The authors developed a legacy-wells screening method for potential CO2 storage sites. The screening method enables project teams to evaluate qualitatively, in a consistent manner, the legacy wells risks at potential Carbon Capture and Sequestration (CCS) candidate sites. It does so by identifying the legacy wells current status, preliminary corrective action requirements and associated risks based on a specific data set that can be extracted from public well records.

For CO2 storage sites, legacy wells are existing wells that are located within the predicted CO2 plume and/or pressure front area. When assessing the feasibility of potential storage site candidates, legacy wells risk is an important variable for ranking the feasibility of potential locations against each other. Legacy wells risk is a result of not only the number of wells but also their leakage potential and the complexity of the corrective actions required. Often, only legacy well density (number of wells per squared kilometer) is evaluated, disregarding elements of risk such as well status, well age, well architecture, well mechanical status, etc.; if or when a risk level is assigned to the wells, it is done so in a subjective manner. A systematic method will be illustrated in this paper that evaluates existing well conditions and intervention requirements to evaluate legacy wells risk for potential storage sites.

To evaluate the legacy wells risk beyond only well density and to eliminate the subjectivity of screening, the authors used a method based on a ranking system that considers a few, critical, well characteristics to assign a risk score that can then be compared equally between potential locations. The ranking system assigns risk scores to legacy wells based on the availability and quality of well data, and in parallel the complexity and risks of each potential remediation or corrective action.

The paper will present a case study of a CCS site feasibility study where the workflow was applied to provide a consistent approach to legacy well assessment and qualitative risk evaluation for potential storage sites. Future CCS operators will benefit from this approach to improve decision making before selecting and moving forward with a storage site.

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