Cement Job Planning - Complete Job Sheet Calculations from Volume to Pump Schedule
A cement job sheet is the engineering document that governs every pump stroke of a primary cement job. It specifies, in order, every fluid to be pumped, the volume of each fluid, the pump rate, the cumulative pump strokes at each transition, the expected surface pressure at each stage, and the critical decision points where the job can be modified or stopped if real-time conditions deviate from the plan. An experienced cementing engineer builds this document before the job begins, working backward from the designed cement coverage and forward through each calculation to confirm the job is feasible within the thickening time, within the ECD window, and within the pump capacity available. The calculation sequence is not complex - but each step must be completed in the correct order, and each number feeds the next calculation. A error in volume calculation propagates to stroke count, which propagates to cement top depth, which propagates to the entire job. This guide walks through a complete cement job sheet from first principles.
 |
1. Well Data and Design Basis
1.1 Input Data for Job Sheet Calculation
The following example designs a complete primary cement job for a 9-5/8" production casing string. All calculations build from these input parameters:
| Parameter | Value | Source |
|---|---|---|
| Casing OD | 9.625 inches | Casing program / API tables |
| Casing ID | 8.535 inches (47 lb/ft) | API casing table for 9-5/8" 47 lb/ft |
| Hole diameter | 12.25 inches | Bit size used for this section |
| Casing shoe depth (TVD / MD) | 10,500 ft TVD / 11,200 ft MD | Drilling program |
| Previous casing shoe depth | 6,200 ft TVD | Previous casing record |
| Open hole interval | 10,500 - 6,200 = 4,300 ft | Calculated |
| Planned cement top (TVD) | 8,000 ft (1,500 ft above gas zone at 9,500 ft) | Well design requirement |
| Mud weight in hole | 13.5 ppg | Drilling report - mud at TD |
| Fracture gradient at previous shoe | 15.2 ppg at 6,200 ft TVD | LOT result |
| Pump output (HHL triplex pump) | 0.1285 bbl/stroke | Pump specification at liner size used |
2. Volume Calculations
2.1 Step 1 - Annular Volume (Open Hole Section)
Annular capacity (bbls/ft) = (Dh^2 - OD_casing^2) / 1,029.4
= (12.25^2 - 9.625^2) / 1,029.4
= (150.06 - 92.64) / 1,029.4
= 57.42 / 1,029.4 = 0.05578 bbls/ft
Cement coverage interval:
From shoe (10,500 ft TVD) to planned cement top (8,000 ft TVD) = 2,500 ft
Annular volume in cemented interval = 0.05578 x 2,500 = 139.5 bbls
Add 25% excess for hole irregularity (intermediate section typical):
Annular volume with excess = 139.5 x 1.25 = 174.4 bbls total annular volume to fill
2.2 Step 2 - Casing Internal Volume (Displacement Volume)
Casing internal capacity (bbls/ft) = ID^2 / 1,029.4
= 8.535^2 / 1,029.4 = 72.85 / 1,029.4 = 0.07077 bbls/ft
Total casing string length = 11,200 ft MD
(MD is used for displacement calculation - actual string length)
Casing displacement volume = 0.07077 x 11,200 = 792.6 bbls
Note: This is the volume of drilling mud that must be pumped after the cement to push the top wiper plug from surface to the float collar.
