Friday, 24 April 2015

Evaluating the role of hydraulic fracturing in causing the 2013-2014 Azle, Texas, Earthquakes.

In November 2013-January 2014 the United States Geological Survey recorded a series of 27 Earthquakes to the north of Azle and Reno in northeast Texas, the largest two of these events had Magnitudes of 3.6 and were widely felt across the region. This compares to one single recorded Earthquake in the previous 150 years in the area, and was widely linked to the practice of hydraulic fracturing (fracking) which has been used in the Newark East Gas Field since 2008. Geologists have known that hydrocarbons extraction can lead to seismic movements since at least the 1960s, and hydraulic fracturing, which involves blasting impermeable rocks with high pressure water, chemicals and sand to free oil and gas has been connected to a rise in Earthquake activity in Texas and other parts of the US. However correlation does not necessarily imply causation, and while in some cases fracking has been associated with a rise in seismic activity, in other instances the method has failed to produce any such affect. This has led many within the industry have strenuously denied that any connection between fracking and earthquakes, particularly as no mechanism by which the process could be causing the events has been established.

In a paper published in the journal Nature Communications on 21 April 2015, Matthew Hornbach and Heather DeShon of the Huffington Department ofEarth Sciences at Southern Methodist University, William Ellsworth of the United States Geological Survey, Brian Stump and Chris Hayward of the Huffington Department of Earth Sciences at Southern Methodist University, Cliff Frohlich of the Institute for Geophysics at the The University of Texas at Austin, Harrison Oldham, also of the Huffington Department of Earth Sciences at Southern Methodist University, Jon Olson of the Department of Petroleum and Geosystems Engineering at The University of Texas at Austin, Beatrice Magnani and Casey Brokaw, again of the Huffington Department of Earth Sciences at Southern Methodist University and James Luetgert, also of the United States Geological Survey, discuss the causes of the 2013-14 Azle Earthquakes, and the likelihood that these quakes were caused by hydrocarbons extraction in the region.

The study area forms part of the Fort Worth Basin, and is underlain by the deeply buried Newark East Fault Zone within the basement crystalline rocks (i.e. ancient volcanic rocks, underlying all sedimentary units in the basin), which comprises a primary fault dipping to the northwest at an angle of ~66˚-70˚, and a shallower conjugate fault dipping to the southeast at an angle of ~70˚-80˚. However there is no reason to believe that there has been any activity on this fault zone for millions of years. Only a single Earthquake had been recorded in the region prior to 2008 (and this is regarded as a somewhat unreliable record), and while there are some surface features in the region that appear to have been caused by ancient Earthquakes, they appear to be related to karstification (limestone being dissolved by percolating water) in the Ellenburger Aquifer.

The area has also been subject to increased changes in water level, both within Eagle Mountain Lake, a large local reservoir, and the shallow Trinity Aquifer. Both of these had suffered sharp drops in water level prior to the onset of seismic activity, and such changes have been linked to Earthquakes on other areas. However both of these are very shallow sources of geologic stress, and could not have caused significant pressure changes at the depths of the 2013-14 Earthquakes.

Natural and anthropogenic stress changes that may trigger earthquakes in the Azle area. Several natural and anthropogenic (man-made) factors can influence the subsurface stress regime resulting in earthquakes. Natural stress changes that promote earthquakes include intraplate stress changes related to plate tectonics and natural water table or lake levels variations caused by changing weather patterns or water drainage patterns with time, and in some instances (not pictured) the advance or retreat of glaciers. Anthropogenic stress changes that promote earthquakes include human generated changes to the water table (including dam construction) and industrial activities involving the injection or removal of fluids from the subsurface. The figure is not to scale. Hornbach et al. (2015).

Hydraulic fracturing is carried out from two wells in the area. Both pump fluids into the hydrocarbons-bearing deposits in bursts, then pump out a mixture comprising both the injected fluids and the desired hydrocarbons. The process generates a considerable amount of briny wastewater, which is disposed of by injection into the underlying Ellenburger Salt-water Aquifer. No correlation was found between pumping times and seismic activity, implying that the process was not directly causing the events.

However Hornbachet al. found that the closeness of the pumping activity to the inclined Principle Newark East Fault resulted in a situation that while the majority of the extraction was occurring to the northwest of the fault, the wastewater was being injected on the opposite, southeastern side of the fault, despite originating at the same point on the surface. This resulted in a steady increase in pressure across the fault zone, from the onset of drilling in 2008, which eventually expressed itself as a series of Earthquakes in 2013-14, as the fault moved to accommodate the changes in pressure.

Modelled pressure changes in the Ellenburger caused by injection and production. Map view of modelled excess pressures at a depth of ~2,500m for May 2009 (a), January 2010 (b), January 2011 (c) and December 2013 (d,e). The model uses average monthly reported water injection rates and the Dupuit–Theim equation to estimate bottom-hole pressure values. Pressure above hydrostatic averages 0.58MPa for injector well #1 and 0.28MPa for injector well #2 during injection.Ellenburgerpermeability is assumed constant at 5 ´ 10-14m2; boundary conditions are open along the side and closed at the top and bottom. We apply an average rate of brine production based directly on reported Texas Railroad Commission G-10 waterproduction values for the 70 largest water producing production wells in the region. The images show the system before injection (a) through the onset of seismicity (e). Black lines, the Newark East Fault Zone location at the top of the Ellenburger Formation; red squares, injector locations; pink arrows, approximate location of two large brine production wells that are located both near the faults and near reported earthquakes swarms within the Ellenburger (grey circles with white outlines). Note that the most significant amount of brine removal occurs along the fault trend (a). Hornbach et al. (2015).

This study therefore for the first time establishes a direct connection between the process of hydraulic fracturing and seismic activity in the Fort Worth Basin, and in addition establishes a method by which the industry could be seen to be directly causing Earthquake events. However by demonstrating how the events were being caused it also creates the potential to avoid such problems in future, through a better understanding of how the process is impacting the regional geology.

See also… 3.1 Earthquake in Snyder County, Texas.                                                                          The United States Geological Survey recorded a Magnitude 3.1 Earthquake at a depth of 4.5 km, roughly 7 km north of the city of Snyder in Scurry County.. Magnitude 3.3 Earthquakes in Parker County, Texas.                                                        The United States Geological Survey recorded a Magnitude 3.3 Earthquake at a depth of 5 km, in northeastern Parker County, Texas, roughly 30 km...

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