Exploration Drilling

The second step is to drill into the geological structure to ascertain what it contains. Drilling is conducted from a jack-up rig in shallow waters of less than about 100m, or from a semi-submersible rig or a drill ship in deeper waters. The legs of a jack up rig rest on the seabed while the semi-submersible rig/drill ship floats and is anchored in location. The well is drilled in a series of steps of decreasing size with increasing depth. As each step is drilled the hole is lined with steel pipe which is cemented in place to prevent collapse and the flow of liquids into or out of the well. As the drill bit penetrates the rock, a fluid termed "drilling mud" is pumped around it to lubricate, cool and carry away the rock cuttings. The drilling mud normally contains a variety of minerals and chemicals designed to give it required properties such as viscosity to aid the removal of rock chippings, and weight to balance the pressures being encountered in the well.
The possible sources of environmental effects from exploration drilling are illustrated in the figure below. Of these, the prime concerns are generally regarded as the effects of discharges of drilling mud and rock cuttings on seabed fauna and if the well contains hydrocarbons, a possible accidental oil spillage.