Key Factors in Choosing Shale Drilling Motors
Selecting a motor for a shale project is more than just torque or horsepower. The motor should be examined as part of the whole drilling system comprising bit, BHA, drilling fluid, directional tools and projected trajectory.
Torque and Power Output
A positive displacement motor uses the pressure and flow of the drilling fluid that passes through the power part of the motor to create a mechanical spin. The shape of the rotor and stator controls crucial operational properties such as the connection between flow rate, rotational speed and differential pressure.
For a shale application the needed motor output should be consistent with the chosen bit and anticipated drilling conditions. The motor should have the right torque and speed range to keep the bit in its desired drilling window, although larger torque does not always guarantee better performance.
The choice of motor must thus evaluate the bit speed necessary, the flow rate available, the predicted pressure drop, the weight on bit, and the formation response. These aspects must be evaluated together not looking at torque as a stand-alone criterion.
Directional Control Capabilities
Directional control is one of the primary reasons for the broad usage of downhole motors, including the drilling mud motor, in directional drilling. A steerable motor assembly may be fitted with a bend setting that allows the drilling crew to modify the trajectory as they drill.
However, the success of this arrangement hinges on more than the bend angle itself. Directional response may be affected by motor length, BHA design, stabiliser location, bit selection, formation behaviour and drilling parameters.
The motor should consequently be chosen in accordance with the trajectory requirements of the given well, especially in shale wells, where the anticipated trajectory may contain vertical, curve and lateral parts. The aim is to get the desired directional reaction, without imposing undue mechanical or operational constraints.
Durability and Reliability
Downhole motors are working in harsh settings. Motor life may be affected by temperature, drilling fluid characteristics, differential pressure, vibration, abrasive particles and operation methods.
For PDMs, the power section is a critical portion, as the elastomer stator is in close contact with the drilling fluid and the rotor. Consequently, material selection and compatibility with the drilling-fluid system must be addressed in choosing a motor for a specific application. Industry motor manufacturers also recognise elastomer performance as a key element in downhole motor dependability.
Reliability should thus be judged by the overall motor design and by its operating environment, rather than by a simple comparison of the number of moving parts.
Positive Displacement vs. Turbine Motors: Shale Performance Comparison
Both positive displacement motors and turbine motors convert drilling-fluid energy to mechanical rotation, albeit in distinct ways. The decision should be made according to the drilling target and circumstances, not based on the general idea that one design is better than the other.
Torque and Speed Characteristics
Positive displacement motors are based on the interaction between the rotor and the stator. Depending on the particular power-section arrangement and the operating circumstances, their performance qualities enable them to generate meaningful torque at quite low to moderate rates of spin.
This may be advantageous in cases when the bit and formation chosen need regulated rate of rotation and enough torque. However, the actual operating point is a function of flow velocity, differential pressure, motor design and bit loading.
Turbine motors employ numerous turbine stages to transfer the energy in the drilling fluid to rotation. Turbine motors typically function at greater rotational speeds. This feature might be beneficial for situations where greater bit rate is desired.
So the real-world comparison is not “high torque versus high speed.” Operators must evaluate the motor’s working range to the bit requirements, formation, available hydraulic power and intended drilling parameters.
Directional Drilling Capabilities
PDMs are often used in directional drilling as they may be integrated in steerable assemblies and utilised with bent-housing designs. mud motors are defined by SLB as positive displacement drilling motors that convert hydraulic horsepower from drilling fluid into rotary force to the bit and are used extensively in directional drilling.
When drilling a directional shale well, the issue to be asked is whether the motor and BHA chosen can provide the requisite trajectory control across the anticipated build, turn and lateral sections.
The turbine motors may also be used directionally, however their high speed operating characteristics and the particular arrangement of the assembly may make them more useful for other drilling aims. Therefore the decision should be based on the desired directional response and drilling performance rather than on the kind of motor alone.
Reliability in Shale Environments
The reliability of a motor relies on its design and operating circumstances, drilling-fluid system, and maintenance methods. Just because a PDM has a different mechanical arrangement than a turbine, doesn’t mean it will automatically have a longer service life.
The state of the elastomer, rotor-stator contact, bearings and other gearbox components should be addressed for PDMs. Temperature and drilling-fluid compatibility are of special relevance since they may impact the power section.
