Swabbing In A Kick

When the drillstring is pulled up out of the hole during a trip, mud must flow down past it to fill the space left behind. Energy is needed to make the mud move, and this is shown as a pressure drop as the mud flows — a reverse 'annulus pressure loss'. The effect is that the total pressure exerted by the fluid column is reduced slightly.

The effect of the bit and bottom hole assembly can be compared with a loosely fitted plunger in a syringe.

Since the pressure drop is related to the energy required to move the mud into place, the principal factors encouraging swabbing are:

  1. Pipe Pulling Speed: It takes more energy to move mud quickly, so the faster the string is moved, the greater the pressure drop.
  2. Small Hole Clearance, or Slim Hole Geometry: It takes more energy to move the same volume of mud through a smaller space, so the less the annular space available, the greater the pressure drop.
  3. High Mud Viscosity, or Gel Strength: Evidently, it takes more energy to move a thick mud than thin one, hence the more viscous the mud, the greater the pressure drop.

Every time the drill string is moved, it follows there must be a pressure variation, however small, and this must be allowed for. The normal overbalance, or 'trip margin', is intended to overcome this potential problem.

The likelihood of swabbing in a kick can be reduced by good drilling practices, including use of an adequate trip margin. These include:

  1. Circulating the hole clean before starting a trip.
  2. Noting the pressure and position of 'tight-spots' from previous trips.
  3. Conditioning mud to as thin a condition as well circumstances permit.
  4. Careful observation of pipe pulling speed.

The swabbing-in kick is particularly hazardous since often a brief swabbing episode is followed by normal tripping practice. If the small discrepancy in string displacement volume is not noted at once, it will probably be overlooked thereafter. No other warning sign of the presence of a kick in the well may be seen.

An overall influx of gas, for instance, swabbed into an open annulus, may displace only a very short head of mud. The net decrease in bottom hole pressure is small and likely to be well below the normal range of 'trip margin' overbalance. No further flow of gas will occur into the well and, if the well is shut in, no pressures will show on either drill pipe or casing pressure gauges since the well is still in balance.

If a gas influx has been swabbed in, it will slowly migrate up the Well and expand as it does so. At first this expansion is very slow, and it is unlikely that any significant flow will be seen at the surface unless the influx is very large, or very close to the surface. The greatest swabbing action generally occurs when tripping through a 'fresh drilled' hole. This is because the initial mud filter cake deposited on a new section of hole is soft and thick. It has not been 'wiped' thin by the bit. Particular care should be taken at the start of a trip to ensure that no swabbing occurs.

As previously mentioned, bit and stabiliser balling are common causes of swabbing, especially in soft 'gumbo' shales and clays. The best protection here is to circulate 'bottoms up' or until the hole is relatively clean.

The only reliable method of detecting a swabbed-in kick as it occurs is proper hole fill procedure. Once again the trip volumes must be care fully monitored.

Swabbing Well

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Swabbing

Figure 4 - Swabbing Effect

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Responses

  • palmira
    How to reduce swabbing?
    5 years ago
  • Lleyton
    How is a kick swabbed in?
    4 years ago
  • KAROLIINA
    What to do when having swabbing effect in well?
    4 years ago
  • BEVERLY MCELWEE
    How to control swabbing in well?
    2 years ago

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