Turbine vs PDM

Turbine vs. PDM Farshad Asgari 93134020 Supervisor: Dr.Fazaeli

Contents Positive Displacement Motors  Turbines  Downhole motor and bent sub  PDMs vs. Turbines with a Bent Sub  References 

Positive Displacement Motors 

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A positive displacement motor (PDM) is a hydraulically driven downhole motor that uses to rotate the bit, independent of drill string rotation. The PDM is made up of several sections: By-pass valve or dump sub. Motor section. Universal joint or connecting rod section. Bearing section with drive sub.

Types of Positive Displacement Motors PDMs come in various configurations. the stator will have one more lobe than the rotor.  The first types of PDMs, and the simplest, are 1/2 motors. These generally give medium to low torque output and medium to high rotary speed.  Torque output is directly proportional to pressure drop across the motor.  The 1/2 motors have good applications in performance drilling with a PDC, diamond, or TSP-type bits. Some shorter models are used for directional purposes. 

Types of Positive Displacement Motors 

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Multi-lobe motors have high torque output and relatively slow speed. Therefore, they have good applications with roller cone bits. Such motors are also suitable for use with PDC bits, especially the large cutter types which require a good torque output to be efficient. also have good directional applications with bent subs as the deflection device.

Positive Displacement Motors  PDM

Observations:

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LCM can be pumped safely, though care should be taken that the material is added slowly and evenly dispersed. The system should not be slugged.

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Sand content in the drilling fluid should be kept to a minimum.

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Temperature limits are around 270°F to 130ºC, but higher temperature stators have been developed.

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Pressure drop through the tool while working is typically around 50 psi to 800 psi.

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Allowable wear on bearings is of the order of 1mm - 8mm, depending upon tool size.

Positive Displacement Motors  Characteristics: 

Torque is directly proportional to the motor’s differential pressure. This makes the tool a very simple to operate.

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RPM is directly proportional to flow rate, through somewhat affected by torque output.

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Hydraulic horsepower consumed = {(P x Q) ¸ 1714}, where P is the pressure drop (psi) across the motor and Q is flow rate (gpm).

Turbines A turbine is made up of several sections: 

Drive stages or motor section.

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Axial thrust bearing assembly and radial bearings.

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Bit drive sub.

Turbines 

Turbine Observations:

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Turbines do not readily allow the pumping of LCM.

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Sand content of the drilling fluid should be kept to a minimum.

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Due to minimal rubber components, the turbine is able to operate in high temperature wells.

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Pressure drop through the tool is typically high and can be anything from 500 psi to over 2000 psi.

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Turbines do not require a by-pass valve.

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Usually, the maximum allowable bearing wear is of the order of 4mm.

Turbines  Turbine

Characteristics:

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Torque and RPM are inversely proportional (i.e. as RPM increases, torque decreases and vice versa).

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RPM is directly proportional to flow rate (at a constant torque).

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Torque is a function of flow rate, mud density, blade angle and the number of stages, and varies if weight-on-bit varies.

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Optimum power output takes place when thrust bearings are balanced.

Downhole motor and bent sub 

A common method of deflecting wellbores is to use a downhole motor and a bent sub. the bent sub is placed directly above the motor and the bent sub which makes this a deflection assembly.

PDMs vs.Turbines with a Bent Sub 

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For directional work with a bent sub, PDMs offer several advantages over turbines: When drilling with a PDM, the directional driller can use pump pressure as a weight indicator. If the pump pressure is constant, the differential pressure across the PDM is constant, so torque and WOB are constant. PDMs give a longer bit life than turbines because of the slower rotary speed. They can tolerate LCM whereas turbines cannot. instead of using a bent sub, a PDM with a small bend at the U-joint housing can be used. This reduces the problem of the bit riding the side of the hole while tripping in and out.

PDMs vs.Turbines with a Bent Sub The only real advantage of turbines is that they can operate at higher temperatures than PDMs.  Also turbines DO NOT have a dump valve.  short deviation turbines could handle higher flow rates than PDMs, but this is no longer the case. Nowadays, it is quite rare to use a turbine with a bent sub. 

Advantages and disadvantages of turbines vs PDMs

References 

Richard S. & Robert D. - Directional Horizontal Drilling Manual – PetroSkills.

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Directional Drilling, Course Manual, Baker Hughes.

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Drilling Engineering Workbook, A Distributed Learning Course, Baker Hughes INTEQ, 1995.