http://www.washingtonpost.com/wp-dyn/content/article/2010/05/30/AR2010053002195.htmlBP says crude may continue flowing into gulf until August
By David S. Hilzenrath and Matt DeLong
Washington Post Staff Writers
Sunday, May 30, 2010; 12:59 PM
... The administration and the oil company say the flow ultimately can be stopped by drilling relief wells, a slow but proven process that is intended to relieve the pressure from the vast undersea oil field. Despite recent setbacks, Browner expressed confidence that the relief wells will work, saying the worst case scenario is that the spill continues until their expected completion in mid-August...
http://www.itulip.com/forums/showthread.php?p=162998"Relief well" is actually a bit of a confusing misnomer. A relief well is not designed to "relieve pressure" from the existing blowout wellbore. Quite the opposite actually. A relief well is drilled in a similar way to a conventional well but it targets to intercept the blowout wellbore above the producing formation (using directional drilling techniques). In this case the blowout well is roughly 15,000 feet deep (measured from the wellhead on the ocean floor) so the relief well will be targetted to intercept at perhaps 8,000 - 10,000 feet sub-seafloor depth. As the relief well is drilled the crew will have to carry out substantially the same procedures as during the drilling of the original well...for example setting and cementing intermediate steel casing at intervals on the way down. Until they get near the intercept point there normally won't be any significant differences in the drilling procedures.
A little dissertation on drilling mud might be worthwhile here. The "mud" isn't a mixture of soil and water like the stuff we played with in the back yard as kids. It's actually a complex chemical soup that has a specific gravity greater than one (e.g. it is denser than fresh water) that can be controlled by changing the chemical mixture and water content. It serves several purposes. In addition to holding back the formation pressure as the well is being drilled the drilling mud also cools and lubricates the drill bit, and picks up and holds the rock cuttings from the bit in suspension to carry them back to the surface during drilling operations (the drill cuttings have to continuously be removed from the well or the bit risks being jammed in the bore). The higher the density of drilling mud the greater the margin of safety over formation pressure but also the greater the viscosity. The greater the viscosity the slower the rate of drilling (think of it as turning the long drill string and bit in a thicker "sludge"). This is one dilemma facing BP now - do something quick dammit, but make sure you don't have another screw up.
For this reason normal mud weights will be used to drill most of the depth of the relief well (contrary to some reports BP really would like this nightmare to end sooner rather than later). I do not know the details of BP's relief well drilling program, but typically a relief well is not drilled all the way into the blowout wellbore. As it approaches the intercept point, but before actually hitting that target, drilling is often suspended and the relief wellbore is circulated to the "kill fluid", which in this case is likely to be a "weighted up" mud (sometimes in shallow, lower pressure wells, salt water alone is enough to kill the well). The combination of the hydrostatic head from the heavy kill fluid and hydraulic pressure applied to the relief wellbore from surface is used to fracture the rock creating a pathway from the end of the relief well to the blowout wellbore, through which the kill fluid is forced at high rate in an effort to overcome the blowout formation. This is seriously complicated by the fact that the original wellbore is cased and cemented all the way down...so they may have to use shaped explosive charges to make the connection from the relief well to the blowout bore.
A relief well is the only reasonably high-probability-of-success way to kill a blowout like this one. That is the reason that starting the first relief well was done as quickly as possible by BP. So reliant are they on the success of this technique that for back-up a second relief well is drilling about one week behind the first one...just in case something goes wrong with the first relief well (stuck drill bit, hole collapse, etc.). They simply cannot afford to wait to start a second relief well until after they have any sort of problem with the first one. If the first attempt with a relief well fails, then there will be a second attempt, and if necessary a third and a fourth...because there is no other better option to stop the blowout...