3.3 Tripping Operations & Trip Tank Monitoring

The Tripping Process

Tripping means pulling the drill string out of the hole and running it back in, usually to change the bit or BHA:

• Pulling out of hole (POOH): stands are pulled, broken out and racked in the derrick. Steel leaves the well, so the hole must be filled with mud to replace it. The main risk is swabbing.
• Running in hole (RIH): stands are picked up and run back in. The pipe pushes mud out of the well into the trip tank. The main risk is surging, and a string with a float must be filled at intervals.
• A round trip is a complete trip out and back in. The well must be monitored on the trip tank throughout, including while the bit is out of the hole.

The Trip Tank: Engineering Design and Principles

Active mud pits are poorly suited for monitoring tripping operations. A typical active mud pit holds between $200$ and $500 \text{ bbl}$ of fluid across a wide surface area; a volume change of $1$ to $2 \text{ bbl}$ represents a liquid level variation of less than a tenth of an inch, making precise detection virtually impossible.

The trip tank is an engineered monitoring vessel specifically designed to solve this limitation:

• Capacity and Dimensions: Small total volume (typically $20$ to $50 \text{ bbl}$), constructed as a tall, narrow rectangular or cylindrical tank.
• High Measurement Resolution: Because the cross-sectional area is small, a $1 \text{ bbl}$ volume change causes a prominent liquid level shift of several inches. Calibrated digital transducers and mechanical float pointers provide resolution to within $0.1$ to $0.5 \text{ bbl}$.
• Continuous vs. Batch Filling:
  • Continuous Filling: A low-rate centrifugal pump continuously circulates mud from the trip tank to the bell nipple, with an overflow line returning excess fluid back to the tank. As pipe is pulled, fluid fills the wellbore automatically, and the trip tank level drops continuously in direct proportion to displaced steel.
  • Batch Filling: The well is filled periodically after pulling a fixed number of stands (e.g., every 3 to 5 stands of drill pipe, or after every single stand of drill collars), recording the exact volume required to refill the hole to the overflow line.

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Trip Sheets: Stand-by-Stand Displacement Tracking

A trip sheet is an active accounting ledger maintained by the driller, mud logger, and derrickman throughout every tripping operation. It records cumulative theoretical metal displacement against actual measured mud fill from the trip tank.

$\text{Discrepancy} = V_{\text{actual fill}} - V_{\text{theoretical displacement}}$

Interpreting Trip Sheet Discrepancies

• Case 1: Actual Fill < Theoretical Displacement: The well took less mud than calculated metal displacement. Formation fluid has entered the wellbore (swabbed influx) and is displacing mud volume. Tripping must stop immediately to conduct a formal flow check!
• Case 2: Actual Fill > Theoretical Displacement: The well took more mud than calculated metal displacement. Drilling fluid is leaking into the formation (lost circulation), risking a drop in annular hydrostatic level. Tripping must halt to monitor the well and prepare remedial action.

Any deviation from the expected trip-sheet value that cannot be explained is treated as a warning: stop the trip and flow check. Many operators also set a numeric tolerance in their procedures, but a Level 2 crew member's duty is simply to report any difference to the Driller at once.

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Pulling Dry Pipe vs. Pulling Wet Pipe

Accurately calculating theoretical displacement requires distinguishing between dry and wet pipe:

1. Pulling Dry Pipe

When pulling dry pipe, fluid inside the drill string drains freely through the bit nozzles back into the well. Only the volume of the steel pipe wall leaves the borehole:

$V_{\text{fill, dry}} = L_{\text{pulled}} \times \text{Steel Displacement Factor (bbl/ft)}$

Example Calculation: Pulling 5 stands ($465 \text{ ft}$) of 5-inch, $19.5 \text{ lb/ft}$ drill pipe with a steel displacement factor of $0.00753 \text{ bbl/ft}$ requires:

$V_{\text{fill}} = 465 \text{ ft} \times 0.00753 \text{ bbl/ft} = 3.50 \text{ bbl}$

2. Pulling Wet Pipe

When the bit nozzles are plugged, or when no heavy slug is pumped and the mud cannot drain fast enough, drilling mud is pulled out inside the pipe. The pipe comes out full of mud and dumps onto the rig floor. Both the steel wall and the internal fluid capacity leave the well:

$V_{\text{fill, wet}} = L_{\text{pulled}} \times (\text{Steel Displacement} + \text{Internal Capacity}) = L_{\text{pulled}} \times \text{Closed-End Displacement}$

Example Calculation: The same 5-inch pipe has an internal capacity of $0.01776 \text{ bbl/ft}$. The closed-end displacement is:

$\text{Closed-End Factor} = 0.00753 + 0.01776 = 0.02529 \text{ bbl/ft}$

$V_{\text{fill, wet}} = 465 \text{ ft} \times 0.02529 \text{ bbl/ft} = 11.76 \text{ bbl}$

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Slugging the Drill String: Physics of U-Tubing

To avoid the hazards, mess, and volumetric confusion of pulling wet pipe, crews pump a heavy slug into the drill string prior to tripping out.

