Microseismic monitoring of both overburden and reservoir/basement induced seismicity is an essential part of almost any C02 sequestration project. Most of the induced seismicity is triggered by increased pore pressure due to injected C02 (e.g., Zoback, 2012) and re-activation of pre-existing faults. Hence, the observed microseismicity in the overburden is an indicator of seal breakage due to C02 penetration into the seal. Microseismicity in the basement is usually interpreted as the C02 plume causing increased pore pressure on pre-existing faults in the reservoir or basement (e.g., Williams-Stroud, et al., 2020). However, our understanding of the usual size and frequency of the induced seismicity that results from C02 sequestration is constrained by limitations of currently deployed induced seismicity although these are monitored (e.g., Sleipner, Aquistore, Otway). Furthermore, different types of monitoring arrays are used, from regional networks (e.g., Sleipner, Snøhvit) through local arrays with surface or shallow borehole receivers (e.g. Aquistore, Cranfield), downhole arrays (e.g. Weyburn-Midale, In Salah) to hybrid arrays with combined downhole and surface receivers (e.g. Lacq-Rousse, Decatur). As the injections of C02 need to last for exceptionally long periods of time and involve large volumes, microseismicity may occur at great distances from the injection wells.

Microseismic monitoring of both overburden and reservoir/basement induced seismicity is an essential part of almost any C02 sequestration project. Most of the induced seismicity is triggered by increased pore pressure due to injected C02 (e.g., Zoback, 2012) and re-activation of pre-existing faults. Hence, the observed microseismicity in the overburden is an indicator of seal breakage due to C02 penetration into the seal. Microseismicity in the basement is usually interpreted as the C02 plume causing increased pore pressure on pre-existing faults in the reservoir or basement (e.g., Williams-Stroud, et al., 2020). However, our understanding of the usual size and frequency of the induced seismicity that results from C02 sequestration is constrained by limitations of currently deployed induced seismicity although these are monitored (e.g., Sleipner, Aquistore, Otway). Furthermore, different types of monitoring arrays are used, from regional networks (e.g., Sleipner, Snøhvit) through local arrays with surface or shallow borehole receivers (e.g. Aquistore, Cranfield), downhole arrays (e.g. Weyburn-Midale, In Salah) to hybrid arrays with combined downhole and surface receivers (e.g. Lacq-Rousse, Decatur). As the injections of C02 need to last for exceptionally long periods of time and involve large volumes, microseismicity may occur at great distances from the injection wells. For example, Williams-Stroud, et al., (2020) show induced microseismic events more than 2 kilometers from the injection wells. Microseismic monitoring networks should be able to detect such seismicity and reliably map its depth to differentiate between overburden, reservoir and basement locations of induced events. The detectability of monitoring networks is dependent on the monitoring array geometry and types of sensors. This is a challenging task because downhole monitoring wells (Maxwell et al, 2010) while surface or near surface monitoring provides wide areal coverage but poorer vertical resolution (Duncan and Eisner, 2010). Hence optimization and costs of various monitoring networks need to be considered. 

We have developed the software to model the performance of a seismic monitoring network which consists of arbitrarily placed seismic stations (Hallo, 2012). We investigated combinations of various seismic monitoring networks including downhole, surface and near surface receivers. For each network we evaluated both the sensitivity and location accuracy and compared it with the cost estimate. The sensitivity was evaluated using the P- and S- wave arrival times of seismic wave propagation required to detect and reliably locate induced seismicity in a velocity and attenuation model of reservoir and overburden combined with noise levels that corresponded to receiver locations (Eisner et al, 2010). In summary, optimized microseismic monitoring networks for C02 sequestration allow accurate evaluation of induced seismicity at reasonable costs.