Minerals change the equilibrium condition and water transformation ratio of methane hydrates

Abstract

Natural gas hydrates(NGHs), as a promising clean energy source, are primarily found in seabed sediments and permafrost layers. Efforts are underway globally to develop efficient and economical methods for their extraction. However, improper drilling during extraction can lead to geological disasters and contribute to climate change due to large release of the green gas methane, which does not help with target selection, extraction strategy planning, and on-site production design. To investigate the influence of minerals on the phase equilibrium of methane hydrates and the characteristics of pore water conversion into hydrates, this study conducted quantitative analyses using a high-pressure differential scanning calorimeter (HP DSC). The experimental materials included feldspar and carbonate—the mainly mineral components in the hydrate reservoirs of the South China Sea, and foraminiferal sediments. The results reveal that the phase equilibrium curves of methane hydrates in feldspar, carbonate, and foraminiferal systems shift toward lower temperatures or higher pressures. The foraminiferal system exhibits the most significant shift, up to 3.51 K, which is attributed to its abundant surface structures. Thermodynamically, hydrate formation is inhibited within mineral systems, with the phase equilibrium shifts being more pronounced at lower water saturation conditions. Additionally, feldspar and carbonate systems achieve the highest water-to-hydrate conversion ratio (ranging from 80 % to 95 %) at low water saturation (S_w = 10 %), but the foraminiferal system attains its highest conversion ratio (81.43 %) at highest water saturation (S_w = 80 %). This work not only effectively explains the hydrate formation mechanism but also provides critical insights into the hydrate drilling and production.