Having known Whipstock to be one of the most commonly used deflecting tool in sidetracking a well, there are other four different types of well deflecting tools. Whip-stock was formerly used but today, it is not regarded as one of the most commonly used deflecting tool but are usually use to other well operations. Other types of deflecting tools used in sidetracking a well are Jetting Bit, Stabilizers, Mud motor and Turbine.
Mud Motor OR Positive Displacements Motors
The first mud motor was introduced to the petroleum industries in the late 1960s. Since then, the use of mud motor has increase greatly for directional drilling operation. A mud motor sometimes called drilling motor is a progressive cavity positive displacement pump (PCPD) placed in the drilling string to provide additional power to the bit while drilling. The PCPD pump uses drilling fluid (Commonly referred to as drilling mud) to create motion in the power section of the motor which is transferred as concentric power to the drill bit. The mud motor uses different rotor and stator configurations to provide optimum performance for the desired drilling operation, typically increasing the number of lobes and length of power assembly for greater horsepower. In certain applications, compressed air or others gases can be used for mud motor input power. Normal rotation of the bit while using a mud motor can be from 60rpm to over 100rpm.
Normal mud motor consists of a top sub, which connects the mud motor to the drill string, the power section, where the eccentric power from the rotor is transmitted as concentric power to the bit using using a constant-velocity-joint. The bearing assembly which protects the tool from off bottom and on bottom pressures and the bottom sub which connects the mud motor to the bit. When the bit is bottomed and the motor is effectively working, there is a notable increase in the pressure in the fluid system. This is caused by a restriction within the motor and is termed differential pressure. If this differential pressure is too high then the motor can stall, which means the bit has stopped turning and this can cause severe damage to the internal surface of the stator.
A mud motor is described in terms of its number of stages which is usually three namely Lobe, Ratio and External diameter. Stages are the number of full twists that the stator makes from one end to the other and the lobe ratio is the number of lobes on the stator to the number of lobes on the rotor. (The stator always has one more lobe than the rotor). A higher number of stages or the number of lobes indicates a more powerful motor, capable of producing more torque. A lower number of lobes indicates a reduction in the torque produced. The operation parameters include flow-rate, bit rpm and torque. The relationship between the rotor and the stator geometry determines the rotational speed and torque. The rotational speed is proportional to the flow-rate and torque is proportional to the pressure drop in the fluid as it flows through the motor. The more lobes the higher the torque and the slower the rpm.
The use of mud motors is greatly dependent on financial efficiency. In straight and vertical holes, the mud motor may be used solely for increased rate of penetration (ROP) or to minimize erosion and wear on the drillstring, since the drill string does not need to be turned as fast.
Uses of Mud Motor
- The majority use of mud motor is in the drilling of directional holes.
- Mud motor can be configured to have a bend in them using different setting on the motor itself.
- Mud motor can be modified from 0 degree to 4 degrees with approximately six increments in deviation per degree of bend. The number of bends is determined by rate of climb needed to reach the target zone.
- Using measurement while drilling, a directional driller can steer the bit to the desired target zone.
- Steerable motors are used to drill kick off point. When drilling the kick off point be sure to avoid drilling a soft formation immediately below the hard ones because its can cause severe bit shank wear. In as such, the kick off point should be selected in a non abrasive homogeneous formation.
- Hard rocks formations can be drilled with motors using diamond or PDC bits.
The steerable motor assemblies are versatile and are used in all sections of directional wells starting from kicking off and building angle to drilling tangents sections and providing accurate trajectory control. In trajectory control, the mud motor is used because it contain PDMs (Positive Displacements Motors) with bent subs or bent housing.
The bent sub and bent housing uses bit tilt and bit side which force the drill string to change hole direction and inclination. Bent housing is more effective than bent sub because
- Shorter bit to bent distance, which effectively reduces the bit offset and creates a higher build rate for a given bend size.
- Its also reduces the bending stress and moment arm at the end.
Because of these reasons above, the bent housing PDM is easier to orient and allows a long rotation period. Now the question is how can we obtain the build rate? Before when computer wasn’t widely used and its applications was limited, the build rate was assumed and predicted through some mathematically calculation equations. Given that
Rb= Build rate in degree/100ft
Ѳ=Bend angle in degree/100ft
L1= Distance from the first contact point (bit) to the second (bend) in ft
L2= Distance from the second contact point to the third (motor top stabilizer) in ft
For more accurately result, a BHA anlysis program is usually used to calculate the build turn rate and the drop turn rate of the motor assemblies.
Jetting bit is the most common deviation tools for directional drilling and this technique is apply only in soft formations. This technique was developed in the mid 1950 after the usage of whipstocks and its superseded the use of whipstocks as the primary deflection technique as by then. Although Mud motor has sequentially over powered the jetting bit but it is still used frequently and its offers several advantages which makes it the preferred method in some situations. A special jet bit may be assigned to be used but to be on the safe side, it is a common practice to use a standard soft formation tri-cone bit with one very large nozzle and two smaller ones.