Drilling guides › Buoyancy factor, hookload and weight on bit
Buoyancy factor, hookload and weight on bit
The weight indicator is the driller's most-watched gauge after the standpipe pressure. It shows the load hanging from the hook, and almost every decision about tripping speed, overpull, weight on bit and stuck pipe starts from it. To read it properly you need to know what the string should weigh in the mud you have in the hole, and how that number changes between hanging free, tripping and drilling. That starts with buoyancy.
Why a string weighs less in mud
A drill string hanging in mud is pushed upward by the fluid pressure acting on its lower end and on every shoulder where the cross-section changes. Archimedes' principle gives the same answer in a simpler form: the string loses weight equal to the weight of mud its steel displaces. Because steel and mud fill the same volume, the ratio of buoyed weight to air weight depends only on their densities.
65.5 ppg is the density of steel (about 7.85 sg). In metric units, BF = 1 − mud density (sg) ÷ 7.85. Some references use 65.4 ppg; the difference is negligible in the field.
| Mud weight (ppg) | Buoyancy factor | Weight lost |
|---|---|---|
| 8.33 (fresh water) | 0.873 | 12.7% |
| 10.0 | 0.847 | 15.3% |
| 12.0 | 0.817 | 18.3% |
| 15.0 | 0.771 | 22.9% |
The simple factor assumes the same mud inside and outside the pipe and full columns of it. When they differ, such as with a heavy slug in the drill pipe or during cementing, the string's apparent weight changes with the pressure difference, and you have to work with the pressures acting on each area instead.
Use the right weight per foot
Drill pipe has a nominal weight (for example 19.5 lb/ft for a common 5 in size) that excludes the tool joints. For string weight sums, use the adjusted or approximate weight from the pipe data sheet, which includes the tool joints and is a few pounds per foot heavier. Using the nominal figure makes the predicted hookload too light.
What the weight indicator shows
The hookload includes the travelling block, hook and top drive unless the indicator was zeroed with them hanging. Make sure everyone on the floor knows which convention is in use, or a 50,000 lb block weight can be mistaken for string weight.
- Hanging still, off bottom, not rotating: close to the buoyed string weight plus block weight. This is the baseline.
- Picking up (pulling out): buoyed weight plus friction from the hole wall. The difference from the baseline is the drag or overpull.
- Slacking off (running in): buoyed weight minus friction.
- Rotating off bottom: most of the axial friction disappears, so this reading is usually closest to the true free-hanging weight in a deviated hole.
- Drilling: the string weight minus the weight you have set on the bit.
Take the off-bottom reading with the same pump rate and rotation you will drill with, just above bottom, so that pump-off forces and rotating friction cancel out. Re-zero it regularly, because the string gets heavier with every stand drilled.
Worked example 1: expected hookload and drilling weight
A vertical 12,000 ft well in 12.0 ppg mud. The string is 600 ft of 8 in × 2-13/16 in drill collars (147 lb/ft), 900 ft of 5 in heavy-weight drill pipe (49.3 lb/ft) and 10,500 ft of 5 in drill pipe whose adjusted weight on the data sheet is 22.5 lb/ft. Block and top drive weigh 50,000 lb and are not zeroed out.
To drill with 35,000 lb on the bit, the driller slacks off until the indicator reads about 351,250 − 35,000 = 316,250 lb. If the hole shows 30,000 lb of drag pulling out and 20,000 lb running in, expect about 381,250 lb picking up and 331,250 lb slacking off. Readings that drift further from these numbers over several stands are an early sign of a dirty or tight hole.
Weight on bit and the neutral point
In a vertical well, weight on bit should come from the drill collars, not from the drill pipe. Drill pipe in compression buckles and its tool joints and body fatigue quickly. The point in the string where the axial load changes from compression to tension is called the neutral point, and it should stay inside the collars with a margin.
A design factor of 0.85 is a widely used rule of thumb, which keeps the neutral point in the lower 85% of the collars. In example 1 the neutral point at 35,000 lb WOB is 35,000 ÷ (147 × 0.8168) ≈ 292 ft above the bit, well inside the 600 ft of collars, and the maximum weight under the 85% rule is 0.85 × 88,200 × 0.8168 ≈ 61,200 lb.
In a deviated well only the component of weight along the hole axis pushes on the bit, which is roughly the buoyed weight times the cosine of the inclination, and many directional BHAs deliberately put heavy-weight pipe in compression where the hole angle stops it buckling. Use a torque and drag model for those wells.
Worked example 2: how many collars for the planned weight
The next section is planned with up to 40,000 lb on the bit in the same 12.0 ppg mud, using 8 in collars at 147 lb/ft in 31 ft joints and a 0.85 design factor.
The planned 600 ft of collars more than covers this. If the mud is weighted up later, the buoyancy factor drops and the same collars give a little less available weight, so repeat the check after any large change in mud weight.
Common mistakes
- Using nominal drill pipe weight instead of the adjusted weight with tool joints.
- Forgetting whether the block and top drive weight is included in the reading.
- Taking the off-bottom reference without pumps on, then drilling with pumps on.
- Not re-checking buoyancy and available bit weight after weighting up.
- Running drill pipe in compression in a vertical hole to get more bit weight.
- Ignoring a slow increase in pick-up weight over several connections; it is often the first sign of cuttings building up or a tightening hole.