energies Article Influence of Water Saturation, Grain Size of Quartz Sand and Hydrate-Former on the Gas Hydrate Formation Yulia Zaripova 1 , Vladimir Yarkovoi 1 , Mikhail Varfolomeev 1,2, *, Rail Kadyrov 2 and Andrey Stoporev 1,2,3,4, *   Citation: Zaripova, Y.; Yarkovoi, V.; Varfolomeev, M.; Kadyrov, R.; Stoporev, A. Influence of Water Saturation, Grain Size of Quartz Sand and Hydrate-Former on the Gas Hydrate Formation. Energies 2021, 14, 1272. https://doi.org/10.3390/ en14051272 Academic Editor: Alexei Milkov Received: 9 January 2021 Accepted: 19 February 2021 Published: 25 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Physical Chemistry, Kazan Federal University, Kremlevskaya Str. 18, 420008 Kazan, Russia; yu-ya98@yandex.ru (Y.Z.); waldemaryarkovoi@gmail.com (V.Y.) 2 Department of Petroleum Engineering, Kazan Federal University, Kremlevskaya Str. 18, 420008 Kazan, Russia; rail7777@gmail.com 3 Nikolaev Institute of Inorganic Chemistry SB RAS, Ac. Lavrentiev Ave. 3, 630090 Novosibirsk, Russia 4 Department of Natural Sciences, Novosibirsk State University, Pirogova Str. 1, 630090 Novosibirsk, Russia * Correspondence: mikhail.varfolomeev@kpfu.ru (M.V.); stopor89@bk.ru (A.S.); Tel.: +7-843-233-7977 (M.V.) Abstract: The development of technologies for the accelerated formation or decomposition of gas hydrates is an urgent topic. This will make it possible to utilize a gas, including associated petroleum one, into a hydrate state for its further use or to produce natural gas from hydrate-saturated sediments. In this work, the effect of water content in wide range (0.7–50 mass%) and the size of quartz sand particles (porous medium; <50 μm, 125–160 μm and unsifted sand) on the formation of methane and methane-propane hydrates at close conditions (subcooling value) has been studied. High-pressure differential scanning calorimetry and X-ray computed tomography techniques were employed to analyze the hydrate formation process and pore sizes, respectively. The exponential growth of water to hydrate conversion with a decrease in the water content due to the rise of water–gas surface available for hydrate formation was revealed. Sieving the quartz sand resulted in a significant increase in water to hydrate conversion (59% for original sand compared to more than 90% for sieved sand). It was supposed that water suction due to the capillary forces influences both methane and methane-propane hydrates formation as well with latent hydrate forming up to 60% either without a detectable heat flow or during the ice melting. This emphasizes the importance of being developed for water–gas (ice–gas) interface to effectively transform water into the hydrate state. In any case, the ice melting (presence of thawing water) may allow a higher conversion degree. Varying the water content and the sand grain size allows to control the degree of water to hydrate conversion and subcooling achieved before the hydrate formation. Taking into account experimental error, the equilibrium conditions of hydrates formation do not change in all studied cases. The data obtained can be useful in developing a method for obtaining hydrates under static conditions. Keywords: gas hydrates; methane; methane-propane mixture; quartz sand; water saturation; gas storage 1. Introduction Gas hydrates have been known since the end of the 18th century. They are clathrate compounds, composed of small molecules located in the cells built by hydrogen-bonded water molecules, which can be formed under low temperature and high pressure condi- tions [1]. By structure, gas hydrates can be divided into three main groups, namely sI, sII and sH. Unit cell of sI hydrates is formed with 2 tetrakaidecahedron and 6 dodecahedron cavities; the unit cell consists of 46 water molecules; such a structure is characteristic of hydrates of methane, carbon dioxide, hydrogen sulfide, and ethane. sII structure consists of 16 dodecahedron and 8 hexadecahedron cavities; the unit cell contains 136 water molecules; such hydrates are formed by propane, isobutane, as well as nitrogen, oxygen, argon. Finally, unit cell of sH hydrates consists of 3 dodecahedron, 2 irregular dodecahedron and one irregular icosahedron cavities; the cell consists of 34 water molecules; sH type hydrates are Energies 2021, 14, 1272. https://doi.org/10.3390/en14051272 https://www.mdpi.com/journal/energies