OREANDA-NEWS. February 18, 2016. Oil and gas producers in northwestern Oklahoma must reduce their drilling wastewater well injections by more than 300,000 b/d to curb potentially damaging earthquakes, according to state regulators.

The announcement followed a 5.1 magnitude earthquake on 13 February, the largest since 2011.

The plan applies to a 5,281 square-mile area and is the broadest effort so far to restrict the injection of produced water in disposal wells. The Oklahoma Geological Survey (OGS) said it believes a recent increase in earthquakes is linked to increased injection well volumes.

The most recent reduction is in addition to a previously announced 191,000 b/d reduction in the same region near Fairview, Oklahoma. The two plans will result in a total volume cutback in the region of more than 500,000 b/d or about 40pc of the 1.25mn b/d of wastewater injections, said the Oklahoma Corporation Commission, which regulates the industry.

The plan will be phased in over two months as recommended by researchers, who caution against sudden pressure changes.

The area includes parts or all of the Oklahoma counties of Alfalfa, Grant, Garfield, Major, Woods, Woodward and Harper. It will affect 245 disposal wells in the Arbuckle formation, which pose the highest potential risk for causing damaging earthquakes, the commission said.

A sharp increase in earthquakes in Oklahoma has coincided with a surge in oil and gas drilling activity in recent years. The OGS concluded last year that that the majority of earthquakes in Oklahoma are likely triggered by the injection of produced water in the wells. Researchers in Texas have made similar connections between the increased frequency of earthquakes and injection wells.

The number of earthquakes in Oklahoma above a magnitude 3.0 increased to more than 900 in 2015, compared to about 580 in 2014 and about 100 in 2013.

Earthquakes near Cushing, Oklahoma, have been of special concern because it is a major crude storage hub, with about 64.7mn bl of oil stored there currently, according to the US Energy Information Administration.

The 5.1 magnitude earthquake on 13 February was 21 miles (34km) northwest of Fairview. It was the largest in Oklahoma since a 5.6 magnitude earthquake in Prague in 2011, according to the US Geological Survey (USGS). The Prague earthquake was the largest ever recorded in the state.

The Oklahoma Corporation Commission has previously taken action after earthquakes above a magnitude 4.0, instructing producers to shut certain wastewater wells and cut back on others in the region. But the plan released today is the largest volume reduction plan so far.

"There is agreement among researchers, including our partners at the Oklahoma Geological Survey, that the data clearly underscored the need for a larger, regional response," said director of the state's Oil and Gas Conservation Division Tim Baker

A USGS official told Argus in December that it was advising Oklahoma to distribute the oil and gas wastewater injection wells over a larger area that is far from major infrastructure, such as the Cushing storage facility, and large urban areas such as Oklahoma City.

The Oklahoma Corporation Commission said today that its staff "continues to work on other areas of the state."

The Sierra Club and Public Justice filed a federal lawsuit today against three producers who drill in Oklahoma alleging that they are contributing to the earthquakes. The lawsuit filed in the US District Court for the Western District of Oklahoma alleges that Chesapeake, Devon Energy, and New Dominion have violated the Resource Conservation and Recovery Act and "are placing people and the environment in Oklahoma and Kansas at significant and immediate risk from major man-made earthquakes."

The lawsuit urges the court to order the companies to immediately reduce the amount of production waste they are injecting into the ground.

New Dominion said it does not comment on pending litigation. Chesapeake and Devon did not immediately respond to requests for comment regarding the lawsuit.