Pore-scale analysis of relations between seepage characteristics and gas hydrate growth habit in porous sediments

Abstract

Seepage characteristics of gas hydrate in quartz sands play important roles in gas production from hydrate-bearing sediments. Reported seepage characteristics varied from different experimental conditions due to different growth habits of gas hydrate in pores. It is critical to capture the essence of seepage variation in the presence of gas hydrate as can be employed in the prediction of long-term hydrate exploitation. In this work, the gas hydrate with different growth habits (grain-contacting and pore-filling) is synthesized in quartz sands porous media. The pore structure characteristics of the hydrate-bearing porous media has been identified by X-ray computed tomography (X-ray CT). Experimental results show that the formation and growth of gas hydrate in pores lead to the occurrence of non-interconnected pores, further limiting the validity of pore structure analyzation and the accuracy of permeability estimation. Hence, the interconnectivity degree with definite physics meaning is proposed to clarify the variation characteristics of pore structure. According to the linear correlation of interconnectivity degree with hydrate saturation, a modified permeability reduction model based on Kozeny grain model has been proposed to estimate the permeability variation of hydrate bearing porous media with different hydrate growth habits. The modified model yields a good prediction performance as is evaluated by a number of permeability measurements from experimental studies and the normalized mean squared error (NMSE). Given that the gas production from hydrate dissociation is a more complicated process which involves multi-phase fluid flow, the modified model can be further applied in permeability estimation during gas production from hydrate-bearing sediments.