Effect of stress on permeability of clay silty cores recovered from the Shenhu hydrate area of the South China Sea

Abstract

Hydrate saturation and effective stress are the most important key factors in controlling permeability by altering the pore structure of hydrate reservoirs. However, the understanding of the dependence of the permeability on these factors in fine-grained hydrate reservoirs is not comprehensive due to limited experimental conditions. In this study, clay silty cores recovered from the Shenhu hydrate area of the South China Sea were used as skeletons to synthesize tetrahydrofuran (THF) hydrate-bearing sediments with hydrate saturations of 0–0.6. A series of step-by-step loading and water permeability measurements were conducted on a customized permeability measurement system. The results showed that the water permeability was in the range of 0.005–4 mD, approximately 0.03–1.47 mD near the value of the in situ effective stress of ∼2 MPa. The water permeability increased with increasing effective porosity and conformed to a modified Kozeny-Carmen equation, while the effective porosity increased exponentially with hydrate saturation and effective stress. Permeability stress sensitivity analyses showed that the normalized permeability stress sensitivity coefficient increased with the increase of hydrate saturation. Pore space compression and pore throat clogging were the important factors in the reduction in the water permeability, which dropped by approximately 95% when the effective stress increased from 0 to 8 MPa. This study provides insight into the evolution of pore space and permeability during gas recovery from hydrate-bearing clay silty sediments.