How carbon dioxide improves recovery

Following Yankee's story about carbon dioxide injection, it appears that not everyone understands one of the ways in which carbon dioxide will help enhance oil recovery (EOR). I am therefore going to just list some of the previous posts that include carbon dioxide, which was discussed here, and here, not to mention here and here.

Below the fold, however, I am going to repeat, with a little update, the post where I described what carbon dioxide injection can do to an existing oil well, and that itself followed an earlier post. These were pre-cursors to what later became the weekend techie talks, and these really relate to those, and from now on I will include this post in that listing. Since the topic largely relates to oilwell production, the listing this week will be for those sites. For those new to the site, on most weekends (though not next week) I will post a simplified explanation of one aspect of fossil energy extraction. So far we have been covering coal this year, after covering aspects of oil and gas production last year.

Carbon dioxide injection is a current DOE program for enhancing oil recovery from an older oil reservoir that has already produced the bulk of the primary oil that it will yield. Just recently Glencoe have started injection in central Alberta, and though the OGJ article on this is behind their wall, a short quote:

Glencoe Resources Ltd., private Calgary independent, is using the gas to improve recovery of primarily light oil from multiple formations in several depleted oil fields about 100 miles north-northeast of Calgary.

The company hopes to boost the recovery factor to as high as 40% from 10-20%. All of the formations are deeper than 1,300 m.

Glencoe has long-term agreements to purchase CO2 from two industrial plants. It operates about 50 miles of CO2 pipelines and has begun injecting gas from the MEGlobal Canada Inc. plant at Prentiss. A second CO2 separation facility being built near the NOVA Chemicals Corp. petrochemical plant is to go into service in early 2006.

The original post related to cleaning up after elephants*, and was written during the time when I frequently compared Saudi Arabia to a sandwich shop (sorry but no-one every noticed the pun!)

So you're feeling cheap and don't feel like going to the sandwich shop, huh? So lets see what's in the refrigerator. An apple, some cheese, some butter - that will do. You put them on the table and.... darn, you got a blob of butter on the apple. Rather than have it roll all over the table making a mess, you stick it under the tap.

With the water running on cold it seems to take forever to wash the butter off the apple, but if you turn the tap to hot, the butter runs off very quickly. The same sort of thing happens when you apply hot water or steam to the oil left on the sand grains of a rock after the primary and secondary recovery of the oil is over. The oil is a lot thicker than butter and you generally have to heat the water a bit hotter (it works best above 185deg F) but you can still clean the oil from the rock that way. There is, however, a bit of a snag. (And from this point on DO NOT TRY THIS AT HOME).

Think of it as little Johnnie (helped of course by Jessica) having raided the orchard and spread butter onto all the apples, gluing them together and filling the kitchen full, right to the ceiling. How do we clean the butter off and get it back without taking all the apples out and cleaning them one by one (which is sort of what they do with the oil sand up in Canada).

We could just stand in the hall and stick heaters up against the wall of apples, hoping that the heat would melt the butter and work its way back to the ones further into the kitchen. That sort of works, but burns the local apples and doesn't reach all that far. (They have tried setting fires inside oil wells). You could fill the kitchen with hot water, but while that washes out some of the butter, a lot of the heat goes into the apples and the water is cold before it reaches the back of the room. And the water doesn't have that much pressure to push the remaining butter off the apples. (In oil wells this is the secondary recovery that might get us back another 10 - 20% of the oil, except that they use cold water and some rocks have clays that swell when wet and this stops the oil from flowing).

What we need is something that will get through the gaps between the apples and keep its heat. So how about steam? You get a steam cleaner (such as you use for carpet cleaning) and blow the steam into the apples. That works but as the butter starts to flow out it clogs the gaps and starts to re-harden except when the steam is right there. So you start to run the steam for a bit, stop and collect the butter that comes out, run the steam for a bit, etc. You can do this in an oil well and it has the exciting technical name of "Huff and Puff" (would I kid you?). (Ask Your Government). (pdf file) To make the steam more effective it is heated to between 150 and 300 deg C. Where the rock is very permeable and the steam can, in time, work its way back through the particles (apples) this can recover a lot of our butter. But you still lose a lot of heat, which is expensive to generate, just in heating the apples.

