What is a Microseismic Event?
Unlike large-scale earthquakes which are caused when energy is released as a result of rock failure along a fault, microseismic events are caused when human activities such as mining or oil and gas production change the stress distribution or the volume of a rock mass. When the solid earth particles attempt to redistribute the stress within the rock mass, they will suddenly slip or shear along pre-existing zones of weakness such as along faults or fracture networks. The movement results in the release of energy in the form of seismic waves and are known as microseismic events.
A microseismic event, characterized by energy propagating through the Earth's subsurface, typically registers a magnitude of less than 2.0 on the Richter scale. These events often stem from natural processes like the movement of rock and soil underground or the release of stress along geological faults. Additionally, human activities such as mining, hydraulic fracturing (fracking), and injecting fluids into the subsurface for waste disposal or enhanced oil recovery can also induce microseismic events.
Microseismic events usually do not cause any noticeable shaking at the surface and are not felt by humans. However, the seismic and acoustic waves can be detected and recorded using sensitive seismometers. Monitoring and analysis of microseismic events can provide valuable information about the Earth's subsurface structure, as well as help in understanding and mitigating the potential risks associated with human-induced seismicity.
What is Microseismic Monitoring?
Microseismic monitoring is the passive observation of very small-scale earthquakes, seismic waves, which occur in the ground as a result of human activities or industrial processes such as mining, hydraulic fracturing, enhanced oil recovery, geothermal operations or underground gas storage. Microseismic science grew out of earthquake seismology and focuses on micro-earthquakes (i.e. magnitude less than zero). These micro-earthquakes are too small to be felt on the surface, but they can be detected by sensitive seismic equipment such as geophones and accelerometers.
What can Microseismic Monitoring Tell Us?
Microseismic monitoring is a valuable tool that provides crucial insights into the Earth's subsurface processes and the impact of human activities. At its core, microseismic monitoring seeks to address three fundamental questions about microseismic events: (1) when did the event occur, (2) where did it take place, and (3) how significant was the event in terms of magnitude. Traditional microseismic mapping techniques are employed to determine the location and magnitude of these events, enabling a better understanding of the subsurface dynamics.
By observing microseismic activity over time, operators can identify patterns of seismicity related to production activities, such as hydraulic fracturing, mining, and fluid injection. Recognizing these patterns allows them to optimize their operations, minimize environmental impacts, and mitigate potential risks associated with induced seismicity. Furthermore, continuous microseismic monitoring can contribute to a better understanding of natural seismic activity and help assess the stability of underground structures and infrastructure projects. In essence, microseismic monitoring plays a vital role in advancing our knowledge of the Earth's subsurface processes and ensuring the safety and sustainability of various industrial activities.
Microseismic fracture mapping schematic, unlike traditional 3D seismic technologies which measure acoustic reflections from an energy source, microseismic monitoring is a passive method, meaning that it listens for seismic energy which is already occurring underground. Passive seismic monitoring is also commonly referred to as "induced seismicity."
Passive wave energy methods provide a continuous 4D record of seismicity in the monitoring region, rather than individual snapshots in time obtained by conventional 3D seismic methods. Microseismic results are often delivered in real-time, and can literally offer a video recording of what is happening deep underground as a result of industrial operations.
Why is Microseismic Monitoring Important?
Microseismic monitoring is important for hydraulic fracturing (fracking) operations for several reasons:
- Optimizing the fracking process: By monitoring microseismic events during fracking, operators can gain insights into the effectiveness of the process. The data helps them understand how the fractures are propagating through the rock and whether they are reaching the target zones. This information allows operators in the energy industry to optimize the injection parameters, such as fluid pressure and volume, to maximize hydrocarbon recovery and minimize the risk of induced seismicity.
- Environmental and safety concerns: Microseismic monitoring helps identify any unintended consequences of fracking, such as the activation of nearby faults or the potential for induced seismicity. This information is crucial for ensuring the safety of the operation and addressing concerns about the environmental impact of fracking. Monitoring can provide early warning signs of potential issues, allowing operators to take corrective actions before problems escalate.
- Regulatory compliance: In some regions, regulatory agencies require oil and gas companies to conduct microseismic monitoring during fracking operations. This is to ensure that the operations are conducted safely and with minimal impact on the environment and surrounding communities. Monitoring can help demonstrate compliance with these regulations and provide valuable data for further research.
- Public perception and acceptance: The oil and gas industry faces increasing public scrutiny and opposition due to concerns about the environmental and social impacts of fracking. By implementing comprehensive microseismic monitoring programs, companies can demonstrate their commitment to transparency and responsible practices, which may help address public concerns and build trust.