Drilling at sea poses a few obvious problems.  Drilling crews have to operate through the depth of the sea water. Some environments,  the North sea for example, have hostile climates, inhospitable to man and corrosive to machines. To deal with these specialized problems, drilling contractors have fashioned new tools.  This machinery is employed once suitably promising locations have been pin pointed by  cheaper geophysical methods.  The operating company sets up their exploration drilling program, usually using either a Jack-Up a Semi-Submersible or a Drill Ship.

How Is It Done Then? Once the drilling unit is in position, the well is started, or "spudded", typically by drilling a 36 inch hole in the sea-bed. To prevent the hole from caving in, heavy steel pipe casing is cemented in to form a solid lining. The casing becomes progressively narrower as the hole deepens so that, for example, a 4,000 ft well may start with 30 inch casing and finish with 7 inch casing (Imperial measures remain standard for most drilling activities). Formation pressure can be unpredictable, and is therefore potentially hazardous. If the bit penetrates a high pressure zone unexpectedly, oil or gas,  sometimes a mixture of both, may rush up the bore hole, producing an uncontrollable gusher at the well head. This is known as a blow-out, and to help reduce this danger all wells are fitted with an emergency valve called a blow-out preventer (BOP) 

A production test involves the drill crew attaching a perforating gun to the bottom of the drill string and lowering it to the target zone. The gun fires off explosive charges which perforate the casing, allowing hydrocarbons to flow into the bore hole and up to the surface 

Drilling techniques have been constantly improved to maximize performance and reduce costs. For example, directional drilling is a widely used technique that allows the drilling crew to reach different parts of the reservoir from a single drilling location. The crew use special fittings to deviate the drill string to specific predetermined angles. Horizontal drilling is now being carried out in the North Sea. This technique is primarily used to improve recovery from low permeability reservoirs.

Rigs and what they are capable of:

      This Should Give You An Idea What They Look Like

   From Left To Right

   Land Based Oil Exploration Rig

   Platform Rig

   Jack-Up

   Semi-Sub

   Drill Ship

   Deep Water Semi-Sub

Reproduced with kind permission from the UK Offshore Operators Association”.

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