Mechanical power to the bit / corehead


This article describes  the various ways of transmitting mechanical power to the bit by rotation. There must be sufficient HHP at the bit to keep the cutters or diamonds clean, and that an increase in mechanical power, applied to the bit with minimum vibration, will increase penetration rate.

Mechanical power is the product of torque and speed. For a given bit, once the threshold WOB necessary to make the bit teeth or cutters start to dig in has been overcome, then for a given WOB increasing the rotational speed will increase penetration rate.

1 Rotary drilling

Rotary drilling implies turning the whole drillstring from surface and with conventional clearances this can lead to severe vibrations. If the vibrations can be tolerated or minimised without impairing other aspects of the overall drilling system, and pipe/borehole friction is not too great, then it becomes a cheap and effective method of getting power downhole.

The additional pressure drop up the annulus due to drillpipe rotation can be quantified. As a consequence, in a well where the drillpipe is eccentrically located in the hole, the dynamic pressure drop up the annulus can be doubled in some cases, when the pipe rotation is taken from zero to 100 rpm. For many drilling situations this is not a significant effect but for deep HP/HT wells where the pore pressure is close to the formation strength this can restrict either penetration rates as the pipe cannot be turned fast without mud losses, or the circulation rate, leading to poor bit/hole cleaning.

When using the continuous coring technique, the DP is only slightly smaller than the hole and vibrations are minimised if the hole remains in gauge. However the small annulus can give problems with mud hydraulics.

2 Downhole hydraulic motor drilling

Downhole hydraulic motor drilling involves a hydraulic motor which converts energy in the mud flow into bit rotation. The product of the pressure drop across the motor and the flow rate through the motor represents the hydraulic power put into the motor. Similarly, the product of the torque and rotational speed represents the mechanical power produced by the motor and put into the bit. The ratio of output power divided by input power is known as the efficiency of the motor.

The maximum flowrate is usually limited either by the maximum available standpipe pressure, or by the maximum ECD which can be tolerated during drilling. When designing a hydraulics programme the available standpipe pressure has to be allocated between the bit, motor, MWD, and parasitic losses, i.e. those in the DP, DC, annulus and surface facilities. This implies a balance must be struck between bit and motor pressure drops so that there is sufficient mechanical power to drive the bit and to keep it clean. In most circumstances, lowering the parasitic losses enables penetration rate to be increased for a given standpipe pressure limit.

Motors may be straight hole or steerable. The steerable motor is so configured that when it is rotated it drills straight ahead, and when it is oriented, using an MWD or steering tool, it drills a curved hole in the desired direction. It plays a key part in the drilling of practically all horizontal wells and of most deviated wells.

The available downhole motors are the hammer, the Moineau motor (mud motor), the Turbine.

3 The Moineau motor (mud motor)

The Moineau motor (mud motor) has a number of advantages for use in slim hole drilling:

1.It is a fundamentally simple machine, with essentially only one moving part.

2.The power output of a Moineau motor (mud motor) is, to a first approximation, linearly proportional to tool diameter, thus the mechanical power per area of hole increases dramatically as the hole gets smaller. In formations where the limitation on penetration rate is mechanical power to cut the rock, this is reflected in increased penetration rates as the hole gets smaller. In vertical shallower wells in softer formations, rotary drilling may well provide a more cost-effective bit drive system.

3.It is a positive displacement device. Thus the output rpm is directly related to flow rate through the motor and the differential pressure across the motor is directly related to the output torque. This means that you can see from the surface if it is working and by observing the stand pipe pressure you can monitor the output torque, and thus monitor the WOB which is related to the motor torque by the bit aggressiveness.

4.If the standpipe pressure is maintained at a constant value when drilling, the motor torque is constant. This is critical when drilling oriented, i.e. with no string rotation, the reactive torque and hence the tool face can be held constant.

5.The pressure drop across the motor is independent of mud weight, thus permitting a limited range of motors to be used in a wide variety of wells.

6.The motor can pass large quantities of LCM without plugging up.

7.Moineau motor (mud motor)s can be designed with an increasing number of lobes on the rotor which lowers the bit rotational speed, increases the torque, and slightly lowers the efficiency. The higher torque of multi-lobe motors allows higher WOB to be run. They are mainly used with roller cone bits to give longer bearing life.

The Moineau motor (mud motor) also has a number of disadvantages:

1.It costs money to rent.

2.It has a limited life due to universal joint failure, rubber stator wear, and hard facing on the stabilisers. All rubbers, and other downhole components, wear rapidly when hematite is used as a weighting agent.

3.The motor is weaker than a drill collar of the same OD and contains threads which, due to slip-stick vibrations, can backoff.

4.Moineau motor (mud motor)s are a source of lateral vibrations. These are larger with multi-lobe motors.

4 Turbines or turbodrills

Turbines or turbodrills are not significant in slim hole drilling. Since then their share of the overall downhole motor market in all hole sizes has declined as Moineau motor (mud motor)s have become more reliable and powerful.

The only advantage turbines might have in slim hole drilling is for work in very hot wells, but unless there is a requirement to drill at static formation temperatures much above 200°C Moineau motor (mud motor)s will suffice.



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