Pressure, surely not in the oil business ?

There are some myths growing up about oil and the energy business, on both sides of the aisle. Here, on Saturdays I am trying to give a little of the technical background so that those interested can understand more about the realities of production. The posts are a simplification of what goes on, but give enough detail that, hopefully, it is understandable (and if not then you should ask questions).

I also have said that I would explain the scientific background that justifies the depletion argument (in rebuttal to Lynch – though I am not the first). That is going to take more than one post, but as it turns out I can lay some background for both, in what I want to cover today. It all comes down to something called Differential Pressure, but to explain that, let's remember High School.

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. (UPDATE - I added rest which is a special case of uniform motion since that is specific to the oil we want to talk about).

And though we had our drill, mud, derrick and casing all working away, 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 pressure bit.

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 of 6,000 psi. (As will be, initially, any fluid in that rock).

Now water does not weigh as much as rock, but can be approximated to roughly half the weight. So that, by the same argument, under water, for every foot of depth the pressure goes up half-a-psi. So that at 6,000 ft under water the pressure is 3,000 psi (roughly twice the water pressure in the wand you use at a car wash).

Now why do we need to know this before we reach our layer of oil-bearing rock? Well first let's go and interpret that first law a little more.

If a person on either side of you pushed you with equal force at the same time, you don't move, because the two forces balance out. It is only if there is one force, or if one of the two pushes harder, that you move. In other words, where there are a number of forces acting on a body, it is the size of the difference in pressures, and the direction of that difference, that controls the movement.

Consider, here we are drilling merrily away ( and have cased the well near the surface, and hit no more fluids on the way down)and at 6,000 ft. we penetrate the rock that is capping the well, and enter the rock with the oil in it. The oil (in the rock) is at 6,000 psi , the fluid in the well (under water) is at 3,000 psi. There is a difference of 3,000 psi. We are drilling a hole some 6-5/8th inches in diameter. That has an area of about 34.5 square inches. The total force we have suddenly applied to the bottom of the well (bit and fluid) is thus 35 x 3,000 = 100,000 lb (or 50 tons). Oops!

It's called a blow-out, and they still happen. The one capped in Alberta last week killed one of the crew and injured three, and left the well on fire.

This is why we approach the oil/gas producing zone of the rock with caution. And bear in mind that the driller that is controlling the progress of this well is at the surface, trying to guide the bit at the bottom of the hole, with, historically, little immediate information to help.

Based on the surveys that brought the crew to the site in the first place he knows roughly how thick the layers of rock are, and probably what rock they are, but the only real information on where the bit is in that sequence, is from the returns (cuttings) that come out of the well, and there is the lag, we mentioned before, while those chips make their way up the 6,000 ft pipe. (This is why Measurement While Drilling [MWD] has been such a relatively recent boon to the industry – though not all rigs have it).

By monitoring a number of pressure gages the driller can gain a sense of what is happening at the bottom of the well. If he senses that there is going to be a problem, then he can do one of several things, based on the way the well is set up.

The first thing is to increase the density of the mud. By making the fluid in the well weigh more, the difference in the pressure across that face is reduced, and the change in conditions is easier to handle. However weighting up the hole has the disadvantage that it becomes much slower to drill with a heavier mud (it is a poor bottom-hole cleaner among other things). And, if done during drilling, bear in mind that once the heavier mud is added to the well it won't be fully effective until it has had time to get down to the bit and then fill back up the annulus between the drill string and the casing all the way to the surface.

So that is an expensive and slow option. Let us take the game a little more interesting and say that there is a gas pocket above the oil. Gas will enter the well at the down-hole pressure, but as the bubble rises, that pressure is reduced, and the gas expands, pushing the mud above it out ahead of itself. Another potential source for big-time trouble. And this one (which is known as a kick in the well) happens much faster, so there is less time to react.

How do we handle this? The answer is to invert the problem. Gas or oil flows into the well because the well is at a lower pressure than the fluid in the rock. The fluid in the well is, initially at the pressure created by the depth, and by the weight (density) of the mud in the hole. However, if we put a restriction on the flow of fluid out of the well (such as when you put your finger over the end of a garden hose so that the stream becomes smaller and shoots out further) we can increase the pressure in the well.

