Re: Update on BP's top kill efforts, now largest spill in US History
Pressure is important, but not the main issue; flow rate is. Let us suppose that the blowout well could be closed off completely at surface right now. In that situation the maximum pressure at the bottom of the non-flowing well would be the shut-in reservoir pressure...no more than that. The mud weight that will be used to kill the well must result in a column of fluid with a gradient that creates a pressure at the bottom of the well that is higher than the shut-in reservoir pressure in order to prevent any flow from the reservoir into the wellbore.
The first problem is that the primary stress pressure in these deep water Gulf of Mexico wells is close to the reservoir pressures. The primary stress pressure is the pressure at which the reservoir rock breaks down and fractures. The problem in this situation is that if the mud weight is too high and the bottom hole gradient exceeds the primary stress limit, the rock breaks down creating the risk that the mud leaks off into the reservoir through the fractures that are created, thus losing the mud column in the wellbore that is required to maintain the pressure gradient needed to keep the well controlled. [I hope my explanation makes sense so far]. This narrow difference between reservoir pressures and primary stress limits is one reason these wells are quite complex to drill and why you have heard in some reports that drilling these wells is like a "balancing act"...the drilling mud weights simply cannot be too high.
Now let's imagine the situation when the relief well is completed. The blowout well is flowing up from the bottom of the original wellbore. The relief well will intersect that original wellbore several thousand feet above the bottom. The relief well will be used to pump drilling mud into that flow stream...but the problem is that the mud cannot be "too heavy" and the desire is to have the mud flow downward to the bottom of the well...which is against the high flow rate of the oil and gas that is flowing upward from the bottom of the well. This is the reason that all the engineers I know gave the [ultimately failed] "top kill" an almost zero probability of working - they were trying to inject the mud through the small diverter connection on the wellhead into a fast flowing oil and gas stream.
The ideal situation would be to momentarily stop the oil and gas flow completely at the moment that kill fluid mud begins to be pumped into the wellbore from the relief well to ensure that it flows down...and not up. If the flow cannot be stopped completely*, then choking the well at the seafloor to reduce the flowrate as much as possible is the next best thing. This, by the way, was the theory behind the "junk shot". If they could have plugged up the failed BOP enough to choke the flow they thought there might have been a chance to pump enough kill fluid mud through the small diverter connection on the wellhead to kill the well. Instead the 'junk shot" didn't work, and the mud got carried upward by the high flow rates and ended up in the ocean.
Hope this all makes sense.
* One reason that the well may not be able to be shut-in at the wellhead with the new BOP rams is because the casing or wellhead may have been compromised and is now incapable of containing the shut-in pressure from the reservoir.
Originally posted by we_are_toast
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The first problem is that the primary stress pressure in these deep water Gulf of Mexico wells is close to the reservoir pressures. The primary stress pressure is the pressure at which the reservoir rock breaks down and fractures. The problem in this situation is that if the mud weight is too high and the bottom hole gradient exceeds the primary stress limit, the rock breaks down creating the risk that the mud leaks off into the reservoir through the fractures that are created, thus losing the mud column in the wellbore that is required to maintain the pressure gradient needed to keep the well controlled. [I hope my explanation makes sense so far]. This narrow difference between reservoir pressures and primary stress limits is one reason these wells are quite complex to drill and why you have heard in some reports that drilling these wells is like a "balancing act"...the drilling mud weights simply cannot be too high.
Now let's imagine the situation when the relief well is completed. The blowout well is flowing up from the bottom of the original wellbore. The relief well will intersect that original wellbore several thousand feet above the bottom. The relief well will be used to pump drilling mud into that flow stream...but the problem is that the mud cannot be "too heavy" and the desire is to have the mud flow downward to the bottom of the well...which is against the high flow rate of the oil and gas that is flowing upward from the bottom of the well. This is the reason that all the engineers I know gave the [ultimately failed] "top kill" an almost zero probability of working - they were trying to inject the mud through the small diverter connection on the wellhead into a fast flowing oil and gas stream.
The ideal situation would be to momentarily stop the oil and gas flow completely at the moment that kill fluid mud begins to be pumped into the wellbore from the relief well to ensure that it flows down...and not up. If the flow cannot be stopped completely*, then choking the well at the seafloor to reduce the flowrate as much as possible is the next best thing. This, by the way, was the theory behind the "junk shot". If they could have plugged up the failed BOP enough to choke the flow they thought there might have been a chance to pump enough kill fluid mud through the small diverter connection on the wellhead to kill the well. Instead the 'junk shot" didn't work, and the mud got carried upward by the high flow rates and ended up in the ocean.
Hope this all makes sense.
* One reason that the well may not be able to be shut-in at the wellhead with the new BOP rams is because the casing or wellhead may have been compromised and is now incapable of containing the shut-in pressure from the reservoir.
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