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45 comments on Saudi Arabia's Ghawar Isn't Sinking (but has apparently moved)
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k Nation (Jim Kunstler)
GAIA Host Collective
Re: Using the the technique of Synthetic Aperature Radar (SAR) Interferometry, which is capable of measuring millimeter vertical movements via satellite, they found not subsidence but actually a slight rise in one area
Is such a surface rise possible? I know that subsidence over old oil fields has been observed:
http://pubs.usgs.gov/fs/fs06903/
My guess is that water injection would be the likely cause but Ghawar being at nearly 1500-2000 m deep, pressure from the above ground is probably around 1000 bars.
Well if subsidence is possible from removing something, then it is reasonable that putting something back in would restore it.
Back in the late 80s, when production in Haradh was shut down, pressure started rising back up (naturally, since it was not being water injected as it was produced) but then had some strange pressure drops in wells across the field. This has been interpreted as fractures -- which had been shut off as the oil was produced -- re-opening during the stoppage, temporarily reversing the pressure buildup. Of course, it is not know if this translated into anything measurable on the surface, and no unusual seismic activity was noted.
The idea is interesting, but ECREE (extraordinary claims require extraordinary evidence). If you postulated subsidence and then measured a (growing) depression shaped like Ghawar, you might have something. If you found nothing, most likely your hypothesis is wrong. If you instead see a slight rise that includes part of Shedgum but doesn't conform at all to where Shedgum is, your hypothesis is likely still wrong. Getting to enamored with one's idea (that any rise or fall is due to Ghawar subsidence or inflation) and then tying the fate of Ghawar to what you measure is amazingly delusional.
Dr. Joules, that completely ignores Khebab's point. That is the weight of the overburden rock. It is a lot easier to pump oil or water out that is under tremendous pressure than to push it back in. It is a lot easier to let 7000 feet of solid rock fall than it is to lift it.
Injection wells, from a link I posted yesterday, have an injection pressure of about 1500 PSI. Some of them in Ghawar may be slightly more but that is not even close to the amount of pressure it would take to lift 7000 feet of rock. Khebab says it would take 1000 bars, or about 14,500 psi. Someone on Drumbeats yesterday said it would be slightly less or about 1 psi per foot. But at any rate, 1500 PSI is not even near the pressure it would take to raise such a heavy load.
I would take the position that lifting 7000 feet of rock with an injection pump is an impossibility.
Ron Patterson
My understanding is the lifting force is actually due to intergrain water sheets, which move by capillary action. This force is incredibly powerful--it can considerably multiply the measured pressure.
Another thing to consider is the solute potential of material left in the rock. Even injecting sea water, if there are water soluble compounds (sulfur, acids, iron, etc.) in the field the water activity will want to equilibrate. Water will flow from a dilute solution to a concentrated solution until the concentration of solutes is balanced or the sources migrate away from the rock.
Plants grow through this mechanism. They raise the solute potential in cells (using any small molecule, salts, sugars, acids, proteins, etc), water flows in from outside, raising the internal pressure and forces the cell walls to physically deform/stretch in one direction along a fault zone in the cell walls. The power of osmotic potential can be very large, getting into the 1000's of PSI. Plant roots break granite rocks apart using this mechanism as they grow. I am not saying this mechanism is happening in this field, just that it shouldn't be discounted in a water drive oil field.
according to wiki, this is the mechanism at work in ekofisk, mentioned below. subsidence due to solution cavitation. i have never heard of this happening in ghawar, but if bernstein is to be believed, it is not happening.
Dr Joules,
It is not uncommon for oil fields to subside when the oil is removed though this does not always happen. A famous example is the Ekofisk Field in the North Sea where sea bed subsidence meant they had to reconstruct the platform which was in danger of sinking to within the reach of large waves.
I think the main processes here are depressurisation combined with the removal of bouyant oil that kind of holds things up - the pressure and bouyancy are of course both related.
Like Ron, I very much doubt that the Ghawar structure could be pumped up by water injection. The Arab D is very well connected down dip (unless it is sealed by a tar mat) and so injected water would rather travel down into the "infinite" volume of the aquifer than raise the ground.
It is well known that the Arabian Plate is experiencing compressional tectonics. It is in fact that compression that has given rise to the large structures that the oil is trapped beneath. I heard at a talk many years ago in Baharain that this compression and structural growth continues to the present day. Thus if measurable changes in land height have been made I imagine this might be one cause.
