(632f) Understanding the Phase Saturation Heterogeneity in CH4 Hydrate-Bearing Sediments from Formation to Dissociation

Methane hydrates have been considered as the future source of clean energy because of its vast resource volume (~3,000 trillion cubic meter) and high energy density (170 vCH₄/vH₂O). Energy recovery from this exciting energy source has attracted intensifying research and industrial interests. Past geological surveys and production tests have confirmed that hydrate saturation , S_H could vary significantly from 0 to 80% in geological media. More recently, there has been a paradigm shift from assuming a homogeneous region with uniform properties to a detailed depiction of multi-layered heterogeneous regions with serval sets of properties in assessing the fluid production potential from hydrate reservoir. However, the cause of the phase saturation heterogeneity in hydrate-bearing sediments is still lacking because of the limited number of hydrate cores extracted from the field and the lack of prior knowledge of hydrate formation under reservoir condition. Thus, it necessities the study of the kinetic behavior of methane hydrates in sandy media holistically from formation to dissociation under controlled conditions in laboratory by combining approaches of experimental observation and numerical modeling.

Herein, we conducted a series of experiments synthesizing aqueous-rich MH-bearing sediments with consistent high S_H > 40% and subsequently a series of dissociation experiments induced by depressurization. All the time-series data of observed pressure, temperature, and cumulative production of fluid (gas and water) were numerically analyzed by employing the state-of-the-art numerical simulator (TOUGH+Hydrate v1.5) coupled with a global optimization algorithm, i.e. particle swarm optimization. A series of key thermophysical parameters of hydrate-bearing sediments (e.g. absolute and relative permeability, specific heat, thermal conductivity, kinetic rate parameters, etc.) were identified through the history-matching process. Heterogeneous spatial distribution of hydrate was successfully derived through the process, which varies from S_H = 5% near the reactor top warm region to S_H = 60% near the reactor cooling boundary with average S_H = 42%. Through a sensitivity analysis, the relative importance of key transport parameters is identified and the controlling mechanism of fluid production from heterogeneous hydrate-bearing sediments is analyzed. The methodology developed in our study could provide high-resolution in-situ phase distribution of the hydrate-bearing core samples in the absence of direct visualization instruments. Moreover, the key thermophysical properties of hydrate-bearing sediments are important inputs for reservoir simulation predicting fluid production potential in future production tests.