Ambient Seismic Imaging Throughout the Unconventional Field’s Life Cycle

Abstract

Technology Update Newly developed ambient seismic imaging methods provide valuable information throughout the life cycle of an unconventional field. Methods developed by Global Geophysical Services enable new insights for exploration, development planning, fracture and refracture design, reservoir management, production forecasting, and reserves estimation. Ambient seismic imaging has also been used to monitor conventional reservoirs, waterfloods, CO2 sequestration, mine stability, and water influx into mines. Additional applications include geothermal systems and induced seismicity monitoring. Many assume that the term “ambient seismic” (imaging artificial or natural seismic emissions) is synonymous with “microseismic” (imaging earthquakes with a magnitude less than 0, i.e., microearthquakes or MEQs). However, advances in the understanding of natural and induced seismic emissions and new imaging and analytical methods have differentiated these terms. Microseismic methods typically locate MEQs by use of classical seismological methods: polarization angles and arrival times of P and S waves. Thus, the term “microseismic” implies methods relying only on MEQs, the “dots-in-a-box” long familiar to fracturing engineers and development geologists. In recent years, research has shown that the majority of the seismic energy radiated during a fracture treatment is in seismic phenomena other than MEQs. Other emission sources include longperiod, long-duration (LPLD) activity (Das and Zoback 2013a and 2013b), long-duration signals (LDS) (Sicking et al. 2014), and fluid resonance in the fracture system (Tary and van der Baan 2012). These types of emissions cannot be analyzed with classical methods because they do not have distinct, noticeable first arrivals or recognizable P/S wave separation. Such emissions can persist or fade in and out for minutes to hours, rather than milliseconds, and therefore cannot be classified as distinct events such as MEQs. Furthermore, there may be other seismic phenomena that have yet to be identified and categorized. Seismic emission tomography (SET) is an alternative to locating earthquakes with P and S wave arrivals and polarization angles. It is in wide use for surface monitoring and most recently for downhole monitoring. SET focuses total trace energy by means of ray tracing through the velocity model in short time steps (on the order of 100 ms), thus imaging energy from many types of seismic emissions, including MEQs, LPLD, LDS, fluid resonance, and possibly unknown phenomena. Each computed SET time step produces a voxel-by-voxel depth image of seismic emissions in the same way that a 3D surface reflection seismic survey produces a 3D time image. MEQs located in individual time steps are termed “proxy hypocenters” because they represent brief, high-energy events. However, they are not identified uniquely as MEQs by classical methods.