New Ultrasensitive Techniques for Reservoir Characterization and Monitoring of CO2 Sequestration Sites Offshore
Authors: Schrynemeeckers Rick 
Published: April 2022

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 and plugged and abandoned (P&A’d) wells, prior to the injection of CO2. Current offshore CO2 detection and monitoring falls into two basic categories: geophysical methods and CO2 testing methods at existing wells.
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 the majority of offshore CO2 monitoring methods are performed at or in existing wells. What happens when there are no wells in close proximity to potential spill points? CO2 leakage could be missed.
A new approach has been developed with passive geochemical sorbers which have been used onshore for reservoir seal evaluation and CO2 monitoring for some time. This new approach involves attaching passive geochemical sorbers to the bottom of Ocean Bottom Seismic (OBS) nodes or Autonomous Underwater Vehicles (AUVs) (Figure 1). The weight of the OBS node/ AUV pushes the geochemical sorber into the seabed floor, which is then left in-place for approximately 3 - 4 weeks. This provides time for the acquisition of high resolution OBS imaging as well as for CO2 molecules, CO2 tracer molecules, and hydrocarbon molecules to concentrate on the geochemical sorbers.
The end product is high resolution seismic data with ultrasensitive CO2, CO2 tracer, and hydrocarbon seepage data acquired simultaneously from co-located sites. The ability to overlay these disparate data sets allows for improved understanding of faults, natural fractures and CO2 leakage to a high degree of accuracy.
Two onshore case studies will be presented to illustrate the efficacy and potential of the tandem deployment mechanism. A site characterization survey in northwestern Oman demonstrates the ability of passive geochemical sorbers to identify and map elevated hydrocarbon signatures along certain fault segments, showing that an associated reservoir was unsuitable for sequestration purposes.
A CO2 monitoring program in Algeria 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. While CO2 tracers were detected in some production wells, the surface geochemical survey detected no CO2 tracers above spill points or around injection wells.
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, this system allows for movement and redeployment of seismic and geochemical sensors directly over potential spill points.
Additionally, passive geochemical sensors can be placed around P&A’d wells during the site characterization studies to determine the potential for leakage due to improper plugging or cement deterioration that may occur over time.