by polandeze
Tech Talk: Revisiting Oil Well Pressures and Blowout Preventers after BP’s Gulf of Mexico Oil Spill
[Via The Oil Drum]
I thought that it might be useful to explain where and how at least part of the problem with the Transocean Deepwater Horizon fire and sinking spreading oil across the Gulf of Mexico might have started. I am going to start by repeating one of my previous technical posts, where I explain what a blow-out preventer is, then I will add some comments in an update relating to the current leak.
Pictures of the oil flows ( Drillingahead )
This post is going to deal with some of the problems that a driller encounters as he reaches the layer of rock (the reservoir) in which the oil or gas is being held. And what I want to talk about is something called Differential Pressure, but to explain that, I need to drag you back to High School for just a minute.
Let’s, in fact, go back to Newton’s Three Laws. And, for those who slept through that part of the Physics class in school, don’t be too ashamed – I have seen the desk where Newton whittled his name, being similarly bored. Let’s start with the first law, which is probably the most relevant.
Every object in a state of (rest or) uniform motion tends to remain in that state of (rest or) motion unless an external force is applied to it.
Except that I want to change external force into pressure (which is force divided by area) since it is the way we normally think of it. (Note: I added rest which is a special case of uniform motion since that is specific to the oil we want to talk about). In other words, nothing is going to move unless something pushes it. It is what does the pushing and what does the moving that this is all about.
And now our drill, is down through the casing, drilling the well open hole and using the circulating mud to carry away the cuttings as it continues to go deeper. I had stopped progress last week just before we went down to total depth (TD) of the well, or into the pay. And the reason I did has to do with this differential pressure. But first, the bit about how you calculate pressure.
As you go deeper into the earth, the rock at any layer is carrying the weight of all the rock vertically above it. For rough calculations we generally consider that this rock weighs 144 lb a cubic foot. So that 10 ft down the weight of the overlying column on a square foot would be 144 x 10 = 1,440 lb/sq ft. But through convention we reduce the area that we talk about to a square inch (144 sq in= 1 sq ft) so with this division the weight on a square inch would be 10 lb. A remarkable resemblance to the depth number (grin). This means that we can assume, as we go deeper into the earth, that the pressure on the rock increases by 1 lb/sq. inch (psi) for every foot we go deeper. This means that at 6,000 ft, the rock is under a pressure, from the rock above it, of 6,000 psi.
[More]
We should all gain a better understanding of the drilling process, as I expect we will hear a lot more about them soon. This will tell you much of what you want to know about drilling.
Then if you want more about cementing a pipe and well completion, there is more. In particular, I wonder about the suggestion that the high pressure of the escaping oil (over 10,000 psi) could have eroded the blowout preventer enough to allow some oil to get out. The worry would be that further erosion of the system could release even more oil.
But it also means that there is a possibility that a new blowout preventer could be lowered onto the surviving riser pipe to completely cut off all the flow or at least restrict it enough for the relief wells to have a chance.
There really is a chance that the drama eventually made about this could have a relatively happy ending. I hope so.
