Cost-effective Baseline Studies for Induced Seismicity Monitoring Related to CO2 Storage Site Preparation

Abstract

Induced seismicity has been identified as one of the key showstoppers for the safety of geological CO2 storage sites. The selection procedure for adequate storage sites therefore needs to include the assessment of the seismic hazard. Such studies include the mapping of active seismic faults and understanding their maximum magnitude potential. In addition, the injection and related pore pressure changes will result in stress field changes in, around and outside of the reservoir. These stress field changes can reach several kilometres outside of the field and have the potential to trigger seismicity on faults that are critically stressed, i.e. close to failure. To better assess the stress field conditions in certain regions, knowledge on the seismic activity of regional faults is important. This is generally conducted by recording a baseline of the seismicity. Having established such baseline seismicity, subsequent changes in the seismicity level can be rapidly identified during continuous monitoring. Hence, seismicity that is induced or triggered due to the injection, can be separated from the baseline seismic activity and evaluated accordingly. Based on the location, strength, mechanism and pattern of the seismicity, an estimate of both the short-term and long-term hazard can provide a decision basis on how to proceed with an injection programme. The establishing of a stable baseline-seismicity-estimate may require many years of recording within the area of interest, given a sufficiently dense deployment of seismometers. How long the actual recording time should last is dependent on the earthquake activity in the region and on the expected magnitude range that the baseline should cover. Ideally, a dense network of sensors can be deployed directly above the anticipated storage site, with sensors in shallow boreholes. In most cases of offshore CO2 storage, the deployment of a network of seismometers or geophones above the target storage site with the sole purpose of establishing a baseline is economically prohibitive. The main costs for such an offshore deployment are related to the shipping and sensor deployment, and different solutions for power consumption, which need to be properly evaluated. There is a variety of sea-floor sensors available, ranging from industry standard 4C geophones to broad-band Ocean Bottom Seismometers. Deployment methods also vary from rope installation to free-fall and pop-up to ROV operated installation. Noise from seismic interferences (shooting), platform noise and vessels also need to be considered, along with fishery/trawling routes and other offshore infrastructure. Consequently, a method to establish the baseline seismicity from land would be preferable. A very cost-effective, alternative approach to monitoring seismicity from far distances is the use of seismic arrays. By placing a number of seismometers in a certain spatial pattern, this technology takes advantage of the local coherency of the seismic wavefield while simultaneously reducing the noise. This methodology is well-known to and applied in the Comprehensive Nuclear-Test-Ban Treaty Organization, where seismic arrays are used for global monitoring. This technology is especially useful if a deployment of sensors close-by or above an area of interest is not possible. For offshore CO2 storage sites, the only cost-effective long-term installation would be onshore, which must take into account the distance to the target region, anthropogenic noise sources, property rights and other borders. To demonstrate this technology, a temporary seismic array has been installed on the Norwegian coastline with the sensor setup evaluated for both its ability for noise reduction and signal improvement. Together with single sensors along the coastline, this temporary array installation is analysed in terms of its detection capabilities and thus ability for providing an adequate seismicity baseline. Ultimately, such an array can easily be converted into a permanent installation and hence provide a continuous record of the seismic activity throughout the CO2 injection period and during the post-injection phase. The first results from a small sub-set of sensors provide encouraging results towards improved detection capabilities.