Integrated Monitoring Approach for Offshore Geological CO2 Storage

Measurement, monitoring and verification (MMV) are vital to ensure the conformance and containment of geological carbon storage (GCS). This requires cost-efficient and multidisciplinary approaches. To investigate this challenge in an offshore environment, we have studied and tested different monitoring approaches, covering seismic, electromagnetic, micro-seismic, active and passive sonar, and chemical sensing methods. The studies in the manuscript are based on laboratory- and field-scale tests. The data of our current interest are various as mentioned above, and for both deep- and shallow-focused monitoring. We measured laboratory geophysical data in the scenario of CO2 flowing through a fracture in a sandstone core sample (De Geerdalen Formation, Svalbard, Norway) to see the possibility of detecting leakage. The field-scale feasibility was also demonstrated through a synthetic modeling study. Laboratory acoustic emission tests were performed with North-Sea relevant rock samples to evaluate the micro-seismic applicability to offshore GCS monitoring. Acoustic and chemical sensor technologies are considered essential for marine monitoring of the seabed and water column, but knowledge and documentation on how to optimally use and combine these technologies is scarce. During a recent controlled CO2 release experiment, we have investigated the performance of different acoustic and chemical technologies for application to GCS monitoring. By quantifying the capabilities and limitations of different acoustic and chemical technologies, we aim to provide operators with the knowledge needed to maximize monitoring performance while minimizing the number of sensors and costly operations. First, it was learned through a laboratory rock physical test that electromagnetic signal is relatively sensitive to CO2 flow through fracture (and potentially faults as well) compared to seismic. The acoustic emission tests showed that reservoir sandstone core samples are subjected to induced seismicity, whereas the cap-rock or shale are rather quiet during these tests. To be conclusive, more tests and data analysis are required. Nevertheless, the up to date result indicates that detection of leakage in shale only via micro-seismic might be challenging. Initial results from the cotrolled experiments releasing CO2 to the water column indicate that a small amount of CO2 in gas phase may be detected from a large distance (100s of meters) using a broadband echo sounder. Passive acoustic detection of a small leak (1.15 l/min) was feasible from a distance of 10m. A plume of dissolved CO2 was detectable using chemical CO2 and pH sensors placed 4-10 m from the origin of the leak, when releasing CO2 at a rate of 5-6 l/min. Finally, we have investigated how to integrate the deep-focused geophysical and shallow-focused seafloor monitoring techniques. In our study, we have used a set of leakage scenarios (leakage path, rate, etc.) available in the literature. In addition, we have included into our discussion additional datasets e.g. surface/seafloor heaving and gravity not directly acquired in the current study but available through literature. We conclude that integrating different datasets and different disciplines are necessary to maximize the extracted information and eventually to save cost as well. In addition, relevant future R&D task candidates have been identified.