How Is a Drilling Operation Carried Out?

Prospects must be drilled to see whether a hydrocarbon deposit has extraction potential. This multi-stage process involves searching deep underground to determine if the selected site contains gas or oil. This means using specific equipment and one indispensable ingredient - drilling mud.

How a drilling operation is carried out?
© Keblow

A Large-Scale Operation

Drilling provides information on whether the hydrocarbon prospects, i.e. the potential deposits detected underground, actually contain oil or gas.
These prospects are buried deep beneath the rock. To reach them, a 20-50 cm diameter borehole is drilled.

The drill bit looks just like a huge household drill.

Boreholes do not go further than 2000 meters deep.

False. While most boreholes go less than 2000 meters deep, some go as deep as 4000 or even 6000 meters - twenty times the height of the Eiffel Tower!

   • A drill bit is attached to the end of the first drill rod. This looks just like a household drill - only much bigger. This three-part tool, called a trepan, has very hard steel teeth. When drilling in highly resistant rock, a drill head- which is a single block set with diamonds- is used instead of a trepan because it is far more robust.

   • Once started, the trepan rotates at high speed, crushing the rock into tiny pieces.

   • As it goes deeper underground, a new drill rod is screwed on to the previous one and so on until the right length is reached: this is called a drill string.

To stop the borehole from collapsing, large hollow cylinders are inserted along its full length - this is the drive pipe. Like the drill rods, these pipes are inserted in sections attached to each other as the bore goes deeper underground. The thickness of these pipes can reduce the initial bore diameter by over 50%.

Drilling Mud - An Indispensable Ingredient

Drilling mud is continuously injected into the hole while it is being drilled. This is a mixture of water and solid clay particles, to which chemical products are added to increase its density and stabilize it.
Its composition varies according to the type of rock being drilled.

Drilling mud is a key factor in ensuring the success of the operation.  It:

   •  cools down the drill bit and prevents it from overheating continuously circulating in the borehole.

   •  helps the trepan attack the rock and cleans out the bottom of the well as it is injected under pressure.

   • brings rock fragments ripped out by the trepan (debris) up to the surface when it comes back up along the drive pipe. Geologists examine these bits of rock to see if there are any traces of hydrocarbons.

   •  maintains the size of the borehole as it consolidates the bore walls.

   • provides the right pressure inside the drive pipe to stop oil, water, or gas from one of the rocky layers from gushing out too quickly.

To work properly, the drilling mud must be accurately prepared and its density continually monitored, otherwise:

   • If the mud is too heavy, it could enter reservoirs it encounters too quickly because the pressure there is lower than inside the pipe.  This causes "lost circulation" - instead of returning to the surface, the mud disappears into the borehole.

   • But, if the mud is too light, the water pressure in the geological formations being drilled through could exceed that in the mud column, causing the water to flow into the borehole and rise suddenly to the surface. If this happens, the teams above ground have to act very quickly to prevent a blowout.

This is why drilling mud must be carefully prepared by a specialist, known as the "mud man" in oil jargon. This engineer plays a key role on the drilling site, constantly ensuring that the mud remains homogenous and stable. If a problem arises, he has to act very quickly to change its composition as quickly as possible.