Carbonates, chalk and oilfield subsidence

This is a part of the continuing series of technical talks that I post on Sundays, and which are listed on the right side of the page. Gradually these are getting a little more technical, so I would suggest if you are new to these that you start at the beginning and work your way up.

Having just written about sandstones and permeability, and then about water flooding, I want to go on, this time to talk about carbonate rocks, as the general topic and secondary porosity, with some comments at the end on formations in chalk and the problems that this can bring to places such as Ekofisk. (Added for clarity - when I talk about carbonates I mean calcium carbonate and the related rocks such as limestone, chalk and dolomite, to name but three). I again want to emphasize that, life being what it is, the true situation is often a bit more complex than I describe in this simplistic overview, and that I am very grateful when folk give more specific information about some of their experiences in the field.

When I wrote about sandstones, I mentioned that (as a general rule) carbonates are different to the conditions found with sandstone. For a start the rock itself is much finer grained than a typical sandstone. Even if there were oil in the body of the rock it would be more difficult to get out. Most of the oil is found in what is called secondary porosity. This is the network of fractures and bedding planes that are formed in the rock as it is compressed and moved after it has first been formed. The rock is largely limestone, though it may also be dolomite or related rocks. The rock is usually not a massive, solid rock (such as you might want to build a house or cathedral with).

Exposed limestone

Rather oil will collect where there are spaces in the rock. These can be where there was a coral reef, or a lot of change in rock structure. You can see that sort of thing in some new road cuts

such as this one.

It is easier to explain some of the problems of getting the oil out, if one looks at a road cut that has weathered a bit

Weathered rock wall

You can see how the passage of water has opened the joints and dissolved small holes (or vugs) in the rock. These provide the spaces in the rock into which the oil can move and collect and be trapped. Perhaps it might be easier if I used a simpler sketch of a section through the rock to make the next point.

Section through a fractured rock, showing how the oil is in fractures that the well does not reach.

With the oil painted green, as it fills one crack system, but not them all, the well is just a little bit too far to the left to tap into the major fissure system and get all the oil. If only we could reach out a few feet and hit those cracks! And we can and do.

This is part of the reason that we send down the small explosive shaped charges that not only penetrate through the well case and the concrete, but also extend a hole out into the rock. By driving these perforations out into the crack system in the rock a path can be created from the oil-filled cracks to the well. Those cracks can also be cleaned up a bit (since they often have very small particles of carbonate filling them in the zone around the well) by having an acid pumped down into the completion zone. This acid will also open up some of the finer fissures in the rock so that a free path will develop from the well to the oil-filled cracks.

And so we can begin to produce oil. However after a while, for one of several reasons, the flow from the well will begin to decline. I mentioned last time that the underlying reason will be that the pressure in the reservoir will drop as the oil is withdrawn. But there are other reasons.

If the oil is removed too quickly we can generate, before we need to, a gas cap in the well. Fine rock can be carried through the cracks towards the well, and block the passages. Some of the oil contents, such as the waxes, can settle out of the oil, and fill the holes in the well casing. As the oil flows to the well, underlying water can follow it and cut-off blocks of oil in the rock. And the pressure difference between the oil further away from the well, and the well itself might not be enough to move that more distant oil towards the well. (Are you really sure you want to make this investment ?)

Before I leave Carbonates let me go back to the other kind, where there is a reasonable amount of primary porosity, and where the oil is spread through the rock. This can happen when, for example, the rock is a chalk (another form of calcium carbonate or limestone).

Chalk, somewhat similar to that in which the oil at Ekofisk is found

Chalk poses some different problems in production. Let me illustrate them with an abbreviated history of the Ekofisk field. The field was found in 1969, and started production in 1971. It produced from a fractured chalk that has “high porosity but low permeability.” When it was originally produced the driving force to move the oil to the wells was through pressure depletion of the oil, and in this way they were able to recover between 17 and 18% of the oil in place. In the process, however, there was a relatively unexpected problem.

To explain it let me make a very very simple illustrative example.

Representation of Oil (grey) filling pores in a rock.

Now in the original condition the pressure of the oil in the pores (holes) in the rock above is equal to a significant portion of the the pressure of the overlying rock. At Ekofisk the pore pressure was 7,135 psia at a depth of 10,400 ft. Porosity can be as high as 48%.

