In a report commissioned by United Kingdom-based Cuadrilla Resources, researchers concluded that a series of earthquakes in Lancashire, UK were likely caused by hydraulic fracturing. Two relatively large earthquakes, with magnitudes 2.3 and 1.5, and 48 smaller events occurred in the hours after several stages of the Preese Hall 1 well were fracked. A separate report written by a seismologist at the Oklahoma Geological Survey concluded that a swarm of about 50 earthquakes in Garvin County, Oklahoma, ranging in magnitude from 1.0 to 2.8, could also have been induced by hydraulic fracturing.
The occurrence of so-called “induced seismicity” – seismic activity caused by human actions – in conjunction with fluid injection or extraction operations is a well-documented phenomenon. However, induced earthquakes large enough to be felt at the surface have typically been associated with large scale injection or withdrawal of fluids, such as water injection wells, geothermal energy production, and oil and gas production. It was generally thought that the risk of inducing large earthquakes through hydraulic fracturing was very low, because of the comparatively small volumes of fluid injected and relatively short time-frame over which it occurs. As the controversy over hydraulic fracturing has heated up, however, researchers and the public have become increasingly interested in the potential for fracking to cause large earthquakes. These two reports underscore the need for more data and research into this topic.
In the meantime, it is important to determine what steps can be taken immediately to prevent this from happening in the future. One obvious and easy strategy is avoidance – don’t frack wells that intersect or may link up to faults. In the case of the UK earthquakes, it appears that fluids were injected directly into a single fault or fault zone. The researchers determined that this fault was already highly stressed and the disturbance caused by hydraulic fracturing was enough to cause it to fail.
This highlights the importance of performing thorough site characterization prior to any drilling activity and collecting appropriate data during operations. Hydraulic fracturing is a complex process taking place in an even more complex natural system and it is crucial to understand as much as possible about that system before disturbing it. In an image log of the well, investigators could identify the fault that likely caused the UK earthquakes. A more thorough understanding of geologic conditions at the well location, including the nature of the fault, may have allowed the operator to avoid this incident altogether.
These incidents also underscore the need for appropriate monitoring during hydraulic fracturing operations. A technique called microseismic monitoring can provide detailed, real-time data, giving a picture of how and where the fractures are growing. As suggested by the UK report, monitoring seismic activity during hydraulic fracturing can provide the necessary data to make decisions on if and when to shut down the operation to avoid any potential problems, including induced seismicity or out-of-zone fracture growth. In practice, microseismic monitoring is sparsely used, due to costs or technical challenges. This kind of data is, however, invaluable for understanding and predicting how the rocks will respond to fracking. Operators should develop a plan to obtain a statistically significant sample of microseismic data for each field, in order to monitor frac jobs in real-time and gather the data necessary for accurate modeling.
In an attempt to allay concerns about the unknown risks related to hydraulic fracturing, representatives of the oil and gas industry often state that hydraulic fracturing has been used for over 60 years. Yet these and other incidents make it clear that there is still a great deal that is not understood about this process.