Category Archives: Directional Drilling

What are Turbine Drilling Motors?

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Turbine Motors work by harnessing the energy of a continuous flow of steam which passes through them. More specifically, drilling fluid travelling down the drillstem is deflected by the blades of a stator which is connected to the housing. This deflected fluid then flows over the blades of a rotor, which causes the drive shaft itself to rotate. The blades of both the rotor and stator are configured in the same way as a standard ventilation fan, but with the blades positioned in reverse. This is because a fan is designed to propel air outwards with a motor, whereas a turbine requires an input of air or liquid to turn its motor.

Mud or drilling fluid is pumped down the drillstring from the surface, until it enters the power section of the turbine. It then comes into contact with the stator blades, which cannot move since they are fixed to the turbine housing. The fluid’s momentum is therefore redirected to the rotor blades. This then moves the drive shaft to the drill bit, causing it to rotate. When the rotor blades perform their exit turns, the liquid is then directed into the next rotor/stator stage. Each turbine may include up to 400 of these stages, although a more typical figure is 100-250. Every stage will transmit the same amount of torque to the drive shaft, and uses up an equal amount of the total energy.

Figure 1 - Turbine Motors, (oilandgasproductnews.com, 2015)

Figure 1 – Components of Turbine Motors, (oilandgasproductnews.com, 2015)

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What are Positive Displacement Mud Motors in Drilling for Oil and Gas?

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Positive-Displacement Motors (PDM) make use of a power generation section which is made up of a rotor/stator combination. In order to move a rotor part,  a PDM requires hydraulic power from drilling fluid flowing through the power generation part. With a PDM, the stator and rotor work in tandem in the same way that gears do. The stator acts as the outer gear, and is made from a moulded elastomer featuring at least two lobes. The OD of the elastomer is protected by a secure metal casing. The rotor is positioned within the stator, and acts as an internal gear. This rotor is made of metal, and will have one less gear or lobe than the stator. Because of this difference, a cavity is created which is filled with drilling fluid when the PDM is downhole. This cavity acts as a wedge when it is put under pressure, and because the drilling fluid itself can’t be compressed, the force applied to the top of the wedge causes the rotor to move.

Mud Motor

Figure 1 – Mud Motor

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What are Hole Openers for Drilling?

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Hole openers are used to increase size of well bore and there are two broad categories of hole openers: fixed diameter hole openers, and under-reamers.

Fixed diameter hole openers are usually made up of three “cutters” arranged around a mandrel, and mounted on “saddles” by strong retaining pins. Cutters may be milled tooth, PDC, or TC inserts, which will vary depending on the formation to be cut.

Fixed Diameter Hole Opener

Fixed Diameter Hole Opener (Getech Equipments International, 2018)

Under-reamers, on the other hand, are hydraulically actuated hole openers that possess two or three arms. They are primarily used when a hole needs to be opened to a diameter larger than the casing which has already been set. Both of these forms typically feature a series of fluid passages, or “jets”, which are arranged to keep the cutters lubricated and help with the removal of cuttings. These need to be set up properly before use, to ensure a balanced mud flow both through and out of the hole opener.

Crew sets up the under reamer to enlarge the hole,

Crew sets up the under reamer to enlarge the hole, (tamu.edu, 2003)

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What are Shock Subs and Its Applications?

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Shock subs, also known as vibration dampeners, are used to absorb vibrations and bit shock loads in drill collar strings. They usually feature long integral elastomeric elements, which serve to transmit torque and weight to the bit simultaneously. When drilling is being carried out at shallow depths, intermittent hard and soft streaks, along with broken formations, can transmit vibrations to the surface, where they are easily detected. With greater depths, though, these vibrations might not be detected because the drill string cushions them. However, they will still cause damage to the bit, as well as bottom hole assembly components and the drill string.

Shock Sub (Vibration Dampeners), Hunting (2018)

Shock Sub (Vibration Dampeners), Hunting (2018)

Some advantages of using a shock sub include:

  • Offering faster drilling rates, since optimum bit weight and rotary speed may be used on the bit constantly.
  • Increasing the bit length by reducing shock loads.
  • Cutting damage to drill collars, drill pipe, and downhole tools by reducing bouncing.
  • Reduced connection damage, because the elastomeric element absorbs both torsional and axial loads, so that connections are not at risk if the bit stalls.
  • Reduced damage to surface equipment, including swivels, blocks, and wirelines.

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What are Drilling Jars?

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Most modern drilling jars are hydraulic. They are also usually double acting, meaning they can deliver an extra-heavy impact should the bottom hole assembly become stuck. They are intended to work as an integral part of the drill string, and can withstand high pressures and temperatures over a long period of time, making them suitable for long-term use.

With almost the same length and diameter specifications as standard drill collars, and with a similar connection strength and slip setting area, they may be used as a component of a stand of drill collars without difficulty.

Usually, jars will be used alongside accelerators, which are run above the jar and work automatically. They serve to amplify the impact force of the jar, and can even double it in some cases. They commonly use the compression of silicon to give added stored energy and optimize jar impact and free-travel distance in both directions. They also have the added benefit of dampening the dynamic load in the drillpipe, since they transmit shock waves poorly, thus helping reduce damage to both string and surface equipment.

Drilling Jars Diagram (Slideshare, 2017)

Drilling Jars Diagram (Slideshare, 2017)

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