But as the oil is drawn off through the well that pressure reduces, and the load transfers to the rock columns that are one either side of the circular pores. (The well pressure was reduced to below 5,000 psi). When the load on these columns becomes higher than the strength of the rock then the pillars will collapse. This helps a little to squeeze some of the oil out, but it also (when it happens over a large enough volume) compresses the entire volume of the rock holding the oil. This can close some of the passage ways to the well (permeability), and make it harder to get the oil out, though that does not appear to have happened. But it also lowers the sea bed, on which some of the recovery oil platforms were standing. That subsidence has been up to 30 ft in places. (The platforms were elevated in 1987) but new platforms were ultimately required that would allow a subsidence of up to 66 ft. (20 meters).

In order to stop the collapse, and also to increase the flow of oil to the wells, a program using water injection at pressure was started in 1987, by injecting 820,000 barrels of water a day, it was anticipated that the field would allow the recovery of about 36% of the oil, and provide an additional 300 million barrels of oil. Current estimates are that it will increase overall recovery to 50% of the oil in place.

There are, however, now some concerns about the dissolution of some of the chalk by the water that is being used for the injection, weakening it further, One of the problems in analysis, as it turns out, has been in disregarding the temperatures at which the extraction is occurring. (For those who go to the reference the exchange rate is roughly 5 kr to the dollar).

From which you will see that there is still lots to talk about. But, as before, if I have glossed over stuff, just ask a question and I (or others who contribute) will be glad to give an answer.

The Nile, The Guardian and disaster without climate change

There are times when the corruption of the press by the mantra of “climate change” becomes a little more obvious than usual. So it is with the piece that appeared in the Guardian last Friday concerning the coming disaster to Egypt from the changes going on in the Nile Delta. To condense the story into a nutshell, it deals with the declining prospects of the Delta - the main agricultural resource for Egypt - both as it is currently evolving and looking forward from that into the future. You can see the influence of the Nile, and the Delta, (the green bits) in this overview shot of Northern Egypt (mostly the brown bits) from Google Earth.

Nile Delta and Egypt from Google Earth

The story begins with the story of the farmer Maged Shamdy, and his perceived fate.

"We are going underwater," the 34-year-old says simply. "It's like an occupation: the rising sea will conquer our lands."

Maged understands better than most the menace of coastal erosion, which is steadily ingesting the edge of Egypt in some places at an astonishing rate of almost 100m a year. Just a few miles from his home lies Lake Burrulus itself, where Nile flower spreads all the way out to trees on the horizon. Those trunks used to be on land; now they stand knee-deep in water.

Maged's imperial imagery may sound overblown, but travel around Egypt's vast, overcrowded Delta region and you hear the same terms used time and again to describe the impact climate change is having on these ancient lands.

The only problem is that the rest of the story documents how it is everything but climate change that is causing the problem – which doesn’t of course stop the author of the piece, Jack Shenker, from making the claim. So let me, as I did for the Bangladesh Delta, explain, with the aid of the odd peer-reviewed journal article, what is causing the problem – it isn't climate change - but in a word or four it comes down to overpopulation and the Aswan High Dam.

Let’s start by explaining how a delta system works –whether in Egypt, Bangladesh or Louisianna. The delta lies at the seaward end of a long river that picks up eroded soil carried into it from its feeding and surrounding tributaries, or eaten away from upstream by its own passage. Seasonally the river floods over the delta, and in so doing, as the water slows, it deposits soil on the surface of the delta. You can actually see effects of previous climate change by the changing nature of these sediments. In the case of the Nile, the floods come about following heavy rains in the Ethiopian highlands and Sudanese basin typically in July. August and September. Lands could be flooded to a depth of up to 5 ft, and would be inundated for about a month and a half. In that time the sediment in the water would settle out as silt, the water would flush out any residual salts in the soil, and would prepare the soil for the subsequent planting of crops. This process has provided fertility and water to the Delta for thousands of years. On average the rains in the headwaters of the Nile removed around 0.2 mm/yr of soil and this was deposited in the Delta to an average thickness of around 1 mm/year. Interestingly across the Mediterranean at Venice, Day et al showed that this type sedimentation is anticipated to provide enough land build-up over the next 100 years to mitigate even the sea rise anticipated by the IPCC at some sites.

However, as the article notes, in 1970 the High Dam at Aswan was built, and this captures all the Nile sediment (between 40 and 132 million tons a year) which is now filling Lake Nasser behind it. Although, with the Lake being some 300 ft thick, and 500 miles long, it may take a long time to do so. But now that fertile material is denied the Delta.

