OKLA. CITY —
A state agency says misinformation related to the debate about the cause of more earthquakes across Central Oklahoma includes oil well types, well numbers and injection pressure.
The Prague sequence of 2011 along the Wilzetta Fault zone included a significant foreshock, a main shock of magnitude 5.7 and numerous aftershocks. It has been suggested that this sequence represents tremors triggered by fluid injection.
More recently, earthquakes have been recorded in the vicinity of Jones, Arcadia Lake, Edmond, Guthrie, Langston and Crescent. Regulators and scientists are working together to better understand what’s causing all the shaking.
The Oklahoma Corporation Commission, which regulates the oil and gas industry, and the Oklahoma Geological Survey are collecting data in an effort to determine if man-made or natural causes are behind the increased seismic activity.
OCC spokesman Matt Skinner said one of the common misperceptions about the issue is the type of well involved. Skinner said the possible culprits being studied are the state’s roughly 4,000 saltwater disposal wells.
Disposal wells inject brines and other fluids associated with the production of oil and natural gas or natural gas storage operations, according to the U.S. Environmental Protection Agency. When oil and gas are produced, brine is also brought to the surface. The brine is segregated from the oil and then injected into the same underground formation or a similar formation. Disposal wells represent about 20 percent of Class II wells.
Oklahoma has about 4,000 total with 53 of those existing in Oklahoma County and 83 in Logan County, Skinner said.
Skinner said another common misperception is that all disposal wells inject at very high pressure.
“Not so,” he said. “In fact, there are some in Oklahoma that have no injection pressure at all, although they’re still classified as a Class II injection well. They operate on a natural vacuum. Others need only low pressure to get the wastewater to the injection formation.”
Skinner said another misperception is the belief that the average depth of disposal wells is 20,000-25,000 feet. Skinner said the average depth is more along the lines of 2,500 feet, with a few being deeper.
Recently, motivated by a desire to gain more real time data, the Oklahoma Corporation Commission approved a rule requiring daily reporting of injection well mechanical integrity testing and other data-gathering requirements. Previously it was being reported on a monthly basis. The Legislature and governor must both approve the rule.
Via position statement on triggered seismicity, state geologist G. Randy Keller said it is possible there have been incidents of “triggered seismicity,” or manmade earthquakes, in Oklahoma during the past few years.
Keller stated that “triggered seismicity” is more accurate than “induced seismicity.” It means stress released in an earthquake was accumulated through natural processes, but inadvertently caused by human activities.
“If cases of triggered seismicity are identified, our goal is to adequately and thoroughly document and research these cases, so that proper steps can be taken to mitigate the likelihood of future triggered seismicity,” Keller stated.
Skinner said when earthquakes occur the OCC is plotting them on a map as part of the research effort.
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