Transient fluid flow simulation and optimization of cleanup in offshore low-permeability wells: Integration of core flooding and nuclear magnetic resonance characterization

Drilling fluid invasion into low-permeability gas reservoir pores results in a water block effect. The cleanup operation is essential at the start of gas well production, as it helps displace near-wellbore invaded fluids via gas flowback to restore reservoir productivity. However, a few studies have attempted to optimize the cleanup operations, and post-cleanup gas production rates often decline below expectations due to a lack of understanding of transient fluid flow mechanisms. The purpose of this paper is to clarify the physics of drilling fluid flow in near-wellbore porous media and provide effective designs for cleanup operations. To optimize the cleanup design, we first clarified the critical conditions for fluid mobilization across pore scales. By combining core flooding experiments with nuclear magnetic resonance analysis, this study revealed a hierarchical pore-size displacement mechanism: larger pores (>1 μm) are activated first, followed by intermediate pores (0.1–1 μm), while smaller pores (<0.1 μm) remain immobile. This pore-size-dependent mobilization results in a pressure gradient-dependent flow characteristic of invaded fluids in porous media. Accordingly, a transient gas flow model incorporating dynamic permeability was developed to describe the transient displacement behavior. Numerical simulations demonstrate that once the flowback gas rate exceeds a critical threshold (corresponding to the minimum pressure gradient for 0.1 μm pore activation) and cleanup time is also sufficient, all invaded fluids can be thoroughly cleared. This study provides a quantitative framework to optimize cleanup operations, particularly in offshore low-permeability gas reservoirs. The findings enable operators to balance flowback rates and duration, reducing near-wellbore damage and restoring formation productivity.