Recent evidence of induced seismicity near Basel, Switzerland, and Youngstown, Ohio, in proximity to injection activity highlights the need to minimize the risk of anthropogenic seismicity during pumping operations.
According to the Environmental Protection Agency’s numbers, approximately 151,000 oil and gas and waste injection wells are pumping more than 2 billion gallons of liquid under the US every day. These Class II wells are performing fracturing and waste injection.
To date, more than 2 million wells have been stimulated through fracing, which employs a small volume of fluid at high pressure in short durations. Induced seismic events are most often between -3 and -1 on the Richter magnitude scale – magnitudes so small they are only detected with sensitive monitoring equipment.
Wastewater injection for disposal purposes involves larger fluid volumes than fracing, pumped in at lower pressures with slower rates over a longer time. The fluid is intended to disperse aseismically in the target formation.
Despite this high level of activity, any report of seismicity associated with either process is rare. In the few instances where microseismic (magnitude 2+) events have occurred, it appears that the injection well was situated near a fault or other zone of weakness that was under tectonic stress.
The key to safe practices is understanding the processes that cause seismic events. The movement of continents, gravity, and planetary forces dictate that the Earth’s crust is critically stressed almost everywhere. As a result, there are literally millions of earthquakes worldwide annually, though fortunately only a few are large enough to be felt and fewer still cause damage. When fluids are injected into the crust, the pore pressure that pushes back against the tectonic forces compressing the rock increases. If the pore pressure becomes high enough, and if there is a stressed fault currently bound fast by friction, the pore pressure may overcome the friction and allow the fault to slip.
Seismic imaging can be used to screen potential well locations for proximity to fault zones. Microseismic monitoring allows continual observation of the reservoir’s seismic response during injection procedures. A “traffic light” response protocol for injection monitoring has been developed for enhanced geothermal system projects in the US and has been carried over to fracing operations in the UK. The system posts a green light when observed seismic events are less than a prescribed threshold (typically 0). A yellow light (typically between 0 and 1.7), a caution condition, obligates closer observation of well activities. A red light condition exists for events detected above the highest threshold, calling for cessation of pumping activity and flowback of the well until the seismicity returns to an acceptable level.
An important aspect of risk mitigation is community awareness. It is essential that local residents be informed of what processes are taking place and be educated about the real risks of seismicity so that wholesale panic does not set in if weak events are felt.
According to the Lawrence Berkley National Laboratory’s Earth Science division, “All experience to date has shown that the risk [from induced seismicity], while not zero, has been either minimal or can be handled in a cost-effective manner.”
With appropriate monitoring and controls the industry can continue to be a good steward of the environment and provide the new sources of energy that the global market requires.