The U-Tube Slugging Procedure

1. A high-density mud pill (typically $2.0$ to $4.0 \text{ ppg}$ heavier than active mud weight) with a volume of $15$ to $30 \text{ bbl}$ is mixed in the slug pit.
2. The slug is pumped into the top of the drill pipe and displaced down the string until it is positioned in the upper drill pipe, well above the bit.
3. The mud pump is disengaged and the drill string is vented to atmosphere.
4. Hydrostatic U-Tubing: Because the high-density slug column exerts greater hydrostatic pressure than the surrounding active mud in the annulus, the fluid inside the drill string falls, pushing annular fluid upward and out into the surface pits until hydrostatic balance is achieved at the bit nozzles:

$HP_{\text{string}} = HP_{\text{annulus}}$

As the fluid falls, the internal liquid level drops below the rotary table by a vertical distance $\Delta h_{\text{drop}}$:

$\Delta h_{\text{drop}} = \frac{V_{\text{slug}} \times (\text{MW}_{\text{slug}} - \text{MW}_{\text{mud}})}{\text{MW}_{\text{mud}} \times \text{DP Capacity (bbl/ft)}}$

This drop creates an empty interval of $150$ to $300 \text{ ft}$ inside the pipe below the rig floor, allowing stands to be disconnected and racked dry without spillage.

Surface Pit Gain Accounting

The mud displaced out of the drill string flows up the annulus and discharges into the active surface pits, generating a predictable pit gain:

$V_{\text{pit gain}} = \Delta h_{\text{drop}} \times \text{DP Capacity (bbl/ft)} = V_{\text{slug}} \times \left(\frac{\text{MW}_{\text{slug}}}{\text{MW}_{\text{mud}}} - 1\right)$

Crew pit discipline requires calculating this expected gain in advance. Any pit gain in excess of the theoretical U-tube volume indicates that an influx has entered during the slugging process!

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Trip Margin

A trip margin is an intentional safety increment added to the drilling mud density prior to pulling out of hole, designed to offset the calculated swab pressure drop ($P_{\text{swab}}$):

$\text{Trip Margin (ppg)} = \frac{P_{\text{swab anticipated}}}{0.052 \times \text{TVD}}$

The trip margin is set in the well programme (a few tenths of a ppg is common), providing a hydrostatic buffer that prevents transient underbalance during pipe withdrawal.

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Flow-Check Protocols During Tripping Operations

A flow check requires halting all pipe motion, lining the well up on the trip tank (or visual observation at the bell nipple), and verifying that the well is completely static.

Flow-Check Duration and Points

The observation period is set by company procedure (commonly 10 to 15 minutes, and longer in some conditions, for example with oil-based mud or in deep, hot wells). Typical flow-check points are:

1. On Bottom Prior to Tripping Out: After circulating bottoms up and confirming mud properties are homogenous and gas-free.
2. At the Casing Shoe: When the bit reaches the casing shoe, isolating the open-hole section from the surface casing.
3. Before the BHA Enters the BOP Stack: Drill collars and stabilizers have minimal clearance in the BOP bore; checking flow before entering the stack ensures the well is static before rams are restricted.
4. Whenever the Trip Sheet Shows a Discrepancy: any time the hole takes more or less mud than the calculated displacement and the difference cannot be explained.
5. Whenever Overpull or Swabbing Indications Occur: Any tight hole drag encountered during hoisting.

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Actions When the Trip Sheet Shows an Influx

The IWCF syllabus rates this outcome at the highest importance level. When the trip tank shows the hole has taken too little mud:

1. Stop the trip and flow check: set the slips, install the open full-opening safety valve, and watch the trip tank.
2. If the well is flowing: shut the well in immediately using the tripping shut-in procedure (Chapter 5).
3. If the well is static: the swabbed influx is still in the hole. Run back to bottom, checking the trip tank displacement on every stand, and then circulate the influx out (bottoms up), taking returns through the choke and mud-gas separator if gas is expected. Only then is the trip restarted, often with a higher trip margin or by pumping out of the hole.

Running back to bottom matters because an influx can only be circulated out from below; circulating with the bit high in the hole would leave the influx beneath the bit.