Wouldn't fracking (removing heavy oil out of recesses in the earth, leaving gaping open spaces once the water drains) create more areas in the crust
for collapse once quakes hit?
First: no, I'm not in the oil industry nor does my family depend on the oil industry. I did become a geophysicist by both education and employment
but voluntarily left that behind to pursue a more personally interesting field: semiconductors.
It's a common misconception that oil is found in pools or lakes under the ground. That's not the case. Oil is trapped in solid rocks which allow
the oil to seep slowly through them. Sometimes oil is trapped in more porous rocks where it can move more easily. Depending on the type of rock,
and the porosity of the rock (the ability of water to move through them), one can either remove a larger or smaller percentage of the oil contained in
the rock. In most cases, the vast majority of the oil remains trapped in the rock because the oil is too thick for the pores or the pores do not
connect well enough for the oil to move.
If you have a sandstone honing tool to sharpen knives, you can see how this works for yourself. Add a drop of honing oil to it, and some of it will
sink into the sandstone. That oil is very light -- much lighter than oil in wells. Anyway, most knife sharpening kits provide a fine Arkansas
sandstone which is not very porous. So not much oil gets into the sandstone, but some does. This sandstone is like the rock that we find oil in --
and it's very difficult to get the oil out. Imagine trying to remove all the oil from the Arkansas sandstone and you will see the problem. It's
not going to happen, but you can get some.
There is always pressure from the overburden (the rocks above weigh a great deal) and sometimes there is pressure from trapped gas too. Just like
oil, gas can be trapped in the rock. Drilling a hole into the rock creates a space for the pressure to push the oil into. That happens more rapidly
in formations with high porosity (fluid moves more easily) than low porosity (fluid moves slowly if at all). In all cases, some oil sticks to the
rock and some of the pores clog up with oil. The rock layer itself does lose some density and will often be compressed a little by the overburden as
the oil is removed. This is where subsidence after oil removal (same as water from water wells for drinking) comes from. If you return fluid, like
water, there is less subsidence.
To get more of the oil out, there are only a couple of different things which can be done (cost is a big constraint). Sometimes hot water is injected
to increase the pressure and improve the flow -- hot oil flows more easily than cold oil. Place a jar of oil in your fridge and see how this works
yourself. The injections can also locally increase the pressure in the formation, repalcing the lost "drive" as the oil is removed. Pressure is
important because only very high pressure (many thousands of pounds per square inch) can cause the oil to move. Oil does not like to move through
rocks very much!
Other times the porosity isn't very good or the pores get clogged up. The rock can then be fractured around the drill hole by applying a high
pressure fluid. The heavier the fluid, the easier it is to make it high pressure (gravity helps do the work). In many cases some material, something
like a sand, will be added to the fluid to help prop open the fine cracks. Sand does not help make the fluid heavy (it's light) so there are other
"engineered" materials which can be used but they are usually more expensive. Anyway, the fractures do not extend very far from the borehole, but
there can be 10 or more boreholes in the same surface well if there is enough oil to make it worth drilling that many.
Now, we do expect some bulk movement due to subsidence. If any of the overburden, or the oil bearing formation cracks in a major way, there can be
a small EQ. Because fracking mostly occurs in the immediate neighborhood of the borehole, the only way that fracturing could influence a fault would
be to add lubrication along the fault itself. Large EQs require "breaks" possibly hundreds of mile long..... so influencing a major fault isn't a
local thing at all. Also, keep in mind the formation we are taking the oil out of is already saturated with a fluid (oil) -- and oil is a lot better
lubricant than mud or water.
It is still possible, at least theoretically, and a correlation with local small EQs has been claimed to have been observed. I don't know of any
experiments though where the conditions were controlled to start and stop local, small EQs at will by adding and removing fluid. Any other claim
which is based merely on timing or number of events over a short history (not hundreds or thousands of years of equally sensitive observation) is just
Many oil bearing formations do have oil trapped against an old, local fault of some type and limited extent. It would be very difficult to decisively
know, without an experiment, that a small EQ in the area of such a well was due to fracturing the rock as opposed to subsidence or even natural EQs.
Most oil fields are not near faults with large historical EQs and there is not even a short seismic history to refer to. We can get suspicious, but
if we are paying more close attention or adding a seismometer because there is fracking in the area we can easily mistake natural occurrences from
fracking related occurrences of EQs. The only way to know for sure is to do an experiment where small EQs can be started and stopped at the will of
the experiment. Physics doesn't favor fracking as a possible "cause" of an EQ big enough to do much damage, but it also doesn't totally rule it
out either. I'd personally suspect subsidence first if there is no prior history. Then again, faults exist because something in the past caused the
fault to form..... so no prior history even over a hundred years is less than meaningful evidence per se. You need to know something about frequency
of occurrence, and magnitude distributions, to know what you think you are observing is rare.
I have no doubt speculation is rampant about fracking, but the reality is most people passing on these stories have no clue. The few who might have
the knowledge of the basic science (a geophysicist but not a geologist or hydrologist) could easily have other motivations or a grudge against a
particular company. I don't follow this area because it's mostly uneducated nonsense, so I can't point to individual cases or people. But I'm
well aware that there has been, since the 1980's, an almost continuous contraction (reduction) in the total number of geologists and geophysicists
in US companies through mergers and decline of exploration -- read, lots of jobs lost due to layoffs. Alternative jobs for geologists and
geophysicists, who are no longer wanted by the companies, usually pay much less and often are environmental in nature. These then, are a natural
source of opponents of fracking. I'd check the credentials and read between the lines before blindly believing a theory that is not published in
the journals. And without an experiment providing statistically significant evidence, I'd consider any journal publication very weak evidence --
basically speculative theory only. Not real science, yet, despite the claims.