Production Behaviors of Water-Saturated Methane Hydrate Deposits during the Depressurization with/without Thermal Water Compensation Process

Abstract

Marine methane hydrate is an important source of energy for the future, attracting great attention of global researchers and many governments. The water-saturated hydrate deposit is more similar to the real environment of marine sediments. Yet, the production behaviors during the depressurization with/without thermal water compensation process are still unclear. In this study, the water-saturated hydrate deposits with initial conditions of 6 MPa, 3 °C, and 20% hydrate saturation are remolded. The basic decomposition method is simple depressurization to 2 MPa, and the compensation process proceeds by injecting a certain thermal water of different temperatures (20, 30, or 40 °C) into the inner deposit. The injected thermal is used for both the deposit temperature increase and the part hydrate decomposition. There are three stages of release, equilibrium, and free in the production process of water-saturated hydrate deposit by simple depressurization and another stage of compensation during the water injection. Irrelevant to the thermal compensation, the same amount of hydrates decompose in the most important equilibrium stage, in which the real-time pressure and temperature conditions change along the hydrate phase equilibrium line. Due to the effects of excess water on the depressurization rate and thermal supply, the effective temperature increase based on the phase equilibrium temperature is confirmed to be the deciding factor of decomposition rate of hydrates in the equilibrium stage. In addition, the gas/ water production of water-saturated hydrate deposit is jointly controlled by the water injection and hydrate decomposition. It is found that the decrease in water saturation is in favor of the gas production, and the gas saturation should be controlled over 25% by drawing water for a high gas production rate. The results of this study are significant to guide the spot production process of marine water-saturated hydrate deposits under depressurization or combined methods.

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