Failure to keep the hole full during a trip

If the fluid level in the hole falls as the pipe is removed, a reduction in bottom hole pressure will occur. If the magnitude of the reduction exceeds the trip margin or safety overbalance factor a kick may occur.

Swabbing and surging

Swabbing is when bottom hole pressure is reduced below formation pressure due to the effects of pulling the drill string, which allows the influx of formation fluids into the wellbore.

Swabbing can also be caused by the full gauge down hole tools (bits, stabilisers, reamers, core barrels) being balled up. This can create piston like effect when they are pulled through mud.

Surging is when the bottom hole pressure is increased due to effects of running the drill string too fast in the hole. Down hole mud losses may occur if care is not taken and fracture pressure is exceeded while RIH.

The factors affecting swabbing and surging are pulling speed of pipe, mud properties and hole geometry

Loss of circulation

If returns are lost, the resulting loss of hydrostatic pressure will cause any permeable formation containing greater pressures to flow into the wellbore.

Insufficient mud weightThis may be caused by loss of the weighting agent at the solids removal equipment or from errors in the design. Formation pressure exceeds the existing hydrostatic pressure

Abnormal pressured formationskicks may occur when drilling into abnormally pressured permeable zones. This may be indicated by high penetration rates.

Shallow gas sandsKicks from shallow gas sands are caused by charged formations, poor cement jobs, casing leaks, injection operations, improper abandonments, and previous underground blowouts can produce charged formations

Special situations

Drill stem testing (DST): during the course of the test, the bore hole or casing below the packer, and atleast a portion of the drill pipe or tubing, is filled with the formation fluid. At the end of the test if this fluid is not removed by proper well control techniques, a kick may result.

Drilling into an adjacent well: if the drilling well penetrates the production string of a previously completed well, the formation fluid from the completed well will enter the wellbore of the drilling well, causing a kick.

Excessive drilling rate through a gas sand/ limestone: when drilling a gas bearing formation, the mud weight will be gas cut due to the gas breaking out of the pore space of the cuttings near the surface. The severity of the influx will depend on the penetration rate, porosity and permeability. if the percentage of gas in mud increases, it allows greater bottom hydrostatic pressure reductions