What if we could use something else that does chemically what we have done with the heat? How about a soap? Yes that might work, the only thing is that soaps cost a lot of money and there really isn't that much butter, so unless we can get our soap back we really can't afford it. And what we have to do is to put in the soap, wait for it to work, and then push some water through to move the water:soap:butter to a place that we can collect and separate them.

Hmmm, but what if we had a chemical that could mix with the butter, and make it melt, so that it ran off without heat? That might be a lot simpler. Well it turns out that carbon dioxide, that gas we all love to hate, does exactly that.

(But now we have to talk just about oil). When carbon dioxide is mixed with oil, then it is absorbed into the oil and the resulting liquid has a larger volume, which means that it is thinner. (You could think of this as being the same sort of change as when you add a teaspoon of hot water to honey). The carbon dioxide also makes this thinner oil less sticky, so that it comes away from the rock more easily. Thus the oil will come away from the sand more easily, and will flow more readily to the well, being thinner. And tests, at the UT Austin , show that CO2 will move right through a rock of the right sort, reaching all the oil that might be there, and coughing up a lot more of it. But that also the rock would retain the CO2, if sequestration was desired.(Note that this does work when the process takes place under water).

We can do the same sort of Huff and Puff approach that is used for steam injection, except using liquid CO2 and, in the right rocks we can get much more of the oil out. Or we can drill one well and pump in the carbon dioxide, and collect the oil and gas from the next well over. But there is an additional advantage. When we get through with pumping out the oil, we just pump all the carbon dioxide back into the rock and it stays there. So not only have we got the oil out, we have replaced it with the gas that is causing global warming, and once put into the rock it will stay there, in just the same way as the oil did.

This technique is being used in Liaohe, the third largest oilfield, in China, which is now declining in production. By pumping in flue gas from a nearby power plant and combining it with steam the recovery of oil from the reservoir was increased from the 20 -30% achieved with steam, to around 50 - 60%. Since the gas was not otherwise treated it only contained about 10 - 14% CO2, the rest of the gas being largely nitrogen. What the experiments showed is that we can use the gases from power stations, without separating the components out, which would otherwise be a lot more expensive. What is also interesting is that the Chinese gases did not appear to have been liquefied, and that there might have been some production gain (up to perhaps 40% of the oil recovered) by combining the flue gas with steam.

But the benefits extend beyond that. An experiment that the Globe and Mail referred to has been going on in the Weyburn oil field in Saskatchewan for the last five years. CO2 from a synthetic fuel plant in North Dakota is piped to the oilfield and injected. While the reservoir holds the gas, the mix with the oil, means that the oil flows out of the well more easily.

Carbon-dioxide injection will allow EnCana to extract another 140 million barrels of oil from its 51-year-old Weyburn field, an enormous volume at a time when the average new well drilled in Western Canada yields a mere 50,000 barrels.
The technique has promise in a number of sites where the rock layer does not otherwise allow much oil to be recovered. The potential gain for Canada can be quite large.
Now, better technology and high crude prices are about to shift an enormous amount of oil into the grasp of the industry. As many as five billion barrels could be added, according to Mr. Issacs. That would more than double Canada's conventional oil reserves.
There is now an updated page for the Weyburn site

A significant gain can also be achieved in the United States where the process has seen some limited use since it was first tried in Scurry County Texas in 1972. It is being used particularly in the Rockies and West Texas where it is currently producing over 190,000 bd of oil .

The benefits from this are two-fold, since the gas can be stripped from the oil and reinjected, thereby trapping it back underground rather than releasing it to the air. The only downside to that is that, to be economic and useful, a power plant already in existence should really be used for the gas, and it needs to be relatively near the oil to be economic.

The OGJ had an article on this last April. The article is now archived, and thus requires a password to access, but begins:

The latest technology for enhanced oil recovery by injection of carbon dioxide holds the potential to recover 43 billion bbl of oil "stranded" in six mature US producing regions, says a study conducted for the Department of Energy.

DOE's Office of Fossil Energy calls the volume, estimated in the study by Advanced Resources International, "technically recoverable potential."

It identifies as "state-of-the-art CO2 EOR technologies" horizontal wells, 4D seismic to track injectant flow, automated field monitoring systems, and injecting larger volumes of CO2 than were used in earlier EOR projects.he study says state-of-the-art CO2 injection might recover 5.2 billion bbl of 22 billion bbl of oil unrecoverable by conventional production methods in California. The stranded oil is in 88 large reservoirs amenable to CO2 injection.

