Tripping · well control · field units

Swab and surge pressure, and the trip margin formula

Surge and swab are the pressure changes in the well caused by moving pipe. Running pipe in pushes mud up the annulus and raises bottomhole pressure (surge), which can break down a weak formation and cause losses. Pulling pipe out drags mud with it and leaves a space below that mud must fill, which lowers bottomhole pressure (swab), which can let formation fluid in and start a kick. Both grow with pipe speed, mud viscosity and yield point, and with a tighter clearance between pipe and hole. A quick field estimate treats the moving mud as laminar Bingham plastic flow; the trip margin, the extra mud weight carried to cover swab when pulling out, is commonly taken as trip margin (ppg) = YP ÷ (11.7 × (Dh − Dp)). This page gives the formulas, a worked example and how to read the trip sheet for signs of swabbing.

Reference material only. Tripping speeds, trip margin and flow checks must follow your company's well control policy and the drilling programme. Worked examples are illustrative arithmetic, not a design.

1. What surge and swab do to the well

A 2022 review in Applied Sciences puts it simply: lowering the string creates a positive pressure change and pulling it out a negative one, and the size depends on the tripping speed, so tripping too fast risks fracturing the formation while tripping too slowly adds rig time.

2. The quick estimate: Bingham plastic, steady state

The field method treats the mud moving past the pipe as steady laminar flow and uses the same Bingham plastic equation as the annular pressure loss formula. What changes is the velocity: it is the mud velocity created by the moving pipe, not by the pumps.

Vp is the pipe speed in ft/s, Dh the hole or casing ID and Dp the pipe OD in inches, PV in cP and YP in lbf/100 ft². Kc is the clinging constant: the share of the pipe speed carried by mud that clings to the pipe wall. Burkhardt's value of 0.45 is the usual default; charts give lower values for wide annuli. For open-ended pipe part of the mud goes up the inside of the string, so use the steel area (OD² − ID²) in place of Dp² and add the pipe bore area (ID²) to the annulus area (Dh² − Dp² + ID²).

Use the maximum pipe speed, not the average. A common rule (Lapeyrouse) is maximum speed ≈ 1.5 × average stand speed, because the pipe accelerates and decelerates on every stand. For a full estimate, work out the change over each section, drill pipe and BHA, and add them, exactly as for annular pressure loss; the BHA section is short but the clearance is small, so it adds a lot.

3. Worked example: surge and swab on a trip

5 in drill pipe, closed end (float in the string), in 8.5 in hole at 10,000 ft TVD. Mud 12.0 ppg, PV 25 cP, YP 15 lbf/100 ft². A 93 ft stand is run in about 60 seconds. For simplicity the whole length is treated as drill pipe; including the BHA would make the answer larger.

If the pore pressure at the bottom is 11.4 ppg equivalent, the static overbalance is 0.6 ppg but only 0.1 ppg remains while pulling at that speed. Add the BHA section, or a balled bit, and the well can be swabbed in. On the surge side, compare 12.5 ppg with the leak-off or FIT equivalent at the shoe and with any known loss zone.

Notice how much of the answer is the yield term, which in this model does not depend on speed at all. That is the main weakness of the steady Bingham estimate: it overstates surge and swab for slow trips. Power-law and Herschel-Bulkley models (API RP 13D) and dynamic surge-swab software, which also include mud compressibility, inertia and gel strength, are used when the margins are tight.

4. Maximum tripping speed

Turning the equation around gives the fastest the pipe can move for an allowed pressure change:

Same well, allowing 0.6 ppg of surge: G = 0.6 × 0.052 = 0.0312 psi/ft; V = (0.0312 − 0.0214) × 1000 × 12.25 ÷ 25 ≈ 4.79 ft/s; Vp = 4.79 ÷ (0.529 + 0.45) ≈ 4.89 ft/s maximum, or about 3.26 ft/s average. A 93 ft stand should therefore take at least about 29 seconds. If the allowed change is smaller than the yield term alone (0.41 ppg here), the quick formula gives no safe speed at all, which is a sign to use a better model, condition the mud or both.

