I worked them rigs a year or so,
‘Til the day I saw them headers go
‘Nd all that pipe come flyin’ through the floor.
I knew right then I couldn’t stay
It ain’t like a man to die that way.
—- From an unpublished song of mine ca. 1986
When it comes to exploring for oil or gas, there’s only one constant – uncertainty – which means that however informed by experience, surveys, proximity to the proven, or intuition, it’s all guesswork. Looking back on my first three hundred consecutive days of 12 on an 12 off as a Logger, I can tell you that being wrong about where to spud in is just about the only bad guess that can’t get somebody, if not everybody, killed. It’s all a matter of what lies below and every inch of depth poses greater risk.
Wellbore integrity is critical. The hole must be kept clean and the downhole formation pressure must be kept at bay but not entirely overcome. It’s a difficult balancing act and the armory in chief consists of casing, drilling fluid, and the blowout preventer.
Casing is steel pipe that lines the drilled hole to keep it open and to seal off the different geological strata so that the hole doesn’t collapse and there are four types, each of which is intended to counter a specific threat and each of which is cemented in place.
Conductor casing, which can be 
more than a meter wide, is the first to be set and it’s done by either pounding the casing into the ground and then removing the material inside or by drilling a hole wide enough to allow the pipe to be inserted. With the conductor in place and the risk of unconsolidated minerals, organic matter or preexisting rock falling back into the hole eliminated, the drilling equipment is moved into place over it and drilling begins.
Surface casing comes next and is intended both to protect the well from contamination in (relatively) shallow water and gas layers and to keep the drilling fluid from moving out into those layers. It also supports the wellhead equipment and sustains the weight of the other layers of casing.
Intermediate casing is used to isolate different layers of formation pressure to facilitate normal circulation of the drilling fluid and also to protect the production casing. Intermediate casing also facilitates the installation of blowout preventers, anti-leakage devices, and other tools in the well.
Production casing (aka the oil string) is the casing that is run across the reservoir in sections through which the well will be drilling. It’s one of the final intervals of the casing which is performed during the casing of a well. The production casing is the deepest section of casing in a well just above the hydrocarbon formations. It is used to isolate the zone which contains gas from other subsurface formations.
Drilling fluid called mud is pumped into the hole through the drill pipe (aka tubing) and does several things. It cools and lubricates the drill bit, flushes cuttings up to the surface where they can be examined to determine nature of the substrata, and by its weight, pushes back on formation pressure below the casing.
Finally, the blowout preventer is the tool of last resort in controlling the flow of mud, gas, or when a well kicks, which is to say when the formation pressure overcomes the weight of the column of mud and it all heads back to the surface – A potential blowout. In the event, the blowout preventer can literally sever the pipe and move into place to stop the flow or even pinch it off like a drinking straw, but there’s no guarantee it will work.
Thing is, drilling is not continuous.
Going in, the drill string is created by adding drill pipe in sections about thirty feet in length and mud flow stops each time a section is added. When taking the string out of the hole, it’s taken out in stands of three sections at a time and again the mud flow stops until drilling resumes. The act of pulling the entire drillstring out of the hole or replacing it in the hole is called “tripping” and is usually done because the bit has dulled or has otherwise ceased to drill efficiently and must be replaced.
And there’s the rub.
Drilling may stop, but downhole formation pressure doesn’t and, owing to the time it takes, the deeper the hole, the greater the danger of taking things out and putting them back in.
I actually worked as a Logger on a drill site out in the middle of a sorghum field a few minutes north of Corpus Christi that suffered a blowout because the surface casing was set too short to clear an aquifer. When the drill penetrated a pocket of hydrogen sulfide gas below the aquifer, but unexpectedly close to the surface, the mud and casing kept the wellbore intact, but the gas found its way to the surface by bypassing everything and following weak seams in the aquifer to the surface – The path of least resistance. As a result, the water in the aquifer, which provided both drinking water and agricultural irrigation, was poisoned for six miles in every direction. Ultimately, it was determined that the casing was about ten feet short.
But my then supervisor was the Logger on a near-legendary blowout which killed eight people and injured a ninth.
As my supervisor tells it, the following happened in less than seven minutes:
A drill string eleven thousand feet long had been pulled in order to change the bit and all was well until the last stand was attached and mud flow was about to resume when they punctured an accumulated gas bubble at the bottom of the hole so powerful that it blew absolutely everything out of the hole, tossed the blowout preventer aside at the surface, threw ten tons of machinery attached to the top of the drill string more than a hundred yards away from the drilling floor, and caught fire.
The driller and two roughnecks on the drilling floor were killed instantly. The derrick man, however, was at first uninjured and shinnied down corner of the derrick keeping the steel between himself and the fire and leapt into the reserve mud pit, probably hoping to avoid burning to death, only to have the derrick heat up to the point of failure, twist and fall on him.
As the eleven thousand feet of pipe weighing about twenty pounds per linear foot came out of the hole, it was heated to the point of crystallisation, which made it flexible enough to move like a snake and wreak havoc on its own. A roughneck who was the designated “mud man” ran into the mixing hopper of a mud tank and was cut in half by the flying pipe, which then doubled back on itself and ripped a three-bedroom mobile home in half lengthwise, killing the on duty paleontologist who was seated at his desk doing paperwork.
The last to die were two of three roughnecks who were running essentially single file but with distance between them. When the flying drill string lost its momentum, it fell on the unfortunate pair and clipped the third’s left leg causing a compound fracture and throwing him off to the side injured but able to tell the tale.
My supervisor, however, was stopped by a rancher in a pickup who came up on him running down the road and wanted to know what he was running for. He was more than a mile form the site and all he could do was to look back and say, “Well. I guess I made it.”
The link below leads to footage of a similar, but smaller blowout taking place in under three minutes. Note that the drillstring gets tied into a knot.