Optimization of passive seismic monitoring network for CO2 sequestration in Saudi Arabia

Abstract

This study assesses the effectiveness of various seismic monitoring arrays, including surface-based, shallow borehole, and Distributed Acoustic Sensing (DAS) arrays, for detecting microseismic events at a potential sequestration site in Saudi Arabia. The analysis focuses on two key parameters: the sensitivity of the arrays in detecting seismic events and the accuracy in locating detected events. Sensitivity is quantified by determining the minimum detectable moment magnitude at three depth intervals of interest: the seal, reservoir, and underburden layers. Results indicate that surface-based and shallow borehole arrays are more effective at detecting weak (around moment magnitude 0.5 and weaker) seismic events at shallow layers, whereas DAS arrays exhibit significantly reduced sensitivity at greater distances from the monitoring borehole due to increased attenuation. The study also examines location uncertainty caused by variations in seismic wave arrival times and the impact of different array configurations. While DAS arrays can detect seismic events, their ability to accurately locate events is limited by their sensitivity to horizontally propagating waves, particularly at greater distances from the monitoring borehole. For effective microseismic monitoring, the study concludes that DAS arrays should be spaced between 2 km and 4 km, while surface and shallow borehole arrays are preferable for monitoring the top seal formation. Furthermore, surface and near surface arrays are able to differentiate between seismicity from the seal, reservoir, and underburden subject to accurate velocity model.