Scientists investigate whether drilling, Oklahoma earthquakes are linked
11/12/2011 12:00 AM
11/12/2011 11:42 PM
Scientists were swarming around central Oklahoma last week planting seismometers and digging through records in an effort to answer the question of the moment: Did oil and gas fracking cause last week’s unnervingly strong earthquakes?
On the face of it, it seems ridiculous that the small amount of activity involved in drilling a well could unleash the vast energy that shook the ground across seven states. The tremors might, in fact, turn out to be purely natural events. Oklahoma has geological faults and a history of earthquakes. But while scientists doubt that the fracturing of rock around a well to unleash oil and gas by itself could cause earthquakes as powerful as the 5.6 magnitude Oklahoma temblor, the deep injection wells associated with oil and gas fracking, along with mining and other industrial uses, are believed by scientists to have caused earthquakes nearly that powerful.
There have been 30 earthquakes in Kansas, mostly in the north central part of the state, since 1973. Oklahoma had a similar rate until 2007, according to the U.S. Geological Survey. In 2008 Oklahoma had more than a dozen earthquakes, nearly 50 in 2009 and even more and more big ones since then, and they tended to be clustered in the area between Oklahoma City and Tulsa.
Last week’s earthquakes and aftershocks are centered in rural Lincoln County in an area about 30 miles east of Oklahoma City. Scientists are in the area trying to pinpoint the exact epicenters, the location and size of wells in the area, and a precise grasp of the geology They want to see if any of the recent activity can be traced to a man-made cause.
“We are very much interested in answering that question,” said Art McGarr, a seismologist at the USGS’s western center in Menlo Park, Calif. and an expert on man-made earthquakes.
He said the agency hopes to have a preliminary answer within a couple weeks. The National Academy of Sciences is studying the seismic effects of energy drilling and mining and will issue a report next spring.
Injection wells are designed to make millions of gallons of wastewater go away forever. The wells take waste fluids generated by drilling, mining and other industrial type uses all day and all night, for years, pushing the fluid into tiny pores and cracks in rock thousands of feet underground. In one horizontal oil well that was recently drilled in Barber County, a contract fracking crew with a fleet of massive pumper trucks forced 4,200 gallons a minute of water - enough to fill an average swimming pool in eight minutes - into the well. The water pressure cracks the rock in the oil or gas bearing layers to allow flow. Most of the water eventually is eventually pumped back out along with the oil or gas. The fluid is separated out and pumped to a wastewater injection well.
There are there are 181 injection wells In Lincoln County, according to Matt Skinner, spokesman for the state agency that oversees oil and gas production.
The link between injection wells and earthquakes has spurred action elsewhere. In July, the Arkansas Oil and Gas Commission voted to shut down existing injection wells and impose a moratorium on new ones in more than 1,000 square miles of a gas field north of Little Rock. Based on scientists’ testimony, the commission decided that the hundreds of recent earthquakes, including ones of 4.7 and 4.1 magnitude, were caused by the weight and pressure of wastewater from fracking. Fracking itself wasn’t banned, but the move required drilling companies to truck the wastewater out of the area.
The science connecting injection wells and earthquakes is pretty solid, McGarr said. There are documented case in Texas, California, Ohio, England, Germany and Switzerland. In one of the best documented cases, a deep injection well at the Rocky Mountain Arsenal near Denver caused a series of earthquakes in the 1960s. What’s particularly interesting is that scientists say the fluid migrated about six miles from the site before causing an earthquake of 4.85 magnitude and the earthquake happened months after further injection of fluids was stopped.
There is also scientific evidence that the fracking itself causes earthquakes, but nothing of the size of what happened in Oklahoma last weekend. A recent study by seismologist Austin Holland, a seismologist with the Oklahoma Geological Survey, said that it’s possible that hydraulic fracking caused a series of small earthquakes, peaking at 2.8, in an area south of Oklahoma City earlier this year. When lots of liquid is injected into the ground it changes the stress and pressure in a place that probably already was a fault, Holland said. It’s similar to injecting water between two adjacent bricks, it allows them to slide more easily and "the water under pressure is helping push the bricks apart ever so slightly," Holland said.
But Holland doesn’t believe fracking caused the big Nov. 5, 6 and 8 earthquakes. He compared a man-made earthquake to a mosquito bite.
"It's really quite inconsequential," Holland said.
What’s that mean for Kansas?
Geology is very different in different places, so the interaction between water and rock is quite different. As McGarr noted, there are tens of thousands of injection wells in the country, including many in Oklahoma, and nothing has ever happened.
Kevin White, senior vice president of SandRidge Energy, an Oklahoma City company with nearly a million acres of leases in northern Oklahoma and southern Kansas, said he didn’t think the issues has much application to Kansas. The oil-bearing formation in northern Oklahoma and southern Kansas, the Mississippian limestone, is different than the shale formations that have generated much of the earthquake concerns. Their wastewater goes into a formation called the Arbuckle in virtually unlimited quantities without increasing the pressure.
“It’s a 1,000 feet thick and it just takes the water like a sponge,” White said.
He did say that if producers were no longer able to use injection wells, it would raise the cost of production. But, he said, at today’s prices, it’s not enough to keep his company from drilling.
The problem for many place, though, is that the places that are prone to earthquakes are also places where oil and gas flow along fractures, experts said. In some studies, scientists have taken earthquake data and, like detectives, tracked its causes to deep injections of lots of liquid under high pressure, such as ones that peaked at magnitude 3.3 at the Dallas-Fort Worth airport in 2008 and 2009, said USGS geophysicist William Ellsworth.
"How big an earthquake might we trigger? That is an open question at this point," Ellsworth said. "We do know we can trigger magnitude 5 earthquakes."
Contributing: the Associated Press
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