Formation mechanism of heterogeneous hydrate-bearing sediments

Abstract

Lack of knowledge on the formation mechanism of heterogeneous hydrate-bearing sediments (HBS) leads to difficulties of forming nature-like HBS, further resulting in unknown implications of HBS heterogeneity to the hydrate field exploitation. This work aims to reveal the unaddressed formation mechanism of heterogeneous HBS by theoretical hypothesis, model derivation, and experimental verification. The hypothesis that the water migration (including water diffusion and transportation) during hydrate formation plays a significant role in controlling HBS heterogeneity is firstly proposed. Since the dual effects of water migration on hydrate formation rate is non-negligible, a modified hydrate formation kinetic model is established by integrating the effects of water diffusion and water transportation on dynamic reaction area. Furthermore, three typical nature-like HBS (lumpy, layered, and homogeneous HBS) with the same hydrate saturation of 0.673 but different heterogeneities are formed by changing the initial water distribution. The proposed kinetic model successfully predicts that the average formation rate of lumpy HBS is 16 times faster than that of homogeneous HBS. This model solves the problem in describing the variations of formation rates of homogeneous and heterogeneous HBS. The hydrate distribution and morphology are also captured by X-ray Computed Tomography (X-CT) scans. The X-CT results further confirm that the spontaneous water transportation against gravity results in heterogeneous hydrate distribution and various hydrate morphologies in pores. Hence, the newly proposed formation mechanism and formation method of heterogeneous HBS in this work provide theoretical support and research method for future studies on efficient exploitation of naturally occurring heterogeneous HBS.