What Well Control Is

Deep under the ground, rock is not solid like a brick. Many rocks have tiny spaces between their grains. These spaces hold fluids: water, oil and gas. The fluids are under pressure, because the weight of all the rock and water above them presses down.

When a rig drills a well, it opens a path from those pressured fluids to the surface. If nothing holds them back, they will flow up the well. Gas is the most dangerous, because it rises fast, expands as it rises and can burn or explode at surface.

Well control is everything we do to stop formation fluids from entering the well in an uncontrolled way, and to bring the well back under control if they do enter. It includes the drilling fluid, the equipment, the procedures and, above all, the people.

How a well is normally kept safe

While drilling, the well is full of a heavy liquid called drilling fluid, usually called mud. The weight of this column of mud pushes down on the bottom of the hole. As long as the mud pushes down harder than the formation fluids push up, the fluids stay in the rock.

If the mud stops doing that job, the crew must close the well at surface with strong valves called the blowout preventer (BOP). Then they pump heavier mud to get the well back in balance.

So well control is not one device. It is a chain: correct mud weight, a full hole, careful watching for warning signs, quick shut-in, and a planned kill. A weak link anywhere in the chain can lead to loss of control.