Application of full-waveform event location and moment-tensor inversion for Groningen induced seismicity

Abstract

Induced seismicity from gas production has been identified as a significant problem within the Groningen Field, located in the northeast corner of the Netherlands. A key concern is the location, source mechanism, and magnitude of the microearthquake events, which are triggered by stress changes induced in the reservoir through pressure depletion. The resulting seismic energy release has been linked to building damage and social unrest in the area. The situation is compounded by the structural complexity of the subsurface with many hundreds of faults that crisscross the area, making earthquake location estimation challenging. An accurate methodology to determine both location and seismic mechanism is required. We present the results of an integrated full-waveform, 3D elastic, event location, and moment-tensor inversion workflow that has been applied to the regional, shallow borehole-monitoring array. This workflow utilizes all event arrivals to improve the accuracy of locating earthquakes and offers a reliable estimate of the earth's sense of motion during an earthquake. It is also demonstrated that the biggest source of error in locating earthquakes at Groningen lies in the vertical depth accuracy, which is correlated to the azimuthal angular coverage, and the fact that small earthquakes are only detected by a relatively small number of stations.

3-D mechanical analysis of complex reservoirs: a novel mesh-free approach

“…extensional stress field, such low-angle faults are very difficult to reactivate from reservoir compaction. In order to modify the structural model to be more representative of seismically reactivated, steeply dipping fault structures during gas depletion, such as observed in the Groningen gas field (Willacy et al 2018), the model was vertically scaled by a factor 3. Afterwards the z-coordinate was shifted upwards by 3 km in order to have the reservoir depth at approximately 3 km.…”

Small-aperture array as a tool to monitor fluid injection- and extraction-induced microseismicity: applications and recommendations

“…In fact, the average velocity model for the Northern Netherlands used to derive slownesses is not describing the complex structure of the Groningen gas field very well. The Rotliegend gas reservoir (average P-wave velocity v P =3.8 km/s) is overlain by anhydrite with a much higher velocity of v P =5.9 km/s and underlain by the Carboniferous with v P =4.25 km/s (Willacy et al, 2018). These high-impedance contrasts channel earthquake energy within the reservoir and result in significant mode conversions (Willacy et al, 2018).…”

“…The Rotliegend gas reservoir (average P-wave velocity v P =3.8 km/s) is overlain by anhydrite with a much higher velocity of v P =5.9 km/s and underlain by the Carboniferous with v P =4.25 km/s (Willacy et al, 2018). These high-impedance contrasts channel earthquake energy within the reservoir and result in significant mode conversions (Willacy et al, 2018). In addition, there are strong impedance contrasts between the Zechstein reservoir seal and the overburden as well as within the overburden itself, further complicating the propagation of seismic waves, such that seismograms recorded at the surface contain considerable P-to-S and S-to-P conversions (Willacy et al, 2018).…”

“…These high-impedance contrasts channel earthquake energy within the reservoir and result in significant mode conversions (Willacy et al, 2018). In addition, there are strong impedance contrasts between the Zechstein reservoir seal and the overburden as well as within the overburden itself, further complicating the propagation of seismic waves, such that seismograms recorded at the surface contain considerable P-to-S and S-to-P conversions (Willacy et al, 2018). Including single and multiple reflections seismograms are difficult to interpret (Willacy et al, 2018) and beamforming may stack converted instead of direct arrivals due to a wrong phase association.…”

Microseismic Event Estimation Based on an Efficient Wavefield Inversion

“…This penalty function, used to measure the source image focusing property, is applied as an objective function to optimize the source image and the velocity model [18]. Besides the source location and velocity inversion, FWI is also used to invert for the moment tensor [19], [20]. Most of the work mentioned above is based on the conventional FWI [21].…”