Shaking All Over - Hydraulic Fracturing and Earthquakes

The Earth’s crust is a dynamic affair, with tectonic plates ever shifting, converging, diverging and rubbing against one another. As two plates interact, seismic pressure is stored and released in the adjacent rock. The release of this pressure is manifested in earthquakes, with the size depending on the amount of pressure, and how well the intervening layers of rock absorb or conduct the vibrations. It’s not just the adjacent rock which gets in on the act, as tectonic plates can converge, causing “crumpling”, or diverge and leave tension in rocks far from the tectonic fault itself. To complicate matters further, the rock can quite happily remain under pressure for an extended period, until a change in equilibrium causes the pressure to be released. So, does injecting large quantities of water into the ground facilitate the release of stored seismic pressure? Undoubtedly yes, but the hydraulic fracturing process is not the biggest offender here.

The path of fluid from an injection site to
an adjacent fault. Image: Davies et al. 2013

It’s worth noting that disposal of flowback water is not permitted in the UK or anywhere in Europe, so concerns about seismicity should be focused on the events which can result from the hydraulic fracturing process itself. As it stands, the magnitude of earthquakes resulting from hydraulic fracturing are not adequate to cause much in the way of property damage, but a deep understanding of the surrounding geology is critical to ensure that it stays this way.

Induced earthquakes (those which are caused by human activity) result from changes in the loading state of the Earth’s crust such as the removal of material during mining, or extraction of large quantities of oil or gas. Due to the “tight” formations which are generally targeted during hydraulic fracturing operations, this is not considered to be a high risk, as the tight rocks (shale, for example) are dense enough to maintain structural integrity. Faults (interfaces between separate rock faces) can be “activated” by environmental changes, such as reduced friction from increased fluid pressure, resulting in and stored pressure being released as the rock elements slide against each other.

The quantity and duration of water injection are significant risk factors for induced seismicity, and wastewater disposal wells tend to beat fracking on all of these fronts. They’re also utilised in various forms of mining, even in the absence of hydraulic fracturing. That’s not to say that hydraulic fracturing doesn’t have an associated risk. The Preese Hall 1 exploration well near to Blackpool in the UK experienced two earthquakes, of magnitude 2.3 and 1.5, chronologically congruent with water injection. It was concluded that an inactive fault was reactivated by the increased water pressure, but that the fault in question was perhaps unusually susceptible to this sort of event due to its steep angle.

Coming up:

• UK policy in contrast to the US