Sunday, June 30, 2013

Well control for non-drillers ...

Well control is one of the most important aspects of offshore shallow or deepwater drilling
operations. Improper handling of kicks in well control can result in blowouts with very grave consequences, including the loss of valuable resources such as in recent Deep Horizon Macondo incident losing millions in revenue. Even though the cost of a blowout (as a result of improper/no oil well control) can easily reach millions or billions of dollars, the monetary loss is not as serious as the other damages that can occur: irreparable damage to the environment, waste of valuable resources, ruined equipment, and most importantly, the safety and lives of personnel on the drilling rig.

In order to avert the consequences of blowout, the utmost attention must be given to oil well control. That is why well control procedures should be in place prior to the start of an abnormal situation noticed within the wellbore, and ideally when a new rig position is sited. In other words, this includes the time the new location is picked, all drilling, completion, workover, snubbing and any other drilling-related operations that should be executed with proper oil well control in mind. This type of preparation involves widespread training of personnel, the development of strict operational guidelines and the design of drilling programs — maximizing the probability of successfully regaining hydrostatic control of a well after a significant influx of formation fluid has taken place.

One concern is the increasing number of governmental regulations and restrictions placed on the hydrocarbon industry, partially as a result of recent, much-publicized well-control incidents. For these and other reasons, it is important that drilling personnel understand well-control principles and the procedures to follow to properly control potential blowouts.

The key elements that can be used to control kicks and prevent blowouts are based on the work of a blowout specialist and are briefly presented below:

Quickly shut in the well.

When in doubt, shut down and get help. Kicks occur as frequently while drilling as they do while tripping out of the hole. Many small kicks turn into big blowouts because of improper handling.

Act cautiously to avoid mistakes—take your time to get it right the first time. You may not have another opportunity to do it correctly.

Many well-control procedures have been developed over the years. Some have used systematic approaches, while others are based on logical, but perhaps unsound, principles. The systematic approaches will be presented here.

The drilling mud forms the first line of defence against kicks and blowouts. The second, and last, line of defence is the blow-out preventer stack. This is a collection of large, high-pressure valves which is fitted on the top of the wellhead in a vertical tier and which can be controlled remotely from any of several positions on the drilling unit. Although outwardly the BOP stack on a deep-water floater appears fairly unremarkable, it is an enormously expensive precision tool that can withstand pressures of up to 15,000 psi.

Because of the intricacy of its numerous working parts a dedicated ‘sub-sea engineer’ is employed by the drilling contractor to maintain it and its control system in top condition. Through the middle of the BOP stack is a hole wide enough for large drilling tools to pass up and down during the course of normal operations. The width of the opening is determined to some extent by the stage at which the stack is intended to be first used in the well programme. An 18-3/4” stack is quite a popular size, but this can obviously not be used until wide-diameter bits have drilled 36” and 26” hole.
When a kick or blow-out threatens the rig and the BOP controls are operated, large and powerful devices are closed together to seal off the hole and prevent the passage of well fluids up to the rig. Arrangements have to be made for sealing the hole either when drill pipe is in it, or when it is empty,
and different types of preventer are incorporated in the stack for use in everydifferent situation.The topmost preventer in the stack looks like a large steel pot from the outside and is called the ‘annular preventer’ or, sometimes, the ‘bag preventer’or ‘spherical preventer’. This can seal off the annulus between the preventer housing and any type of tubular that happens to be inside it. It can also seal off the hole completely if there is nothing inside running through the preventer at the time.
With the constant-bottomhole-pressure concept, the total pressures (e.g., mud hydrostatic pressure and casing pressure) at the hole bottom are maintained at a value slightly greater than the formation pressures to prevent further influxes of formation fluids into the wellbore. And, because the pressure is only slightly greater than the formation pressure, the possibility of inducing a fracture and an underground blowout is minimized. This concept can be implemented in three ways:

One-Circulation, or Wait-and-Weight, Method. After the kick is shut in, weight the mud to kill density and then pump out the kick fluid in one circulation using the kill mud. (Another name often applied to this method is “the engineer’s method.”)

Two-Circulation, or Driller’s, Method. After the kick is shut in, the kick fluid is pumped out of the hole before the mud density is increased.

Concurrent Method. Pumping begins immediately after the kick is shut in and pressures are recorded. The mud density is increased as rapidly as possible while pumping the kick fluid out of the well.

If applied properly, each method achieves constant pressure at the hole bottom and will not allow additional influx into the well. Procedural and theoretical differences make one procedure more desirable than the others.

Process suitability partially depends on the ease with which the procedure can be executed. The same principle holds true for well control. If a kick-killing procedure is difficult to comprehend and implement, its reliability diminishes.

The concurrent method is less reliable because of its complexity. To perform this procedure properly, the drillpipe pressure must be reduced according to the mud weight being circulated and its position in the pipe. This implies that the crew will inform the operator when a new mud weight is being pumped, that the rig facilities can maintain this increased mud-weight increment, and that the mud-weight position in the pipe can be determined by counting pump strokes. Many operators have stopped using this complex method entirely.

Subsea well control

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