Primary Well Control is hydrostatic pressure provided by drilling fluid more than formation pressure but less than fracture gradient while drilling. If hydrostatic pressure is less than reservoir pressure, reservoir fluid may influx into wellbore. This situation is called “Loss Primary Well Control”.
Not only is hydrostatic pressure more than formation pressure, but also hydrostatic pressure must not exceed fracture gradient. If mud in hole is too heavy, it will cause a broken wellbore, you will face with loss circulation problem (may be partially lost or total lost circulation). When fluid is losing into formation, mud level in well bore will be decreased that will result in reducing hydrostatic pressure. For the worst case scenario, you will lose the primary well control and wellbore influx or kick will enter into wellbore.
Typically, slightly overbalance of hydrostatic pressure over reservoir pressure is normally desired. You must keep in mind about the basic of maintaining primary well control that you must maintain hole with drilling fluid that will be heavy enough to overbalance formation pressure but not fracture formations.
References
Coleman, S. (2018). Well Control Quiz Online. [online] Well Control Quiz Online – Test Your Well Control Knowledge for Free. Available at: http://wellcontrolquiz.com/ [Accessed 2 Aug. 2018].
Cormack, D. (2007). An introduction to well control calculations for drilling operations. 1st ed. Texas: Springer.
Crumpton, H. (2010). Well Control for Completions and Interventions. 1st ed. Texas: Gulf Publishing.
Grace, R. (2003). Blowout and well control handbook [recurso electrónico]. 1st ed. Paises Bajos: Gulf Professional Pub.
Grace, R. and Cudd, B. (1994). Advanced blowout & well control. 1st ed. Houston: Gulf Publishing Company.
Watson, D., Brittenham, T. and Moore, P. (2003). Advanced well control. 1st ed. Richardson, Tex.: Society of Petroleum Engineers.
