Characteristics Analysis and Stage Division of Class Ⅰ Gas Hydrate Deposit Developed by Vertical Well and Depressurization Based on Numerical Analysis

ABSTRACT

Class Ⅰ gas hydrate deposits are regarded as the most promising type for commercial development due to the underlying free gas layer, and they offer a relatively cheap and safe method for developing gas hydrates using a vertical well and depressurization. Therefore, this paper studies the production characteristics of Class Ⅰ gas hydrate deposits developed by vertical wells and depressurization, to provide a basis for productivity prediction and stimulation. On the basis of the gas production rate, a method is proposed to stage the gas production process. And then a new index is built to evaluate the gas hydrate contribution to gas production in different stages and analyze the influencing factors. The research shows that the gas production rate and decomposition gas decrease with time. The contribution of gas hydrate decomposition to gas production cannot be ignored. The gas and water saturations in the hydrate layer increase over time, while the hydrate saturation decreases. Deposit energy is rapidly consumed during the early production stage, which is one reason for the low gas production rate in the later stage. The production process can be divided into two stages according to the relationship between the gas production rate and the decomposition gas rate. The ratio of cumulative decomposition gas to cumulative gas production can be used to evaluate the contribution of the gas hydrate layer. The contribution ratio of gas hydrate is over 1 in Stage Two because the decomposition gas not only directly contributes to gas production but also sustains it by replenishing the deposit energy. High gas saturation reduces cumulative gas production due to the low effective permeability of the hydrate layer; therefore, heat or fracture stimulation can be used to enhance productivity. The gas production rate and cumulative gas production increase with the gas layer thickness. An increase in deposit energy can enhance productivity by accelerating hydrate decomposition in Stage One. Lower production pressure can promote gas production. To release productivity, production pressure can be kept as low as possible in Stage One, and methods for supplementing deposit energy can be considered in Stage Two.