Trip monitoring: the trip sheet, surge and swab

A trip is when a well is easiest to lose. The pumps are off, the circulating pressure is gone, and every stand pulled either takes the mud it should or lets something into the hole. Two tools keep it safe: the trip sheet, which checks the hole's fill stand by stand, and a surge and swab model, which says how fast the pipe can move. This article works through both on a deep, narrow-window well: the equations, a hand check, the safe speed by depth, and a trip sheet that stopped a trip after three stands.

What the trip sheet checks

Pull a stand out of the hole and the mud level drops by the volume of steel that came out. Pull it wet, with the mud inside it, and the level drops by the pipe's whole closed-end volume. Either way the hole must take that volume back from the trip tank, or the bottom-hole pressure falls. Running in, the hole must give the same volume back.

The trip sheet compares the two, stand by stand: the expected fill from the pipe pulled, and the measured fill from the trip tank. The running total of the difference is the number to watch. A hole that takes less than the pipe displaced has something else filling the space: formation fluid swabbed in, or an influx. A hole that takes more is losing mud. Rigs set two alarm levels on the running total, and also watch single stands, because a swab shows on one stand before the total reaches its limit. Three short stands in a row are the classic swab signature, even when the total is still small. The answer to any of them is the same: stop and flow check.

A flow check is a static period, pumps off and pipe still, long enough to see whether the well flows. Plan them before the trip starts: off bottom after the first few stands, at the casing shoe before the pipe goes into casing, and before the BHA enters the BOP.

Surge and swab

Moving pipe pushes mud. Pulled out, it leaves a space below the bit that mud from above must fill, so mud flows down past the string and its friction lowers the pressure below it: swab. Run in, the pipe pushes mud up the annulus and the friction raises the pressure: surge. Swab can bring in formation fluid; surge can fracture a weak zone.

The standard steady-state model, after Burkhardt (1961), turns pipe speed into an annular velocity, then into a friction gradient for the mud's rheology:

v_p, v_a

pipe speed, and the mean mud velocity in the annulus it causes

K_c

Burkhardt's clinging constant, the share of mud dragged along with the pipe, about 0.45–0.5 in a typical annulus

d, D, D_h

pipe outside diameter, hole or casing inside diameter, and the gap D − d; the displacement term is for closed pipe

τ_y, K, n

Herschel–Bulkley yield stress, consistency and flow index, fitted from the Fann readings at low shear as API RP 13D does: τ_y = 2θ₃ − θ₆

Adding the gradient over every section of annulus above the bit gives the pressure change at the bit; below the bit it is the bit's change. Divided by TVD it becomes an equivalent density, set against pore pressure for swab and fracture gradient for surge, each with a margin. The safe trip speed is the fastest speed that keeps every open-hole point inside its limit. The model is steady-state: the pressure spike when gels break at the start of a pull, and the acceleration of the string, come on top of it.

The example well

The example is a deep vertical well set in conditions typical of onshore Kuwait: heavy oil-based mud in a small hole below a liner, in overpressured rock, with a narrow window between pore pressure and fracture gradient. The trip in question is the trip out from TD in the 5⅞″ hole.

The mud fits to a yield stress of 3.83 Pa (8 lbf/100 ft²), a flow index of 0.865 and a consistency of 0.122 Pa·sⁿ. The float means the pipe cannot drain while it is pulled, so the trip is wet and the swab is a closed pipe's.

A hand check

1. Expected fill. A stand of 5″ pipe pulled wet displaces its closed-end volume: π / 4 × (5 × 0.0254)² × 28.5 = 0.361 m³, or 2.27 bbl.
2. Overbalance. At 4,500 m, (1.92 − 1.86) × 9.81 × 4,500 = 2.65 MPa, 384 psi. The 0.02 SG margin takes 128 psi of it, so swab may use 256 psi.
3. Annular velocity at 600 m/h. In the open hole around 4″ pipe, v_p = 0.167 m/s; K_c = 0.476 and the closed displacement ratio is 16 / (5.875² − 16) = 0.864, so v_a = 0.167 × 1.340 = 0.223 m/s.
4. Friction. The gap is 1.875″, 0.0476 m. γ_w = 12 × 0.223 / 0.0476 × 1.052 = 59.2 s⁻¹, so the viscous stress is 0.122 × 59.2^0.865 = 4.16 Pa. The gradient is (4 × 4.16 + 6 × 3.83) / 0.0476 = 832 Pa/m, 12.1 psi per 100 m.

DrillIQ's engine gives the same 832 Pa/m for that section. The hand check also shows something important: 58 % of that friction is the yield-stress term, which does not depend on speed. A thick mud swabs as soon as the pipe moves.

Swab, surge and the safe trip speed

With the bit at TD, swab costs 191 psi of bottom-hole pressure at a crawl, almost all of it the mud's yield stress, and rises with speed: 264 psi at 480 m/h, 280 psi at 600 m/h. The swab line crosses pore pressure plus margin, 1.88 SG, at 418 m/h; that is the safe pulling speed off bottom. Running in, surge at the bit reaches only 1.965 SG at the crew's 670 m/h limit, against a limit of 2.03, so here the crew limit binds long before surge does. On this well, pulling out is the dangerous direction.

