WELL CONTROL
Well control is one of the most important systems in Oil & Gas Production either in offshore or onshore drilling. It is mostly about preventing incidents such as kick, loses and blowout from occurring on the well.

kick and blowout occur when there is formation fluids flow into the well in the presence of drilling fluid. In order to prevent formation fluid from flowing into the hole, the pressure exerted by drilling fluid column must be greater than the pressure of formation fluid at any depth. The pressure of formation depth is usually proportional to the height of the column and density of drilling fluid.

Lost circulation occurs when the drilling mud been pumped is not returning to the surface as much as been pumped down to the well. This also can lead to blowout.

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Detection of kick:

-Increase in pump stroke rate.

-Sudden increase rate of penetration.

-Surface indications from increased active mud pit level.

-Gas Kick Warner(GKC)

  • A tool that been designed to measure the propagation time of pressure pulse which travel through the mud system.

  • However, this tool has some limitations such as it can only detect gas and only functioning when there is mud flowing in the hole.

- Wave processing of the parameters

  • Basically has the same principal as GKC but it uses theory of acoustic wave and flow rate wave through the drilling fluid.



Causes of kick and blowout:

I. Reduction in fluid level by lost circulation.

  • Natural lost circulation which is the formation itself that results the loss of drilling fluid. For example, loss of drilling fluid into highly permeable sands or gravel beds.

  • Induced loss circulation occurs when the pressure required to prevent the formation fluid flowing into the hole is in excess such that it will imposed on the walls of the hole and causes the drop of fluid level thus loss of circulating fluid.

II. Insufficient Drilling Fluid Density.

  • Drilling fluid density (mud gradient) must be kept between pore pressure and fracture gradient.

  • If mud gradient is lesser than pore pressure, blowout could occur.

  • If mud gradient is greater than fracture gradient, the whole well could collapse.

III. Reduction in Static Pressure by Pipe Withdrawal.

  • Reduction of static pressure when removing the pipe is not related to the fluid level or density.

  • It is usually occurs during the cementing operation.

  • After plugging with the cement, the drilling fluid generally has high gel strength. As the result of it, pipe withdrawal could cause big pressure reduction.

  • If density of the drilling fluid does not give enough excess pressure, then it may cause a blowout.

IV. Failure to Keep Hole Full of Fluid.

  • The cause can be divided into those occurring during pipe withdrawal and during other operations.

  • During other operation such as, suspension of drilling equipment to be repaired or for economic reason, the slow drop of drilling fluid often been neglected. With the exposure to the permeable formation, gradual but slow loss of fluid can lead to significant decrease in static pressure thus can cause blowout.




Prevention:

I. Reduction in fluid level by lost circulation.

  • the consideration of the method of overcome lost circulation need to be done before starting to drill the well such as the addition of remedial materials, use of rapidly, high-viscosity mud and the cementing of loss zone.

  • When lost circulation cannot be overcome before reaching the blowout zone, casing must be done and cemented to prevent blowout.

  • After penetrating the potential blowout zone, lowering mud viscosity and gel strength is required prior running and cementing of casing because high gel prior strength fluid in small annular space will results to high running pressure.


II. Insufficient Drilling Fluid Density.

  • Drilling fluid density is increased so that it will be greater than the pore pressure. However, it is essential to maintain the fluid density such that it will not be greater than fracture gradient (in mud window).

  • If higher mud density is required during drilling the well, the casing and cementing process must be done in order to avoid the well from collapse.

  • Figure 2 below shows that increased drilling fluid density can only be done if casing been set otherwise the drilling fluid density will exceed fracture peressure or less than pore pressure.


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Figure 2

Figure 2: impact on casing points of reduced margin.

III. Reduction in Static Pressure by Pipe Withdrawal.

  • The pressure reduction of pipe during withdrawal is proportional to the gel strength, the pipe length on the hole and the annulus size.

  • Lowering the gel strength when penetrating the potential blowout zone is essential since the gel strength is usually at the maximum after the cementing casing or plugging especially when using large diameter drilling tools.

IV. Failure to Keep Hole Full of Fluid.

  • The hole is kept full at all times to ensure that continuous circulation of fluid into the annular space is maintained.

  • When there is mechanically impossible to keep the hole with available drilling equipment, it is compulsory that the maximum draining pipe can be withdrew is five and closed pipe is only two without stopping to fill the hole.

Killing the kick

There are 3 methods to kill the kick:

  • Balanced method

  • Drillers method

  • Concurrent method

Blowout Preventer (BOP)

In case of when there are indications that kick or lost circulation will occur, there will be necessary to seal off the well to prevent an uncontrollable flow or blowout of formation fluid. Blowout preventer (BOP) is the equipment that will be used to perform this operation.

There are 2 types of BOP:

1. Annular.

2. Ram.

Annular type.

Annular type of BOP is function by hydraulic pressure that been applied to the piston, causing the piston to move upward and force the sealing component to extend into the wellbore. Then, the steel segment molded into the element partially close over the rubber to prevent excessive extrusion when sealing under high pressure. The advantage of annular BOP is that it designed such that it has the capability to close and seal on any size and shape of equipment. However, the disadvantage of this type of BOP is that it will not working effectively under very high pressure.

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Figure 3: Annular Blowout Preventer

Ram type.

There are usually four types of ram BOP:

1. Pipe rams.

2. Variable-bore rams.

3. Blind rams.

4. Shear rams.

The similarities that have in all the ram types are the sealing process occurred by forcing two elements (rubber packing seals) to seal the annulus. Each type of ram BOP can only work in specific size and cannot be used in a variety of applications. This type of BOP is more dependable in high pressure condition and it is easily serviceable.