeze. Operators routinely pay for this testing because a failed primary job and follow-up squeeze can cost CAD 150,000 or more.

Related Terms

Slurry yield is one of three numbers that always travel together in job design. It is set against slurry density, since the water and additive volumes that raise yield simultaneously lower the pounds-per-gallon weight of the column. It depends on the cement class and its additives, each contributing absolute volume to the blend. And it serves the larger cementing operation, where yield converts the annular volume into the sack count that achieves zonal isolation across the casing string.

Sizing a Surface Casing Job near Grande Prairie

An operator setting 244.5 mm surface casing to 620 metres in a Montney development well near Grande Prairie gauged the open hole with a caliper and found an average 12 percent washout, giving an annular plus shoe-track volume of about 14.8 cubic metres after a 30 percent excess factor. Using a Class G slurry yielding 1.15 ft3/sk, or about 0.0326 cubic metres per sack, the cementer divided to arrive at roughly 454 sacks, or near 21 tonnes of blend, and confirmed the yield on rig water in the service company lab before mixing.

The lab returned a yield within 2 percent of the design figure, the job placed cement to surface as required by AER Directive 008 for surface casing, and the bond log confirmed isolation across the shallow groundwater interval. Total cementing cost for the surface string came in near CAD 38,000, with the accurate yield ensuring no costly top-up job was needed.