Martin Rotich
Geothermal Development Company
Directional drilling, a key technology for exploration and exploitation of deep geothermal resources is conducted using special tools and directional measuring instruments that guide the directional driller in orienting the directional tools down hole. The measuring instruments contains magnetometers and accelerometers that measure the magnetic direction and gravitational pull respectively. When kicking off a directional well, the orientation of the mud motor bend can be set relative to Magnetic North (Magnetic Tool Face, MTF) or with respect to the High Side of the hole (Gravity Tool Face, GTF). From vertical until approximately 3-5° inclination, gravity forces are minimal and the wellbore does not have a well-defined high side or low side. Until this point, the tool face is set relative to magnetic North. Above 3-5° inclination, the tool face is set using the high side of the hole as the reference (High Side Tool Face or Gravity Tool Face setting). Exactly the same convention applies whether single- shot/multi-shot surveys, MWD or a Steering Tool is being used. Effective interpretation of the measured data is therefore key in guiding the well in the right directional and in achieving the desired displacement as required by the geologist. A poor interpretation and orientation will lead to several correction runs to correct the well course and, in some cases, lead to serious hole problems such as stuck pipe, key seating and pipe severing at the weakest point.
This paper highlights how effective interpretation of directional measured data led to faster directional drilling of Menengai well MW-35A.The well was kicked off at 390 metres with the mud motor bend (measured as MTF) pointed in the direction N330W.The well was drilled while maintaining this constant tool face until the well reached an inclination of 3 degrees. Thereafter the GTF was set to between 270-330 degrees to continue steering the well in right direction and build inclination up to a depth of 700 metres where the BHA was changed after attaining the proper desired azimuth and inclination.
Key words: Directional Drilling, Orientation, Gravity tool face, Magnetic tool face, azimuth.
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