Abstract
Hydrocarbon wells are permanently plugged and abandoned upon the closing of production from a field. The physical domain surrounding the well—comprising all materials external to the casing and cement—can be defined as the external well environment. This environment includes geological formations, faults and fractures, pore fluids, and associated pressure regimes. To ensure long-term well integrity and prevent potential post-abandonment leakage, the well plug system must be capable of withstanding future variations in subsurface pressures. Such variations may arise from natural pressure re-equilibration processes, including overpressure development or dissipation of pressure from adjacent compartments within the reservoir. In this study, we developed and implemented a basin modelling approach to simulate water flow between reservoir compartments across semi-permeable flow barriers. This approach enables the estimation of new equilibrium pressures expected to develop in depleted reservoirs following well abandonment and provides a means to quantify the timescales associated with re-establishing pressure equilibrium. The methodology is demonstrated using a regional model comprising two major reservoir units located in the Norwegian sector of the North Sea. A series of simulations was performed to evaluate the effects of long-term pressure changes around plugged and abandoned wells, assuming depletion in three production wells. The simulations cover a period of 1,000 years post-abandonment. Both deterministic and stochastic modelling approaches were applied, incorporating variations in fault properties to account for uncertainties in lateral flow barriers, particularly fault transmissibilities. The results suggest that, for the target wells represented in the model, pressure redistribution following field depletion and well abandonment in the studied North Sea setting is generally limited in both magnitude and spatial extent. Consequently, the risk of long-term pressure-driven leakage along these abandoned wellbores appears low under the simulated conditions. However, the findings also indicate that pressure redistribution may exert a more significant influence in certain parts of the basin, particularly in areas characterized by high fault transmissibility, strong hydraulic connectivity, or contrasting pressure regimes between neighbouring compartments. These areas warrant further investigation to better constrain local pressure evolution and assess implications for well integrity and long-term subsurface containment. The presented basin modelling framework provides a “simple and fast” tool for evaluating post-abandonment pressure evolution, integrating geological uncertainties, and supporting risk assessments for permanent well abandonment and future subsurface utilization.