A comprehensive study of fines migration in internally unstable natural gas hydrate reservoirs

Evaluating unconsolidated reservoirs' internal stability concerning fine migration is essential for the commercial development of natural gas hydrate resources. In this pursuit, we use our newly developed visualized sand production cell and validated CFD-DEM model to study the fines migration dynamics in unconsolidated reservoirs. Our approach leveraged the novel Analysis Software for the Internal Stability of Granular Soils (ASISGS), equipped with three distinct criteria. Experimental observations revealed reservoirs with smaller fine particles experience intensified sand production, characterized by extended production durations and a heightened presence of fine particles in the produced sand. The fine particles move faster than the coarse ones. The ‘coarse matrix with floating fines’ is a premise for fines migration. The flow channel severely clogged by fine particles may reopen, leading to erosive bursts. Kezdi law and the Kenney and Lau criterion in ASISGS software successfully assessed reservoirs' internal stability, consistent with our experimental results. However, the Sherard criterion sometimes overestimated the reservoir's internal stability. Rather than a binary distinction, our research posits a continuum between internally stable and unstable reservoirs. From a CFD-DEM perspective, we discerned two predominant failure modes leading to sand production: erosion of fines and destabilization of the primary sand arch subsequently triggering a sand skeleton reorganization. The smaller the fine particle size, the slower the coordination number increase, the lower the coordination number the platform reach, and the more significant the fine particle migration. With the hydrodynamic drag force exerted by the fluid, fine particles move faster and farther than coarse particles. The formation process of the sand arch includes initial, migration, and equilibrium stages. Overall, this study offers valuable insights into fines migration and its impact on sand production, providing a foundation for effective sand management in natural gas hydrate reservoirs.