A kick is defined as flow of formation fluids or gas into the welbore, a blowout is the uncontrolled release of the fluid or gas, gained through the kick. A blowout can take place at the surface or into another formation( underground blowout). Formation fluids that enter the welbore can be crude oil or brine, gas entered can be any kind of naturally occurring gas. During a kick, drilling mud is displaced by the fluid or gas entering the borehole. To deteck a kick, this is one of the easiest warning signs, others are discussed below. Some causes of kicks may be:
- Lost circulation, thus reduction of hydrostatic pressure,
- Abnormally high pressured horizons drilled with to low mud weight,
- Reduction of hydrostatic pressure while swabbing,
- Failure to keep the borehole full of drilling fluid while tripping out.
A well control system permits:
- Detecting a kick,
- Closing the well at the surface,
- Circulating the well under pressure to remove the formation fluids and increase the mud density,
- Moving the drillstring under pressure,
- Diverting the flow away from the rig personnel and equiptment.
To prevent a well from blowing out, it is essential to detect a kick as soon as possible. Especially at wildcat wells and wells drilled in areas of abnormal formation pressures, the various kick detection parameters have to be observed continuously.
Kick Detection Parameters:
- Gain in pit volume,
- Increase in mud return flow rate while circulating at constant rate,
- Mud-return even when the pumps are shut down,
- Well is taking less ( tripping in) or giving more ( tripping out) mud than calculated,
- Increase in drilling rate,
- Decrease in the circulation pressure,
- Increase of the chlonicle content of the mud,
- Increase of trip, connection of background gas.
It should be noted that some of these parameters alone are sufficient to indicate a kick ( e.g. pit gain), others (e.g. reduction of chlonicle content) are additional observations and should only be used as kick detection when multiple other kick detecting observations are present as well. Small trip tanks have proven to provide the best means of monitoring hole fill-up volume. Trip tanks usually hold 10 to 15 bbls and have 1 bbl gauge markers. On the site, the top of the gravity-feed trip tank must be slightly lower than the bell nipple to prevent mud from being lost to the flow line. In case that a trip tank is not installed, hole fill-up volume should be determined by counting pump strokes each time the hole is filled.
When a kick is detected at the surface , a device called » blowout preventers» (BOP) is used to stop`the fluid flow from the well. To cover all possible scenerios and handle different kick situations, different BOPs are collectively attached to the well head. This series of BOPs is called BOP stack. The BOP stack should enable the rig personnel to perform following actions when a kick is shut in:
BOP must be capable to termine flow from the well under all drilling conditions.
When the drillstring is in the hole, movement of the string without releasing well pressure must be possible.
BOP stack must allow fluid circulation through the well annulus under pressure.
To achieve these objectives, a series of so called » ram preventers» and «annular preventers» are combined.
Ram preventers consist of two packing elements on opposite sides that close the well by moving toward each other.
Pipe rams have semicircular openings which match the diameter of the drillpipe seize for which the are designed. Thus when the drillpipe seize is changed during the well, the closing elements of the pipe rams have to be changed as well. When the drillstring consists of drillpipe with more than one diameter seize, additional ram preventers have to be used in the BOP stack.
Blind rams can close the well when there is no drillpipe in the hole. When blind rams are activated while drillpipe is in the well, the drillpipe will be flattened but the flow of the well will not be closed in.
Shear rams are special designed blind rams that cut the drillpipe. When the shear rams are closed and drillpipe is in the hole, the pipe will be separated, the lower part will drop into the hole and the flow of the well will stop. Thus shear rams are activated in emergencies only when all pipe rams and annular preventers failed.
Annular preventer or also called bag type preventer, close the flow of the well with a ring of synthetic rubber that contracts in the fluid passage. Annular preventers are designed in such a way that when they are closed, the pressure in the annulus helps to keep it closed. The primary method of closing both ram and annular preventers is hydraulically by accumulators. In case the hydraulic system fails, the ram preventers have a screw-type locking device that can be applied to close them.
Well Control Operation:
To be able to circulate out a detected kick, a minimum amount of information is required. Some of this information is obtained prior to the kick and concerns the equiptment in use and operations performed, other information is gathered after the kick is shut-in the borehole.
Maximum allowable casing pressure ( pressure rating of the BOP and burst strength of the casing).
Cpacity of the drillpipe, drill collars, and annuli,
Volume of active mud circulation system,
FRacture gradient of the drilled formation,
Reduced circulation rate and pressure,
Pump factor and efficiency.
After a Kick is detected and the well shut-in, the following parameters are obtained:
Stabilized shut-in casing pressure,
Stabilized shut-in drillpipe pressure,
Pit gain ( is assumed to be equal the kick volumen),
TVD and MD depth of the bottom of the hole.
Kick Tolerance and Kill Mud Weight
The kick tolerance determines the ability to control a kick at the current situation without fracturing a formation and causing lost circulation. By definition, the Kick tolerance is the difference between the maximum pore pressure of any formation penetrated and causing the kick, and the drilling mud weight at this depth.
Tk = (ρe − ρm),
- Tk: Kick Tolerance
- Pe: Pore pressure at this depth.
- Pm: Mud Pressure at this depth.
To establish safe drilling condition, it should be ensured that the kick tolerance is never below 1 ppg.