Page 187 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 163

2.3.2
Directional Planning [ ]

Motors incorporate a bearing pack above the bit sub. Normally a
small amount of mud is diverted past the bearing pack to cool it. The
seals across the motor will have a maximum pressure rating (i.e.,
pressure differential inside and outside the motor), and the bit pres-
sure drop will provide most of this drop. The bit nozzles must there-
fore be chosen to give a pressure drop within limits at the anticipat-
ed drilling flow rates. (For specific details on a motor, consult the
manufacturer’s literature.)

2.3.2. Directional Measurement and Surveying

Survey intervals. The survey tools are your eyes. They tell you
where the wellbore is relative to the planned position, and they tell you
how much to correct the wellbore path if it strays enough to place the
wellbore outside of the required targets.
How often surveys need to be taken may be dictated by govern-
ment regulations or company policy. Otherwise, the following general
advice can be followed.

Directional plan for the interval > 81/2 in hole size ≤ 81/2 in hole size
Vertical (<5˚) 200 m / 600 ft 100 m / 300 ft
Tangent section 100 m / 300 ft 50 m / 150 ft
Build/drop/turn section 20 m / 60 ft 20 m / 60 ft

Surveys can be taken on wireline as drilling progresses; either
Totco surveys (which only give inclination) or magnetic single shot
(which require monel drill collars and stabilizers to be run). These
wireline surveys take time, which, apart from the cost, can be a prob-
lem if wellbore conditions are not good, and produce a risk of the wire-
line breaking and problems with the wireline winch.
UBHO sub. The universal bottom hole orientating sub is like a
short sub of the same connections (pin x box) as the drill collars.
Inside the sub is a sleeve with a key in it. The orientation of this key is
set up and locked on surface so that it is aligned with the toolface
azimuth, such as the inside of the bend on a bent sub or bent housing.

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