Antallet af jordskælv som er forbundet med injektionsbrønde er røget til vejrs fra en håndfuld per år i 1970erne til mere end 650 i 2014, ifølge doktorgradskandidat Matthew Weingarten fra CU-Boulder, som ledte undersøgelsen. Stigningen omfattede flere ødelæggende jordskælv i 2011 og 2012 som rangerede i størrelse fra 4,7 til 5,6 i Prague, Oklahoma; Trinidad, Colorado; Timpson, Texas; og Guy, Arkansas.
"Dette er den første undersøgelse som ser på sammenhængen mellem injektionsboringer og jordskælv på en stor, næsten national skala," sagde Weingarten fra CU-Boulders geologisk naturvidenskabelige fakultet. "Vi så en enorm stigning i jordskælv forbundet med disse høj-rata injektionsbrønde, specielt siden 2009, og vi tror at beviserne er overbevisende derom at de jordskælv vi ser tæt ved injektionsstederne er fremkaldt af olie og gas aktiviteten."
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A paper on the subject appears in the June 18 issue of Science.
The researchers found that "high-rate" injection wells -- those pumping more than 300,000 barrels of wastewater a month into the ground -- were much more likely to be associated with earthquakes than lower-rate injection wells. Injections are conducted either for enhanced oil recovery, which involves the pumping of fluid into depleted oil reservoirs to increase oil production, or for the disposal of salty fluids produced by oil and gas activity, said Weingarten.
Co-authors on the study include CU-Boulder Professor Shemin Ge of the geological sciences department and Jonathan Godt, Barbara Bekins and Justin Rubinstein of the U.S. Geological Survey (USGS). Godt is based in Denver and Bekins and Rubenstein are based in Menlo Park, California.
The team assembled a database of roughly 180,000 injection wells in the study area, which ranged from Colorado to the East Coast. More than 18,000 wells were associated with earthquakes -- primarily in Oklahoma and Texas -- and 77 percent of associated injection wells remain active, according to the study authors.
Of the wells associated with earthquakes, 66 percent were oil recovery wells, said Ge. But active saltwater disposal wells were 1.5 times as likely as oil recovery wells to be associated with earthquakes. "Oil recovery wells involve an input of fluid to 'sweep' oil toward a second well for removal, while wastewater injection wells only put fluid into the system, producing a larger pressure change in the reservoir," Ge said.
Enhanced oil recovery wells differ from hydraulic fracturing, or fracking wells, in that they usually inject for years or decades and are operated in tandem with conventional oil production wells, said Weingarten. In contrast, fracking wells typically inject for just hours or days.
The team noted that thousands of injection wells have operated during the last few decades in the central and eastern U.S. without a ramp-up in seismic events. "It's really the wells that have been operating for a relatively short period of time and injecting fluids at high rates that are strongly associated with earthquakes," said Weingarten.
In addition to looking at injection rates of individual wells over the study area, the team also looked at other aspects of well operations including a well's cumulative injected volume of fluid over time, the monthly injection pressure at individual wellheads, the injection depth, and their proximity to "basement rock" where earthquake faults may lie. None showed significant statistical correlation to seismic activity at a national level, according to the study.
Oklahoma had the most seismic activity of any state associated with wastewater injection wells. But parts of Colorado, west Texas, central Arkansas and southern Illinois also showed concentrations of earthquakes associated with such wells, said Weingarten.
In Colorado, the areas most affected by earthquakes associated with injection wells were the Raton Basin in the southern part of the state and near Greeley north of Denver.
"People can't control the geology of a region or the scale of seismic stress," Weingarten said. "But managing rates of fluid injection may help decrease the likelihood of induced earthquakes in the future."
The study was supported by the USGS John Wesley Powell Center for Analysis and Synthesis, which provides opportunities for collaboration between government, academic and private sector scientists.
Story Source:
The above post is reprinted from materials provided by University of Colorado at Boulder. Note: Materials may be edited for content and length.
Journal Reference:
M. Weingarten, S. Ge, J. W. Godt, B. A. Bekins, J. L. Rubinstein. High-rate injection is associated with the increase in U.S. mid-continent seismicity. Science, 2015 DOI: 10.1126/science.aab1345
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