How blowout at the oil rig can happen?

Blowouts happen when a pressurized underground zone is encountered while drilling, and the weight of the drilling mud is insufficient to hold back the pressure. Natural gas and fluid then travels from the highly pressurized zone to an under-pressured zone, whether it is the surface or a less pressurized zone above it that has already been drilled through. 

In the case of a "kick", the BOP can be shut in and a blowout situation prevented. If the pressure is extreme, or the crew is caught off guard when drilling under-balanced, a kick can result in a blowout. The threat of oilfield blowouts and wild well fires is very real. Mud engineers have to constantly monitor their mud weight to be ready for pressurized zones. 

The man on the mud pits must monitor mud weight coming out and going in, and the trip tank needs to be watched for gain. If the level in the trip tank, where displaced fluid goes when drill pipe is put into the hole, is greater than the displacement of the drill pipe, the well may be about to take a kick. 

 Drillers must keep the hole full when removing pipe from the well, since the weight of the whole fluid column, or hydrostatic pressure, helps hold back gas from pressurized zones entering the well bore. A good mudlogger can help prevent a blowout by interpreting indications of higher pore pressure, such as heaving shale and increases in connection gas, drilling breaks and other clues.