current well control paradigm

Drillers circulate drilling fluid, consisting from sea water to highly specialized mud, down the drill string and up the annulus of the well bore. Primary Well Control is achieved by maintaining the effective circulation density of the drilling fluid in the well such that the hydrostatic pressure in the well is more than the formation pressure, but less than the fracture gradient. Under normal drilling circumstances this prevents an undesired influx of hydrocarbons into the wellbore.

Unanticipated hazards such as shallow gas, a loss of circulation, gas cut mud, or higher than expected formation pressure can result in an undesired influx of hydrocarbons into the well bore known as a “kick.” Once drillers detect a kick, they close the annular BOP and/or pipe ram BOP which increases hydrostatic pressure and prevents further influx of hydrocarbons into the well. During this activity known as Secondary Well Control, adjustments are made to circulate the kick out of the well, increase mud weight to match formation pressure, and continue drilling the well.

Coil Tubing, Hydraulic Workover or Snubbing, and well intervention operations are regularly performed with hydrocarbons present in the well under full formation pressure. In these cases, the BOP is an integral part of the operation and is used continuously to control the well.

In rare cases, the secondary well control means are insufficient to maintain well control. In these situations, the blind shear ram, a special BOP ram outfitted with cutting blades and seals, is activated to shear the drill string and shut-in the well.

Today’s BOPs are “Blowout Preventers” as opposed to “Blowout Stoppers.” Typical blind shear rams require 30-45 seconds to close and cannot seal once full flow has been established due to a tendency for rushing wellbore fluids to washout the seals. This limitation creates an urgent situation where the driller has as little as minutes to determine whether he must sever the drill string and shut-in to prevent the potentially deadly consequences of a blowout. There is also the risk of an unshearable section of the drill string such as a tool joint, bottom hole assembly, or a large casing passing through the BOP wellbore when the blind shear ram is activated which would result in failure to close and seal.

These unshearable and unsealable situations result in a complete loss of well control, known as a blowout. The consequences of a blowout include: loss of the well, loss of life and equipment, criminal and civil litigation, and untold environmental damage including potential release of toxic gases like hydrogen sulfide (H2S.) Due to these hazards and consequences, operators frequently pay expensive rates for OSRVs, capping stacks, and other emergency teams and equipment up to and including a stand-by drilling rig for a potential intercept well.