Forecasting seismic activity induced from hydraulic fracturing operations

As the world transitions towards a carbon-neutral economy in order to meet the Paris climate change accords, many countries are utilising natural gas as a transition fuel while the renewable energy sector continues to develop. As part of this transition in the UK, it is the intent that the use of domestic natural gas, including gas from unconventional reservoirs such as shale, is fully realised. The extraction of natural gas from shale is not without environmental risk and seismic events induced by the hydraulic fracturing process are cited as the reason for the current suspension of hydraulic fracture operations in the UK. The reactive control approach, of which the ‘traffic light system’ procedures are part of, is the most widely used method to forecast seismic events. This ties the likelihood of a seismic event occurring to a single seismological derived parameter. There have been challenges in this approach, and newly developed probabilistic forecasting methods that are capable of predicting seismic events likely to occur in the future are still in development and yet to be the primary decision making system to control the injection schedule. The primary objective of this thesis was to research forecasting approaches that alleviates the disadvantages posed by these current methods. A software system was developed based on relating a forecasting model to real-time changes in the fracture network from the two causes of induced seismicity; an increase in pore-pressure re-activating fault lines and the transfer of stress from other seismic events. This software system analyses microseismic records using four geophysical signal analysis methods which when combined produces two maps updated in real-time; a fracture map highlighting hydraulic connections and a Coulomb stress change map. To verify the software system, the causes of a magnitude 3.9 earthquake on the 12 January 2016 from a shale gas production well in Fox Creek, Canada were retrospectively investigated and the usage of the system to forecast seismic events evaluated. The fracture map generated from the microseismic records indicated a hydraulic connection between stage 23 of the hydraulic fracture process and a legacy fault line. The input of fracture fluid increased the pore-pressure on the fault line, ultimately causing slip and the magnitude 3.9 earthquake. There was no evidence to show that static stress transfer from other seismic events in the area was affecting the triggering process. The forecast model was validated by comparing the fracture maps in the time leading up to 12 January event to the forecast model. Although numerous events were positioned to be part of a transition between the hydraulic tensile fractures to the fault line, it was not possible to analyse these events due to the low signal to noise ratio and therefore not viable to fully validate the forecast model with this case study. Further research with different case studies where the acquisition geometry is closer to the events is recommended to fully validate the forecast model before field implementation