(For those who want to know why, if the same volume has to go through a smaller hole in the same amount of time it has to go faster. This means it has to be pushed harder. Bernoulli explained it, and you can read the explanation from Tufts or, if you are really mathematical, from Berkeley).

What it means is that by adjusting the flow out of the hole, the driller can adjust the internal pressure, and thus "kill the kick", or if gets to be too much of a problem, "kill the well". But it is not completely that simple. Bear in mind that there is the Kelly and all the rotation gear connected to the drill pipe at the top of the well. None of which can stand much pressure. Another piece of equipment has to be placed at the top of the well.

This is the Blow-out Preventer, which is essentially a ram that very rapidly shuts off fluid flow at the top of the well. A more modern one can be seen here. These have to be well designed, since they are generally the line of last defense against a blowout, and when they fail serious problems arise. They also form the basis for the well-known structures, often referred to as Christmas Trees, that sit at the top of producing wells. By themselves, however, these aren't enough, since their main function is just to slam the door shut, before all the oil gets out and we have a gusher. The more critical tools are the chokes on the well. There are generally several, both hydraulically operated and manual (in case the power dies) which are simply large valves that can be turned to increase or reduce the size of the flow path out of the well over to the mud pits. By adjusting these, in real time, the driller can control the well pressure, and thus the dynamics of the behavior at the bottom of the well. And after the rig leaves, an operator can adjust well pressure, and thereby the production from the well and its long-term performance.

If the operator is well trained (and you find drilling simulator equipment in Petroleum Engineering Departments so that students can understand how to do this – I last tried some decades ago) the well pressure will be controlled, so that any kicks can be handled, and the drill can now penetrate safely into the rock containing the oil/gas, which we call the reservoir, or the pay. And you think the hard part is over ?

There is still well completion, and then production (which will be where the answer to Lynch comes in), but for now – as usual comments, questions and criticisms are welcomed.

BTW – if you're impatient with the speed of these posts, there is a lecture series on all this available from Rigzone, with videos. I haven't seen it, but I noticed it while looking for sources of pictures.

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I love this series you're doing on the practical matter of getting oil out of the ground, HO.

The mystery grows. What does Lynch know, and when did he know it?

I just don't get Lynch's arguments at all. Somebody should introduce him to Mr. Bayes one day.

To HO:
With that kind of technical setup, I hope to see some sort of TKO coming up.

So HO, how much energy (roughly) does it take to transport the casings and all from Texas to ANWR ? How much energy does it take to pump the cold oil back south?
How much energy is consumed in building our next generation Thunder Horses? How deep is "deep" for deep offshore drilling? Can we go way out beyond the continental shelf and expect to find oil? Lay people seem to think it's no problemmo. What's the truth?

I suspect that the largest energy consumption with ANWR might be in building the roads and pads out to the site, given that there are rigs up in that part of the world already. And since (and this is referred to in the "mud" post) the oil comes out of the ground hot, one of the larger costs of the Alaskan Pipeline was to ensure that it remained hot all the way down to Valdez.

I don't know how much energy is required for the Thunder Horse platform, though there are information pages on them that we have cited before, and for about $1 billion I expect there was a fair amount used. But putting it into the context that the platform will produce around 250,000 barrels of oil a day, it is all relative.

In regard to deep offshore you might want to read the piece at abscom/Abstract_Lisbon_Bruhn.pdf
it gives some basic data. There are wells around the world now drilling in deep water and finding oil, but I would be the last to say that this can be done without problems.

"I just don't get Lynch's arguments at all."

There's no "there" there.

Once you read through the whole thing, and subtract out the ad hominum, innuendo, and logical fallacies all you are left with is the nearly useless observation that discovery, production, and depletion do not exactly follow a smooth mathematical curve, and therefore, you cannot make a mathematically precise prediction of the date of peak production from a plot of historical data.

