Abstract

A two-phase, three-dimensional numerical model was developed to simulate the dissociation and formation of hydrates (composed of water and any mixture of methane, ethane, or propane). This model was developed in order to address fundamental questions regarding the feasibility of production schemes for the utilization of this unconventional energy resource, gas hydrate, whenever a gas reservoir is in contact with a hydrate cap. These questions are also important in the design of conventional hydrocarbon resource production schemes from geologic zones in which hydrates are production schemes from geologic zones in which hydrates are predicted to be thermodynamically stable. Results of the predicted to be thermodynamically stable. Results of the simulations indicate that massive hydrate can be dissociated without an external heat energy source, and that the water evolved from hydrate disassociation does not impose limits on the producibility of the reservoirs. Additionally, gas from hydrates producibility of the reservoirs. Additionally, gas from hydrates was shown to contribute significantly to the produced gas stream. These results support the continued evaluation of gas hydrates as a potential resource, and indicate that conventional in situ recovery schemes may be effective in releasing gas from zones of hydrate stability.

Introduction

Gas hydrates exist in a solid, ice-like form that consists of a host lattice of water molecules that enclose voids, each of which may contain one molecule of a guest gas. Many different gases such as Ar, N2, CO2, and H2S, are capable of combining with water to form hydrates. Of primary interest as a fossil fuel resource are the hydrates that contain combustible, low molecular weight hydrocarbons such as methane, ethane, and propane. Hydrate resource estimates vary widely; however, in all cases, a vast amount of naturally occurring hydrate is predicted. Estimates suggest that conditions below as much as 90 percent of the Earth's ocean bottoms are suitable for hydrate formation, as are regions in and beneath the permafrost, which covers more than 23 percent of the Earth's land mass. Consequently, areas underlying approximately 75 percent of the Earth's surface have the potential for hydrate occurrence. Recently, it has been shown that hydrates exist in association with free gas and heavy oils which may further enlarge the resource estimate and which will have an immediate impact on the production of these conventional resources.

In order to exploit the hydrate resource, it is necessary to convert the solid hydrate into its fluid components. Such a conversion is represented by the following transformation:

(1) (Equation)

Where CnH2n + 2 is a hydrocarbon (n = 1-3), and m is the hydrate number (or the number of moles of per mole of gas in the hydrate phase). Typical values for m are given in Table 1. phase). Typical values for m are given in Table 1. A temperature-pressure phase diagram for the gas-water system, which summarizes the thermodynamic relationship which governs this process is in Figure 1. Above the equilibrium curve, hydrate is the stable form, while below it free gas coexists with either water or ice. This diagram can be to demonstrate the dissociation mechanism modeled in the present study; depressurization corresponds to a vertical crossing of the equilibrium curve. The simulation discussed in this paper is of a conventional gas reservoir with a hydrate cap (Figure 2). Production of the free gas results in a reduction in the gas pressure at the hydrate-gas interface, which destabilizes the hydrate.

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