Slim hole, coiled tubing and under-balanced drilling surveying
Since the mid eighties there has been an interest in drilling wells with smaller hole sizes, typically 4-1/8", 4-3/4" or 5-7/8" hole sizes, with the smaller holes taking a 3-1/2" liner.
1. Slim hole surveying
There are a number of gyro survey tools and steering tools that have an OD of 1.75" and are used inside conventional drillstrings. These are directly applicable in slim hole drilling where the bore of the drillstring or casing inner diameter is sufficient. Scientific . . .