5. Trip margin

Trip margin is an extra increment of mud weight above the weight that just balances the formation, carried so that the well stays overbalanced when the pumps are off and the pipe is being pulled. The formula is the yield-point part of the laminar annular pressure loss written as a mud weight, using the older 225 constant in place of 200. Example: YP 15 in 8.5 in hole with 5 in pipe gives 15 ÷ (11.7 × 3.5) ≈ 0.37 ppg; YP 14 in 12¼ in hole with 5 in pipe gives about 0.16 ppg. Wider holes need less margin, high-yield muds more.

Trip margin is often added to the kill mud weight after a kick so that the well can be tripped safely once it is dead, and some operators set it as a fixed figure in the well programme instead. Too much margin costs rate of penetration and raises ECD and the risk of losses and differential sticking, so it is a balance, not a free safety factor.

6. Reading the trip sheet for swabbing

• Hole taking less mud than it should. When pulling out, the hole should take the steel displacement of the pipe pulled (pulling dry) or the closed-end displacement (pulling wet). If the trip tank shows less fill than calculated, the missing volume may be formation fluid. See drill pipe capacity and displacement for the volumes.
• Hole giving back too little when running in. Less return than the displacement of the pipe run means mud is being lost, often to surge.
• Flow checks. Typical practice is a flow check before pulling off bottom, at the shoe, before the BHA enters the BOP and whenever fill is wrong. If the well flows, it is shut in and handled as a kick.
• Swab indicators while pulling. Overpull with mud level dropping in the annulus or mud coming up with the pipe (a pulled wet string or a balled bit) are signs the pipe is acting as a piston. Slow down, pump out of the hole or circulate.

7. Reducing surge and swab

• Set and follow maximum running and pulling speeds for each section, slower through the BHA and liner laps and in tight clearance.
• Keep PV, YP and gel strengths no higher than hole cleaning needs, and break gels by rotating or circulating before tripping after a long static time.
• Pump out of the hole or back-ream through swab-prone sections, within the allowed ECD.
• Run casing and liners slowly with auto-fill float equipment where the programme allows, since closed-end casing in a small annulus gives the largest surge.
• Record every trip with a trip sheet. Comparing actual fill against calculated displacement is the most reliable early warning available on the rig floor.

8. In DrillX

DrillX daily drilling reports log trips with IADC-style time codes, so tripping time, flow checks and any well control time show up in the NPT view. The engineering calculators include hydrostatic pressure, ECD, annular pressure loss, annular velocity and the kill sheet. This page explains the arithmetic; it is not a surge-swab simulator.

Sources and further reading

• A. Mohammad and R. Davidrajuh, Modeling of Swab and Surge Pressures: A Survey, Applied Sciences 12(7) 3526 (2022): direction of the pressure change, dependence on tripping speed, review of steady and dynamic models.
• Drilling Formulas, Surge and swab calculation, method 1: maximum pipe velocity = 1.5 × average, drill pipe and collar sections added, surge added to and swab subtracted from hydrostatic pressure.
• Drilling Manual, Swab and surge in drilling: Bingham laminar worked example for a liner run, clinging constant chart (Burkhardt default 0.45), maximum allowable surge.
• J. A. Burkhardt, "Wellbore Pressure Surges Produced by Pipe Movement", Journal of Petroleum Technology (1961): the clinging constant.
• A. T. Bourgoyne Jr. et al., Applied Drilling Engineering, SPE Textbook Series Vol. 2 (1986), chapter 4: surge pressure from pipe movement and Bingham plastic annular flow.
• N. J. Lapeyrouse, Formulas and Calculations for Drilling, Production and Workover, 2nd ed. (Gulf Professional Publishing, 2002): surge and swab methods and the trip margin formula.
• API RP 13D, Rheology and Hydraulics of Oil-well Drilling Fluids: surge and swab with power-law and Herschel-Bulkley rheology.

Reference summary only. Worked examples are illustrative arithmetic. Tripping speeds, trip margin and flow checks are set by your operator's drilling programme and well control policy.