Oil Well Pressures and Blowout Preventors (TheOilDrum)

Tech Talk: Revisiting Oil Well Pressures and Blowout Preventers after BP's Gulf of Mexico Oil Spill

Posted by Heading Out on May 2, 2010 - 11:01am
Topic: Supply/Production
Tags: blowout, blowout preventer, deepwater horizon, gulf of mexico, leak, oil, oil well, tech talk, well pressures [list all tags]

I thought that it might be useful to explain where and how at least part of the problem with the Transocean Deepwater Horizon fire and sinking spreading oil across the Gulf of Mexico might have started. I am going to start by repeating one of my previous technical posts, where I explain what a blow-out preventer is, then I will add some comments in an update relating to the current leak.

Pictures of the oil flows ( Drillingahead )

READ FULL POST HERE

Update

The oil spill is continuing to get worse, and there are growing questions that have been raised on what could have gone wrong, and how it can be fixed. I am in the same position as most, in regard to getting information – it comes from news reports, in the main. But there are some points that can be picked out as the focus of those reports switch to the impact that the oil is going to have on the coast and businesses that are going to be severely damaged.
An early story noted that the BOP had recently been tested.

Mr. Hayward said the blowout preventer was tested 10 days ago and worked. He said a valve must be partly closed, otherwise the spillage would be worse.
There are a number of things that can go wrong with a blowout preventer, said Greg McCormack, director of the Petroleum Extension Service at the University of Texas, which provides training for the industry.
The pressure of the oil coming from below might be so great that the valves cannot make an adequate seal. Or in the case of a shear ram, which is designed to cut through the drill pipe itself and seal it off, it might have encountered a tool joint, the thicker, threaded area where two lengths of drilling pipe are joined.
Still, Mr. McCormack said, “something is working there because you wouldn’t have such a relatively small flow of oil.” If the blowout preventer were completely inoperable, he said, the flow would be “orders of magnitude” greater.

However oil is now flowing through the BOP and out into the water immediately above the well site on the sea bed. When the site was visited by a small remotely operated vehicle with cameras they showedthat the riser, the pipe that normally carries the oil from the sea bed to the surface, had kinked over when the rig sank, and oil was coming from three places:

The Coast Guard said it had not detected oil coming from the well Friday and assumed post-accident efforts to activate the blowout preventer “a huge stack of valves sitting atop the wellhead on the sea floor” had been successful.
But later trips by the remotely operated vehicles (ROV’s), discovered oil shooting from the end of the pipe-like riser that had connected the rig to the blowout preventer.
A second, smaller leak was found in a section of drill pipe near the wellhead. That 21-inch-diameter riser had become detached from the rig when it sank. In the process, it was folded over at a 90-degree angle just above the wellhead, which had the effect of kinking it like a garden hose and constraining the flow of oil from the well. It now sits in a long, meandering mess on the ocean bottom. This helps explains why oil was not initially thought to be seeping. , . .
The preferred option, he said, is still to find a way to engage the blowout preventer. That fix, if it works, could be handled in a matter of days, he said.
But if that doesn't work, the other option is to drill a deep “relief” well into the damaged well and stem the flow of oil, though that option could take several months, Suttles acknowledged. He said his team would spend the next several days trying to determine the best method.

The problem lies, in part, with the capabilities of the ROVs and their ability to get access to the well site on the sea-bed.
There is a report from a survivor (h/t Fractional Flow ) that says that the well was shut in and they were going through the process of separating the rig from the well, and moving it off. They began by cleaning out the drilling mud from the riser, replacing it with sea water. However, when they re-opened the valves at the top of the well, the pipe in the well had become filled with gas from the well, under considerable pressure, and this “Kicked” the well as the valve opened. Gas, as the pressure gets less as it moves up the pipe, expands much more than oil. And unfortunately in the process of disconnection, the pressure to hold the gas, which comes from the density of the drilling mud in the riser initially, had been removed as part of the disconnection process.
So the high pressure gas was able to blow all the sea water in the riser out onto the deck of the rig. (This happens extremely quickly, well below a minute) The gas then followed, and as it flowed out of the pipe at the top of the well there was some hot source that caused it to ignite. (This could even be from a static electricity spark). Because of the depth of the well, the pressure in the bottom of the well was in the 30-40,000 psi range.
Part of the problem that arises with flows at that pressure is that any abrasive particles (such as small pieces of rock) will cut through metal at the speeds at which it is carried. (Such jets were used to remove the damaged tops of the wells in Kuwait after the Gulf War, for example). So that it is possible that as the BOP started to function the high-velocity flow may have eroded part of the system to allow some fluid to bypass the plug that the BOP inserted. If that happened then the continued flow would just enlarge the passage again fairly quickly, so that the BOP will become ineffective.
However there are pictures of the leaks available.



Pictures of the oil flows ( Drillingahead )
At this stage there does not appear to be that great a driving pressure for the oil coming out of the well. (If there were, the flow would be more directed horizontally) This suggests that the BOP did at least partially function, and that the passage may have been eroded by the particles in the gas and oil now escaping.
There is a recent report that the accident may have been caused by a poor cementing job by Haliburton:

After an exploration well is drilled, cement slurry is pumped through a steel pipe or casing and out through a check valve at the bottom of the casing. It then travels up the outside of the pipe, sheathing the part of the pipe surrounded by the oil and gas zone. When the cement hardens, it is supposed to prevent oil or gas from leaking into adjacent zones along the pipe.

As the cement sets, the check valve at the end of the casing prevents any material from flowing back up the pipe. The zone is thus isolated until the company is ready to start production.
The process is tricky. A 2007 study by the U.S. Minerals Management Service found that cementing was the single most-important factor in 18 of 39 well blowouts in the Gulf of Mexico over a 14-year period. But at the time of the accident, "well operations had not yet reached the point requiring the placement of the final cement plug, which would enable the planned temporary abandonment of the well," the Halliburton statement said.

However it is hard to see from what is known, that this was a cause in this case.

READ FULL POST HERE

Some other earlier Tech Talk posts that might be of interest in following the ongoing discussion about the problems encountered include:
Casing a Well
The Drilling Part of Creating an Oil Well
Completing and Perforating a Well