Plugging and abandonment of wells is an enormous undertaking, with an estimated 25-30 million wells worldwide currently requiring plugging and abandonment (P&A). While the vast majority of these wells were for hydrocarbon extraction, the search for better solutions to this fundamental problem can be considered industry-agnostic. In addition to future oil and gas exploration and production, several other use cases are fully integrated and dependent on the life-cycle of the wellbore (e.g., geothermal, critical resource extraction, CO2-storage, H2-storage, nuclear waste disposal). Each of these potential applications presents different fundamental problems. However, at the end of the well’s life-cycle, the baseline goal remains to create a seal that prevents interaction between the surface and subsurface. Shale, which is prevalent within these systems, could offer a solution that is simultaneously more reliable and cost-efficient than a manufactured solution (e.g., cement). Given the cost of P&A, we propose a systematic workflow for evaluating whether and how shale may provide a solution in advance of closure, or preferably even in advance of drilling. Furthermore, we demonstrate key components of this workflow utilising legacy data from a single field on the Norwegian Continental Shelf, with a specific focus on the Kimmeridge formation.
Introduction
P&A range from 10-30% of total well costs (Vrålstad et al., 2019; Raimi et al., 2021) with onshore wells averaging around $75,000 per well. Offshore, P&A alone can represent 40-80% of abandonment expenditures, and costs increase nonlinearly with depth. Average P&A costs for an offshore well is c. $28 million (Vrålstad et al., 2019; Raimi et al., 2021). Given these economics, abandonment methods should be considered early in the decision-making process.
Approach to P&A varies with geology, geography, and the applicable standard. Nonetheless, the basic elements for acceptance criteria are effectively the same. Seals are expected to be impermeable, long-term stable, non-shrinking, and ductile (e.g., NORSOK D-010, OEUK Decommissioning, ISO 16530-1:2017, APR RP 65-3, AER D-020). All standards require a primary seal and a surface plug, and many also require a secondary seal (e.g., NORSOK D-010, OEUK). The secondary seal can have the same properties as the primary seal but must act independently.
Interest in using shale creep for abandonment is growing, typically as an annular primary or secondary seal (Brun-Lie, 2017; Fjær et al., 2023; Arjomand et al., 2026). Its appeal is that it can, in some cases, meet or exceed required criteria at a lower cost (Brun-Lie, 2017; Fjær et al., 2023). However, determining when shale can serve as an appropriate primary, secondary, or dual seal requires further study, partly due to the complexity of shale itself (Brun-Lie, 2017; Fjær et al., 2023).
Shale spans a wide range of compositions (Ougier et al., 2016; Johnson, 2022), varying by as much as ~70% on a ternary diagram while still classified as the same rock (Johnson, 2022), compared to ~30% for typical sandstones (Johnson et al., 2025). Many components are mechanically and chemically complex and behave differently under a variety of thermal-hydro-mechanical-chemical (THMC) conditions (e.g., Huang et al., 2023). We therefore propose a workflow for incorporating P&A considerations into the exploration process and use a North Sea Kimmeridge Formation example to examine part of that workflow and the factors that make ‘shale creep’ a potential solution.