The technology does require that the gas be liquified (which requires a pressure of about 1,000 psi) so that it can better mix with the oil, and make it easier to extract. It even gets the CEO of Shell excited.

Which leaves us with that apple, and the butter. Why don't you spread the butter on two slices of bread, slice the apple on top of one of them. Sprinkle blue cheese crumbs on the top of the apple, put the second slice of bread on top and have lunch - you deserve it, and (like me) you don't have to go the sandwich shop today.

(Technophiles can read a less dramatic version of the above here.).
*From a play here.

This is part of an ongoing weekend series on technical aspects of oilwell (and natural gas) drilling. Previous posts can be found at::
the drill

using mud

the derrick

the casing

pressure control

completing the well

flow to the well

working with carbonates

spacing your well

directional drilling 1

directional drilling 2

types of offshore drilling rigs

coalbed methane

workover rigs

Hydrofracing a well

well logging

seismic surveying

gravimetric surveying

As ever, if this is not clear, or if there is disagreement then please feel free to post, and I will try and respond.

Maybe I'm cynical but this strikes me as bogus. In the case of low pressure gas why not just use air onsite? Both flue gas and air are mainly nitrogen. On the other hand liquid C02 requires tremendous energy to scrub and compress. A food processing plant near home uses it to make beer flavouring extract worth $$thousands per ton. Since neither the US nor Canada have carbon taxes there must more reasons than EOR to use up perhaps 40% of the surface plant's power output. Could it be there is a government handout to 'prove' it works? You also need to factor in the extra CO2 when the EOR is eventually burned.
The reason for using CO2 os probably its low critical pressure (ie, relatively easy to "liquefy"). Lots of gases will probably do but CO2 is the easiest to work with for supercritical extraction.
Liquid CO2 has solvent properties that N2 and oxygen do not have.  For example, liquid CO2 is used in industry to extract caffein from coffee beans.  This is the so-called "natural" process of decaffination.

Actually, it is not exactly correct to classify the CO2 used in petroleum wells as "liquid".  It would be more correct to call it a supercritical fluid.  

Thanks for your links to the previous articles on the topic--I did a search in the archives, but I must have missed the fact that they're in the classic section.

In any case, HO, can you address the petroleum coke question I had in the previous post? Since I don't know what it's used for, I was rather curious about why BP/Edison would ship it to China instead of using it for something else in the US.

A review of what petroleum coke is  (the carbon residue when you have driven off the hydrogen molecules) also describes how it is produced
Petroleum coke is a by-product of the oil refining process. Delayed coking, the most widely used process, uses heavy residual oil as a feedstock. During delayed coking, heavy residual oil is introduced into a furnace, heated to about 480 °C, and pumped into coking drums. The coking process initiates the formation of coke and causes it to solidify on the drum wall. Thermal decomposition drives off gases, which are removed continuously. When this reaction is complete, the drum is opened, and coke removal begins. Water spraying thermally shocks the coke and allows it to break off. Coke that remains on the drum walls is subsequently cut from the drum with a high pressure water jet. After the water drains, coke is transported for use or storage.
. The volume of coke generated is also given
As of January 1, 1998, total worldwide production of coke was reported to be approximately 46 x 106 tonnes per annum [6]. Of this, North America (predominantly the U.S.) accounted for approximately 66.5 percent; Europe, about 17 percent; the Asia-Pacific region, about 9.5 percent; South America/Caribbean, about 4.5 percent; and the Middle East/Africa, about 2.5 percent [6]. Nearly 90 percent of the total coke produced is delayed petroleum coke. Of the petroleum coke produced in the U.S., about 66 percent is exported. Japan, Turkey, Italy, Spain, Belgium, The Netherlands, and Canada consume 75 percent of U.S. exported coke [4]. Of the approximately 10 x 106 tonnes of petroleum coke consumed domestically, approximately 2.5 x 106 tonnes (which is equivalent to about 1,000 MW of electric power) are used for power generation. No hard statistics are available as to how much of the world's annual petroleum coke production is currently used for power generation. However, projections of major boiler suppliers indicate that between 1,500 and 2,000 MW of additional petroleum coke-based power is expected to come on line within the next 5 years. With the maturation of petroleum coke-based power generation technologies and increased production of petroleum coke, it is expected that these numbers will grow.
The paper also explains how to get the pollutants out of the flue gases, and ends
A deterioration in the quality of crude oil and improvements in oil refining technology have led to increased production of petroleum coke. The mismatched supply and demand situation has caused the price of petroleum coke to drop, and experts predict that this situation will continue for the near-term. This has created an opportunity for increased use of petroleum coke as both a primary and a co-firing fuel for power generation. Today four technologies are available to successfully use petroleum coke.
 I suspect that the US may now have as many boilers in operation as can use the product, and the Chinese may be building them, and looking for additional material.

