Cutting-Edge Advances in Offshore CO2 Monitoring

Abstract Carbon Capture Utilization and Sequestration (CCUS) entails sequestration of carbon dioxide (CO2) which is injected into underground geologic sinks. Critical to the success of sequestration projects is detailed site characterization that includes reservoir assessment and evaluation of potential spill points such as intersecting faults, plugged and abandoned (P&A'd) wells as well as leakage up the CO2 injection wellbore. Current offshore CO2 detection and monitoring falls into two basic categories: geophysical methods and CO2 testing methods at wells sites. Geophysical methods:are critical for the evaluation and identification of potential spill points,can image the opening of natural fractures as pressure increases during CO2 injection,can map the CO2 plume as it moves across the field. However, geophysical methods do not answer the critical question is CO2 actually leaking? Rather, these methods highlight where CO2 may be leaking. The ability to determine if subsurface structures have adequate seal and whether those seals remain leak-proof is difficult since most offshore CO2 monitoring methods are performed at well sites. What happens when there are no wells near potential spill points? CO2 leakage could be missed. A new approach has been developed with passive geochemical sorbers which has been used onshore for reservoir seal evaluation and CO2 monitoring for some time. This new offshore approach involves attaching passive geochemical sorbers to the bottom of Ocean Bottom Seismic (OBS) nodes in deep water or using divers to place passive geochemical modules on the seabed floor in shallow transition zones. The modules are left in-place for ~21 days. This provides time for the adsorption of CO2 molecules, CO2 impurities, and hydrocarbon molecules to concentrate on the geochemical sorbers. The innovation of mobile deployment provides the ability to provide ultrasensitive detection of CO2 seepage at any potential spill point across the field. Impurities in industrial CO2 serve as a surrogate to validate that the CO2 source is subsurface and not ambient. The passive geochemical data can then be co-located with seismic data during the life of the program to provide an improved understanding and correlation between faults, natural fractures, and potential CO2 leakage. Two case studies will be presented to illustrate the efficacy and potential of the tandem deployment mechanism. A site characterization survey in 2012 in northwestern Oman demonstrates the ability of passive geochemical sorbers to identify and map elevated hydrocarbon signatures, from a depleted gas reservoir, along fault segments, identifying current leak points that rendered the reservoir unsuitable for sequestration purposes. A CO2 monitoring program in Algeria from 2015 – 2021 presents a case in which 3D seismic data indicated CO2 injection had activated a deep fracture zone several hundred meters wide, extending ~150 m above the reservoir. CO2 tracers were detected at the production wells, but no CO2 tracers were detected at the surface. This indicated that the opened natural fracture did not provide a sub-seismic pathway to the surface. The tandem technologies provide a unique capability to measure CO2 directly over potential spill points, not miles away. Furthermore, once CO2 injection has begun, and changes occur in the subsurface, the mobile deployment of the modules allows for movement and redeployment of seismic and geochemical sensors directly over areas of concern. Additionally, passive geochemical sensors can be placed around P&A'd wells as well as around the injection well to evaluate potential leakage due to improper plugging or cement deterioration. The paper will close with a summary of the lessons learned in performing real-world studies over the last 12 years.