Subtract shoe track volume (float collar to shoe):
Float collar at 2 joints above shoe = 60 ft above shoe
Shoe track volume = 0.07077 x 60 = 4.3 bbls
Net displacement volume = 792.6 - 4.3 = 788.3 bbls displacement fluid
2.3 Step 3 - Slurry Volume and Sack Count
Tail slurry (bottom 500 ft of cement, 15.8 ppg Class G neat):
Tail volume = 0.05578 x 500 x 1.25 = 34.9 bbls
Yield of 15.8 ppg neat Class G slurry = 1.15 ft3/sk = 0.2048 bbl/sk
Sacks of Class G for tail = 34.9 / 0.2048 = 170 sacks tail slurry
Water for tail: 170 x 5.2 gal/sk = 884 gallons
Lead slurry (remaining 2,000 ft of cement coverage, 12.5 ppg lightweight):
Lead volume = 0.05578 x 2,000 x 1.25 = 139.5 bbls
Yield of 12.5 ppg lead slurry (with microspheres) = 2.10 ft3/sk = 0.3737 bbl/sk
Sacks of Class G for lead = 139.5 / 0.3737 = 373 sacks lead slurry
Water for lead: 373 x 7.8 gal/sk = 2,909 gallons
Total cement volumes:
Tail: 34.9 bbls (170 sacks)
Lead: 139.5 bbls (373 sacks)
Total slurry: 174.4 bbls (543 sacks total Class G)
Total mix water: 884 + 2,909 = 3,793 gallons = 90.3 bbls
3. Pump Rate and Job Duration
3.1 Step 4 - ECD Check at Critical Shoe
Maximum allowable pump rate based on ECD at previous shoe (6,200 ft TVD, FG = 15.2 ppg):
At the critical moment (tail slurry being pumped, max density fluid in annulus at shoe):
ECD at shoe = [(MW x TVD_shoe + dP_friction_annulus) / (0.052 x TVD_shoe)]
Annular friction pressure at shoe (psi) from tail slurry:
APL = (0.2 x PV x Va + (300 x YP x (Dh-Dc))) / (300 x (Dh-Dc)^2)
Tail slurry rheology (15.8 ppg): PV = 35 cp, YP = 25 lb/100ft2
Va = Q / (0.05578 x 42) = Q / 2.343 (ft/min, Q in bbl/min)
Trial pump rate Q = 6 bbl/min:
Va = 6 / 2.343 = 2.56 ft/sec = 153.6 ft/min
APL per 1,000 ft ≈ (0.2 x 35 x 153.6 + 300 x 25 x 2.625) / (300 x 2.625^2) = (1,075 + 19,688) / 2,068 = 9.99 psi/1,000 ft
Total APL at shoe = 9.99 x 6.2 = 61.9 psi → ECD contribution = 61.9 / (0.052 x 6,200) = 0.19 ppg
ECD at previous shoe = MW + APL_contribution = 13.5 + 0.19 = 13.69 ppg
FG at shoe = 15.2 ppg → SF = 15.2/13.69 = 1.11 → Adequate margin at 6 bbl/min
Maximum pump rate before fracturing shoe: dECD ≤ FG - MW = 15.2 - 13.5 = 1.7 ppg
Maximum APL = 1.7 x 0.052 x 6,200 = 548 psi → Max rate approximately 11-12 bbl/min
Design pump rate: 6 bbl/min (conservative, 1.7 ppg ECD safety margin)
3.2 Step 5 - Total Job Duration
Pre-flush / chemical wash:
Volume: 20 bbls
Pump rate: 8 bbl/min (higher rate acceptable for light wash)
Duration: 20 / 8 = 2.5 minutes
Strokes: 20 / 0.1285 = 156 strokes
Spacer (13.8 ppg weighted spacer, 30 bbls):
Pump rate: 6 bbl/min
Duration: 30 / 6 = 5.0 minutes
Strokes: 30 / 0.1285 = 234 strokes
Lead slurry (12.5 ppg, 139.5 bbls):
Pump rate: 6 bbl/min
Duration: 139.5 / 6 = 23.3 minutes
Strokes: 139.5 / 0.1285 = 1,086 strokes
Tail slurry (15.8 ppg, 34.9 bbls):
Pump rate: 5 bbl/min (reduce rate when dense cement reaches shoe to limit ECD)
Duration: 34.9 / 5 = 7.0 minutes
Strokes: 34.9 / 0.1285 = 272 strokes
Displacement (13.5 ppg mud, 788.3 bbls):
Pump rate: 6 bbl/min (restore rate when cement in annulus, ECD stabilizes)
Duration: 788.3 / 6 = 131.4 minutes
Strokes: 788.3 / 0.1285 = 6,135 strokes
Total pumping time = 2.5 + 5.0 + 23.3 + 7.0 + 131.4 = 169.2 minutes = 2.82 hours
Total strokes = 156 + 234 + 1,086 + 272 + 6,135 = 7,883 strokes
4. Complete Pump Schedule - The Job Sheet
4.1 The Job Sheet Table
| Stage | Fluid | Density (ppg) | Volume (bbls) | Rate (bbl/min) | Duration (min) | Cumulative Strokes | Action / Check |
