Factors influencing the likelihood of seismicity associated with wastewater disposal include the rate of injection and the volume of the fluid to be injected compared to the pore space available in the target formation. Available pore space depends on the porosity and thickness of the formation and the initial pore pressure of the fluids in the pore space. Injecting at a higher rate in a well will result in more viscous drag of the fluids going into the formation, and consequently higher pressures. To illustrate these factors and their relation to earthquakes, let’s take a look at wastewater disposal in the Fort Worth Basin, a sedimentary basin in north Texas containing the hydrocarbon-rich Barnett Shale.
Starting in 2005, natural gas production in the Barnett Shale has increased dramatically, and increased water production followed. Some of this water is the return of the fluids injected during hydraulic fracturing, while the rest is the associated extraction of original subsurface waters deposited with the Barnett Shale and surrounding formations. These produced waters have salinities on the order of ten times that of seawater, thus they are unusable by humans and can cause environmental surface damage if improperly managed. Many operators dispose of produced water and similarly contaminated wastewater by returning it to the subsurface. Disposal depths are deep, far below freshwater aquifers, and sometimes below hydrocarbon formations, in order to avoid contamination problems. In the case of the Barnett Shale, operators dispose of wastewater beneath the shale into a section of carbonate rock called the Ellenburger Formation.
According to a December 2016 study, in the years starting in 2006 through September, 2014, 270 million cubic meters (or 1.7 billion barrels) of fluid were injected into the Ellenburger Formation.¹ Since the Ellenburger pore space originally contained brine, typically at hydrostatic pressure, the only way to accommodate the added fluids was for the pore pressure to increase. This increased pressure essentially compressed the fluids already in the Ellenburger to make room for the new injection volumes. Pressures would be even higher in the vicinity of wells, particularly when injection rates are high. The connection of this injection to earthquakes is that raising the pore pressure in the vicinity of a fault will reduce its resistance to slip. Consequently, as the cumulative injection volume in the Ft. Worth Basin increased, the M3-earthquake frequency also increased. (M3 earthquakes are minor, barely felt if at all by humans.)
The graph below shows injection rates from 2005 through 2015 into the Ellenburger Formation.
1. Matthew J. Hornbach et al., 2016, “Ellenburger Wastewater Injection and Seismicity in North Texas,” Physics of the Earth and Planetary Interiors, http://dx.doi.org/10.1016/j.pepi.2016.06.012 (accessed March 9, 2017).
Chart data from the Texas Railroad Commission interpreted by Dr. Bridget Scanlon and the Bureau of Economic Geology, University of Texas at Austin.
Images: “Graphic” by Top Energy Training