Assessment of CO₂ Leakage Risk in the Annulus of Wells

Background
Williams et al. demonstrated in 2009 that certain rock types can creep, and can completely seal the annulus outside the casing against potential fluid flow. This phenomenon is now utilized in the permanent abandonment of oil and gas wells, as creeping rocks have significantly better sealing properties than, for example, conventional cement. Additionally, this simplifies the abandonment operation considerably. Recently, both Equinor and AkerBP reported that creeping rocks in the North Sea can be identified based on their smectite content (> 45%, “Deal with the seal”, 2023).

Physiq AS have developed a new method for predicting smectite content from well logs. The technology has been validated on more than 30 wells from 9 different fields in the North Sea and Norwegian Sea. With this method, we can determine in-situ smectite content at much higher resolution (~1 meter) than the standard method (~10–50 meters), which is X-ray diffraction (XRD) analysis of cuttings. This enables highly accurate identification of creeping rocks.

Areas being considered for CO₂ storage often contain old oil and gas wells. The annulus in these wells can represent a potential leakage path for CO₂. Cement logs from older wells have shown that there are often leakage paths between the cement and the formation, meaning these plugs do not always function as tight barriers. The annulus in new wells drilled for CO₂ injection can also be a potential leakage path.

The main idea of this project is to utilize information about smectite content along the wellbore to assess the likelihood that the annulus is, or will become, sealed by rock creep, and to use this to evaluate the risk of leakage.

References:
Williams, S., Carlsen, T., Constable, K., Guldahl, A. (2009): Identification and Qualification of Shale Annular Barriers, Using Wireline Logs during Plug and Abandonment Operations, SPE/IADC 119321.
Deal with the Seal (2023), presented at NPF webinar, September 2023.

Goal
Sub-goal 1: Determine the smectite content in selected wells within an area relevant for CO₂ storage.
Sub-goal 2: Assess the likelihood that the annulus is sealed by rock creep.
Sub-goal 3: Evaluate whether, and if so how, this affects the risk of leakage from potential CO₂ storage sites.
Sub-goal 4: Develop a simple graphical user interface to demonstrate and visualize the results.

Activities
Activity 1: Identified smectite content in selected Volve wells.
Activity 2: Assessed the likelihood that the annulus in these wells is sealed by rock creep.
Activity 3: Evaluated how this affects the risk of leakage from the Utsira Formation, Ty Formation, and Grid Formation.
Activity 4: Developed a simple GUI for visualizing creep in the wells described above.

Results
Found that there is a low probability that creep seals the annulus above the Utsira Formation. Also found that there is little likelihood that the annulus above the Grid and Ty Formations is sealed. However, there is considerable uncertainty regarding the model and associated parameters that best describe creep in shale layers.

Further Work
Physiq, found a more suitable creep model for shale-rich rocks.