Pumping changes the picture. Pumping out at 400 L/min sends mud down the string to fill the space below the bit, and the safe pulling speed rises to 1,909 m/h, far beyond anything a crew would run. The pumped flow has its own friction, so the pressure at the bit now sits above the mud weight, at 1.979 SG at a crawl and 1.968 SG at 600 m/h, inside both margins. Where the swab margin is this thin, pumping out of the open hole is often worth its time, at a rate checked against the fracture side.

The safe speed is not one number. As the bit comes up, there is less annulus above it to create friction, and the open hole below it sees the bit's lower pressure change. The engine's schedule rises from 418 m/h at TD to 495 m/h at 4,350 m and 625 m/h at 4,200 m, and reaches the crew limit at the liner shoe. A trip plan that gives the driller one speed for the open hole either wastes time near the shoe or swabs near TD. A schedule by depth does neither: the 13 open-hole stands at their safe speeds took 44.4 minutes of pipe movement, against 37.1 at a flat 600 m/h. Slowing down cost seven minutes.

The trip sheet that stopped the trip

The first trip out of the hole was pulled at the crew's usual 600 m/h. The engine checked each stand as it was entered.

• Stand 1 took 0.251 m³ against 0.361 expected, 0.110 m³ short. Two warnings: the stand alone was over the 0.08 m³ limit, and the swab at the bit, 1.876 SG, was inside the 0.02 SG margin, with a safe pulling speed of 418 m/h.
• Stand 2 was 0.105 m³ short. The running total, 0.215 m³ (1.35 bbl), passed the 0.16 m³ warning.
• Stand 3 was 0.125 m³ short: three short fills in a row, each more than a quarter of the expected fill, the swab signature. The total stood at 0.340 m³, 2.14 bbl.

The crew stopped and flow checked. Note what the model said: at 600 m/h the swab left the bottom-hole pressure 0.016 SG, about 104 psi, above the planned pore pressure. Steady-state swab alone should not have brought anything in, yet the trip tank showed 0.34 m³ that the hole did not take. Either the pore pressure at TD is higher than the plan's 1.86 SG, or the transient at the start of each pull swabbed harder than the steady value. The trip sheet does not need to know which. It measures the hole directly, and it is the one to believe.

The well was static on the flow check. The crew ran back to bottom, within the safe running speed, circulated bottoms up and conditioned the mud. The second trip was pulled to the engine's schedule, 410 m/h on the first stand and rising to 600 m/h by stand 11, and every stand took its fill within a reading's error. The fill alarms stayed quiet through the open hole and into the liner.

On the rig

Plan the trip before pulling off bottom: the safe speed by depth in both directions, the flow-check points, the expected fill per stand for the pipe and fill mode in use, and the alarm levels. Give the driller the schedule, not a single number. On this well the standard checks fall at 4,357.5 m (off bottom after five stands), at the liner shoe at 4,150 m, and at 230 m before the BHA enters the BOP.

Pull the first stands slowly whatever the model says. Gels break and the string accelerates at the start of every stand, and that transient is not in a steady-state number. Circulating and conditioning the mud before the trip lowers the yield stress, and with it the part of the swab that speed cannot fix. Where the margin is thin, pump out of the open hole.

Read the trip tank every stand, and treat any short fill as an influx until a flow check says otherwise. Running in, watch for the opposite: returns beyond the pipe's displacement are a gain, short returns are losses, and surge is highest with the bit at the shoe and in the open hole below it. With a float, fill the pipe at planned intervals and account for it on the sheet.

The pore pressure and fracture gradient article covers where the window comes from, and the kick tolerance and kill sheet article covers what to do if the flow check shows flow.

A checklist for the rig team

• Work out the expected fill per stand for the pipe and fill mode in use, dry or wet.
• Agree a safe speed by depth for pulling out and running in, not one figure.
• Plan flow checks: off bottom, at the shoe, before the BHA enters the BOP.
• Read the trip tank every stand and keep the running total on the sheet.
• Stop and flow check on a single bad stand, three short stands in a row, or the running-total alarm.
• Pull the first stands off bottom slowly, and condition the mud before the trip.
• Pump out of the open hole where the swab margin is thin.
• When the trip tank and the model disagree, believe the trip tank.

How DrillIQ handles it

DrillIQ's Trip Monitoring module builds the trip sheet from the rig's time log, stand by stand: it finds each stand and its speed, reads the fill from the trip tank, works out the expected fill from the string and the fill mode, and raises alerts for a single bad stand, the running total, the swab signature, over-speed, missed flow checks and stale data. Its own surge and swab model, with the mud's Herschel–Bulkley fit, Burkhardt clinging, every section of annulus and the survey's TVD, checks each stand against the pore pressure and fracture gradient from Pore Pressure & Geomechanics, or the section's own window, gives the safe speed there, and builds the trip schedule by depth for both directions. Every number above came from that engine.

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References

1. Burkhardt, J.A. Wellbore Pressure Surges Produced by Pipe Movement. Journal of Petroleum Technology 13 (6), 1961, 595–605.
2. American Petroleum Institute. API RP 13D, Rheology and Hydraulics of Oil-well Drilling Fluids.
3. American Petroleum Institute. API RP 59, Recommended Practice for Well Control Operations.
4. Bourgoyne, A.T., Millheim, K.K., Chenevert, M.E. and Young, F.S. Applied Drilling Engineering. SPE Textbook Series Vol. 2, 1986.

About this example. The well in this article is a representative scenario set in typical onshore Kuwait conditions and worked through with DrillIQ's engines. It is not a client's well, and its figures are not field results.