So that is the first part of the problem. The second part is that the sediment of a delta will normally compact over time, forcing water out of the lower members, and thus gradually lowering the top of the overlying surface. Where the land is regularly flooded that sinking is matched by the new soil that floods over it, but it is now about 40 years since the soil stopped flooding over the land, and the amount of soil missing is becoming significant. Hence, as the quote above notes, the gradual sinking of the trees into the water of the lake.

The lowered land levels also make the land more vulnerable to sea erosion. Smith and Kader showed that this can be tied to the reduction in sedimentation.

Although coastal erosion is a serious problem along the Egyptian Mediterranean Coast, it is localized at specific areas. These areas have undergone slow to moderate erosion since the turn of this century as a result of natural decrease of the River Nile flow and as a result of increased number of structures across the Nile. In a post High Dame phase, these areas eroded at accelerated rates (3-5 times the rates before the Dam).

Lake Nasser from Google Earth – at the other end of Egypt (the yellow line is the border) The High Dam is at B, and this used to be Nubia.

And to get back to the original article for the remaining problems

Today, however, Nile water barely reaches this corner of the Delta. Population growth has sapped its energy upstream, and what "freshwater" does make it downriver is increasingly awash with toxins and other impurities. Farmers such as Maged now essentially rely on waste water – a mix of agricultural drainage and sewage – from the nearby town of Sidi Salim.

The result is plummeting fertility; local farmers say that whereas their fathers spent just a handful of Egyptian pounds on chemicals to keep the harvests bountiful, they now have to put aside between 25 and 80% of their profits for fertilisers just to keep their crops alive.

As the article itself notes the increased population is taking the water that used to irrigate the lower parts of the Delta. This has nothing to do with climate change (except in that the milder conditions of a Warming Period has historically led to population surges) But that doesn’t stop the charge being made.

Experts believe the problem is only going to get worse. "We currently have a major water deficit in Egypt, with only 700 cubic metres of freshwater per person," explains Professor Salah Soliman of Alexandria University. "That's already short of the 1,000 cubic metres per person the UN believes is the minimum needed for water security. Now, with the population increase, it will drop to 450 cubic metres per person – and this is all before we take into account the impact of climate change."

Much before any problem that might be related to climate change shows up, Egypt has a much larger problem, which is the root cause of the above, and which the article points out

With Egypt's present-day population of 83 million set to increase to more than 110 million in the next two decades, the seemingly unstoppable spread of bricks and mortar over the soil is both the most visible symptom of the country's demographic time-bomb and an inevitable response to it.

Perhaps that should, more logically, be addressed first?

Ah, well, enough said.

Some thoughts on statistics

Back when both my arms were fully functional (a time that hopefully will return soon) I would spend one of the two posts I wrote on Saturdays looking at what was being said on some of the Climate debate websites. I visit sites on both sides of the debate, being particularly interested in the evidence that is called forth, and the conclusions drawn from it. (The current list is given by the CC sites on the blogroll). These usually lead on to others, and so one can, to a degree, filter out new evidence from that which is being rehashed. However it can require a bit more due diligence, when both sides claim that the rational answer lies only with their side of the argument.

Consider, for example, the little debate this week between Real Climate and Climate Audit on the analysis of the data from the Antarctic that I have mentioned in an earlier couple of posts. The debate relates to how many Principal Components (PCs)* should be considered in deriving the predictive equation from which to generate the model results that decide whether the Antarctic is warming or cooling.

In the original paper Steig et al had used 3 PCs in order to generate the model and thus to conclude:

Here we show that significant warming extends well beyond the Antarctic Peninsula to cover most of West Antarctica, an area of warming much larger than previously reported. West Antarctic warming exceeds 0.1 °C per decade over the past 50 years, and is strongest in winter and spring. Although this is partly offset by autumn cooling in East Antarctica, the continent-wide average near-surface temperature trend is positive.

The paper had a number of flaws in it, based on the source data, but I initially assumed that the statistical analysis was carried out using reasonably widely accepted methods, a conclusion that I am increasingly unsure of.

In an earlier post at Climate Audit the number of PCs used was shown to have a very dramatic effect on the conclusions that could be drawn from the data with Antarctica getting either warmer or cooler depending on how many PCs were chosen (ranging up to 15). And so it seems that it is time to “bite the bullet” and talk a little about what proxies, PC’s and the terms that crop in these discussions mean. (And then to point to the misdirection of a couple of stories where they aren't).


