Fluid Injection Rates

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.

Fort Worth Basin and the Barnett Shale

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. Although geologists discovered the Barnett Shale in the early 20th century, drilling in the region did not begin until 1981 when George Mitchell (Mitchell Energy) began his twenty-plus-year search for an economical way to remove gas from shale. Once he applied the combination of horizontal drilling and hydraulic fracturing technology in 2002 to allow more gas to escape the rock into the wellbore, the number of wells skyrocketed over the next decade. Let’s examine the earliest years of the Barnett Shale production, starting in 2005, when natural gas production increased dramatically, and increased water production followed.

Injection of produced water from the Barnett Shale

Some of the produced water is the return of 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. As we have seen, operators dispose of produced water and similarly contaminated wastewater by returning it to the subsurface through underground injection. 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.

In the early years of the Barnett Boom, starting in early 2006 through September, 2014, 270 million cubic meters (or 1.7 billion barrels) of fluid were injected into the Ellenburger Formation.1Matthew 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. The graph shown here of early injection rates in the Barnett Shale shows an increasing rate of injection in the early years, culminating in peak injection rates in 2011. Because the Ellenburger pore space originally contained brine, typically at hydrostatic pressure, the only way to accommodate all that 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 ones with high injection rate. Raising the pore pressure in the vicinity of any fault reduced its resistance to slip.

Earthquakes in the Barnett Shale

As the rate of injection continued to increase in these early years, on October 31, 2008, the area surrounding the Dallas/Ft. Worth Airport experienced a magnitude 3.4 earthquake. Although magnitude 3 earthquakes are barely felt by humans, they are extremely troubling to the public. These events kicked off an investigation of the relationship between shale development, hydraulic fracturing, underground injection of produced water, and earthquakes. A little later in this lesson, we look at the results of a study on the specific DFW airport case of low-magnitude earthquakes and their cause.

Citations

2. Chart data from the Texas Railroad Commission interpreted by Dr. Bridget Scanlon at the Bureau of Economic Geology, University of Texas at Austin.

Images: “Graphic” by Top Energy Training