Categories of Well Control

There are 3 important categories of well control that drilling personnel must fully understand. This article will explain basic details of each well control type.

What is Primary Well Control?

Primary Well Control is hydrostatic pressure provided by drilling fluid more than formation pressure but less than fracture gradient while drilling. If the hydrostatic pressure is less than reservoir pressure, reservoir fluid may influx into a wellbore. This situation is called “Lost Primary Well Control”. Not only is the hydrostatic pressure more than formation pressure, but also it must not exceed fracture gradient. If your mud in hole is too heavy causing a broken wellbore, you will face with a loss circulation problem. When fluid is losing into formation, a mud level in well bore will be decreased that will result in reducing overall hydrostatic pressure. In the worst case scenario, you will lose the primary well control and wellbore influx will enter into the well.

Typically, slightly overbalance of hydrostatic pressure over reservoir pressure is normally desired. You must maintain hole with drilling fluid that will be heavy enough to overbalance formation pressure but not fracture formations.

What is Secondary Well Control?

The primary well control is the hydrostatic pressure that prevents reservoir influx while performing drilling operations (drilling, tripping, running casing/completion, etc). When the primary well control is failed, it causes wellbore influx (kick) coming into the well. Therefore, this situation needs special equipment which is called “Blow Out Preventer”, simply called as “BOP”, to control the kick. For this reason, “Blow Out Preventer (BOP)” is the secondary well control. Please also remember that the BOP must be used with specific procedures to control kick such as a driller’s method, a wait and weight method, lubricate and bleed procedures and a bull-heading method. Without proper well control practices to use the BOP, it will just be only heavy equipment on the rig.

What is Tertiary Well Control?

Can you imagine if the primary and the secondary well control are failed? The well is continually flowing all the time, typically called as “blow out”, so how can we deal with this situation? For this situation, you must use the Tertiary Well Control.

Tertiary Well Control is specific method used to control well in case of failure of both the primary and the secondary well control. These following examples are tertiary well control:

  • Drill a relief well to hit an adjacent well that is flowing and kill the well with heavy mud
  • Rapid pumping of heavy mud to control the well with equivalent circulating density
  • Pump barite or heavy weighting agents to plug the wellbore in order to stop flowing
  • Pump cement to plug the wellbore

If you are interested in well control knowledge, please see from this Well Control blog.