Small earthquakes appear likelier with higher underground injection volumes associated with wastewater disposal and carbon capture and storage projects than from hydraulic fracturing to recover oil and gas from tight shales, experts told the US Senate Energy and Natural Resources Committee.
“It really is volume-dependent,” said Murray W. Hitzman, a Colorado School of Mines economic geology professor who chaired a committee which produced a June 15 report on induced seismicity potential in energy technologies for the National Research Council at the National Academies of Science.
“CCS and wastewater disposal, because they involve such large volumes, are in a different league from hydraulic fracturing,” he said June 19.
A second witness agreed. “While there appears to be little seismic hazard associated with the hydraulic fracturing that prepares the shale for production, the disposal of waters produced with the gas does appear to be linked to increased earthquake activity,” said William Leith, a senior science advisor for earthquake and geologic hazards at the US Geological Survey.
Mark D. Zoback, an earth science and geophysics professor from Stanford University, agreed.
“It is important for the public to recognize that the risks posed by injection of wastewater are extremely low,” said Zoback. “In addition, the risks can be minimized further through proper study and planning prior to injection.”
He also called for careful monitoring in areas where seismicity events might be triggered, and he advocated that operators and regulators take “a proactive response” if seismicity occurs.
Prior geologic review
Hitzman said that in contrast to enhanced oil recovery wells, which are sited for precise injection into well-characterized oil and gas reservoirs, wastewater injection wells normally do not have a detailed geologic review performed beforehand. Data are often not available to perform such a review, he added.
Many of the well-documented cases of induced seismicity related to wastewater fluid injection are associated with operations involving large amounts over long periods, Hitzman said.
Most hazardous and non-hazardous wastewater disposal wells do not pose an induced seismicity hazard, although long-term effects of any significant increases in their number in particular areas are unknown, he said.
Susan Petty, president and chief technology officer at Altarock Energy Inc. in Seattle, testified that good project siting can help reduce risks and public concerns.
Projects that potentially could induce even mild earthquakes should be placed away from populous areas until more is know about causes and effects of their underground injections, she said.
More significantly, concerns over possible induced seismicity can divert public attention from more significant issues such as proper water management, Petty said.
Zoback said that the probability of triggering seismic activity can be reduced by avoiding injections into faults in brittle rock, selecting formations for injection (and limiting rates) to minimize pore pressure changes, installing local monitoring arrays when injection potentially could trigger seismic activity, establishing protocols beforehand to define how operations would be modified if seismicity is induced, and preparing operators to reduce injection rates or abandon wells if triggered activity poses any hazard.
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