Drilling guides › Trip sheets: hole fill, pipe displacement and swab checks
Trip sheets: hole fill, pipe displacement and swab checks
Many kicks happen on trips. A trip sheet compares the mud the hole should take with what it actually takes, stand by stand, so a swabbed influx is caught while it is still small.
Trips are risky for two reasons. With the pumps off, the extra pressure from circulation is gone, so the well is held by hydrostatic alone. And pulling pipe can suck formation fluid into the hole (swabbing), especially with balled-up bits, tight hole, thick mud or high pulling speeds. The trip sheet is the record that shows either problem early.
Pulling out dry
When pipe is pulled with no mud inside (dry), the hole must be filled with the volume of the steel removed.
Worked example
5 in drill pipe with tool joints, displacement 0.0075 bbl/ft, 93 ft stands:
Drill collars displace far more. A 6½ in collar with a 2.8125 in bore displaces about 0.0334 bbl/ft, or 3.1 bbl per 93 ft stand, more than four times a stand of drill pipe. Fill the hole after every stand of collars.
Pulling out wet
If the pipe comes out full (a plugged float, or no slug pumped), the mud inside also leaves the hole, so the fill is the closed-end displacement.
A wet trip also brings mud onto the rig floor every time a connection is broken. Mud buckets catch most of it, and the trip sheet must use the wet figure until the pipe starts coming out dry.
Slugs: getting a dry trip
Before pulling out, crews usually pump a small volume of heavier mud (a slug) down the string. The heavy column inside the pipe pushes the level in the string down, so the top stands come out empty. The slug also sends some mud back to the trip tank, which must be allowed for before the trip sheet starts.
Worked example: slug
20 bbl of 12.5 ppg slug in 10.5 ppg mud, 5 in drill pipe (capacity 0.0178 bbl/ft):
Wait for the trip tank to settle after the slug has U-tubed, note the 3.8 bbl, and start the trip sheet from that level.
Pressure lost if the hole is not filled
Pulling pipe without filling drops the mud level and the bottomhole pressure:
Worked example
5 stands dry (3.49 bbl) inside 9⅝ in 47 lb/ft casing (ID 8.681 in, 0.0732 bbl/ft), 10.5 ppg mud:
If the well's trip margin is smaller than that, the hole must be filled more often than every 5 stands.
Working the other way: with a 50 psi trip margin, the level may drop 50 ÷ (0.052 × 10.5) ≈ 92 ft, which is 92 × (0.0732 − 0.0075) ≈ 6.0 bbl, or about 8 stands of dry pipe. Most companies fill far more often than that, or keep the hole topped up continuously from the trip tank.
A trip sheet that shows swabbing
Calculated fill is 3.49 bbl per 5 stands. The measured figures are cumulative from the start of the trip.
| Stands | Calculated (bbl) | Actual this 5 (bbl) | Actual total (bbl) | Difference (bbl) |
|---|---|---|---|---|
| 5 | 3.49 | 3.5 | 3.5 | +0.01 |
| 10 | 6.97 | 3.4 | 6.9 | −0.07 |
| 15 | 10.46 | 3.3 | 10.2 | −0.26 |
| 20 | 13.95 | 2.6 | 12.8 | −1.15 |
| 25 | 17.44 | 2.1 | 14.9 | −2.54 |
The first ten stands are within normal reading error. From stand 15 the hole takes less every time, and by stand 20 the trend is clear: something other than mud is filling the space. The right response is to stop, flow check, and if in doubt run back to bottom and circulate bottoms-up, rather than keep pulling and hope.
Reading the trip sheet
- Hole takes less than calculated while pulling: something else is filling the space. Suspect swabbing. Stop, flow check, and run back to bottom if in doubt.
- Hole takes more than calculated: losses to the formation.
- Running in, returns larger than displacement: the well may be flowing.
- Use the trip tank and record every reading, including the cumulative difference from the calculated figure.
Trip margin
A trip margin is extra mud weight carried before pulling out, to make up for the annular friction that is lost when the pumps stop and for some of the swab effect of pulling. It is usually stated in psi at the bottom of the hole and converted into ppg with the TVD.
Worked example: trip margin
A 150 psi margin at 9,500 ft TVD:
A 10.5 ppg system would be raised to about 10.8 ppg before the trip, provided the shoe and any weak zone can take it with the pumps running.
A trip margin is not a licence to skip filling the hole. It covers small, expected losses of pressure; the trip sheet is still what tells you when something unexpected is happening.
Running in
Going back in, the trip tank should gain the displacement of each stand, or the closed-end displacement if the string has a float and is not being filled from the top. Running too fast surges the hole and can break down a weak zone, which then shows as less return than expected. Fill the string every few stands when running with a float, so the float is not collapsed and the returns stay readable.
Good practice
- Flow check at the shoe, before the BHA enters the stack, and whenever the numbers disagree.
- Use the real displacement from the pipe data sheet, including tool joints, not the tube-only figure.
- Keep pulling speeds to the limits set for the hole, and slow down in tight spots and with large BHAs.
- One person owns the trip tank and the sheet for the whole trip, and hands over in writing.