Page 9 - Drilling a Straight Hole

STRAIGHT HOLE CONSIDERATIONS

Petroleum Extension-The University of Texas at Austin
Straight Hole

Considerations

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In this chapter:
• Definition of a straight hole
• Restricting the total hole angle

• About doglegs and keyseats
• Dogleg and keyseat problems

he term straight hole generally defines a borehole that a drilling
Tcontractor has drilled vertically from surface to Total Depth (TD).
A straight hole does not have to be vertical but must not contain sig-
nificant curvature or angle changes. This means that most straight
holes will be vertical, unless they are drilled with a slant rig. However,
it is virtually impossible to drill a perfectly straight hole. Therefore,
all wellbores will deviate from vertical to some extent. Vertical drill-
ing contracts recognize this fact and allow a variation from the strict
specification. Therefore, straight hole drilling is best categorized as
controlled deviation drilling, where the industry accepts a straight
hole as one that meets two qualifications:
• The hole stays within the boundary of a cone, as designated
by the operator in the deviation clause of the contract. The
total hole angle is therefore restricted (fig. 10).
• The hole does not change direction rapidly, usually no more
than 3 degrees per 100 feet (30 metres) of hole. The rate of
hole-angle change is therefore restricted.

Staying within these allowable parameters, the contractor’s main
objective is to deliver a straight and usable hole to the specified depth.

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