And that's it. He doesn't address at all the fact of peak oil, or the fact that the peak is getting closer, or the implications of declining production on economic expansion, or anything else relevent.

Just, "no one predict exactly when the peak will come".

I can't believe I wasted 15 minutes of my life reading that paper.

Like some other graduates of that otherwise fine school, Lynch gets away with a lot by parading around his MIT credentials. But I don't think a PolSci degree enables you to do much more than get people to swallow logical fallacies in hopes of pushing his agenda (whatever that is).

I would love to see this series of posts put somewhere together on a more permanent basis when you're done. I think it makes for a good introduction to lay people on this complicated subject.

So having just read only the abstract of Bruhn's article at

He is saying that a whopping 70% of Brazil's extraction comes from deep offshore and they are not sure how far out onto the continental shelf they can go and still keep strikling oil --is that correct? (I have not yet read the full pdf)
Thanks !!!

getting through to the "lay people" is kind of like starting to drill for oil

first you got to have the drill bit penetrate through the outermost layer of rock

some people here read me wrong.

in no way do I say that you should not be armed with large masses of technical knowledge --because once the drill bit pierces through the outer denial layers, you are going to have plenty more challenges

it's along fight all the way down to the core, strata by strata

but if you never get through the very first layer --aka "attention deficeit" --aka "eyes glazed over", all those deep bore drill bits that you bring with you to the party are useless

the Bruhn PDF is great

It reminds me of an old Sherlock Holmes joke.
Sherlock awakens Watson in the middle of the night while they are camping out in the fileds.
"Watson, look up at the sky and tell me what you see?"

The ever astute Watson studies the deep details of the Milky Way and starts exhalting brilliance about the distribution densities of the stars, about the scope, size and origins of the Universe ... it's a shaggy dog joke

Finally the flabbargast Sherlock Holmes stops him ...
"No !! Watson you idiot ......

the fact that you can see the stars

means someone stole our freakin tent !!!"

Well it's the same thing with this relentless march of humanity out along the seabed. No Watson, the fact that we our out here on Thunder Horse (or wherever) means it is getting harder and harder to keep the current oil production flows going. Look at page 6 of Bruhn's PDF. Compare 2004 to 2005. What do you see Watson?

Thanks HO

There was a south Atlantic hurricane in March. If it wasn't an anomoly but a sign of things to come with global warming it will be difficult for Brazil to keep production steady.

I think that you will find that a lot of the new exploration is moving offshore - whether Nigeria, Brazil, Algeria, the GOM, the Caspian Sea or Sakhalin Island for the Russians. When you've pretty much explored all the land, all that is left is the offshore. Note that the Saudi's (as I posted last week thanks to J) are now moving another 5 offshore rigs to compliment the 3 they already have in operation.


I'm curious about the idea that the way to determine the pressure in the oil reservoir is the weight of the rock column. Seems like generally the reservoir rock would be carrying the load of the overburden and the fluid in the pores would not be involved in supporting it (unless the rock was compressible and had compressed to the point the fluid was under equal load). If the fluid was in equilibrium with a fluid column throughout the whole overburden, the reservoir fluid pressure would be dependent on the weight of that fluid column. However, presumably there's numerous layers of impermeable rock, meaning no equilibrium, so the pressure in the oil reservoir seems like it would be more a function of how much the various interconnected layers of rock between source and reservoir compacted during geological processes, and how the original contents of the pores changed during the transition from being dead algae and sea water to being oil, etc. No?


you're getting a bit ahead of me, but in the immediate instant when the drill penetrates the cap rock there is no fluid connection upward (otherwise the oil and gas would have followed it and gone many millennia ago) and rock is compressible (but not that much) and it depends on the rock type etc. But as a simplification, AT THE BEGINNING, of oil draw down there is an equivalence between the pressure in the rock and the oil. But your question presages the answer to Lynch, and I will get to it in a couple of posts (since I have to explain permeability, porosity and a couple of other things first).

(And bear in mind this is a simplification, and I am trying to get different things across, one at a time - you don't at this stage need to know all the variants that are found in rock pressure due to the fact that the various bits move against one another etc).