(I have only ever worked with it once, and found it very difficult to ignite).

I have a question to you HO:

Can you use CO2 on field were water injection has already been used extensively?

Imagine you have a well with a water cut close to 90%, would CO2 injection recover more oil from it? How much?

There are some significant constraints on when carbon dioxide can be used, J had some horror stories when I did the first post on this.  But if the circumstances are right (and it does work under water) then you might gain another 10% of the oil.  Remember that in conventional recovery you might only revoer about a third of the oil in place, and as the article I quoted points out, this gives an additional recovery you might not otherwise get.
DOE awards CO2 compression study 12/2005
Southwest Research Institute

CO2 pipeline to recycle greenhouse gas

CO2 pipelines design and risk

CO2 pipeline project list

example CO2 Pipelines
Texas & New Mexico

example North Dakota

CO2 has a critical temperature of 31 degrees centigrade, IIRC, so it dissolves in oil as a gas and makes it more fluid, but it does not dissolve the oil into itself as a solvent because the oil is deep and at a higher temperature than the critical point.
N2 is not very soluble in oil. Flue gas is 80% N2 and 20% CO2, and since you have to compress it to pump it, the CO2 is going to condense anyway so you might as well dump the N2 to atmosphere after using the expansion for cooling.
CO2 liquid is going to dissolve things in your pipeline if you keep it as a supercritical fluid. It also forms hydrates and plugs things up if it gets cold. I don't know how they move it around without it crapping things up. Maybe they line the pipeline with plastic? Or insulate it and keep it hot? If any lurker out there knows, elucidate me.
We keep it dry and supercritical.
Now I have a question for you.  Won't the CO2 stay supercritical at well pressures even though the temp is high?  Or... I guess its only for wells where the pressure has depleted anyway, heh?
In thermo-speak, any temperature beyond 31°C is "supercritical" for CO2.  "Critical" is the point at which the distinction between the liquid and vapor phases disappears; hotter than that, you have one fluid phase regardless of the pressure.

Critical pressure for CO2 is 1072 psia.  As liquid CO2 is more dense than water at reasonable temperatures, it will tend to stay a dense fluid and develop considerable hydraulic head as you go down an injection well.

Do Campbell's, Deffeye's, etc. projections for the date that production will peak take into account growing use of CO2 injection?
From what I know Campbell yes, Deffeyes no. But the last one strongly advises it in his book.

Has I tried to ask HO, the big question is in what fields can one use it. If in just a few, the impact won't be great.

The Hirsch report has some important stuff on this also.

Yes, this form of Enhanced Oil Recovery has been known about for a while.  However the additional benefit of carbon dioxide sequestration has not been considered until recently and this may have a minor influence on the overall use of the technology. Bear in mind it generally comes into play in the declining years of the field after primary production is over. Thus it will be, as a contribution, a bump on the downside of the curve.
That was an interesting quote from that OGJ article, HO. I am wondering about "technically recoverable potential" since that's the first time I've ever heard that term. Now, regarding
...the study says state-of-the-art CO2 injection might recover 5.2 billion bbl of 22 billion bbl of oil unrecoverable by conventional production methods in California. The stranded oil is in 88 large reservoirs amenable to CO2 injection.
At this point in time, late in the game, that's a pretty big bump. Now, when I posted about Weyburn, I had also noted the additional recovery there. But now my question in all these cases is this:

Was this "technically recoverable potential" booked as reserves to begin with? The question matters because one of the common arguments used against peak oil is that EOR increases the URR. And that appears to be the case in these CO2 injection cases if the oil is indeed "stranded" and was never counted as reserves. If that is indeed so, then this would appear to be a case of reserves growth without new discovery due to the application of technology. In other cases, like Yates or the Forties, EOR was applied to get higher recovery rates but it would appear that the URR was never actually affected. As with NO2 injection at Cantarell where it is now expected the higher recovery rates achieved since the mid-90's will be offset by a steep decline in the tail. So, these CO2 injection cases seem to be different. Michael Lynch makes a pretty good living making this argument. I have always found the question of whether technology applied to oil fields increases reserves to be slippery. The answer appears to be sometimes "yes" and sometimes "no".

