Do induced earthquakes occur during hydraulic fracturing after all?

Published: June 19, 2014

This article by Austin A. Holland (2013) is noteworthy for several reasons. It is also important for the discussion on induced seismicity in the context of the production of gas from shale gas reservoirs using hydraulic fracturing methods.

There are very few publications that are concerned with induced seismicity during hydraulic fracturing itself instead of with the events that occurred when disposing production water into permeable layers in the underground. The author states that “earthquakes related to hydraulic fracturing are usually very small events of magnitudes between M-1.0 and M-0.5”. Larger events have only been reported once in the Bowland shale in the UK (2011). Previous cases from Oklahoma are not documented sufficiently to allow a differentiation between natural and induced events. The first case discussed there happened in Carter and Love Counties (1978), the second one again in Love County (1979). The author, considering the fact that the hydraulic fracturing technique has been applied to more than 100,000 wells in Oklahoma alone and only 3 cases in which possibly induced events were reported have occurred, summarizes: “The percentage of hydraulically fractured wells that could trigger earthquakes is small”.
The small events (< M0.0) normally induced during hydraulic fracturing usually originate from the area of the cracks created during the fracturing process and are therefore also used for the mapping of crack formation. The events in South-Central Oklahoma (2011) of between M0.6 and M2.9 mentioned here occurred further away (2 km) from the well with 16 events being >M2. The earthquakes are thus not in direct mechanical contact with the created fractures that only extended to approximately 50m from the well, but can be classified as “triggered“¹. The stress-diffusion caused by the injection of the additionally created pore pressure may very well extend this far, even within a timeframe of up to 40 hours as examined here.

There have been reports of a series of earthquakes in Oklahoma with 16 events > M2.0 that is directly related to hydraulic fracturing. It has been argued that this is because, on a timeline, the events correlate very closely with the hydraulic fracturing procedures and can also easily be classified in terms of location and depth. They are, however, not directly connected to the created hydraulic fracturing wells in terms of location. The impressed pressures of >400 bar have exceeded the 250 bar regarded sufficient for hydraulic fracturing in this area. This may be part of the reason why the pressure anomalies have extended far beyond the fractured area.

¹Explanation concerning the terms “induced“ or “triggered“: Events that are not directly within the hydraulically fractured area are nowadays often described as “triggered”. The term “induced” is also used as an umbrella-term and as a synonym for man-made.