Ok, I'll wait patiently :-)


Please keep writing on this subject. It's something I've very much wanted to become more informed on. Also, can you recommend any websites that have explanations of the various factors that affect the production of oilfields, such as porosity, permeabilty, composition of the reservoir rocks, etc.? You alluded to one website, Rigzone, but their course was about $4,600, just a tad too rich for my blood.-:)

One issue I'd like to know more about is undersea drilling. How exactly does the concept of the "oil window", described by Deffeyes as between 7,500 and about 15,000 play out when we are trying to extract oil from thousands of feet under the surface? Deffeyes' discussion of this in his first book is very sketchy; he seems to be saying something like that in many places the depth of the ocean precludes oil being discovered there. Comment (later in the series)?

I am a geologist, but I deal with water issues (hydrogeology). From my classes though my understanding is that the "oil window " refers to the depth below the land surface (seafloor in this example) to which the organic matter is buried. I could be wrong, but I don't think the amount of water above the sea floor adds much to the heat profile in the subsurface (so Deffeyes was refering to depth below seafloor).

Keep in mind also that the oil window refers to the maximum depth the organic matter was buried. After several million years at the right depths to create the correct oil forming heat and temperature (the oil kitchen) the organic matter could be uplifted out of the oil window. The oil can then migrate upwards to even shallower traps. Thus one does not always drill down to the oil window to find commercial amounts of oil.

Oil is only rarely found in deep water on oceanic plates as the oceanic crust is farther from land and so recieves less organic matter (on average) and the oceanic plates are created at mid-oceanic ridges and destroyed at subduction zones (Ex. off the Oregon coast). The oldest oceanic crust is Jurassic in age which is much younger on average than the continental plates. Thus any oil formed on oceanic plates is likely to get subducted before any large deposits are formed.

As far as I know most of the deep water oil being found is on the edges of continntal plates. The upshot of this is that there is probably very little oil in the middle of the Atlantic or Pacific oceanic plates.

This is interesting. I had thought about the possibility of the oil moving up out of the original "oil window" within which it had formed but couldn't find any discussions of this online (or in either of Deffeyes' books).
Wouldn't the total pressure of the "column" of water equal roughly one-half of the equivalent pressure of the "column" of earth (if I understood the article we are responding to here)? If so, other things being equal, from the surface of the ocean, you could have an oil window that is "deeper" than on land? Just curious. (I get the other factors you mention about organic matter.) Thanks for the info.

Only if the sea had always been sea, and the land always land, and this has not been the case.

I am not a degreed petroleum engineer but I may be able to give you some insights into pressure considerations in drilling. As mentioned in the article there is MWD systems that provide downhole information. That is what I do.

Normally oilfield operations use the unit of pounds per gallon since we are driilling with mud or water. As barite is added the weight of the mud increases. Each geologic area has a normal pound per gallon gradient increase under normal pressure. Abnormal pressure is created when a zone with a impermeable cap that was at a relatively lower depth has been uplifted somewhat. This is kind of like a bag of potato chips at sea level gets all puffed up when you get to Denver.
Otherwise you could spud and drill to TD with a known mud weight. Predicting down hole pressure is a big problem in the GOM and Gulf Coast areas. It is less of a problem in older depositional areas. Other areas there can be problems with loss of circulation where the mud weight is too high. You want to be in balance to drill efficiently.

I think the oil window refers to the depths at which oil can be found before earth temperatures cook it and then hydrocarbons are then found in gasseous state only. I think that the sediments in the GOM are on the order of 50k ft.

BTW pressure is calculated by using the conversion factor of .052 so a well that is 6000 ft deep with water as drilling fluid would be:

6000 x8.3 ppg x.052= 2589 psii

There would be an equivalent circulating density of about .3 ppg for the pumps that would add about 100 psi to the pressure at the bit. I mention this because I've seen wells where circulation would be lost with the pumps on and then the well would kick with the pumps off. A very dangerous situation.

I believe that M. King Hubbert did a lot of research for Shell regarding pore pressure. So you may be able to find additional info by researching his work.