Accurate and Reliable Assessment of Legacy Wells Using Numerical, Risk-Based Models – A CO2 Storage Case Study

Abstract

Abstract Assessing the leakage risks in Temporarily Abandoned (TA) or Plugged and Abandoned (P&A) legacy wells is a crucial step in well decommissioning and remediation projects. Being able to demonstrate the integrity of legacy wells is even more important when repurposing gas or oil reservoirs for carbon dioxide (CO2) storage. It thus, forms a key selection criterion when screening candidate fields. The risk-based, numerical model has been used to evaluate the long-term leakage and crossflow risks for alternative P&A well designs under various reservoir recharge and fluid injection scenarios. Quantified comparisons of multiple scenarios in terms of total risk, greenhouse gas emission or other benchmarks allows selection of the optimum "As Low As Reasonable Practicable" (ALARP) scenario. The model supports project planning and facilitates discussions with the regulators and other stakeholders by clearly quantifying the CO2 storage project's risks in terms of emission from legacy P&A'd wells. This paper describes how the leakage risk was modelled for legacy P&A'd wells located in highly depleted gas reservoirs planned for future CO2 storage. Risks of both leakage to surface and crossflow to overburden layers were evaluated for natural reservoir recharge and CO2 storage scenarios. The model quantifies risks of well CO2 leakage and crossflow and allows probabilistic comparison of alternative scenarios while accounting for the uncertain initial and long-term permeability values for cement. The subsurface well model was extended to simulate fluid migration in overburden layers. The integrated subsurface and well P&A model was used to analyse the impact of crossflow on the risk of leakage to surface from nearby legacy wells. The results allowed the operator to rank the legacy wells based on the risk of leakage. It is shown that legacy P&A wells are expected to have a low risk of leakage in a CO2 storage scenario. The risk of cumulative CO2 emission over 3,000 years even with pessimistic assumptions is expected to be only a very small fraction of the total volume of CO2 stored. These results supported both project planning and discussions with regulators when requesting a storage permit.