Experimental and Theoretical Investigation of Methane-Gas-Hydrate Dissociation in Porous Media

Abstract

Summary. Vast quantities of natural gas deposits exist in the form of solid hydrates. Methane gas hydrate was formed and dissociated for the first time in Berea core samples. A three-phase ID model was developed to simulate the process of gas production from Berea sandstone samples containing methane hydrate by means of a depressurization mechanism. The model closely matched the experimental data of gas and water production, the progress of the dissociation front, and the pressure and production, the progress of the dissociation front, and the pressure and saturation profiles. Introduction Natural-gas hydrates are crystalline, ice-like substances belonging to a class of compounds called clathrates. These chemical compounds have natural-gas molecules bound within almost spherical water cages through physical rather than chemical bonds. Two natural-gas-hydrate crystal structures result from the combination of these water cages. The crystal properties of both structures have been reviewed in detail. When all cavities of either structure are occupied, there are about 15 gas molecules per 85 water molecules. Therefore, the enclathrated gas concentration is much higher than the gas solubility in water (usually on the order of one part per thousand). The effect of this concentration is that each volume of hydrate can contain as many as 170 volumes of gas at standard conditions. Kvenvolden estimated that the total gas reserve in hydrates is 1.87×10(17) std m3. Many schemes have evolved for the recovery of such a large resource of natural gas bound in the solid state. The three most practical schemes are (1) thermal stimulation, in which an external practical schemes are (1) thermal stimulation, in which an external source of energy is used; (2) depressurization, in which the pressure of an adjacent gas phase is lowered to cause decomposition; pressure of an adjacent gas phase is lowered to cause decomposition; and (3) inhibitor injection, in which methanol or some combination of inhibitors is used to disequilibrate the system. Of these schemes, only the first two have been addressed through mathematical modeling. Thermal-stimulation models have emerged from four laboratories. Depressurization, on the other hand, has been the method used in the single example of hydrate production from the Messoyakha field in the U.S.S.R. Verigin et al. proposed an isothermal depressurization model in which heat was assumed to flow instantaneously from the surroundings to the hydrate body. Holder and Angert proposed a methane-hydrate global depressurization model in which the heat of dissociation came from the sensible heat of the reservoir itself. Burshears et al. extended this model to gas mixtures. Yousif et al. proposed a model similar to the Russian moving-boundary model, but allowed for a temperature gradient within the hydrated zone; this model, which has had some limited experimental confirmation, shows the inverse relationship of production with time, relative to the work of Verigin et al. In all the models proposed to date, the flow equations in a hydrate zone were not considered as a complement to the equations of mass and energy. One purpose of this work was to model the hydrate-depressurization process considering equations of change for both mass and momentum for each of the three phases present (gas, water, and hydrate) in the porous medium. It was our purpose to provide experimental measurements of slow, isothermal purpose to provide experimental measurements of slow, isothermal hydrate depressurization in porous media for confirmation or refinement of the model. Depressurization Model Consider a porous medium of uniform porosity occupying the region 0 less than x less than L. Initially, hydrate, water, and gas are at uniform pressure, p0, and temperature, To. At time t=0, the pressure at the boundary x=0 is lowered to a new pressure that is less than the equilibrium pressure, pe, and is held constant thereafter. Hydrate dissociation begins; pressure and saturation distributions develop throughout the system. Water and gas flow through the system because of the resultant pressure gradient and are produced at the boundary x=0. For simplicity, we assume that the depressurization process is carried out under isothermal conditions. Such an assumption is valid only if the boundary pressure is not far enough below the equilbrium pressure to ensure slow dissociation. With this restriction, the pressure to ensure slow dissociation. With this restriction, the transport equations governing hydrate dissociation and the flow of gas and water through the porous medium are (1) (2) (3) The quantity -mH in Eq. 3 represents the local mass rate of hydrate dissociated per unit volume as a result of depressurization. The quantities mg and mw in Eqs. 1 and 2 represent the corresponding local mass rate of gas and water produced per unit volume, respectively. These are related by (4) (5) The local gas-generation rate caused by hydrate dissociation may be obtained from the Kim-Bishnoi model: (6) During the dissociation process, the volume occupied by the gas and water continuously increases with time as a result of depletion of the hydrate phase. Accordingly, the PV occupied gas and water and the permeability of the porous medium will be changing continuously with time. Let phi wg denote the PV occupied by water and gas per unit volume of porous medium. Assuming that the hydrate phase forms a continuous film coating the interior of the solid phase forms a continuous film coating the interior of the solid surface of the porous medium, we may use the parallel-cylinder model to obtain the following relationship for the specific surface area occupied by water and gas: (7) SPERE P. 69