In recent weeks, the number of earthquakes felt in Oklahoma has decreased compared with this time last year, which might reflect measures state officials took earlier this year to severely limit the volumes of oil and natural gas wastewater injected into deep disposal wells, according to the Oklahoma Geological Survey.
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"The incidence of earthquakes is down, and that's attributed to decreased injections," OGS Director Jeremy Boak said in an interview.
Year-to-date the state has seen 403 earthquakes of magnitude 3.0 or above, which puts it on pace to see fewer earthquakes of that magnitude than last year, when Oklahoma saw 907 temblors of 3.0 or higher. However, the current year is still likely to see more quakes than in 2014 when the state saw 569 such earthquakes.
"We believe most of the earthquakes that have been happening since 2010-11 have been induced, although there are a few specific examples where we're not really sure yet," Boak said.
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"In general most of the substantial earthquake activity is due to induced seismicity of deep injection of produced water," he said.
If the upswing in earthquakes was due to injection, then the recent downturn in earthquake activity can be traced to the injection decrease, he said.
Boak said there are about 1 million fewer barrels a day of wastewater being injected than before the Oklahoma Corporation Commission instituted limits on the volume of water being injected into the deep Arbuckle formation in a 10,000-square-mile area of interest, which covers much of the western and central regions of the state.
But while Boak said the decrease in earthquakes could be attributed to the decrease in wastewater injections, operators had begun to inject less water into the ground long before the state acted to compel them to do so.
The peak of wastewater injections occurred in 2014, and for most of 2015, injection volumes had been dropping, he said. This corresponded to a decrease in drilling across the state, due to lower oil and gas prices.
"I would suggest that this started ahead of Corporation Commission's actions, so we know that some of it was market-driven. In other words production was being shut in and therefore injection was declining," he said.
Boak estimated that much as three-quarters of the decline in injections over the past year and a half or so was being driven by market forces.
In February, the OCC's Oil and Gas Conservation Division implemented an injection volume reduction plan for oil and gas disposal wells in western Oklahoma, which covered 5,281 square miles and 245 disposal wells injecting wastewater into the Arbuckle formation.
Then in March the division announced its plan for reducing injection volumes in the central part of the state, which covered more than 5,000 square miles and more than 400 Arbuckle disposal wells.
Taken together, the goal of the two plans is to reduce the total volume of wastewater disposed in the area to 40% below 2014 total disposal volumes.
Boak said it is too early to gauge the impact of the Corporation Commission's disposal targets on actual disposal volumes.
"We're trying to finalize some of those numbers. We're working with the Corporation Commission to get that worked out for the well set for which we have the injection data available," he said.
Average injections of wastewater in the area of interest are now about 800,000 b/d below 2014 levels, OCC spokesman Matt Skinner said in an interview. This reflects 100% compliance with the OCC injection limits from the oil and gas operators in the area, he said.
"We have not had anyone out of compliance; everyone in full compliance. Our target was to get 40% below the totals for a particular operator. Each one was assigned a new limit," he said.
Skinner added that the OCC is likely to announce additional injection volume cutbacks, depending on whether the OGS thinks they are necessary to reduce the continuing risk of earthquakes.
"The Oklahoma Geological Society is our partner. We're joined at the hip on this. We're the people with the hands on the buttons, they're the ones who tell us whether or not what we're doing is good, bad or indifferent," he said.
"In no way do we consider these volume cutbacks the final step. This has been an evolving process to get us to here and there are more steps that need to be taken to get us to where we need to be. Just what those steps are, we'll see," Skinner said.
--Jim Magill, firstname.lastname@example.org
--Edited by Keiron Greenhalgh, email@example.com