Page 84 - Hydrocarbon

Drilling Engineering 71

location on an artificial island in Poole Bay. ERD probably saved a considerable
amount of money and advanced first oil by several years.
Offshore, subsea satellite development may be a viable alternative to ERD wells.

4.5.7. Slim hole drilling

Slim hole drilling has been used by the mining industry for a number of years.
Recently, the oil industry has been developing rigs, drill string components and
logging tools that will allow smaller diameter holes and completions. One definition
used for ‘slim holes’ is ‘a well in which 90% or more of the length has a diameter of
7 in. or less’. In principle, slim hole drilling has the potential to drill wells at greatly
reduced cost (estimates range from 40 to 60%). The cost reductions accrue from
several sources:

less site preparation
easier equipment mobilisation
reduction in the amount of consumables (drill bits, cement, muds, fuel)
less cuttings to dispose of
smaller equipment.

A slim hole rig weighs about one-fifth of a conventional rig and its small size can
open new frontiers by making exploration economic in environmentally sensitive or
inaccessible areas.
The following table highlights the potential of slim hole wells:

Type of Rig Conventional Slim Hole
Hole diameter (in.) 8.5 3–6
Drill string weight (tons) 40 5–7
Rig weight (tons) 80 10
Drill site area (%) 100 25
Installed power (kW) 350 70–100
Mud tank capacity (bbl) 500 30
Hole volume (bbl/1000 ft) 60 6–12
Crew size 25–30 12–15

The greatly reduced hole volume of slim hole wells can lead to problems if an
influx is experienced (see Section 4.7). The maximum depth drillable with slim
hole configurations is another current limitation of this technology.
Some slim hole rigs were adapted from units employed by mining exploration
companies and are designed to allow continuous coring rather than breaking the
formation into cuttings. These rigs are sometimes employed for data gathering wells
in exploration ventures. They are ideally suited for remote locations since they can
be transported in segments by helicopter.