It’s more of a leak than a spill; the source for the oil isn’t a container. Think of it more like poking a hole in an aquifer and having all the water drain out. To explain a bit more about this, we need to explain the geology of oil formations. It’s not like this bubble of liquid is floating within the rocks. Instead, the porous layer in which the oil is located gets trapped by nonporous materials. Considerable pressure can be produced in these formations as a result of the various formations moving relative to each other, compressing the porous material into less porous material and “squeezing” the liquid within into cracks and fissures which form from the pressure. A drilling rig comes along and drills a hole into this formation and prevents oil from coming to the surface with “mud” (drilling fluid) pressure. The pressure of this fluid must be equal to or more than the pressure within the formation (optimally, exactly the same to prevent more cracks in the formation). This serves to maintain the structural integrity of the formation itself and prevent oil from flowing to the surface. When the hole has been drilled and is ready to be capped, a casing is put in and cemented in place with two “plugs” to prevent the oil from seeping out. When the proper quantity of cement is put in place, it is then “squeezed” to remove water. This is done by increasing the pressure which compresses the liquid cement, effectively hardening it. Cement which is not fully “squeezed” is then pumped out by reversing the flow to suck out excess cement.
We know that the casing and first plug were put in place and squeezed before the blowout.We also know that pressure within the riser (the pipe that connects the casing to the surface and allows drilling fluid to return) was increasing rapidly just before the explosion indicating a leak in the plug or casing and they were preparing to move the drilling rig off of the location for a production platform to come in and produce the well (meaning the second plug was set to be put in place in the very near future. The increasing pressure was likely caused by the failure of the plug to be thick enough to contain the oil after the weight of the drilling fluid was removed from the riser. The casing and plug may have become lodged in the BOP and prevented its closing since the shear ram is not designed to cut through cemented casing (which is thicker than a drill string).
Of course, much of this is speculation, but similar instances of BOP failure from a drill collar (thicker than casing) being in the way have occurred before (Ixtoc I). Drill casing is thinner than collars, but much thicker than production pipe and drill pipe, if the cement plug is also in the way of this closing, it is possible the combined resistance of these two components prevented its closure. The “acoustic trigger” which is so hyped right now would have done nothing, since it activates the BOP anyway, just remotely. This is the same BOP which could not be closed by ROVs on multiple occasions.
Here is my account of what likely happened:
- The cement plug fractures due to expanding gas pressure allowing it to escape into the riser pushing the drilling fluid out
- Decreased fluid weight provides a greater pressure differential to further fracture and dislodge the casing and plug
- By the time the problem is detected, the blowout preventer is effectively disabled by lodged material
- This results in a full blowout (zero fluid weight in the riser) and gas and oil reach the surface
- An ignition source (possibly cement or rock sparking as they bounce off of the pipe) ignites the gas and oil
- The crew try to close the blowout preventer manually to no avail
- Heat from the burning oil weakens the structure of the platform causing it to list
- Ballast tanks are breached causing the platform to sink (I’m still confused about this part)