Page 84 - Hydrocarbon Exploration and Production Second Edition
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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.
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