Hydrate Dissociation in Sediment

Abstract

Summary. An analytical model that describes hydrate dissociation under thermal stimulation in porous media is presented. The model views the dissociation as a process in which gas and water are produced at a moving dissociation boundary. The boundary separates the dissociated zone containing gas and water from the undissociated zone containing the hydrate. A similarity solution to the conservation equations is derived, and results are presented in graphical forms that are useful in numerical computations. In particular, heat fluxes, temperature profiles, and gas pressure distributions are presented for two cases that simulate saturated-steam and hot-water thermal stimulation. A parametric study showed that the dissociation rate is a strong function of the thermal properties of the system and the porosity of the porous medium. The energy efficiency of the dissociation process, defined as the ratio of the heating value of the gas produced relative to the heat input, was also computed. For hydrate thermal stimulation, an energy efficiency value of about nine was found, which appears encouraging for natural gas production from hydrate. Introduction Substantial amounts of natural gas hydrates have been found in the earth's sediment beneath the permafrost in Arctic basins and in ocean-bottom sediments along the continental margins of the U.S. Because of this potential resource, the problems associated with the production of natural gas from these hydrate zones have become of greater interest to the hydrocarbon industry. Preliminary simulation studies have been performed for the production of natural gas from hydrate reservoirs through thermal injection, depressurization, and in-situ combustion. Apart from gas production modeling, few studies are available on the heat and mass-transfer rates accompanying hydrate dissociation. Kamath et al. and Kamath and Holder viewed heat transfer during hydrate dissociation as a nucleate boiling phenomenon, while Selim and Sloan viewed it as a moving-boundary ablation process. Experimental results acquired by Ullerich et al. on the dissociation of methane hydrates agreed well with predictions from the ablation model. The heat-transfer models presented by Kamath et al. and Selim and Sloan are useful in modeling pure hydrate dissociation processes. For gas hydrates in the earth's sediment, the heat and mass-transfer rates are strongly influenced by the surrounding sediment. In this paper, an analytical model that describes hydrate dissociation in sediment under thermal stimulation is presented. Results from the model are used to evaluate gas production from hydrate reservoirs by steam or hot-water thermal stimulation. Dissociation Model Consider a uniform distribution of hydrates in the porous medium in Fig. 1, which is initially at a uniform temperature Ti and occupies the semi-infinite region 0 less than × less than infinity. Initially, the hydrates are assumed to fill the entire porosity, phi, so that 1 - phi is the volume fraction of the sediment. At time t=0, the temperature at the boundary x=0 is raised to a new temperature, T0, that is higher than Ti and is held constant thereafter. Hydrate dissociation commences, and as a result, a moving interface that separates the water/gas region from the undissociated hydrate region exists at some distance x=chi(t). Thus, at any time t greater than 0, the water/gas phase [dissociated-hydrates zone (Zone I)] occupies the region 0 less than × less than chi(t), while the undissociated-hydrate zone (Zone II) occupies the region chi(t) less than × less than infinity. Once the hydrate-dissociation temperature is exceeded, the phase interface will move forward at a decreasing velocity. The interface velocity decreases because of the insulating effect of the increasing thickness of the dissociated zone and also because of the energy required to elevate the temperature of the matrix material and resulting gas and water. A sharp density change usually occurs in the pore-filling material across the phase interface. According to the law of mass conservation, the gas will be streaming or transpiring toward the accessible or heated surface. The surface heat flux is consumed in supplying energyto heat the dissociated Zone I,to heat the dissociated gas streaming toward the heated surface and the water resulting from the dissociation process,to dissociate the hydrate at the moving interface, andto heat the undissociated solid matrix. Consequently, the following temperature and pressure distributions develop in the system:a temperature distribution TI(x, t) in Zone I with Td less than TI(x, t) less than T0;a temperature distribution TII(x, t) in Zone II with Ti less than TII(x, t) less than Td; anda pressure distribution p(x, t) for the hydrocarbon gas in Zone I with p0 less than p(x, t) less than pd. Here Td and pd represent the temperature and pressure at the dissociation front, assumed to be in thermodynamic equilibrium. Fig. 1 is a schematic of these distributions. The water resulting from the dissociation process is assumed to remain motionless and is retained within the pores of the dissociated zone. This assumption restricts the analysis to hydrate saturation values of about 0.3. For the sake of simplicity, we assume that the thermophysical properties of each phase are constant. We also neglect viscous dissipation, reversible work of compression, and inertial effects, and rule out the possibility of mutual or external energy transmission. With these restrictions, the differential mass, momentum, and energy balances describing the dissociation process may be written as (1)(2)(3)(4)(5) Eqs. 1 and 2 are the continuity equation and the momentum equation (Darcy's law) of the gas in the dissociated phase. Eqs. 3 and 4 are the energy balances for the dissociated and undissociated phases, respectively. Eq. 5 is the equation of state (EOS) of the gas, assuming ideal behavior; this may be easily modified through use of a more sophisticated EOS or compressibility factors. In the energy balance for the dissociated zone, it has been assumed that the transpiring gas always acquires the local temperature of the sediment--i.e., it reaches an equilibrium heat-transfer condition with the solid. In a similar investigation dealing with the mechanism of heat exchange in sweat-cooled porous materials, Weinbaum and Wheeler demonstrated that the local gas and solid temperatures are nearly indistinguishable throughout the porous structure. The boundary and initial conditions associated with the above equations are (6)(7)(8)(9)(10) SPERE P. 245^