Environmental wire

Aug. 12, 2015

XTO, EnerVest testify to Texas Railroad Commission about disposal wells

XTO Energy and EnerVest executives separately gave testimony during the Texas Railroad Commission's show-cause hearings on whether disposal wells could have played a role in triggering earthquakes that hit Azle in North Texas during 2013-14.

Separately, the commission in June announced it found no link between a 4.0 magnitude earthquake in Venus on May 7 and nearby disposal wells. Previously, regulators temporarily shut five disposal wells within 100 sq miles. Wastewater companies operated those particular wells.

During the commission's show-cause hearings about Azle, XTO Energy of Fort Worth outlined its procedures for a disposal well into which XTO injected wastewater and fracturing flowback from 230 Barnett natural gas wells. Injections into the disposal well reached 18,000 b/d at their peak, showed statistics from XTO, which is a subsidiary of ExxonMobil Corp.

William Duncan Jr., XTO Energy reservoir engineer, said XTO diverted wastewater to other sites when the earthquakes started. More than 30 earthquakes were reported in the Azle area during 2013-14.

The question before the commission was whether the Azle earthquakes were naturally occurring or were induced by human activity.

XTO Energy maintained that a spike in quake activity was naturally occurring, adding that the activity happened miles below where the wastewater was injected into the EllenBurger formation. Earthquake activity was detected in the crystalline basement at least 20,000 ft below the surface.

Andree Griffin, an XTO geologist, said that depth points to "natural tectonism."

EnerVest of Houston and its attorney Brian Sullivan of McElroy, Sullivan, Miller, Weber & Olmstead LLP in Austin suggested that the commission reject a Southern Methodist University study.

A seismology team in a study led by SMU found high volumes of wastewater injection combined with brine extraction from gas wells was "the most likely" cause of earthquakes near Azle during 2013-14.

In an area where the seismology team identified two intersecting faults, they developed a 3D model to assess the changing fluid pressure within a rock formation. They used the model to estimate stress changes induced in the area by two wastewater injection wells and more than 70 gas wells.

While the SMU Azle study added to evidence connecting some injection wells and, to a lesser extent, some oil and gas production to induced earthquakes, SMU's team noted many thousands of injection and production wells are not associated with earthquakes.

University of Colorado researchers study injection wells, quakes

A study by the University of Colorado Boulder and the US Geological Survey said the number of earthquakes associated with injection wells has skyrocketed from a handful per year during the 1970s to more than 650 in 2014.

CU-Boulder doctoral student Matthew Weingarten led the study. He said the increased earthquakes included several damaging quakes in 2011-12, ranging between magnitudes 4.7 and 5.6 in Prague, Okla., Trinidad, Colo., Timpson, Tex., and Guy, Ark.

"This is the first study to look at correlations between injection wells and earthquakes on a broad, nearly national scale," said Weingarten of CU-Boulder's geological sciences department. "We saw an enormous increase in earthquakes associated with these high-rate injection wells, especially since 2009, and we think the evidence is convincing that the earthquakes we are seeing near injection sites are induced by oil and gas activity."

The study determined the injection wells that pumped the most saltwater at the fastest rate were linked to quakes within 9 miles. Researchers found that "high-rate" wells injecting more than 300,000 bbl of wastewater a month were much more likely to be associated with earthquakes than lower-rate injection wells.

A paper on the subject appeared in the June 18 issue of Science.

University of Houston lab measures methane emissions from Barnett shale

Researchers from the University of Houston found some natural gas wells, compressor stations and processing plants in the Barnett shale leak more methane than previously estimated, potentially offsetting the climate benefits of natural gas.

The study was one of 11 papers published in the July 7 edition of Environmental Science & Technology, all looking at fugitive methane emissions in the Barnett shale. The studies were coordinated by the Environmental Defense Fund, with funding from the Alfred P. Sloane Foundation.

Robert Talbot, professor of atmospheric chemistry at UH, noted that CH4, the primary component of natural gas, is a potent greenhouse gas. The paper said methane has a global warming potential over a 100-year time frame as high as 34 times that of carbon dioxide.

Talbot wrote the paper, along with Xin Lan and Azucena Torres, who were graduate students in his UH lab at the time, and former post-doctoral research associate Patrick Laine.

"In the past decade, the horizontal-drilling and hydraulic-fracturing techniques have led to a boom in natural gas production," they write. "However, CH4 emissions associated with the production and transmission of natural gas have raised concern from several parties."

Other sources of methane include wetlands and landfills. The UH researchers measured emissions from a dozen landfills, as well as testing from public roads next to natural gas well pads, compressor stations and processing plants. All testing was done with a mobile laboratory.

The emissions were measured and reported in several ways; the researchers also calculated measurements to gauge what percentage of gas produced escaped via emissions.

Releases at specific installations ranged from 0.01% to 47.8%, and the median was 2.1%.

Methane releases from compressor stations and processing plants were considerably higher than that at the well pads, the researchers reported.

Some emissions can be attributed to human error, compounded by the fact that the sites are often left unattended for long periods of time.

"A lot of them are a broken valve, or someone leaves a hatch open," Talbot said. "It's human error. And nobody goes back to the site for a month or so."

In all, researchers tested 152 facilities during a 15-day test period. They tested 125 well pads, 13 compressor stations, two gas processing plants, and 12 landfills.