Influence of Gas Supply Changes on the Formation Process of Complex Mixed Gas Hydrates

Abstract

Natural gas hydrate occurrences contain predominantly methane; however, there are increasing reports of complex mixed gas hydrates and coexisting hydrate phases. Changes in the feed gas composition due to the preferred incorporation of certain components into the hydrate phase and an inadequate gas supply is often assumed to be the cause of coexisting hydrate phases. This could also be the case for the gas hydrate system in Qilian Mountain permafrost (QMP), which is mainly controlled by pores and fractures with complex gas compositions. This study is dedicated to the experimental investigations on the formation process of mixed gas hydrates based on the reservoir conditions in QMP. Hydrates were synthesized from water and a gas mixture under different gas supply conditions to study the effects on the hydrate formation process. In situ Raman spectroscopic measurements and microscopic observations were applied to record changes in both gas and hydrate phase over the whole formation process. The results demonstrated the effects of gas flow on the composition of the resulting hydrate phase, indicating a competitive enclathration of guest molecules into the hydrate lattice depending on their properties. Another observation was that despite significant changes in the gas composition, no coexisting hydrate phases were formed.

Numerical Simulation of Hydrate Formation in the LArge-Scale Reservoir Simulator (LARS)

“…From micro-to macro-scale, various laboratory experimental devices [3,[12][13][14][15][16][17][18][19][20][21][22][23][24][25] combined with state-of-the-art monitoring equipment [18,[26][27][28][29][30][31][32], have been developed to investigate gas hydrate formation processes based on different hypotheses and to determine the optimum parameters for hydrate production. Evidently, it is impractical and challenging to extract intact and undisturbed NGH samples from hydrate-bearing layers.…”

Heterogeneous and Coexisting Hydrate Phases─Formation under Natural and Laboratory Conditions

“…To further clarify the formation mechanisms of the coexisting hydrate phases in nature, experimental simulations were carried out. In the past, we already studied the influence of the changing gas supply on the hydrate formation process, which did not result in the formation of coexisting gas hydrate phases . Here, we present the results of our investigations on the transformation process of CH 4 hydrates to structure II hydrates when a multiple-component gas mixture was introduced.…”

“…To determine possible conditions for the formation of coexisting phases, in situ experiments were performed in a high-pressure optical flow cell. In our previous study, the effect of gas supply changes on the formation process of gas hydrates formed from complex gas mixtures was investigated . As mentioned before, the depletion of structure-II-forming components in the feed gas phase did not result in the formation of a coexisting structure I hydrate phase besides the thermodynamically preferred structure II hydrate phase.…”

“…They were able to demonstrate in laboratory experiments that, even in a closed system with a limited amount of a complex feed gas mixture, a heterogeneous hydrate phase initially formed, which changed its composition over time. At the end of the experiment, despite the depletion of the gas phase in higher hydrocarbons, only one structure II hydrate phase could be detected . It should also be noted, however, that, during the formation of both simple and mixed gas hydrates, coexisting phases could be observed as metastable transition phases. − Usually, these metastable phases are the thermodynamically less favored or less stable phases.…”

“…If there are no longer sufficient amounts of higher hydrocarbons for the formation of a structure II or structure H hydrate available in the feed gas phase, this may lead to the formation of a structure I CH 4 hydrate as a coexisting gas hydrate phase. This hypothesis of chemical fractionation of the feed gas phase was suggested as a possible explanation for the observation of the coexistence of hydrate phases in nature and laboratory experiments. ,, In contrast, the experimental work of Pan and Schicks showed that the depletion of the gas phase in structure-II-forming components does not necessarily result in the formation of a coexisting structure I hydrate phase . They were able to demonstrate in laboratory experiments that, even in a closed system with a limited amount of a complex feed gas mixture, a heterogeneous hydrate phase initially formed, which changed its composition over time.…”