ADVANCED DRILLING
MODULE-8
DIRECTIONAL DRILLING
POSITIVE DISPLACEMENT MOTORS
By
GAURAV HAZARIKA
Assistant Professor
School of Petroleum Technology
Pandit Deendayal Energy University
Downhole motors
• Downhole motors are designed to turn the bit without rotating
the drill string.
• Drilling by orienting the bit reduces the rig's power
requirements and reduces wear on both surface equipment and
tubular.
Downhole motors come in two basic types
1. Positive displacement motors (PDMs)
2. Turbine motors
Advantages
• Rotary torque was developed at the bit where it was actually
required
• Develop more power than the rotary system could transmit
to the bit
• Produce much faster rotational speed
• In Orient mode, there was less wear on the drill pipe and
casing
PDM
• After World War II, in the late 1950s,
downhole motors (PDM) was developed
by the Smith Tool Company in the USA.
• Also Known as DYNADRILL
• Along with Bent Sub, it is the most
common deflection tool
• Positive displacement motors drive the
bit without rotating the D/S
PDM
• All the PDMs presently in commercial use are of Moineau
type, which uses a stator and rotor arrangement with helical,
spiral passageway.
Metal
Rotor
Cavities
Rubber
Stator
PDM
• When circulating fluid is forced
through this assembly, it
imparts torque to rotor, causing
it to turn eccentrically.
• A universal connection transfer
this rotation through a bearing
and drive–shaft assembly to a
rotating bit sub , which turns
the bit.
Advantages & Disadvantages of PDM
• Used to drill with soft, medium and hard rock formations
• Used with any type of rock bits
• Most surface pump systems can be used
• Normal surface pump systems can be used to operate these downhole
motors
• Can be operated with aerated muds, foam and air mist
DISADVANTAGES
• Elastomer of the stator can be damaged by high temperature and some
hydrocarbons
Bent sub & Downhole motors
• For difficult deflections, a
bent housing can be
installed within the motor.
• Bent housing is often called
as Adjustible Kick-off (AKO)
• It introduces deflection
much closer to the bit and
provides a larger turn.
Bent sub & Downhole motors
Higher BU/Rs are delivered
with a smaller adjustable
kick-off (AKO) angle, since
the bend is closer to the bit.
Bent sub & Downhole motors
Components of PDM
The main components of the PDM
• Dump Valve (allow the fluid
circulation when the pressure below a
certain threshold)
• Motor (Power Unit)
• Universal joint (Connecting rod)
• Bearing Assembly
Dump Valve
• To prevent the motor rotation while running into the hole or pulling out of
the hole, a by pass valve is installed at the upper end of the motor
• The valves has radial ports that allows communication between the drill
string and annulus
Dump Valve
• Optimum pressure differential is required for the dump valve
to function
• If the pressure inside is more than the pressure in the
annulus formed by the wellbore and PDM the by pass
valve remain closed
• If pressure inside the valve is less than the pressure in
the annulus formed by wellbore and PDM , the bypass
valve will remain open
Motor Section
The PDM consist of two basic component
• The Rotor is a steel shaft which is shaped in the form of
spiral or helical. It is free at the top but attached to the
universal joint at the bottom.
• The Stator is a moulded rubber sleeve that forms a spiral
passageway to accommodate the rotor. The rubber body is
fixed with the body of the motor
Power Section
Use the Moineau type principle, a rotor/stator pair converts
the hydraulic energy to rotational mechanical energy
Rotor (steel) and stator (elastomer) profiles are similar with
rotor having one less lobe than the stator
Mud flowing under pressure fills the cavities between
dissimilar shapes of the rotor and stator and under the
pressure of mud , the rotor is displaced and begins to rotate.
Universal Joint
• Since the shaft is rotating eccentrically, the lower end must be
connected to a Universal joint
• A connecting rod is attached to the lower end of the rotor. It
transmits the torque and rotational speed from the rotor to the
shaft and bit
• This joint converts the eccentric motion to concentric motion,
which is than transmitted to the bit
• The Universal Joint than connected to drive shaft, which rotates
within the bearing assembly
Bearing Assembly
Durability of bearing very often determines the operating life of the motor
The bearing assembly fulfil mostly two functions
a. Transmit axial load to bit (Thrust bearing)
b. It maintain the central position of the drive shaft to ensure smooth
rotation (Radial Bearing)
• Thrust bearing supports the WOB while drilling and hydraulic thrust
while circulating.
• Radial bearing counteract the side loads on the bit while drilling
Bearing Assembly
Two radial bearings are normally included in the assembly
• The upper bearing acts as a flow restrictor, diverting a small
percentage of mud through the bearing assembly for lubrication.
• Sealed bearings lubricated by oil will increase the operational life of
the motor.
• Thrust bearing as normally the ball and race bearing where as radial
bearing are journal bearing .
• Mud is utilized to lubricate the bearing, around 5% to 8% of flow is
diverted through the bearings.
POWER CALCULATION OF PDM’s
The maximum predicted mechanical power delivered to the bit is
given by:
𝟐𝛑 × 𝐓 × 𝐍
𝐇𝐏𝐌𝐞𝐜𝐡𝐚𝐧𝐢𝐜𝐚𝐥 =
𝟑𝟑𝟎𝟎𝟎
𝐓×𝐍
𝐇𝐏𝐌𝐞𝐜𝐡𝐚𝐧𝐢𝐜𝐚𝐥 =
𝟓𝟐𝟓𝟐
HPmechanical = Motor mechanical power, hp (1 hp = 746 Watt)
T = Output torque, ft-lbf
N = Drive shaft rotary speed, rpm.
Peak Performance
• Efficiency of the motor is the ratio of the power at the bit to the total
power of the drilling fluid
• This efficiency factor takes into account losses due to:
a) internal fluid leaks along contact surfaces which will increase with
wear (multi-lobe motors have lower efficiencies than single-lobe
motors owing to there being more contact surfaces).
b) friction losses in the bearings and universal joint.
c) entry and exit losses.
• For a single-lobe motor, E may vary between 70 and 90%. The efficiency
tends to reduce with increasing number of lobes, so that for a multi-lobe
motor the value of E may be 40-70%.
