Particle crushing in hydrate-bearing sands

Abstract

The depressurization of natural hydrate deposits to produce methane gas can increase the effective stress in hydrate-bearing sediments, which may induce sand particle crushing. Sand crushing may become detrimental to gas production, because the breakage of sand particles can cause sediment contraction, reduction in permeability, and the generation of finer particles exacerbating solids migration and sand production. This study experimentally investigates particle crushing in tetrahydrofuran hydrate-bearing sands. Sandy specimens with various hydrate saturation are loaded to different stresses (up to 40 MPa) in an instrumented oedometer cell to induce sand crushing. The results show that the presence of hydrate crystals inhibits sand particle crushing. More pronounced sand crushing occurs in specimens with lower hydrate saturation and under higher maximum effective stress. Hydrate crystals can cement neighboring sand particles and constrain the particles from rotating and rearranging during loading. Relatively larger particles in hydrate-bearing sediments experience surface grinding with no significant decreases in particle sizes, and shear-off asperities of the larger particles contribute to an increase in the proportion of smaller particles. The total volumetric strain in hydrate-bearing sediments due to loading can be estimated using the known hydrate saturation and the volumetric strain of hydrate-free sediments subjected to an identical load. The changes in hydraulic conductivity due to sand particle crushing are correlated to the breakage index. The results highlight that gas production from hydrate deposits using depressurization increases the effective stress and the loss of hydrate crystals can exacerbate sand crushing.