Evaluating the Impact of Well Spacing on Depletion, Stress Redistribution, and CO2 Huff-N-Puff EOR Performance in Unconventional Reservoirs

Abstract CO2 huff-n-puff is increasingly recognized as a viable enhanced oil recovery (EOR) method for hydraulically fractured unconventional reservoirs, yet its performance is highly sensitive to operational design and development layout. This study applies a fully coupled compositional–geomechanical simulation framework to evaluate the influence of soaking duration and depletion interference from offset producers on huff-n-puff efficiency. A high-resolution reservoir model was developed in CMG GEM incorporating nine-component fluid characterization, laboratory-derived relative permeability functions, explicit multi-stage hydraulic fracture representation, and geomechanical coupling. The base case consisted of primary depletion followed by cyclic CO2 injection, soaking, and production. Soak durations of 3, 6, and 9 months were assessed in the absence of offset wells, and well-spacing sensitivity was evaluated by introducing a second producer at varying distances under a fixed soak period. The results highlight that both operational parameters and development configuration significantly influence the effectiveness of CO2 huff-n-puff in unconventional reservoirs. Extended soak periods can enhance CO2–oil mass transfer but may encounter diminishing returns beyond an optimal duration. Similarly, offset well interference can substantially reduce recovery when wells are placed too closely, underscoring the importance of spacing strategies in multi-well developments. These findings provide practical guidance for optimizing EOR design to improve recovery while balancing economic and operational constraints.