In 2017-2020, five world-leading research organizations from Europe and Australia executed the research project "Induced-seismicity Geomechanics for Controlled CO2 Storage in the North Sea" (IGCCS) to quantitatively relate induced-seismicity to geomechanical response, and to demonstrate how micro-seismic data can be applied as a key monitoring tool in the North Sea, ultimately ensuring storage conformance and containment. The contributing institutes are Norwegian Geotechnical Institute (Project Leader), NORSAR, University of Oslo, National Oceanography Centre, and Commonwealth Scientific and Industrial Research Organisation. IGCCS was supported financially by the Research Council of Norway (CLIMIT), Equinor and Total.
Micro-seismic monitoring is based on continuous measurement of seismic signals triggered by abrupt subsurface movements such as activation and/or re-activation of faults and fractures. Relating such data to geomechanical response of subsurface requires quantitative understanding of the couplings between geology, geomechanics and seismicity. IGCCS started with defining a representative geomodel for potential CO2 storage sites in the North Sea, based on relevant literature and proprietary database made available by the industry partners. This representative geomodel was the basis to all the lab tests and numerical simulation studies in IGCCS.
Then, we set up an extensive lab program of 16 triaxial tests. We acquired acoustic emission (AE) events and rock mechanical properties of North Sea relevant sediments and analogues, covering shallow, intermediate and deep storages. Then, we updated our North Sea geomechanical database, reduced uncertainty in the existing data-driven correlations, and finally quantified potential micro-seismicity during CO2 injection in the North Sea. We also observed higher strength and stiffness for the sample saturated only with scCO2 than with brine or with brine/scCO2 mix, demonstrating the impact of various fluid and saturations on the strain response. Furthermore, the brine/scCO2 mix-saturated sample produced the greatest number of AE events, compared to the brine and scCO2 fully saturated samples. In addition, the scCO2 saturated sample produced the greatest number of high magnitude AE events. The poorly-consolidated Visund sandstone tests showed no localization of failure plane, but large number of AE were observed starting even at very low strains. The more consolidated rock from Troll showed fewer AE events but a clear localization of failure plane. It should be noted that simulating poro-elastic unloading in sandstone representative for CO2 injection does generally produce no failure and very limited AE. However, large thermal cooling from CO2 injection (thermo-elastic) can increase shear stress and produce more AE especially for deeper and stiffer sandstone reservoirs. In contrast, the Draupne and Nordland cap rocks are aseismic in the observed frequency range and under the entire tested stress conditions, including when tested at extreme shear stresses far beyond what is representative for the CO2 injection scenario and when saturated in a brine and scCO2 mixture for the Draupne.
The numerical modelling workflow developed in IGCCS produced reasonable results and represented well our knowledge of the dominant physical processes and the published literature. Namely, supercritical CO2 injection, in comparison to brine, shows different stress and strain response, resulting from complex couple poro-thermo-elastic processes. We also observed that caprock behavior near injection well (e.g. <1000m) can be so complex that it is recommended first to acquire high-confidence material properties through lab tests so that we can minimize associated uncertainties, and then to apply advanced simulation approach. Our numerical simulation approach for multiphase-flow-driven fracture generation and propagation has shown good potential to improve our understanding of the key governing mechanisms and parameters. The approach can be scaled up to consider field-scale study, including fault re-activation and along-fault flow. We generated synthetic micro-seismic data to investigate slow seismicity and their possible characteristics within seismic records. Our motivation lies in the similitudes of the mechanisms between slow earthquake at tectonic scales and CO2 injection-induced seismicity at reservoir scale.
In summary, IGCCS has filled knowledge gaps that are relevant for micro-seismic monitoring and related geomechanical quantification of CO2 storage candidate sites in the North Sea, through advanced laboratory testing, THM-coupled numerical simulation, and field-scale seismic data processing and interpretation. IGCCS has delivered a new dataset of geomechanical properties from North Sea direct and analogue samples through its extensive laboratory program. This new dataset provides a solid foundation for upcoming research and commercial projects in the North Sea.
IGCCS researchers worked together in the various areas of geophysics, rock mechanics, and geology and across different continents (Europe to Australia), i.e. strong interdisciplinary and international collaboration. Extensive rock mechanical tests in the advanced laboratory produced new valuable datasets and updated existing industrial database, which becomes a solid foundation for various CO2 storage related projects in the North Sea. IGCCS developed a stress inversion of seismicity data and can tell which events are dangerous to avoid, and which are not. Temperature effect due to cold CO2 is found significant for deep storage and complex in cap rock. Northern Lights therefore seriously considers thermal stress for the Aurora storage (around 2.5km depth). So IGCCS suggests that storage operators should plan for advanced laboratory tests to consider thermal effect in cap rock under in-situ condition and reflect into advanced numerical simulation.
The future CCS projects in the North Sea will require a better understanding of the induced-seismicity potential of the representative lithologies in order not only to predict, but also to control any undesired leakage or induced-seismicity in the region, which in turn ultimately would maximize the injectivity (e.g. widening up the pressure limit windows), yet still making the operation secure and confident. The current project work is motivated by this requirement and will be based on advanced acoustic emission (AE) lab tests, combined with geomechanical simulations (coupled with flow and thermal effects). In addition, the upscaling of the AE lab data to the field scale will be investigated, which will be again specific for the North Sea, and can be developed for CCS monitoring applications. We will also focus on two particular fundamental physical process: 1) CO2 injection vs brine injection; 2) thermal effects. Finally, all the outcomes should be able to be implemented into existing industry workflows.
The scope of work in the proposal is established to fulfil the needs mentioned above, which is in turn to address and assist the following main themes in the offshore CO2 storage.
-Better understanding of storage capacity, injectivity and long-term effects of stored CO2
-Better understanding of mechanisms that prevent leakage of CO2
-Increased understanding of how pressure build-up in a CO2 storage can be handled
-Development of new and pioneering methods and equipment for monitoring, measurement and verification of stored CO2
-International cooperation to develop commercial CO2 storage in the North Sea.
The project consists of 3 scientific parts (WPs) as described below.
WP1: AE rock mechanical tests and material models
WP2: Coupled geomechanics modelling of CO2 injection-induced microseismicity
WP3: Application to field-scale monitoring