Euan (and Ron),
Thanks for the comments. I don't believe in Ghawar inflation either, I'm just letting my mind wander. I definitely agree that the water would go sideways, especially since this happened in Harmaliyah.
I'm no geologist but I know that filling aquifers can and do lift the ground above and that this can be measured with InSAR.
I'm no geologist either but I think aquifer depths are usually on the order of tens to hundreds of meters.
Dr Khebab, I'm not sure the focus on Ghawar rising or falling should necessarily be the focus of this debate, since I can't find my copy of this report, and don't really know the details.
The important variable in all this is pressure - hydrostatic pressure, lithostatic pressure and over pressure in the reservoir. My understanding is that Ghawar is "normally" pressured - i.e. the fluids are at some point in contact with the surface, and hence they are not supporting the rock formation. So removing the fluid or pumping in more fluid should not affect the rock strata - that much.
In some reservoirs, the fluids actually support the rock - and here by definition the fluids will be over-pressured. Removing them will cause the rock formation to subside. If the fluids supported the rock then this means there are seals in the sub-surface - and pumping water back in may pump the strata back up.
The other consideration is gravity - lunar and solar cycles do have detectable influence on sub-surface fluid pressure. And so I imagine that changing the fluids may affect the gravitational response of the surface to tides - but you'd need to consult a rocket scientist to get a concrete answer here - I am a humble geologist.
We need to know more about the InSAR analysis that has been performed and whether that has been done correctly. The main source of error in InSAR is the changing water vapor content in the atmosphere, which changes the path-length between the ground and the satellite (refraction). If they've used a long time series of images, this should not be a problem, but if they used only a few scenes and the operator was incompetent, the patterns could be all artefacts.
Since PSI-measurements are relative and local, lunar and solar tides don't matter much at all as they move everything up or down, as do changes in air-pressure (space geodesy is weird).
BTW I've seen a presentation of PSI analysis over some much smaller oilfield in Saudi Arabia, there the ground subsidence was clearly detectable.
To focus on some of the things we know for sure;
Northern Ghawar (Ain Dar, Shedgum and Uthmaniyah) have higher porosity than southern Ghawar (Hawiyah and Haradh).
The north was developed first and has had considerably more oil removed than the south.
Ain Dar is likely near "totally" depleted of dry oil. The same applies to N Uthmaniyah and much of Shedgum. This means that the reservoir now contains mainly a mixture of oil and water in varying proportions.
Haradh and Hawiyah are likely still mainly full of oil.
Further reading:
http://www.theoildrum.com/node/2462
http://www.theoildrum.com/node/2494
http://www.theoildrum.com/node/2507
and by Stuart:
http://www.theoildrum.com/node/2470
http://www.theoildrum.com/node/2563
Should you want to contact any of the staff here who do not have public email they can be reached on:
theoildrum@gmail.com
One qualification here - the 1300 ft oil column in Ghawar does create a massive amount of bouyancy pressure which could just intersect the lithostatic pressure gradient. I'd need more time than I have right now to run up some charts on this but gotta presume that the pros at Bernstein have these angles comprehensively covered.
Euan, any upward pressure caused by oil bouyancy would have had to have happened millions of years ago, when the oil first rose and displaced the water above it. As the oil is pumped out and replaced with water, that bouyancy would be removed.
Remember it always had water in the reservoir, then oil replaced the water and now water is replacing the oil again.
Ron Patterson
it is true that in some cases the fluids support the overburden, geopressured areas.
however the pore volume of all rock is compressible. on the order of 4 x 10^-6 vol/vol/psi and much higher for unconsolidated rock.
hypothetically the removal of 60Gb from ghawar, or about 84 Gb or reservoir volume, could result in subsidence. still the lack of subsidence does not disprove depletion.
depletion is occuring at the rate of 5 million bbls per day.
i think bernstein must have got this idea from a dittohead.
water injection would normally go sideways, either through porous rock or hydraulically created (vertical)fractures, but interestingly, the places where hydraulically created horizontal fractures would occur is in areas of compressional tectonics(black warrior basin of alabama,for example).
the frac gradient for a vertical (hydraulic) fracture is on the order of 0.7 psi/ft. the frac gradient for a horizontal fracture is on the order of 1 psi/ft, implying uplifting of the overburden.
it was my understanding, however that the rift which resulted in the generation of so much oil in the mideast was an extention of the north sea rift. further, i thought that in a rifted area, the rock was undergoing extention.