It is interesting to speculate about a world in which CO2 has a value. Maybe we should be building new coal fired power stations next to LNG terminals, so that CO2 can be shipped back to the oil producing countries in the LNG tankers on their return trips. This gives positive benefits for climate change and EOR, and a more intensive use of existing infrastructure.
That won't be easy.  LNG is shipped at atmospheric pressure, while CO2 is well below its triple point (triple point of CO2 is 5.2 atmospheres and -56.4 C).  Unless you re-engineer the LNG tanks to be pressure vessels, you can't move CO2 in them; you can't pump solid chunks of dry ice, and you probably don't want to temperature-cycle the tanks that much either.
Speaking from someone who will probably have a LNG terminal about a 2 miles from my place, I REALLY don't want a coal-fired plant there too.  But as the LNG speculators tell us, there will be a gas-fired co-gen along with the terminal, although many people familiar with the industry claim this isn't possible. so......
They say NG fires too cleanly to get enough CO2 out of the stack to make it worthwhile.  CO2 extraction is therefore best left for coal fired stacks.
Maybe HO can clarify something.

CO2 is commonly used for EOR. But I'm pretty
sure the Mexicans spent many hundreds of millions
to develop a scheme for injecting nitrogen into
their Cantarell fields, thus doubling production since 1998. (This set of fields is poised for a steep fall in coming years.)

Anybody have beta on why Pemex is using nitrogen, not co2?


What's the EROEI implications of the CO2 injection?  It must take some significant energy to compress the CO2 and pump it, never mind separate it from the nitrogen if that is necessary.  Also, how is the CO2 separated from the oil after recovery?  Finally, it sounded to me from HO's posting  that the CO2 comes out with the oil and is not sequestered unless pumped back underground yet again.  That extra step of course costs money (and energy) and will therefore not be done in the real world, unless there is real political resolve and serious enforcement.  Under conditions of scarce energy and a collapsed economy, that seem unlikely.
  1. Yes, it does take a fair amount of energy to compress the CO2, more than just pumping water.

  2. At the surface under low pressure some of the CO2 comes off the oil and is reused for injection.  The rest stays underground.
As a means of reducing greenhouse gases on a large scale, the injection of CO2 into deep underground formations strikes me as a highly dubious scheme.

Why?  For the simple reason that i) even a medium size coal-fired power plant generates an enormous volume of CO2, and ii) the power required to compress all that CO2 to the high pressures required for subsurface injection constitutes a significant fraction of the original total output of the power plant.

Consider the following simple example:

- A 300 megawatt coal-fired power plant operating at 40% thermal efficiency and using coal with a heat content of about 14,000 BTU/lb and a carbon content of about 80% -

This power plant will use approx. 92 tons/hr of coal and will generate approx 270 tons/hr of C02.

At standard conditions this amount of C02 corresponds to a gas flow rate of approx. 72,900 cubic feet per minute (CFM).  

72,900 CFM is a very large gas flow, and even when you are just pushing it around with a blower operating against a pressure of only several inches of water, a great deal of power is required. But, if on the other hand, you have to compress all that gas to a high pressure, say 500 psi for argument's sake, then the amount of power required becomes truly enormous.

The power required for high-pressure compression depends on many things (the number of compressor stages being but one) and is not all that straightforward to calculate, but a crude back-of-the envelope calculation indicates that something on the order of 80,000 to 100,000 KW would be required.

This represents roughly one quarter to one third of the total original output of the power plant!

So, if you wanted to retain the original amount of power going to the grid, you would have to increase the size of the powerplant accordingly, but when you do that you will have to burn more coal, and in turn generate more CO2, and require more power to compress that CO2, etc, etc.

Somebody out there please check the compression power required to see if I made an error.  And maybe you don't have to compress the CO2 all the way up to 500 psi, but no matter how you slice it, the compression of CO2 for deep subsurface injection will drain a very significant amount of power - power we cannot afford to lose.

