For Class VI project developers evaluating carbon storage in mature oil-and-gas basins, two legacy conditions can materially affect cost, schedule, and permitting strategy:
- Basin-scale pore-pressure changes driven by decades of saltwater disposal (SWD)
- The density and condition of legacy wells within the area of review (AoR).
In basins such as the Permian, high-volume Class II SWD associated with unconventional development has altered pressure regimes across large areas, creating a legacy that Class VI projects must account for during site screening, AoR modeling, and corrective action planning. Lessons from SWD injection also show that when leakage occurs, it is commonly associated with pre-existing wells that intersect or connect pressure pathways, rather than failure of the confining unit itself.
Legacy oil and gas wells also pose a heterogeneous, often under-documented risk of carbon leakage. These penetrations range widely, from plugged-and-abandoned wells to idle wells with or without responsible parties to operating wells completed under outdated construction standards. Basins transitioning from hydrocarbon production to long-term carbon storage can expose weaknesses in cement or casing that were not problematic under prior pressure conditions.
Together, elevated pore pressures from long‑term, high‑volume SWD and the presence of poorly constructed or inadequately documented legacy oil and gas wells represent coupled risks that must be addressed to support the safe development of Class VI projects.
What This Means for Class VI Developers
Early in Class VI project development, even before detailed reservoir simulation, teams can reduce permitting churn and capital risk by treating pore pressure and legacy wells as primary siting constraints:
- Screen for pore pressure up front: compile SWD histories and regional pressure data to identify areas where elevated pore pressure could expand the AoR and corrective action scope.
- Build a defensible legacy-well inventory: compile well records identifying confining unit penetrations, wells with outdated construction, unknown plugging details, or missing mechanical integrity documentation. Developers may even consider a magnetic drone survey to identify undocumented or buried wells.
- Rank wells by likelihood and consequence: focus resources on wells that (a) intersect modeled pressure or plume pathways, (b) have poor cement/casing condition, or (c) are near known faults.
- Plan corrective action as a workflow, not a checkbox: develop cost and schedules for re-entry and remediation, and document early to inform site selection and capital outlay.
- Design monitoring with legacy wells in mind: align monitoring locations with the high-risk well clusters; use these results to refine corrective action planning as the project matures.
Recent studies and corrective action research supporting commercial Class VI permitting show that basin history can strongly influence carbon capture and storage feasibility and corrective action cost. In the Permian Basin, dense well populations and SWD-related increases in pore pressure can expand the areas under review and can drive legacy well remediation costs into the tens of millions of dollars. By contrast, carbon capture and storage projects in parts of the Gulf Coast and the upper Midwest often involve smaller areas of review and lower average mitigation costs due to fewer legacy wellbores. Still, many oil-and-gas basins remain suitable for storage if projects are sited to avoid clusters of higher-risk wells (e.g., high-volume SWD wells or older oil-and-gas wellbores with limited documentation), underscoring the role of early screening.
From a risk perspective, modeling of conservative leakage scenarios often suggests limited CO₂ migration volumes. In many cases, the largest impacts are less about environmental damage and more about operational delays and potential loss of 45Q tax credits. These findings highlight the value of proactive legacy well assessment, targeted remediation, and monitoring strategies tailored to the highest-risk well populations.
As more carbon capture and storage projects are developed, project viability depends heavily on understanding how SWD and legacy oil and gas development have altered pore pressures and confining unit integrity. Establishing best practices for identifying, ranking, monitoring, and remediating wells that penetrate the confining unit is essential. The path forward is not to avoid oil-rich basins but to proactively integrate legacy well management into Class VI project design, informed by decades of Class I and Class II injection experience.
