Features of natural gas hydrates formation of structures I and II in quartz sand with water and polymer solutions

The relevance of investigation of a multicomponent gas mixture hydrate formation in quartz sand with water and polymer solutions is due to the risk of the hydrate formation in the pore space of surrounding rocks under the complex effect of a gas injection and waterpolymer flooding in order to increase oil recovery at fields confined to the Nepa-Botuoba oil-and-gas bearing region (NB OGBR), which are characterized by abnormally low reservoir temperatures. These fields are located in zones of continuous and intermittent distribution of permafrost rocks, which, in combination with low values of heat flow and high heat capacity of the rocks composing their productive horizons, lead to abnormally low reservoir temperatures within 8–17 °C, which is 50–60°C lower than the temperature calculated by the geothermal gradient. Thus, the reservoir conditions of oil occurrence at the NB OGBR fields are in the hydrate stability zone of reservoir gases, and the transition of the gas to the hydrate state is prevented only by the lack of a sufficient amount of salt-free water. In the research, natural gas from the the Srednevilyui field was used as a hydrate-forming gas. Hydrates of this gas were obtained in quartz sand samples with a grain size of 0.4–0.3 mm. The sand moisture content of 17.6% was set by distilled water and the following polymer solutions: 1 g/L polyacrylamide solution, 5 g/L sodium carboxymethyl cellulose solution, and 30 g/L polyethylene glycol solution. Phase transitions during the hydrate formation and decomposition in “the natural gas-sand-water/polymer solution” systems were investigated using thermal analysis. It was shown that hydrates with cubic structures I and II are formed in systems under study. The gas analysis in the hydrate of structure II was carried out by the method of gas-adsorption chromatography. It was established that during the hydrates formation of structure II, the hydrate phase is enriched with hydrocarbons C2-C4, which is the reason for the shift of the thermodynamic conditions of their formation to the region of high temperatures and low pressures.