New Ultrasensitive Techniques for Reservoir Characterization and Monitoring of CO₂ Sequestration Sites Offshore

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.