Microseismic Monitoring
Sometimes, monitoring changes in pressure doesn’t provide all the information crews need. We can use changes in pressure to get a general idea of when fractures form and how much fluid is escaping through them. But what if we want details – how many fractures formed? How are they spatially distributed? Did any fractures extend out of the target formation?
Microseismic monitoring is the best technique we have for answering more detailed questions about hydraulic fracturing activities.
Traditional seismic imaging, employed during the exploration stage of production, requires humans to create an energy event using explosives or the hammering of metal plates mounted on trucks. In contrast, microseismic measurements record the vibrations caused by microseismic events. The creation of a fracture results in a series of small shock waves that spread in all directions. The waves are much too weak to be felt by humans, but by using multiple geophones to record the vibrations, hydraulic fracturing crews are able to pinpoint the location, time, and magnitude of each event.
This information can be used to determine many things about a fracture system, including:
- Fracture geometry and orientation
- Fracture complexity
- The total volume of rock fractured
- Whether any induced seismicity events occurred
Radioactive Tracers
Radioactive tracers are sometimes used to monitor fractures as well. Small amounts of radioactive isotopes can be included in the proppant or in the fracturing fluid itself. Once the hydraulic fracturing process is complete, a gamma ray logging tool can be run in the well. This tool will help the operators assess the movement of proppants and fracturing fluids during the hydraulic fracturing process.
The use of radioactive tracers is controversial from an environmental standpoint, and alternative fingerprints such as chemical indicators and tagged DNA fragments are currently under development.
Images: “Frac Monitoring” by Top Energy Training