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 your mud in hole is too heavy causing 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. In worst case scenario, you will lose the primary well control and wellbore influx or kill 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 formation.
Well Control Book 
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December 27th, 2009 at 1:14 am
[...] is Secondary Well Control? Referring to the previous section, primary well control is hydrostatic pressure bore that prevents reservoir influx while performing drilling operations [...]