First, currently the relief wells are still a couple of weeks away at the earliest. I thought BP might do a quick turn in and attack the well shallower, but for the certainty of the kill, they’re going deeper to make the thing work. While there’s a lot to say about what BP’s engineers are doing right now regarding the relief wells, suffice to say that the specifics are cool but that’ll come in a future chapter.
This post is about the pressure of fluids under the earth. I’m going to generalize this a bit so as to make a bit more accessible.
Feel free to listen to any version of Queen’s famous song, Under Pressure, while you read this.
I prefer the opening number sung by about 75 people in last year’s The Sing Off, located here While it has nothing to do with geology, I still like the tune.
Let’s start with sea water. Think about a plastic milk jug filled with sea water. Minus the weight of the container, it should weigh 8.33 pounds. So, if you do something involving sea water as a fluid, you’re using something that has what they call a mud weight of 8.33ppg (pounds per gallon) or 8.33#. Sea water isn’t that much different from fresh water here, but I’m getting ready to talk about the Gulf of Mexico, so you get the reasoning. The weight of oil differs depending on how dense it is. (The dissolved gas in this oil doesn’t really enter into the problem yet, but it will) The oil in this Blowout Well (BW) is a light crude. 39deg API. They measure it against how much it weighs compared to the same volume of water. Specific Gravity of .830, or that gallon jug of the same oil would be 6.6 pounds. There’s a chart for looking at the different densities of oil here
Okay, these are our starting numbers. Sea water 8.3# and Oil 6.6#
Next, water depth. 5000 ft. That’s the depth to get to the mudline at the Blowout Well.
Now the constant conversion factor. 0.052 if we’re dealing with depths in feet and pressures in pounds per square inch. You know how the language of flying airplanes is English, no matter where you are in the world? Well, with drilling wells, they may measure their depths in meters, but everyone still likes to measure mud weights in pounds per gallon. (The conversion factor for meters is different and won’t be mentioned here.)
Q:What’s the pressure at the mudline, 5000 ft down?
A: 5000ft * 0.052 * 8.33ppg = 2165.8psi (pounds per square inch)
Calculator courtesy of Wolframalpha.com
This means that all that oil coming out of the top of the BOP stack is having to fight against 2165psi of seawater pressing back at it.
Pretty easy to multiply all that to come up with a number, isn’t it? Good, because we’re going to try it again. Ready?
Let’s talk about drilling wells with regards to pore pressure. Look at the lower left portion of this capture from an old time well log. Density is listed as 9.6#
9.6# mud is pretty much the standard for normally pressured wells. Drillers Mud is pretty much water and go figure, mud, mixed in with enough other goodies to keep the mud in suspension and kept thick and luscious. The mud is used to circulate down and bring back the well cuttings, which are filtered out through what looks like a screen door. It’s thick (viscosity) and has weight (density). Many wells are drilled quite deep with a mud weight no higher than 9.8#. You can figure out the pressure the being exerted by the static column of mud by that simple equation above, only substituting out the mud weight of your choice. The goal in drilling a well is to drill as close to “balance” as possible. A safe driller would hedge that balanced line by drilling with a slightly heavier mud weight than he has to, perhaps 0.5# heavier than a balanced well.
What happens when you drill under balanced? Say you’re drilling with a 10# mud weight at 10,000ft. What’s the pressure of the column of mud?
10# * 0.052 * 10,000 = 5200psi.
Your drill rate goes up. You’re drilling faster. Probably a sand. A sand is like a rock sponge, filled with fluids.
But there’s a problem. Your mudlogger tells you the mud is coming back a bit more quickly than it should. You stop the pumps. That stops the flow going into the well, and coming out of the well. But in our case, the well is continuing to flow! How fast? How much? What’s flowing into the well? You can only guess what’s flowing into your well, and you can only guess where it’s coming from.
Let’s say it’s a salt water kick from that new sand at 10,000′. Happens all the time. Your mud is pushing down with 5200psi, and so your sand must be pushing back with a pressure higher than 5200psi. You’re out of balance. It’s time to weight up!
You break out the weight materials and add that to your big tank of your current 10# mud. You increase the weight of mud (let’s say a 0.5# increase to 10.5#), but there’s a time lag while the new mud is being prepared. While that’s going on, you start those mud pumps and you pump your current weight mud down the hole.
You’re only pumping the same mud – why bother continuing to pump? Your well is filling with salt water from below, that’s why. The more 8.3# salt water in your column of 10# mud, the less the whole column weighs. The less it weighs, the more your 10,000 deep sand will flow water into the well bore and right at your drillers. They don’t like it when they’re covered with salt water gushing out of the well. You have to dilute the salt water coming up the well bore with your 10# mud. It might take 60 minutes to circulate out this “kick”, to see that salt water at the surface. It’s a long way up that borehole. Your heavy mud is ready, and you immediately begin pumping it down. Another quarter of an hour for that to get down (the volume of the drill pipe where the mud goes down is much less than the volume of the annulus outside the drill pipe where the mud comes up) so you wait and watch. Your new mud goes down, and then on the way up. You’ll have to check to see if that sand is still flowing in at you. You shut the pumps down. No flow? Good. Back in balance. Being cautious, you increase the mud weight another 0.5# to 11.0# and drill ahead.
You really read this far? Congrats. You’re ready for your next lesson.
In the next lesson, I’ll show you the actual well log over the zone that’s flowing all this oil and gas into the Gulf of Mexico.