Downhole motor

Until downhole motors became available a whipstock (Fig. 3.15) which is a slightly asymmetric steel joint, was inserted in the drill string. The assembly is oriented downhole and a rathole is drilled which is then enlarged to full bore hole size. The technique is still used in wells where hole conditions e.g. high temperatures, are unsuitable for downhole motors (see below). 

However, coiled tubing drilling is limited to slim holes, and the reliability of some of the drill string components such as downhole motors needs further improvement. Presently, the cost of building a new customised CTD rig limits the wider application of this emerging technology. 

Long gage bits, used on downhole motors for drilling ahead in vertical boreholes. 

Flat-bottom, shallow-cone bit designs, used on sidetracking jobs or in sidetracking Jobs with downhole motors. 

Rotary Speed. Diamond bits can usually be rotated at up to 150 rpm without any problem when hole conditions and drill string design permit. Rotary speeds of 200 and 300 rpm can be used with stabilized drill strings in selected areas. Diamond bits have also operated very successfully with downhole motors at 600 to 900 rpm. The actual rotary speed limits are usually imposed by safety. 

Air (or Gas) Downhole Motors. Some positive displacement mud motors can be operated on unstable foam. In general, these mud motors must be low-torque, high-rotalional-speed motors. Such motors have found limited use in air and gas drilling operations where directional boreholes are required. Recently a downhole turbine motor has been developed specifically for air and gas drilling operations. This downhole pneumatic turbine motor is a high-torque, low-rotational-speed motor. 

In 1873, an American, C. G. Cross, was issued the first patent related to a downhole turbine motor for rotating the drill bit at the bottom of a drillstring with hydraulic power [78]. This drilling concept was conceived nearly 30 years before rotary drilling was introduced in oil well drilling. Thus the concept of using a downhole motor to rotate or otherwise drive a drill bit at the bottom of a fluid conveying conduit in a deep borehole is not new. 

The development of positive displacement downhole motors began in the late 1950s. The initial development was the result of a United States patent filed by W. Clark in 1957. This downhole motor was based on the original work of a French engineer, Rene Monineau, and is classified as a helimotor. The motor is actuated by drilling mud pumped from the surface. There are two other types of positive displacement motors that have been used, or are at present in use today the vane motor and the reciprocating motor. However, by far the most widely used positive displacement motor is the helimotor [79,83]. 

In this section the design and the operational characteristics and procedures of the most frequently used downhole motors will be discussed. These are the downhole turbine motor and the downhole positive displacement motor. 

Rather moderate flow rates and pressures are required to operate the positive displacement motor. Thus, most surface pump systems can be used to operate these downhole motors. 

Medium radius and long radius wells are drilled with conventional oilfield tools. Both MWD and LWD are usable in these wells. Downhole motors are mostly used in medium radius wells to avoid fatigue of the BHA. Long radius wells have been drilled with both mud motors and rotary techniques. 

Angle of twist The azimuth change through which the drillstring must be turned to offset the twist caused by the reactive torque of the downhole motor. 

Bent sub Sub used on top of a downhole motor to give a nonstraight bottom assembly. One of the connecting threads is machined at an angle to the axis of the body of the sub. 

True vertical depth (TVD) The actual vertical depth of an inclined wellbore. Turhodrill A downhole motor that utilizes a turbine for power to rotate the bu. Turn A change in bearing of the hole usually spoken of as the right or left turn with the orientation that of an observer who views the well course from the surface site. 

Application of a deflecting tool (e.g., downhole motor with a bent sub) requires determining the orientation of the tool so that the hole takes the desired course. There are three effects to consider when setting a deflection tool ... 

Note With a Dyna-Drill downhole motor there will be a left-hand reaction torque therefore, the tool should be turned counterclockwise from the ideal position. 

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