Exploration

OU study turns scrutiny toward deep faults that cut into oil and gas zones not bound by earthquake regulations

By Corey Jones Tulsa World – A study examining why Oklahoma is susceptible to man-made earthquakes has documented faults in the state’s granite “basement” that cut upward into sedimentary layers not subject to seismicity mitigation regulations.

State regulators have imposed wastewater restrictions on disposal wells in the Arbuckle formation — the deepest layer of sediment — in a 15,000-square-mile area. However, those limitations don’t apply in shallower zones such as the Simpson formation.

Science has pointed to a hydraulic connection between the fault-laced basement and the Arbuckle but not whether structural pathways that can transmit wastewater extend above the Arbuckle. That is a key question because some operators began plugging backward into the Simpson from the Arbuckle as more stringent volume and rate limits were implemented by regulators.

“The existence of such fluid migration pathways is an essential component for fast earthquake triggering by wastewater injection,” the study states. “For example, the foreshocks leading up to the 2016 magnitude 5.8 Pawnee earthquake were near-instantaneous responses to variations in injection rates at disposal wells within a 20 km radius of the mainshock.”

The research, conducted by the University of Oklahoma, was published last week in Nature Geoscience. Brett Carpenter, an assistant professor in the College of Earth and Energy, oversaw the project led by former graduate and undergraduate students.

Carpenter said he would like to see proactive action with robust volume reporting to better study the situation, as well as use of an expert to interpret structures that might be sedimentary faults before regulators permit wells in the Simpson.

“As a state we have not changed our usual method of wastewater disposal,” he said. “We inject it in subsurface formations, and distributing it in a different formation might not represent the best longterm mitigation strategy.”

In a statement, the Oklahoma Corporation Commission said it wasn’t contacted about this research.

“OCC actions that have played a key role in sharply reducing Oklahoma’s seismicity rate have always been based on the science that has resulted from research into induced seismicity,” wrote Matt Skinner, the agency’s spokesman. “The OCC has worked closely with the Oklahoma Geological Survey at the University of Oklahoma and supplies data to all researchers who request it, and will continue to welcome any presentation from researchers who wish to present studies they think are of value in our continuing efforts regarding seismicity.”

Jake Walter, state seismologist for the Oklahoma Geological Survey, said he has moderate concern but that the study isn’t mature enough at this point to inform a regulatory response. He characterized the analysis as in its early stages of whether Simpson injection can drive induced seismicity.

Walter said some fault pathways might represent “super highways” for wastewater to go deeper, while others could present barriers that block fluids from traveling farther down.

“Simpson injection and the link to possibly inducing earthquakes is a topic of current study by the OGS,” Walter said. “And before that happens, I don’t think that the OCC is inclined to direct any change in reporting requirements.”

Carpenter credited swift Corporation Commission action with likely “preventing or stalling out” large aftershocks from the Pawnee quake. However, he holds concerns about the long-term potential for damaging seismicity given that deeper faults can hold larger earthquakes.

Carpenter said market forces — or depressed hydrocarbon prices — were “huge” in dictating the drop in wastewater injected into the Arbuckle. There was an inflection point in early 2015 in which Arbuckle volumes began sharply decreasing and Wilcox volumes started a slower but steady rise.

“But the market force was huge in determining how much (wastewater) was thrown down there, so what happens when the market force goes the other way?” Carpenter asked.

Carpenter noted the Simpson volumes are “radically lower” than the Arbuckle’s peak and haven’t offset the Arbuckle decline. However, the Simpson data lag a year behind, leaving uncertainty.

Arbuckle volumes are recorded daily and reported weekly to keep a pulse on potential seismicity concerns and study the matter in greater detail. Simpson operators only record monthly and report yearly.

“We are playing catch-up on that front,” Carpenter said. “How much (wastewater) is going into that particular reservoir?”

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