Various concerns have been raised about the potential environmental and geologic effects from the use of fluid injection for hydraulic fracturing, or "fracking". Fracking is used for the development of previously unrecoverable shale gas, coalbed methane, and other hydrocarbons. These concerns are often focused on the source of the water used in the fracking operation (the process often requires the use of tens of thousands of cubic metres of water per well) and potential contamination from the use and disposal of fracking fluid. However, concerns have also been expressed about the potential for fracking to cause seismic events.
On August 30, 2012, the British Columbia (BC) Oil and Gas Commission (Commission) released a report that found that fracking in proximity to pre-existing faults was the cause of hundreds of low level seismic events in the Horn River Basin in northeast BC. The Horn River Basin is one of BC's primary shale gas plays. The Commission's conclusion was based on an investigation of seismicity (i.e., seismograph recorded earthquakes primarily caused by fault movement) undertaken between April 2009 and December 2011 in the Horn River area.
Hydraulic fracturing is the process of creating cracks (fractures) in underground geological formations to create pathways along which hydrocarbons, such as shale gas, can flow into a wellbore. For gas-bearing shales, a mixture of water, sand and other chemical additives is pumped into the wellbore at high pressure to fracture the shales. The fractures are then kept open by the sand, or "proppant", to provide pathways for the shale gas to flow back to the wellbore.
As geological rock is fractured during the injection of hydraulic fluids, thousands of micro-seismicity events are created (from -3.0 to 0.5 ML on the Richter scale). The Commission's report did not focus on this activity, but instead on seismic events greater than 1.0 ML.
Prior to 2009, according to Canada's National Earthquake Database, no seismicity had been detected in the Horn River Basin. However, between April 2009 and December 2011, 272 low level seismic events (ranging from 1.0 ML to 3.8 ML) were attributed to movement along pre-existing faults induced by hydraulic fracturing. The Commission used the Davis and Frolich induced seismicity criteria (e.g., timing, location, fluid pressures) to attribute these events to the hydraulic fracturing completions. Of note, disposal wells were ruled out as a source of seismicity, as the four disposal wells in the study area were all injecting recovered fluids into a formation more than 1800 metres above the Horn River Group formations.
The Commission noted that seismic events measuring less than 4.0 ML are considered minor and that, of the recorded seismic events, only one was reported to be "felt" at the Earth's surface; on May 19, 2011, a 3.8 ML event was felt by workers within 10 kilometres of the event epicentre. No injuries or damage to surface structures were reported for this event, or for any of the other events studied by the Commission.
In addition to concluding that the seismic events were the result of hydraulic fracturing, the Commission also concluded that the fractures developed by the hydraulic fracturing operations were confined to the Horn River gas-bearing shales. No effects on shallow aquifers or the environment were identified by the Commission.
Similar Findings Elsewhere
The Commission's report references two public reports on induced seismicity incidents in other locations.
In Blackpool, England, 50 seismic events were attributed to hydraulic fracturing. The strongest event in that case was found to have occurred 10 hours after shut-in, which was interpreted to be the effect of a pressure front spreading out from the hydraulic injection point. The authors of that study (de Pater and Baisch) concluded that the magnitude of the seismicity could be mitigated by "rapid fluid flow back after the treatments and reducing the treatment volume".
The other recently documented case of induced seismicity was in Garvin County, Oklahoma. In that case, there were also 50 seismic events recorded, which began seven hours after hydraulic fracturing operations started. Although Oklahoma has considerable natural seisimicity, the evidence showed a unique origin different from naturally occurring earthquakes. The author (Holland) concluded that the timing and events suggested a possible connection to hydraulic fracturing, although it did not go so far as to state fracking was the cause of the seismicity.
Based on the findings of its investigation, the Commission made a number of recommendations, including the following:
- Requiring the submission of microseismic reports to identify geological features such as faults where there may be increased risk of inducing seismicity beyond 2.0 ML. The reports may also confirm that the fracturing is being confined to the zone targeted by operators. The Commission is going to develop these requirements in consultation with stakeholders and other regulators outside of BC;
- Establishing a notification and consultation procedure between the operator and the Commission. If seismicity is detected, through the Canadian National Seismograph Network (CNSN or "grid"), or through the operator's own seismographs, the Commission would then engage with the operator to investigate and determine if mitigation options are required (e.g., a suspension order);
- Reviewing geological and seismic data to identify pre-existing faults. Where induced seismicity is detected, subsequent wellbores can avoid the active fault or other mitigation procedures, such as bypassing fracturing stages adjacent to the active fault;
- Studying the relationship of hydraulic fracturing parameters on seismicity. The Commission is continuing its analysis of hydraulic fracturing data to attempt to identify correlations (e.g., with pump rates or injection volumes); and
- Upgrading and improving BC's seismograph grid and monitoring procedures. The Commission found that the current grid coverage could not reliably identify smaller seismic events. For example, the current grid did not detect 15 of the 19 events between 2.0 ML and 3.0 ML in the Etscho area of the Horn River Basin, which were only identified due to the more accurate dense array of seismographs temporarily installed for the Commission's study.
The Commission is also undertaking further work with the University of British Columbia with the goal of providing insights into predicting the location and magnitude of seismic events in northeast BC resulting from hydraulic fracturing so as to better predict, detect and mitigate such events.
Further details about the Commission's investigation can be found in the full report ("Investigation of Observed Seismicity in the Horn River Basin") on the Commission's website: http://www.bcogc.ca/document.aspx?documentID=1270&type=.pdf
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