Never mind the current demand for the stuff — what becomes of the void oil drilling creates? If we pump out millions of barrels per day, we can't very well leave giant empty caverns, can we? Is the planet on the verge of caving in or what?
— — Dan, Long Island
Caving in? Please, let's not panic. The fundamentals of our planet are strong. Besides, we're talking about a liquid here, and it's pretty far down. Wholesale removal of solids such as coal or salt close to the surface, now ... no question that can lead to exciting times in the immediate vicinity. Still, even if it's oil at the end of a really long straw, suck out a trillion barrels (roughly what the Exxons of the world have pumped up to date) and you're going to see an impact. I don't say it ranks with the collapse of the world financial system, but 100-plus years of serious petroleum extraction have caused the earth to appreciably shrivel up.
About those voids: The familiar image of a spurting oil gusher may give the impression petroleum is stored in some kind of immense underground spray can. Not exactly. Sometimes there's pressure; compressed natural gas often found in petroleum reservoirs may force the crude to spew after the initial strike. But typically the oil isn't sitting in a subterranean lake but rather in sand, shale, or porous rock. Once the oil is removed, the weight of the earth above flattens out what's left and the land sinks — this is called subsidence. The same thing can happen with groundwater pumpage, as we discussed a while back.
Usually, subsidence can be detected only via sophisticated instruments — one New York investment firm claimed to be able to gauge the depletion of Saudi petroleum reserves by using satellite-based radar to monitor subsidence over Arabian oil fields. But sometimes it can be easy to spot, particularly in the case of shallow, uncompacted reservoirs in coastal areas. Parts of Long Beach, California, for instance, dropped almost 30 feet in the 1940s and '50s due to oil production. The Goose Creek oil field near Houston was a couple of feet above sea level when production began circa WWI; now sizable parts of it are underwater. (Subsidence due to groundwater removal has also been a problem in the Houston area.) And in Venezuela dikes had to be built to prevent flooding of sunken lands around the oil fields at Lake Maracaibo.
One way to control subsidence is to pump something else in after you've pumped the oil out. Following initial extraction of crude, oil companies may flood the reservoir with water to flush out additional petroleum; this process has largely halted subsidence in places like Long Beach. Now experts are trying the same trick using carbon dioxide, both to increase oil production and to sequester greenhouse gases. The Canadians have been stuffing their Weyburn oil field with CO² since 2000, planning eventually to store 30 million tons. Reduced subsidence isn't the primary purpose, but I figure it can't hurt.
It's the removal of solid stuff, though, that you really have to worry about. Some coal mines practice what's called longwall mining, where the roof is allowed to cave in behind the miners as they dig out the coal in front of them. In principle the resulting subsidence is easier to predict and plan for, but still it often causes serious damage as nearby buildings, roads, train tracks, and pipelines sink four to six feet. (Mine owners are supposed to pay compensation.) And worse things can happen. The hamlet of Centralia, Pennsylvania, was rendered uninhabitable after a coal seam underneath it caught fire in the 1980s, opening up steaming hot sinkholes filled with toxic gases. The fire's still going and could burn another 250 years.
Salt mines are a special case: They can slowly squeeze shut as the salt deforms beneath the weight of the rock above, or they can collapse when leaking groundwater dissolves the salt pillars holding up the roof. The Retsof Salt Mine in upstate New York is a prime example of the latter problem: its 1994 cave-in led to gaping sinkholes, a wrecked bridge and utility damage. But for sheer breathtaking stupidity I don't see anything beating the disaster at Lake Peigneur, Louisiana. In 1980, oil drillers in the lake accidentally pierced the roof of the Diamond Crystal Salt Mine 1,200 feet below, ultimately collapsing the bottom of the lake. In the spectacular whirlpool of destruction that followed, 65 acres of land, 11 barges, two drill rigs and a tugboat (but, miraculously, no people) literally went down the drain.
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