Sack

Key Takeaways

• Cement slurry design and calculation using the sack as the base unit allows the cementing engineer to convert between slurry properties (density, yield, mix water ratio) and the physical quantities of materials that must be blended and pumped to achieve the required annular fill: the slurry density calculation begins with the absolute volumes of each component per sack of cement (cement solids at 3.59 gal/sack, mix water at the design ratio in gallons, and additive volumes per sack from their specific gravity), which are summed to give the total slurry volume per sack (the yield), and the total mass per sack (cement weight plus water weight plus additive weight) divided by the yield gives the slurry density in pounds per gallon; API Specification 10A defines the standard API classes of Portland cement and their physical requirements, with Class G being the most widely used base cement for oil and gas wells because of its wide temperature and pressure application range and its compatibility with a broad range of chemical additives; the mix water ratio (gallons of water per sack of cement, typically 4.3 to 7.7 gallons/sack for neat API Class G depending on the target density) is the primary design variable that controls slurry density and water content, with higher water ratios producing lower density, higher yield, and weaker set cement; the API RP 10B cementing calculation procedures provide the standardized algorithms for converting between sack-based quantities and the slurry volume, density, and pressure parameters used in job design and execution.
• Cement sack counting and inventory management at the wellsite ensures that sufficient cement is available for the planned job plus contingency volumes, accounting for the potential requirement to extend the cement column if the planned top of cement is not achieved due to loss of returns, channeling, or calculation errors: the standard contingency for primary casing cementing jobs is 15 to 25 percent additional cement above the calculated theoretical requirement, accounting for washouts in the open hole (which increase the annular volume above the bit size calculation), U-tube effects that redistribute cement during displacement, and uncertainties in the actual caliper log volume versus the bit size calculation; cementing companies deliver bulk cement to offshore locations in cement tanks on supply vessels or in offshore storage tanks, with the total quantity expressed in metric tons rather than individual sacks for large jobs (1 metric ton = approximately 21.3 sacks of API Class G cement at 94 lb/sack); the inventory tracking system records cement consumption in sacks per pump stroke (using the slurry yield per sack and the pump displacement per stroke to calculate sacks pumped), allowing the cementing operator to confirm in real time that the actual cement pumped matches the theoretical sack count based on the return strokes counted from the start of the cement job.
• Additive dosage in cement slurry design is expressed in units per sack of cement (gallons per sack for liquid additives, pounds per sack or percent by weight of cement (BWOC) for solid additives), which allows the additive concentration to be specified independently of the slurry volume and then scaled to the total sacks required for the job: a retarder at 0.5 percent BWOC means 0.5 pounds of retarder per 100 pounds of cement, or 0.47 pounds per sack (94 lb/sack x 0.005 = 0.47 lb/sack); a fluid-loss additive at 0.8 gallons per sack means 0.8 gallons of liquid additive must be metered per sack of cement pumped; the per-sack additive quantities are used to design the mixing unit's additive pump rates (setting the liquid additive pump delivery in gallons per minute for the planned mixing rate in sacks per minute), and to calculate the total additive quantities that must be on location for the job (total sacks x additive per sack = total additive required); the per-sack basis for additive concentration also makes the comparison between laboratory test slurries and field slurries straightforward, since the laboratory test uses the same additive ratios per sack as the field formulation, and the lab-measured thickening time, fluid loss, and compressive strength development at the planned downhole temperature and pressure are expected to replicate in the field job if the sack-based mixing ratios are correctly maintained.
• Bulk cement handling and mixing at the wellsite involves mechanical equipment that meters cement into the mixing unit at a controlled rate measured in sacks per minute, with the cement delivered from bulk storage via pneumatic conveyance (blowing with compressed air from the bulk tank to the re-circulating hopper of the mixing unit) or auger transfer, and the cement feed rate controlled by the mixing unit operator to maintain the target slurry density: the Halliburton-style and BJ-style recirculating mixing systems (the two most common designs) both operate by recirculating the partially mixed slurry through a jet mixer or tub system to progressively hydrate the cement particles and achieve a uniform, fully-mixed slurry before displacement; the mixing rate in sacks per minute determines how quickly the job can be completed and how accurately the slurry properties can be maintained (faster mixing rates increase the risk of density variation if the cement feed and water feed do not track each other precisely); batch mixing (mixing the entire cement job volume in a mixing tank before pumping) provides more uniform slurry properties than continuous mixing but requires a large mixing tank and slows the job because the slurry must be fully mixed before pumping begins; the target mixing density (as determined in pounds per gallon) is monitored continuously during mixing by a nuclear density meter installed on the slurry line from the mixer to the pumps, with the operator adjusting the cement feed rate or water rate to maintain the density within tolerance (typically plus or minus 0.3 pounds per gallon) throughout the job.
• Alternative sizing and packaging for oilfield cement includes 50-pound sacks (used in some European and Latin American markets, where local cement is packaged in 50-kg or 50-lb bags rather than the US standard 94-lb sack), 1-metric-ton super sacks (bulk bags used for offshore cement storage and transfer), and bulk cement deliveries measured in short tons or metric tons that are converted to sacks for job design purposes: the non-US sack sizes require different yield and density calculation factors than the standard 94-lb US sack, and cementing engineers working internationally must confirm the local sack weight before applying standard US calculation procedures; bulk cement quality is controlled by periodic sampling and testing to API Specification 10A at the manufacturing plant and at major distribution terminals, with the test results (fineness, surface area, thickening time, compressive strength, and free water) provided on certificate of conformance documents that accompany each cement shipment; the quality variability of field bulk cement compared to laboratory test cement (from settling and segregation in bulk storage, contamination from previous cement grades, and variation between manufacturing batches) is a recognized source of field-to-lab performance discrepancy in cementing, requiring conservative job design that accounts for the uncertainty in actual field cement performance relative to the laboratory test results that the job was designed around.