re: #759 reine.de.tout

It’s called mud.
But it isn’t really mud.
It’s fluid, water based, or oil based, or synthetic based, that is changed by additives to meet various requirements at various times during the drilling process as well as meet whatever is required for the type of formation they’re drilling into.

Here’s the part about controlling well pressure;

Control formation pressures

* If formation pressure increases, mud density should also be increased, often with barite (or other weighting materials) to balance pressure and keep the wellbore stable. Unbalanced formation pressures will cause an unexpected influx of pressure in the wellbore possibly leading to a blowout from pressured formation fluids.
* Hydrostatic pressure = density of drilling fluid * true vertical depth* acceleration of gravity (m/s2). If hydrostatic pressure is greater than or equal to formation pressure, formation fluid will not flow into the wellbore.
* Well control means no uncontrollable flow of formation fluids into the wellbore.
* Hydrostatic pressure also controls the stresses caused by tectonic forces, these may make wellbores unstable even when formation fluid pressure is balanced.
* If formation pressure is subnormal, air, gas, mist, stiff foam, or low density mud (oil base) can be used.
* In practice, mud density should be limited to the minimum necessary for well control and wellbore stability. If too great it may fracture the formation.