I originally sought wisdom from review of the debate over the use of recognized statistical methods that was performed for the House Energy and Commerce Committee.

The Chairman of the House Committee on Energy and Commerce along with Chairman of the Subcommittee of Oversight and Investigations have been interested in discovering whether or not the criticisms of Mann et al. are valid and if so, what are the implications. To this end, Committee staff asked for advice as to the validity of the complaints of McIntyre and McKitrick [MM] and related implications. Dr. Wegman formed an ad hoc Committee (Drs. Edward J. Wegman – George Mason University, David W. Scott – Rice University, and Yasmin H. Said – The Johns Hopkins University). The Committee was organized with our own initiative as a pro bono committee.

It submitted a report to the Committee on Energy and Commerce.

As part of that report they explained PC analysis thus:

*Principal component analysis is a method often used for reducing multidimensional datasets to lower dimensions for analysis. In this context, dimensions refer to the number of distinct variables. The time series proxy data involved are transformed into their principal components, where the first principal component is intended to explain most of the variation present in the variables. Each subsequent principal component explains less and less of the variation. In the methodology of MBH98/99, the first principal component is used in the temperature reconstruction, and also has the highest explained variance. This method is intended for dimension reduction. In most datasets, the first principal component should be the least smooth (because of the higher variance).

(MBH98/99 refers to the Mann paper that produced the “hockey stick” curve.)

They discuss the effects of temperature change on tree rings, ice cores and coral (of which more anon). but it doesn’t really help those of us who would like this explained in layman’s language. So let me see if I can do that without getting too many folk offended.

When we want to find out the surface temperature of the Earth at some time in the past, and in places where there were neither thermometers, nor folk who kept records, we have to find some other measure that shows what the temperature was. We call these measures “proxies” in part because the behavior of the selected measure approximates the changes in the value we’re interested in. For example, from the list above, the structure, density and width of the ring that a tree grows in a year will vary with the local temperature. And so, by taking a core that recovers the section through a tree that has been around for a long time, we might get an estimate, from the size and structure of the individual tree rings, of the temperature when each ring was formed.

Unfortunately it is not that simple, for example for a Scots pine in Finland.

Earlywood width is controlled by precipitation in June and temperatures in mid winter (December/January) and March. Low mean temperature in April, adequate precipitation in May and a warm July results in wide rings. A long and warm vegetation period results in high latewood density, the strongest correlations occurring with July and August temperature and precipitation.

Tree Ring structure Source NOAA

Since the density and width of the ring can vary from more than one cause, the obvious two cited above are temperature and rainfall, we need to know how much each contributes to the change in the ring. This is not that easy, since the above quote shows that you can’t just use an average value for the year, but have to look at seasonal variables within the year, and this imposes additional variations in the result that we are using as our measure. Further there are some things that may change around the tree that we don’t know about. (from the Wegman report)

Each tree ring is composed of large thin walled cells called early wood and smaller more densely packed thick walled cells called late wood. The average width of a tree ring is a function of many variables including the tree species, tree age, stored carbohydrates in the tree, nutrients in the soil, and climatic factors including sunlight, precipitation, temperature, wind speed, humidity, and even carbon dioxide availability in the atmosphere. Obviously there are many confounding factors so the problem is to extract the temperature signal and to distinguish the temperature signal from the noise caused by the many confounding factors.

Given that complexity the first thing that we need to do is to create a model from data that we already have sufficient information so that we can estimate the impact of changing different values. And this is where our Principal Component comes in. (And for the purists again my apologies I am trying to keep this simple). We can take, for example, data from the last 100 years for a site where we know the variations in temperature, rainfall, relative sunlight etc, and plot the variation in our ring values against these various factors to see which ones best explain the changes in the tree value. The ones that correlate best are called the Principal Components. For example we might find that we explain 60% of the change in tree value by change in temperature, and then when we add rainfall that we have an equation that explains 80% of the variation. By adding further factors (say number of sunny days) we can improve our model so that it predicts with increasing accuracy (smaller variance) the actual values that we found from measuring the rings. So we build our predictive model, adding additional PCs (say soil nutrition) in order of their effectiveness in reducing the variation in the model from the measured data. At some point the improvement is too small to be considered significant, and variables that fall below that value (perhaps insect density) can be neglected.

