Category Archives: Drilling Operation

How to Determine Mud Motor Failure

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Mud motor failure downhole may be happened from time to time. The questions that are usually raised are things like “How do I know if the mud motor fails down hole?” and “What indications will I see that this has happened?” etc. We would like to share our experience regarding mud motor failure and its symptoms.

The following signs indicate that you may be faced with downhole mud motor breakdown.

Frequent Mud Motor Stall – Motor stall happens when the rotor of the mud motor has stopped moving. Typically, the motor stalls only with a high differential of pressure. However, if the motor doesn’t perform as normal, it will get stalled with by a small amount of differential pressure. For instance, a mud motor normally drills at 400 psi differential pressure, but if the motor is stalled out with only 100 psi you can suspect the problem is with the motor.

Pressure fluctuation while rotating –  Rotating with a good mud motor won’t create pressure fluctuations, whereas a bad mud motor will show fluctuation in stand pipe pressure and you may not be able to maintain constant pressure.

Abnormally high surface pressure – A stator is made of rubber. When the stator rubber is worn out and breaking into pieces, small parts of rubber can jam into the flow path in the motor. This situation also results in high stand pipe pressure.

Reduction in Rate of Penetration – If there are no changes in formation and drilling parameters, the decreasing in ROP (Rate of Penetration) may be caused by failure of the down hole tool. Moreover, if the took is severely damaged, you will be able to drill any footage.

What should you do if the problem is clearly identified?

If any failure sign is seen, it is recommended to pull out of the hole and change a new tool. It is almost impossible to drill with a damaged mud motor unless you only have a few feet to the well target depth.

With the mentioned indicators of mud motor failure above, you should be able to identify your suspected problem and begin troubleshooting as soon as possible to minimize non-productive time on a drilling rig.

References

Inglis, T.A. (2010) Directional drilling. Dordrecht: Springer-Verlag New York.

Mitchell, R.F., Miska, S.Z. and Aadnoy, B.S. (2012) Fundamentals of drilling engineering. Richardson, TX: Society of Petroleum Engineers.

Short, J.J.A. (1993) Introduction to directional and horizontal drilling. Tulsa, OK: PennWell Books.

Drilling Bit Types and Drilling Bit Selections

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Currently in the drilling industry, there are two main categories of drilling bits: rolling cutter bits and fixed cutter bits. Plus, bit sizes vary from 3-7/8 inch to 36 inches.

 Rolling Cutter Bits

Rolling cutter bits, which some may also call roller cone bits or tri-cone bits, have three cones. Each cone can be rotated individually when the drill string rotates the body of the bit. The cones have roller bearings fitted at the time of assembly. The rolling cutting bits can be used to drill any formations if the proper cutter, bearing, and nozzle are selected.

There are two types of rolling cutter bits which are milled-tooth bits and tungsten carbide inserts (insert bits). These bits are classified by how the teeth are manufactured:

1. Milled-tooth bits

 

(Mill Tooth Bit)

Milled-tooth bits have steel tooth cutters, which are fabricated as parts of the bit cone. The bits cut or gouge formations out when they are being rotated. The teeth vary in size and shape, depending on the formation. Teeth of the bits are different depending on formations as follows:

Soft formation: The teeth should be long, slender and widely spaced. These teeth will produce freshly broken cuttings from soft formations.

Hard formation: The teeth should be short and closely spaced. These teeth will produce smaller, more rounded, crushed, and ground cuttings from hard formations.

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Break Mud Gel Strength While Tripping In Hole

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When you trip out of hole and leave drilling mud in a static condition for a period of time, mud rheology (PV, YP and gel strength) of drilling mud tends to increase. It is very important that when you trip back in hole you should break the thick mud (most people call “break the gel”).

What can be happened if you don’t condition thick drilling mud while tripping back in hole?

Thick drilling mud tends to consume at lot of energy to move it therefore high frictional factor is occurred that will result in excessive equivalent circulating density (ECD). If formations are not strong enough to withstand the ECD, lost circulation will be happened. The lost circulation will lead to other drilling problems such as well control and well ballooning.

How can you break the gel?

The follow steps are example on how can you break gel of drilling mud.

• Trip in hole to the casing shoe depth

• Rotate pipe slowly 10-15 sec and bring pump up slowly

• Circulate bottom up at reduced speed

• Trip in hole about 1/3 – 1/2 way of open hole

• Rotate pipe slowly 10-15 sec and bring pump up slowly (staging up)

• Circulate bottom up at reduce speed and slowly reciprocate pipe

• Trip in hole to bottom

• Rotate pipe slowly 10-15 sec and bring pump up slowly (staging up)

• Circulate bottom up at reduce speed and slowly reciprocate pipe

• Drilling ahead

In the demonstrated instruction, there are many times that you stop and condition mud which will bring mud properties back to normal.

Ps, this is based on my experience you may need to adjust to suit with your drilling operation.

Underbalanced Drilling – Watch This VDO To Get Clear Idea

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Underbalanced drilling is a drilling technique that hydrostatic pressure from drilling mud is less than reservoir pressure. The underbalanced drilling can be created by using low weight fluids as base oil, fresh water which has less hydrostatic pressure than the expected formation pressure. Additionally, this drilling method is achieved by using low density drilling fluids as gas, foam, combination between conversional drilling fluid and foam/air.

The main advantage of Underbalanced drilling is to minimized formation damage in reservoir. With underbalanced drilling, the reservoir fluid is allowed to flow out into the well therefore the chance of mud invasion into the reservoir is minimal. Hence, the production from the well from this technique is higher than normal wells. Conversely, drilling with conventional method (overbalanced drilling) creates near wellbore damages which affect hydrocarbon production.

The underbalanced drilling is often applied to horizontal drilling wells because long horizontal reservoir will not be damaged with drilling fluid. Only few inches of near wellbore damage in the horizontal section can drastically reduce the reservoir performance.

This video from Shell demonstrates you regarding underbalanced drilling and there is one section showing comparison between conventional and underbalanced drilling. This short VDO will definitely give you clear picture of this technique.