Guidelines to reduce the risk of minor earthquakes during hydraulic fracturing

Researcher Developer Dr Rachel Westwood, Research Fellow Mr Sam Toon, and Emeritus Professor Peter Styles from Keele's School of Geography, Geology and Environment -- together with Professor Nigel Cassidy who is now at Birmingham University -- have published their study advising on hydraulic fracturing safety guidelines for legislative bodies, including governments, environmental agencies, health and safety executives and local planning authorities.

The study, Horizontal respect distance for hydraulic fracturing in the vicinity of existing faults in deep geological reservoirs: A review and modelling study, focused on the safety of hydraulic fracturing -- where millions of gallons of water, sand and chemicals are pumped under high pressure deep underground to deliberately break apart rock and release the gas trapped inside.

In 2014, hydraulic fracturing for shale gas, taking place near Blackpool, United Kingdom, caused two minor earthquakes. This was due to the fluid injected into the rock flowing into a pre-existing natural crack in the rock, known as a fault, triggering a minor earthquake.

Dr Westwood said: "The aim of the research was to investigate the minimum horizontal distance that hydraulic fracturing should occur from pre-existing faults in order to reduce the risk of an earthquake similar to the one that occurred near Blackpool.

"The review looked at seismicity caused by hydraulic fracturing and we created a model that looked at whether it could trigger movement of a pre-existing fault."

This study is the first to advise legislative bodies as there are currently no guidelines set for the horizontal distance required between the fluid injection points and pre-existing cracks in the rock. This research was carried out as part of the ReFINE (Researching Fracking in Europe) consortium, and funded by Ineos, Shell, Chevron, Total, GDF Suez, Centrica and Natural Environment Research Council (UK).

Mr Toon said: "We used data from the Blackpool earthquake and our computerized model which shows the layers within the rocks and the horizontal well. When water is injected into the gas reservoir it creates new fractures that intersect with the natural fractures in the rock and creates a network of open fractures which makes it easier to get the gas out of the rock.

"The study found that the distance required from the injection point depends on the intensity of the natural fracture network, how many and how close together the fractures are, and also what stresses are required to activate a fault. We found depending on the stress trigger threshold and the fracture intensity the distance is up to 433 metres."

The researchers have also published a further study, funded by the Horizon 2020 project SHEER (SHale gas Exploration and Exploitation induced Risks) that analyses the effect of pumping parameters on hydraulic fracture networks and the local stresses it causes during shale gas extraction. The research found for safe fracking there needs to be a compromise of flow distance and fracture area which can be controlled using pumping time and flow rate.