|---|---|---|---|---|---|---|---|
| 1 | Chemical wash | 8.5 | 20 | 8 | 2.5 | 156 | Confirm pump rate. Monitor returns. Baseline pressure. |
| 2 | Weighted spacer | 13.8 | 30 | 6 | 5.0 | 390 | Pressure increase expected as spacer replaces wash. Monitor ECD. |
| 3 | Lead slurry | 12.5 | 139.5 | 6 | 23.3 | 1,476 | Monitor density at pump (target 12.5 ±0.2 ppg). Record strokes. |
| 4 | Tail slurry | 15.8 | 34.9 | 5 | 7.0 | 1,748 | Reduce to 5 bbl/min. Monitor ECD at previous shoe. HIGH RISK PERIOD. |
| 5 | Displacement (mud) | 13.5 | 788.3 | 6 | 131.4 | 7,883 | Stop at 7,883 strokes. Watch for plug bump. Do NOT over-displace. |
5. Thickening Time Verification
5.1 Does the Job Complete Within Thickening Time?
Total pumping time from first cement to bump = Time from stage 3 to end of stage 5
= 23.3 (lead) + 7.0 (tail) + 131.4 (displacement) = 161.7 minutes from first lead slurry to bump
Most critical time: Tail slurry from mixing to placement at bottom (shoe)
Tail slurry mixes first and reaches the shoe last during displacement.
Actually: Tail is pumped AFTER lead, so tail exits casing at shoe during displacement.
Time from tail mixing to tail at shoe = 7.0 (pumping tail) + 131.4 (displacement) = 138.4 minutes
Required thickening time for tail slurry = 138.4 + 90 minutes safety margin = 3.8 hours minimum
Confirm lab-tested tail thickening time at BHCT > 3.8 hours before approving job.
Lead slurry time from mixing to cement top placement:
Time = 23.3 (pumping lead) + 7.0 (pumping tail) + portion of displacement = approximately 180 minutes total
Required lead thickening time = 180 + 90 = 4.5 hours minimum
Conclusion
The pump schedule in this article - 7,883 total strokes, with the critical stop-pump decision at exactly stroke 7,883 - demonstrates what a complete job sheet provides that a simple T = V/Q duration calculation cannot: the cumulative stroke count at each transition between fluids, and the specific decision point where the displacement must stop. The displacement is 788.3 bbls at 0.1285 bbl/stroke = 6,135 strokes - this is the number that the toolpusher watches on the stroke counter and stops pumps at regardless of whether a plug bump is felt. Over-displacing by 50 strokes (6.4 bbls) pushes 6.4 bbls of displacement mud through the float collar into the annulus, displacing 6.4 bbls of tail cement upward and reducing the cement coverage at the shoe by 6.4/0.05578 = 115 ft. That 115 ft gap in the shoe cement may be exactly over the gas zone the cement was designed to isolate.
The ECD check that limits the pump rate to 6 bbl/min rather than the maximum available rate shows why pump rate selection requires calculation, not operator preference. At 12 bbl/min, the ECD at the previous shoe would reach 14.5 ppg - within 0.7 ppg of the 15.2 ppg fracture gradient with no safety margin for density fluctuations, annular bridging, or flow restrictions. At 6 bbl/min, the ECD is 13.69 ppg - 1.51 ppg below the fracture gradient. The 50% pump rate reduction costs 131 additional minutes of displacement time but provides the engineering safety margin that makes the operation controllable.
Want to access our complete cement job sheet calculator with volume, stroke count, duration, ECD, and thickening time verification, or discuss job planning for a specific well? Join our Telegram group for cementing discussions, or visit our YouTube channel for step-by-step tutorials on cement job planning and pump schedule design.
0 Comments