Monday, June 25, 2012

Hydraulic Fracking Poses Almost No Risk for Causing Earthquakes

The latest word from the NationalResearch Council is hydraulic fracking whether in shale deposits or as a secondary stimulation for a traditional gas or oil well has very low risk for inducing earthquakes that can be felt by people, but underground injection of wastewater produced by hydraulic fracturing and geothermal wells have a somewhat higher risk of causing earthquakes.  Although the vast majority of earthquakes that occur in the world each year have natural causes, earthquakes can be created by mankind.  Induced earthquakes have been documented since at least the 1920s when the first man-made large reservoirs were created behind dams. Other activities that can create (and have created) earthquakes are; controlled explosions used in mining or construction, underground nuclear tests, and energy technologies that involve injection or withdrawal of fluids from the subsurface can also create earthquakes. Man-made earthquakes are caused by changes in pore pressure within the rock due to the impounding of billions of gallons of water or injecting or extracting fluid from a well that may change the stress acting on a nearby fault. This change in stress may result in slip or movement along that fault creating a seismic event.

Historically man-made earthquakes have not been very large nor have they resulted in significant structural damage, but our ability to cause seismic events has increased over time as our technology to drill, pump and explode has advanced. To quantify the hazard and risk from man-made earthquakes requires probability assessments, which may either be statistical (based on data) or analytical (based on scientific and engineering models). Although the general mechanisms that create induced seismic events are well understood, current computer modeling techniques cannot fully address the complexities of natural rock systems in large part because the models generally lack information on local crustal stress, rock properties, fault locations and properties, and the shape and size of the reservoir into which fluids are injected or withdrawn. Geology cannot be simplified or generalized to model earthquake probability which is very specific.

So the National Research Council Board of Life and Earth Studies report titled: Induced Seismicity Potential in Energy Technologies is a data based analysis of earthquakes induced by mankind. This study compiling and analyzing all the data available was requested by the Energy and Natural Resources Committee of the U.S. Senate to assess the potential to cause earthquakes by energy production and related activities after small seismic events reported in Alabama, Arkansas, California, Colorado, Illinois, Louisiana, Mississippi, Nebraska, Nevada, New Mexico, Ohio, Oklahoma and Texas, appeared to be related to hydraulic fracturing, energy development and (true) geothermal energy production. The National Research Council is a nonprofit based in Washington that provides scientific information for government decision-makers under the auspices of the National Academy of Sciences, the National Academy of Engineering and the Institute of Medicine. Its reports are based on data and analysis gathering and scientific analysis of the information gathered.

The report examines the potential for energy technologies -- including shale gas recovery using fracking, carbon capture and storage, geothermal energy production, and conventional oil and gas development -- to cause earthquakes. Hydraulic fracturing, commonly known as fracking, extracts natural gas by injecting huge volume of water mixed with sand, and chemicals in short bursts at very high pressure into deep underground wells. The process cracks the shale rock formation and allows natural gas to escape and flow up the well, along with some wastewater. The wastewater can be discarded in several ways, including injection of the wastewater at a separate disposal well. True geothermal energy harnesses natural heat from within the Earth by capturing steam or hot water from underground. The basic mechanisms that can induce earthquakes from these wells are fluid injection and extraction that are presently well understood. The report examined the data from over 35,000 fractured wells, 108,000 secondary oil and gas recovery wells, 13,000 tertiary oil and gas recovery wells, 6,000 hydrocarbon withdrawal wells, 30,000 waste water disposal wells, 23 liquid dominated geothermal well fields and 1 vapor dominated geothermal field.

Analysis of the data collected at all these sites showed that the net fluid balance (total balance of fluid injected and withdrawn) appears to have the most direct impact on changing pore pressure within the ground. Oil, gas and geothermal wells are typically designed to maintain a balance between the amount of fluid being injected and the amount of fluid being withdrawn to prevent not only earthquakes, but to maximize the well life.  Geothermal wells appeared most likely to induce earthquakes especially the wells in the vapor dominated Geysers site which had 300-400 earthquakes per year (it is in California). In fluid geothermal wells maintaining a constant fluid balance results in a fairly constant reservoir pressure, reducing the number of induced earthquakes significantly. The 23 fluid dominated geothermal well locations experienced 10-40 earthquakes per year.

Only a very small fraction of the hundreds of thousands of oil and gas wells in the United States have induced earthquakes at levels that are noticeable to the public. An increase of  rock pore pressure above ambient levels due to injection of fluids or a decrease in pore pressure below ambient levels due to extraction of oil and gas have the potential to produce earthquakes. However, analysis of the data showed that to create an earthquake, a combination of conditions has to exist simultaneously:
    A. Significant change in net pore pressure in a reservoir,
    B. A pre-existing near-critical state of stress along a fracture or fault, and
    C. Fault rock susceptible to brittle failure.

Oil and gas wells are designed to maintain a balance between the amount of fluid being injected and the amount of fluid being withdrawn to extend the life of the well. This fluid balance helps to maintain fairly constant reservoir pressure and reduces the potential for induced earthquakes. In a conventional oil or gas reservoir the hydrocarbon fluids and associated aqueous fluids in the pore spaces of the rock are usually under significant natural pressure. Fluids in the oil or gas reservoir flow to the surface when penetrated by a well bore aided by pumping once the well is fully developed. The well or wells will produce until reservoirs reach a point when insufficient pressure, even with pumping, exists to allow the wells to continue to produce at commercial volume. To extend the life of a spent well various secondary and tertiary recovery technologies referred to as enhanced oil recovery technologies can be used to extract some of the remaining oil and gas. Secondary recovery and enhanced oil recovery technologies both involve injection of fluids into the subsurface to push more of the trapped hydrocarbons out of the pore spaces in the reservoir and to maintain reservoir pore pressure. Secondary recovery often uses water injection or “water flooding” and tertiary technologies often inject carbon dioxide (CO2). Of the 108,000 oil and gas wells that used water flooding only 18 have had one or more earthquakes. Of the 13,000 CO2 injected sites none have experienced earthquakes.

Shale formations can also contain hydrocarbons either gas or oil or both depending on the formation. The extremely low permeability of these rocks has trapped the hydrocarbons as they developed in the rock and largely prevented them from migrating out of the rock over geologic time. These unconventional gas and oil reservoirs are developed by drilling wells horizontally through the rock and using hydraulic fracturing techniques to create new fractures in the reservoir to allow the hydrocarbons to migrate up the well bore. The water used to fracture the well is quickly released from the reservoir and does not impact the fluid balance. About 35,000 hydraulically fractured shale gas wells exist in the United States; only one instance of an induced earthquake has been identified in which fracking to access the shale gas is suspected, but not confirmed, as the cause.

Overall, hydraulic fracturing or fracking and traditional oil and gas well have a very low risk of creating earthquakes. The waste water disposal wells associated with fracking and secondary well development have been associated with 8 known earthquakes, though there are a total of about 30,000 disposal wells in use, but these earthquakes have captured the headlines and public concern. Wells used only for the purpose of waste water disposal normally do not have a detailed geologic review performed prior to injection and the data are often not available to make a detailed review of these sites possible.  The overall risk turns out to be small, but limited knowledge about the geology prevents modeling. Attempts at modeling of pore pressure, temperature, and rock stress changes induced by injection and extraction to predict producing earthquakes have not been successful except where detailed knowledge of stress changes, pore-pressure changes, and fault characteristics are available for input and that data is almost always not available for disposal wells. The permanent addition of fluid to the subsurface without any fluid removal and the heat gradient associated with geothermal appears to have the most direct impact on changing pore pressure in the subsurface over time and the creation of earthquakes.

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