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BP blow-out: Early August top kill PDF Print E-mail
Written by Dr Barry B Pruden   
Tuesday, 10 August 2010 18:17

Tuesday, August 10, 2010

This “kill would be relatively easy if there were a single casing. You could simply start pumping mud into the new BOP, and the measured pressure (PBOP) would drop. This pressure decrease could be calculated according to the amount and density of the mud and the size of the casing. Eventually PBOP would be zero, and at this point the static pressure of the mud and oil would equal reservoir pressure. (All pressures are relative to the sea floor pressure, a logical reference).
Consider now a casing in which there is a smaller pipe. Imagine now that you start pumping mud. This mud will take the path of least resistance, and not conveniently flow equally down both pipes. It will flow more easily* down the path with the highest cross section (I will call this the annulus) because the annulus is much larger than the smaller pipe in the BP situation. Now the static pressure in the annulus will increase, causing a differential which will force oil to flow up the smaller pipe. Hence as mud is pumped into the well, the measured pressure (PBOP in the earlier blog) will stay constant, or nearly so. This oil will mix with the mud, and go back down the annulus. Eventually the mud/oil mixture will reach the entrance to the smaller pipe, and flow up this pipe. Now the engineers will see a decrease in PBOP until it goes to zero. The mud/oil mixture becomes more rich in mud, and the mud pumps can continue until the “circulating” material is mostly mud.
Things are slightly more complicated than this, but I will spare you the discussion about flow pressure drop and/or the effect of reducing the casing size. You are also spared the hardship of mud hang-ups in narrow annular sides (smaller pipe is not perfectly centred), and the effect of telescoping the casing as the well proceeds downward.
With a second pipe in the casing there will be some predictable events as above. The first pipe, then the second pipe will reflux oil from the reservoir. I imagine that a large group of engineers and their computers have modelled this, and are having fun with it. In the end the mud will occupy most of the three conduits, and the kill will be made.

Pouring concrete will be another story. The same flow/static pressure/reservoir pressure considerations will apply, but concrete is not mud. I do hope that BP has plans to deplete the reservoir soon after this well is “permanently” sealed.

*In an 8inch Schedule 40 pipe the flow is at 8.8 ft/sec for water when pressure drop is 1psi/100 ft. In a 20 inch Schedule 40 pipe the flow is at 13.5 ft/sec for the same pressure drop. Clearly the larger pipe will fill more quickly if both pipes are subjected to the same input pressure and are connected to the reservoir. Data is from Crane Handbook, for water at 60F.
PS: This has nothing to do with the above, but the Gulf has an average depth of one mile, and an area of 58,400 sq miles. It holds about 48x10^16 barrels of sea water. The spill was about 6.4 million barrels, using 80,000 barrels per day average for 88 days, less the BP recovery (collection) of about 660,000 barrels. The oil represents only 1.3 billionths of one percent of the sea water, but has created some big problems.
Dr Barry Pruden, Aug 9, 2010
 
July 20th BP blowout well-Well head pressure PDF Print E-mail
Written by Dr Barry Pruden   
Tuesday, 20 July 2010 18:25


There is lots of information about well head pressure which somewhere near 6 850 psi, it is evidently increasing at a rate of 1 psi per hour and the pundits are predicting this and that from changes in this pressure.



The diagram below describes what this pressure is and what the actual pressure is in the reservoir. To start with, the pressure is probably measured utilising a transmitter associated with the new blowout preventer. I will call this pressure PBOP . Because it is probably measured by a pressure transmitter it is probably referred to a sea floor pressure and is not absolute.



PBOP is related to the reservoir pressure .PR. by the formula below:

PBOP =PR-PS

Where all pressures are in psi, and PS is the static pressure created by the column of liquid in the well casing. The value of PS can be found from the formula:

PS=H x SG x 14.69/33.91

Where H in this case is about 18 000ft and SG is specific gravity of the liquid in the casing, probably about 0.95. All this is shown in the diagram below. The important thing to know is that the reservoir pressure, PR, is probably not changing too much. What will be changing is the specific gravity of the fluid in the pipe. The main factor which will influence this SG will be the movement of gas and vapors in the pipe and from the reservoir into the pipe. BP probably has very good models to describe vapour-liquid equalibria and the happenings in the pipe. They also probably know how to correct for drift in the instrument which is important when considering a change of one psi in 6 850 psi. These transmitters probably give changes very accurately but not the actual pressure. In other words the transmitter could be out by a few percent in the pressure to start with but very accurate in the changes of this pressure as time goes by.



So can small changes in PBOP predict small leaks from the well?

I would say No! The well is being fed by a 7 or 9 inch pipe connected to the reservoir and the leak would have to be very large indeed to change PBOP There would probably be more effects from gas movement in the well, especially in the annulus between the production pipe and the casing and possibly gas movement in the annulus between the rock and the casing. (There is some indication that not all cementing efforts were successful, but this is another very complicated story). So. Good luck in predicting small leaks.
 
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