Page 228 - Fundamentals of Gas Shale Reservoirs

208 PASSIVE SEISMIC METHODS FOR UNCONVENTIONAL RESOURCE DEVELOPMENT

Earthquake—An event that generates seismic waves due induced seismicity in a petroleum field that resulted from
to a sudden release of strain energy. injection (Rayleigh et al., 1976). The first successful,
Microearthquake (MEQ)—An earthquake with a magni­ modern, downhole passive seismic monitoring of a frac­
tude less than or equal to zero. ture treatment in an unconventional reservoir occurred in
the Barnett Shale in 2000 (Maxwell et al., 2002). To date,
Passive seismic methods are often taken as synonymous with downhole monitoring has relied on traditional earthquake
microseismic methods. However, the term microseismic seismology methods to locate MEQs. The first applica­
implies methods relying only on small‐magnitude earthquakes. tion of SET to surface or shallow buried array data for
Such studies typically use classical earthquake seismology hydraulic fracture treatment monitoring took place in
methods to locate earthquakes by observing the differences in June 2004 (Duncan, 2005; Lakings et al., 2006). The first
first arrival times of P (compressional)‐ and S (shear)‐waves at attempts at extraction of fracture images from SET data
multiple receivers. followed rapidly (Geiser et al., 2006). Since then, the field
We prefer to avoid the term microseismic to describe of passive seismic monitoring has grown tremendously.
the subject of this chapter because it is restrictive and Downhole methods have grown more sophisticated with
misleading. Recent developments in both the science and new analytical methods and especially with the use of
technology of hydraulic fracture monitoring show the multiple downhole arrays for more precise imaging.
importance of seismic energy sources other than micro­ Surface monitoring using SET is now offered by many
earthquake (MEQs). Such sources include long‐duration, service providers, and MEQ location and focal mecha­
low‐frequency emissions without distinct first arrivals, nism determination using SET are well‐established
and P‐waves produced by fluid pressure oscillations methods (e.g., Duncan and Eisner, 2010; this reference
(water hammering). Also, not all monitoring technologies provides a historical review of surface‐based methods).
rely on classical methods of earthquake identification, Direct fracture imaging methods also have become more
specifically: fully developed (e.g., Geiser et al., 2012) and have been
extended to monitoring ambient seismic activity (natural
continuous background activity) during 3D reflection
• Recent recognition of long‐period, long‐duration seismic surveying (Lacazette et al., 2013). Methods for
(LPLD) seismic activity and other types of activity using passive seismic data for frac production and reser­
shows that MEQs represent only a small fraction of voir simulation have advanced tremendously in the past
the total seismic energy produced during hydraulic few years. Advancements in modeling approaches are
fracture treatments (Das and Zoback, 2013a, b). greatly increasing the utility and hence value of passive
Also, we cannot rule out the occurrence of other as‐ seismic data.
yet‐uncategorized types of seismic activity distinct Other applications of passive seismic methods deserve
from both MEQ and LPLD during hydraulic fracture brief mention but are not the subject of this chapter and
treatments. will not be considered further. Passive seismic methods
• Passive seismic monitoring technologies relying on are widely used in the mining and geotechnical industries
seismic emission tomography (SET) do not identify to monitor the stability of excavations such as mines,
MEQs via classical seismological methods that rely on waste disposal sites, and large structures such as dams
the difference in P‐ and S‐wave arrival times. Also, and their foundations. Monitoring the integrity of water
SET identifies sources of seismic energy other than dams and levees is an especially important application of
MEQs. SET‐based methods are becoming ever more microseismic monitoring. A high‐pressure influx of fluids
widely used. As a result, these methods image energy into a levee or dam causes a significant increase in
from more types of activity than just MEQs, even over pressure in and along the structure. When the pressure
very short time intervals. reaches a failure threshold, preexisting fracture networks
reactivate. The failure of the rocks or reactivation of frac­
In conclusion, the term passive seismic is sufficiently general ture networks acts as a seismic source. Microseismic
to cover all types of seismic activity and the results of all monitoring arrays then detect the seismic signatures of
monitoring and analysis methods in current use. the dam or levee failure. The same applies to monitoring
The geothermal energy industry applied passive seismic volcanos—a large influx of fluids (steam/lava/water)
methods in the 1970s and 1980s to monitor both hydraulic causes a significant pressure buildup in a volcano. Passive
fracturing and natural fracture networks stimulated by seismic monitoring is an important component of volcano
injection and production. The oil and gas industry now warning systems. Passive methods are also used to monitor
uses passive seismic for these same tasks. The first appli­ induced seismicity from oil and gas extraction and seis­
cation of passive seismic in the oil and gas industry was micity related to water disposal and other activities (e.g.,
the Rangely experiment in 1976, which demonstrated Suckale, 2010).