Having got, or “calibrated” the model, we can then go back from the time and values that we know the PC values for, to look at times when we don’t. Knowing the inter-relationship between the factors and the values, it is possible to estimate what the temperatures were back when the tree rings were grown. (The process is also described by NOAA)

At least that is the basis of the procedure. The problem comes in the complexity of some of the proxies that have been used to assess what the temperatures were back then. To make calculations simpler the data that is used does not use the actual values, but rather looks at the variation in the proxy value from the averaged value over the time period. So that the variation in value from that mean is the measured value against which the models are calibrated. (The process is known as centering the model, since it is the variation from the central value of the data that is used in the subsequent analysis). One of the reasons for doing this is to cope with conditions that are below the average. (By using variation from the mean one can more easily note that a drop in temperature below the average can create a negative growth in the ring that year.) It is important that this be carried out correctly since, when it has not been, and the Wegman report notes that the “hockey stick” paper did not do so, then the results obtained can be flawed.

Our committee believes that the assessments that the decade of the 1990s was the hottest decade in a millennium and that 1998 was the hottest year in a millennium cannot be supported by the MBH98/99 analysis. As mentioned earlier in our background section, tree ring proxies are typically calibrated to remove low frequency variations. The cycle of Medieval Warm Period and Little Ice Age that was widely recognized in 1990 has disappeared from the MBH98/99 analyses, thus making possible the hottest decade/hottest year claim. However, the methodology of MBH98/99 suppresses this low frequency information. The paucity of data in the more remote past makes the hottest-in-a-millennium claims essentially unverifiable.

It also comments on something else that I find quite troubling, namely the very small circle that is formed of the “climate scientists” that publish in this area, and their inter-relationships, with most of the major authors in the field being tied to Dr. Mann – but that is an issue for another day. However, this "clique" does not include many with a strong grounding in statistics, and it is this weakness, and the errors generated in the predictions as a result, which make the papers that the groups generate (and which get significant international publicity) very vulnerable to criticism, which may well be justified.

I want to go back, however, to the comments on coral, since the growth of coral is one of the parameters that are used to assess Climate Change. The Wegman Report noted:

Reef-building corals are strongly affected by temperature and, as temperature drops, the rate of calcification drops with lower temperature potentially presaging the death of the colony. Coral growth rates vary over a year and can be sectioned and x-rayed to reveal high- and low-density bands. High density layers are produced during times of higher sea surface temperatures. Thus not unlike tree rings, data on corals also can be calibrated to estimate (sea) surface temperatures.

I draw attention to that since there is a current interest in coral data, with stories commenting on the “ravages” that will occur to coral with increasing temperature. And yet, even in those stories, dealing with coral damage one finds

Sea level rise, which is projected to occur this century as the world's glaciers melt, would not necessarily kill coral reefs, Tamelander said, since the reefs can grow as waters get higher.

"A healthy reef should be able to keep up," Tamelander said.

. Climate Audit points out the inadequate statistical analysis behind the headline, and then when the statistics are applied the conclusion is wrong.

But then these facts, and the impact of land sinking

"It doesn't matter who's causing global warming. Sea-level rise is something we can measure," said Rob Young, a geosciences professor at Western Carolina University. "You can't argue that sea level isn't rising."

And it has been rising faster in the mid-Atlantic because the land here is sinking. Understanding this phenomenon requires thinking of the Earth as an enormous balloon. Push down in one spot on the ball's surface and surrounding areas are raised up. Glaciers did this to Earth's surface during the last ice age: They pressed down on northern North America and areas to the south tilted up, like the other end of a seesaw. Today, thousands of years after the glaciers retreated, the seesaw is tipping back the other way, and the region from New York to North Carolina is falling about six inches per century.

And this is my gripe for the day – we see anecdotal headlines about how this disaster or that is coming, courtesy of global warming, but these warnings are rarely accompanied by an adequate statistical analysis that shows that the facts discussed are in fact real and correlated to global warming in the manner projected.

One can see this, for example, in the recent comment by Kofi Annan story on the level of death caused by global warming. This is rebutted by the statistics as noted in the Wall Street Journal (Hat tip Irv.) There is no mention of the hundreds of thousands of acres of the sub-Sahara that have been converted to agriculture in the last decade, thereby prolonging, instead of shortening life. Rather those stories are hidden in the headlines of starvation

Oxfam expects cereal production across five countries in the dry Sahel belt south of the Sahara -- Burkina Faso, Mali, Mauritania, Niger and Senegal -- will be a record 18.5 million tonnes this year, but the food on sale will be beyond the budget of many in these, some of the world's poorest countries.