Surge and Swab in Drilling

What is Surge & Swab?

During the drilling phase, the drill string is pulled out of the well for various reasons like bit change, coring, logging, etc. After completion of these operations, the drill string is run back into the well. During these tripping operations, the drill string acts as a piston causing sudden transient pressure variations in the wellbore. A similar effect is seen when a casing is run in the well after drilling to the planned target depth. Wellbore hydraulics and transient pressure analysis assume even more significance in narrow pressure margin conditions in deep-water and HPHT (High Pressure & High Temperature) wells. Under these situations, the difference between pore pressure and fracture pressure is very narrow. Hence even low-level variations in wellbore pressures could take the well out of these pressure boundaries.

While running in a drill string or casing in the well, the downward motion of the string forces the drilling fluid out of the flowline. At the same time, the fluid immediately adjacent to the string is dragged downwards. This creates a piston effect and generates additional pressure that is added to the hydrostatic pressure of drilling fluid in the well. This additional pressure is called 'Surge Pressure'. If a drill string or logging tool is pulled out of the hole too fast, the string creates a piston effect pulling some mud out of the hole with it. This tends to reduce hydrostatic pressure in the well. The pressure reduction created by this situation is called ‘Swab Pressure’.

Knowing the maximum allowable surge and swab pressures makes it possible to determine the maximum allowable tripping speed to keep the surge and swab effects within limits. The maximum allowable surge pressure is the difference between the fracture pressure of the weakest exposed formation and the hydrostatic pressure of the mud column. Similarly, the maximum allowable swab pressure is the difference between the hydrostatic pressure of the mud column and the highest formation pressure of the exposed formations.

Effects and significance of Surge & Swab:

It is important to carry out surge and swab analysis at the planning stage because excessive surge or swab could cause downhole complications. The downhole complications increase the Non-Productive-Time (NPT) and therefore the well cost.

  • Excessive surge pressures can increase the borehole pressure. Too high levels of surge pressure induce lost circulation and even create total loss conditions.

  • Total losses will reduce the hydrostatic head. If the hydrostatic pressure of the remaining column of mud in the well falls below the pore pressure of any exposed formation, the formation fluid could flow into the well. This could create a loss-kick situation, which is always difficult to handle.

  • Transient wellbore pressures, such as swabs and surge effects during drilling, may cause wellbore enlargement. Surge pressures can cause a rapid increase in the near-wellbore pressure causing an immediate loss of rock strength, which could lead to hole collapse

  • If the swabbing effect is too much and the net hydrostatic pressure falls below the formation pressure, it may invite a kick (wellbore influx) into the wellbore.

  • Swabbing is considered hazardous whether it is a low-volume swabbing or a high-volume swabbing. Any low-density swabbed formation fluid causes a reduction in effective hydrostatic pressure in the well, reducing the overbalance, which is the planned safety margin to deter any well control event. If the swabbed fluid is gas, even a small amount of gas in the well could expand while moving up the hole and cause a major well control situation.

  • Swabbing action could cause a rapid reduction in wellbore pressure. If the formation has low tensile strength or is pre-fractured, the imbalance between the pressure in the rock and the wellbore can make loose rocks to fall into the well.

Factors affecting surge and swab pressures:

Tripping speed: High tripping-out speeds can cause a greater swabbing effect and a larger reduction in the wellbore pressure. Similarly, high tripping-in speeds induce an increase in the wellbore pressure and cause higher surge pressures. Most companies have guidelines for tripping speeds for drill string, casing, and liner running to minimize surge and swab effects.

Mud rheological properties: A thick mud with high gel strength and yield point significantly increases the surge and swab pressures. Hence, it’s a good practice to condition the mud thoroughly by circulation prior to tripping out of the hole. If the mud in the hole is in good condition, it will have less of a swabbing effect. It will also remain in good condition when the string is tripped back into the well and minimize surge pressures.

Annular clearance: Annular clearance is an important factor for surge and swab pressures. Smaller annular clearance amplifies the surge and swab effects. The Bottom Hole Assembly (BHA) of a drill string has components with a larger diameter than the drill pipes and hence causes higher pressure changes. Bit and BHA size heavily impact surge and swab effects. BHA diameter affects more than the length of BHA. The balling up of bit or BHA further reduces the annular clearance and can cause excessive swabbing while pulling the drill string out of the hole.

Closed or open-ended string: Open-ended pipe strings are defined as those, which have openings connecting the annulus and pipe bore at the bottom of the string, such as a bit on drill string without a float valve or a fill-up shoe on a casing string. While running an open-ended string, mud volume equivalent to the metal volume of the pipe is displaced. Whereas closed-end string usually has a float valve, which disallows mud influx into the string. The viscous drag in both scenarios is similar but running open-ended pipes results in lesser surge pressures than running a closed-end pipe.

e-Learning Resources on Surge and Swab Pressures while drilling

Surge and Swab Pressure Calculation

Drilling Optimization Solutions

References:

[1] https://www.rigworker.com [2] https://www.sciencedirect.com [3] https://www.drillingformulas.com [4] https://www.drillingmanual.com [5] https://www.ijraset.com/research-paper/surge-and-swab-pressure-in-oil-drilling