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The 90 Day Plan

Tuesday, June 1, 2010

A Petroleum Engineer's Explanation

Fun Fact:  The water pressure is about 2400 psi At 1 mile deep (assuming an increase in pressure of 1 atmosphere (14 psi) every 33 feet.)  A firehose can burst at 1200 psi and take out brick walls.

What’s really happening in the Gulf Oil Spill
I appreciate all of the concern with the disaster in the gulf, but there is an awful lot of ignorance concerning the mechanisms both on ATS as well as the myriad of so-called experts in the MSM. I am posting this thread, since I have been in the oil business for over 30 years working as a petroleum engineer and have actually designed drilled and operated oil & gas wells. This explanation may get too technical, but at least it will expose and dissipate myths concerning this disaster.
Any oil well is drilled starting with a large diameter that at certain points in the process the large diameter holes are cased off with pipe and cemented in place for various reasons and the drilling is continued with smaller diameters until it reaches its objective. I will not go into all of the reasons pipe is run and a smaller diameter is initiated, take my word for it happens & is necessary.
The Horizon Explorer had several strings of pipes until the final 7” pipe was run (probably the 7” was run in 9 5/8” OD casing). The well is blowing out of the annulus between the 7” casing & the 9 5/8” casing. There is no flow inside the 7”. This casing has tools & check valves that prevent anything from flowing into it. The 7” was supposed to be cemented in place, but the well was starting to blow out during this process. So the cement was being diluted with oil & gas where it could not solidify and shut off flow from the reservoir @ 18,000+/- feet below the seafloor.
I will not go into the failure of the blowout preventer. Let is suffice to say it didn’t work as designed and with the flow coming out, the oil & gas flowed up the riser (the pipe that connects the wellhead to the drilling rig) and it was game, set & match.
When the rig sank the riser was still attached to the wellhead and fell over & split in a reported 3 places. It w as reported that 16.5 #/gallon drilling mud was in the riser and the well blew out when it was replaced with seawater. To understand the physics I will try to explain hydrostatic head. The pressure created from a column of fluid in an oil well is identical to the reason you have water pressure out of the tap at the kitchen sink. Water which weighs 8.33 #/gallon is put into a water tower 100’ up in the air. Fresh water has a hydrostatic head equal to .433 psi per foot of height. Therefore you get 43.3 psi at your shower head. If water weighed 16 #/gallon then you would have 86.6 psi in the shower. Similarly as an oil well is drilled there is a fluid known as “drilling mud” that is pumped into the drillpipe, through the drillbit & circulated back to the surface where it is screened to remove the soil dug out of the well. This “mud” is carefully monitored to make sure that if encounters oil or gas it is heavy enough to keep it from flowing into the wellbore. The well in question had to have 16.5 #/gallon mud weight to keep the oil & gas from flowing. The hydrostatic head of the mud of this weight as opposed to fresh water is .86 psi/ft. This means that the pressure in the oil reservoir encountered must be nearly 20,000 psi. ((18,000 + 5000) X .86). As the oil replaced the column of mud that would offset this bottomhole pressure it began flowing faster & faster into the annulus with more & more force as the weight of the 23,000 foot column of fluid became lighter & lighter. When the “company man” (the BP guy in charge) removed 16.5 #/gallon mud from the riser with 8.4 #.gallon sea water he immediately reduced the hydrostatic pressure 2000 psi & it was over.
Now BP is trying to recreate this heavy mud column to offset the bottomhole pressure to balance the well using the “top kill”. A “top kill” entails pumping weighted fluid into the top of the well and forcing the oil & gas back into the reservoir and filling the annular space with heavy mud creating a hydrostatic head equal to the reservoir pressure. The oil & gas are not flowing from a void in the earth it is flowing out of the tiny spaces between sand grains. There is a limit how fast the fluid will reverse & flow back into the formation sand. If the mud is pumped at a higher rate than the formation can absorb then the pressure increases and even the heavy mud may be inadequate to hold back the bottomhole pressure and when the pumps stop the mud starts flowing back. In addition, if the injection pressure gets too high it may exceed the burst strength of the casing in the well. If the casing splits & starts blowing out into the ocean bed the party is over, there is no way to control unless the flow in contained inside pipe. So the top kill is a precarious balancing act. To further complicate this situation it appears that 90% or more of the mud is not going down the hole, but is being spewed into the ocean. To get more down the hole one needs to pump at higher pressure which means increased rate which means more escapes instead of going down creating increased hydrostatic head to decrease the flow from the reservoir. The junk shot (rubber, golf balls, etc) is not to solve the whole problem; it is only to try to reduce the amount of mud that is escaping from the holes in the riser. They shut down periodically, to measure pressure to estimate how much is going down the hole & how much is being wasted. If the well initially flowed at 5000 psi & after a day of the “top kill” it’s flowing at 4000 psi then we have offset 1000 psi of the oil & gas with mud. If the mud weighs 16.5 #/gallon with a hydrostatic head of .86 psi/ft then we have successfully gotten 1200’ of mud column in the well if we assume only gas in the wellbore. It’s actually not this much. Having done “top kills” on land with varying degrees of success I can assure that its not easy under any circumstances much less those out here.
A “bottom kill” is vastly easier and is what the relief wells are about. Unfortunately for that to work the relief well must intersect an 8 ¾” diameter well bore 3 miles underground in the dark. It has been and can be done, but the deep intersection is necessary so that the enormous bottomhole reservoir pressure can be offset with heavy drilling fluid. The drilling mud will then displace the oil and gas from the bottom rather than try to force somewhere it doesn’t want to go. Once the heavy fluid balances the bottomhole pressure then cement can be put into place that once set up will end the need for the heavy fluid.
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