Turbine motors have certain operational concerns such high speed rotation and bearing associated stresses. Hence, the motor that is most suited is one that is designed and will function within the envelope for that well.
Optimizing Motor Selection for Improved Shale Well Productivity
The choice of a drilling mud motor should be seen as one aspect of the total optimisation of the drilling operation. Instead of choosing a motor from a catalogue rating, operators may now match the motor’s operational characteristics with the projected well profile, bit requirements, hydraulic circumstances and BHA design.
Customizing Motor Specifications
The motor specs must be chosen to suit the real needs of the well. Relevant criteria may include power section design, motor size, bend setting, connection requirements, operating flow range and the needed torque and speed characteristics.
The number of rotor and stator lobes is one of the factors affecting the operating parameters of the PDM. In general multiple power-section arrangements may be used to provide varied speed and torque combinations. Drilling application determines the optimum design rather than just believing larger lobe count is better for tougher shale.
Bend setting is another critical factor in directional applications. Increasing or decreasing the bend setting may have an effect on the directionality, but the outcome is also a function of the BHA, stabiliser arrangement, hole diameter, formation and drilling variables.
So, motor customisation should begin with the well program and operational needs, not with a fixed motor design.
Integration with Measurement and Logging Tools
The drilling motor is only one element of a contemporary directional drilling system. MWD and LWD equipment may give downhole measurements that allow drilling crews to assess trajectory and formation conditions during drilling.
MWD information may be used to assist directional choices and LWD readings can give formation related information in certain cases depending on the tool setup. By combining these measures with surface drilling data, operators may better advise modifications to drilling parameters and trajectory.
For example, directional and formation data may assist the drilling crew determine whether the well is following the anticipated trajectory and if extra steering choices are necessary. Available actual measurements will depend on the specified MWD/LWD system and BHA configuration.
That is why the motor compatibility must be checked at the BHA level. All the motor, bent housing, bit, stabilisers, MWD/LWD tools, and other components have to perform together within the required working environment.
Operational Considerations for Shale Drilling
Operating circumstances may directly impact the performance of motors. The drilling crew should evaluate drilling-fluid characteristics, flow rate, pressure drop, weight on bit (WOB) and drilling speed before using a drilling mud motor.
The drilling fluid should be compatible with the whole drilling system and the power section materials of the motor. Flow rate is very essential since it affects the speed of the motor and hydraulic performance. Too much differential pressure might also put extra burden on the motor.
The WOB and motor operating conditions should be matched with the bit and formation requirements. The mechanical stress may grow with excessive loading and the motor performance may be affected, whilst the bit may not operate efficiently with inadequate loading.
Directional drilling adds additional issue in that the drilling parameters must be capable of achieving the desired toolface and trajectory response. Thus, motor performance should be evaluated together with drilling settings rather than altered separately.
Conclusion
No single drilling motor is the automatic best option for every directional shale project. A positive displacement drilling mud motor is frequently a good candidate when the application demands regulated bit rotation, useable torque at a suitable speed range, and integration into a steerable directional BHA.
But the decision must be made on the whole drilling program. The hole size, trajectory, formation features, bit requirements, flow rate, pressure drop, qualities of the drilling-fluid, temperature, bend setting and MWD/LWD configuration all impact the final motor selection.
Turbine motors and PDMs also have distinct operating characteristics . Rather than assume one kind of motor is always better than another, well-drilling teams should match each motor’s working range to the needs of the given well.
In shale drilling, a well-matched motor and BHA may help provide more predictable directional control and more efficient transmission of hydraulic power to the bit. The key is to pick the configuration according to the actual drilling circumstances, and use it within the specified operating range.
Call to Action
When considering directional shale projects, motor selection should start with the needs of the well, not a conventional motor setup. Welong can discuss the application requirements and assist clients in evaluating the correct drilling mud motor design for their drilling program.
Useful information for seeking a motor suggestion includes hole size, projected trajectory, formation conditions, drilling-fluid system, estimated flow rate, bit type, and other accessible BHA information. It provides the technical team a better footing on which to assess motor specifications and directional needs.
Welong’s technical staff is available to collaborate with clients to discuss motor design and application needs, rather than a one-size-fits-all decision.
For project-specific inquiries, contact Welong at oiltools15@welongpost.com to discuss your drilling requirements and motor selection.
