Risk Based Approach to Identify the Leakage Potential of Wells in Depleted Oil and Gas Fields for CO2 Geological Sequestration

The selection of depleted oil and gas fields as a potential CO₂ geological storage site has both positive and negative aspects that need to be considered. The positives are that the storage capacity or pore volume can be reliably estimated from field’s production history, and reservoir characterization can be performed with more readily available well, log or seismic data without additional expenses. The main drawback is the presence of wells in the field, as each well may provide a leakage pathway for injected CO₂. The leakage potential of a well is a function of its proximity to injection wells, cement coverage in the potential storage zone, well abandonment conditions including cementing of the annular space, and the nature of any barriers to prevent CO₂ leakage to the surface. Qualitative and quantitative risk-based approaches can be used to identify the wells that have comparatively higher leakage probabilities in comparison to other wells. The objective of this study is to use a risk-based approach to identify and categorize wells based on their leakage potential in depleted oil and gas fields. This will not only help in planning injection strategies but may also help in selection of remediation strategies. The model is based on a Fault Tree Analysis (FTA) technique. It implements screening criteria and tier-based approach in which wells are screened and categorized into different tiers based on different well characteristics. The well characteristics includes physical distance from injection wells, the quality and portion of cement coverage of wells in the target zone and regulations at the time of well completion, the leakage potential of sealing barriers for the targeted zone, number of overlying shale and sand intervals and leakage of either CO₂ or brine to shallower wells, the nature and quality of permanent or temporary well abandonment procedures, quality and length of annular space covered with cement for shallower well casings or sections. An existing model for well leakage rate is coupled with the FTA technique. The risk of leakage is presented qualitatively and quantitatively in the form of a risk matrix. The leakage probability of a well and corresponding leakage rate forms the two axes of the risk matrix and wells having high leakage potential and rate can be identified. The transient nature of the processes involved and deterioration of barriers can be accommodated by discretely modeling the process at desired time intervals. The approach is used for a representative depleted oil and gas field in Louisiana to show an example application of the process. The developed model provides a means to systemically identify the wells that are more likely to leak and have higher leakage rates and may be a useful tool in the planning phase of the CO₂ sequestration process.