Maybe I'm missing something here, but I don't see how this can possibly be a good idea.  

I think you missed a decimal point.  
I used a made up value for Cp, but only got 8000 HP (6000 kW)

you try it again here, see what you get.

Gets IT -

You're right: I did make an error in the power calculation (units).   I wasn't able to run the thing in the website you cited, but I redid my calculation and came up with a tad over 16,000 HP (12,000 KW). Though this is far better than 80,000 KW, it and your number of 6,000 KW still represents a lot of juice.

Mind you, this was based on pumping up the 73,000 CFM to 500 psi.  From comments on a previous post, it appears that one might have to go up to near 1,000 psi if  there is a requirement for the CO2 to be in liquid form where it is to reside in the subsurface pore spaces.

Also, this power does not include the power required for removing the CO2 from the stack gas(not just pumping power but power required in the manufacture of whatever chemicals are used), nor the power required to transport the CO2 to the injection site.

Though not as bad as I originally pictured, disposing of CO2 in this manner is still highly energy-intensive, and I still don't like it.

I don't think the CO2 needs to be in liquid form for injection into a depleted gas field, unless it first requires transport for "long" distances.  If CO2 is just to be stored in a depleted (dry) gas field, the pressure at the well head shouldn't be need ever be higher than 250-500 max.  I had a lot of gas wells in south Texas that got down to 50 psig before they effectively died.  (My boss wanted to start putting in vacuums, but we had so many leaks in the gathering system the pipelines would have sucked in atmospheric air from all the holes, which we would have had to separate later.)  Reinjection must depend on field permeability and residual water levels...  Maybe somebody else knows, I am guessing on this reinjection-storage process.
Ya. Its kinda' the fossil fuel equivalent of what to do with the spent fuel rods.
I don't see how this can possibly be a good idea.
You're right; burning coal in air at atmospheric pressure and then trying to extract pure CO2 from the effluent gas is a very poor idea.  There are a number of better systems out there, some of them already in use.
The brute force way is to separate the oxygen from air and throw away the nitrogen, then dilute the oxygen with CO2 flue gas to keep the temperature down, and get a pure CO2 and H2O exhaust. The H2O condenses as water and the CO2 is left. Transfer the heat to the steam through tubing like the heat exchanger circuit in a nuclear plant.
Of course, it would about double the cost of the plant and give no more power output, so this isn't likely to happen short of Bangladesh nuking a few of our (and the Chinese) cities as an incentive to stop drowning them...
wkwillis -

Your scheme using an oxygen separation plant upstream of the power plant looks like it might be a better way than trying to strip the CO2 out of the stack gas.  

 However, one must appreciate the scale of such a system. If we take the 300 megawatt coal-fired power plant in my previous example, it burns about 93 tons/hr of coal, and therefore would require about 245 tons/hr of oxygen. This would require an oxygen separation plant with a capacity of about 6,000 tons per day.  That is one humongous oxygen plant!

By the way, do you have any numbers on the amount of energy required for oxygen separation, in terms of BTU per ton O2 produced?

Nope. I just assume that half the energy of the power plant will go into the oxygen separator (even using the evaporation of the liquid nitrogen to cool incoming air) and from heat that will be lost in the heat exchanger as the CO2 high pressure exhaust dumps heat to the steam circuit.
Figure the capital cost of the steam circuit as equivalent to the capital cost of the steam circuit in a nuclear power plant as a first order back of the envelope type calculation, and cryogenic separation plant (still cheaper than membrane at large quantities for a coal plant) as turnkey.
Now the problem with doing a closer calculation is that our ability to build big projects is already undergoing strain. Prices are rapidly rising for cement, steel, construction crane rent, and welders. No way I could make any kind of cost estimate closer than "twice as expensive".
How does this address peak oil, the problem of peaking rates of extraction?

As we all know (but often forget) peak oil isn't about the amount of oil it's about the rate it can be extracted with a given amount of investment (hence why the massive tar sands and oil shale reserves don't change the game).

If EOR isn't able to increase the rate of extraction or decrease the investment needed (so we can just do more of it) it doesn't help peak oil.  Does it?

EOR like this sounds like a way to increase the area under the curve by stretching out the tail - useful for sure, don't get me wrong, but doesn't change the problem of not being able to continue to increase or even maintain the rates of extraction for much longer if at all.