Analytical Models for Well Test Analysis in Class 3 Gas Hydrate Reservoirs

摘要

Well testing in gas hydrate reservoirs is challenging due to the complexity of reservoir behavior. Gas hydrates dissociate when the bottom-hole flowing pressure drops below the hydrate equilibrium pressure, a phenomenon also seen in CBM reservoirs below desorption pressures. Relative permeability of reservoir fluids keeps changing at the dissociation front, depending on the prevailing reservoir pressure, a phenomenon also seen in gas condensate reservoirs. If transient rate and pressure behavior of the hydrate formation are to be interpreted accurately, the analytical model for interpretation should consider both phenomena. This makes well testing in these reservoirs more challenging, not forgetting the multiphase flow and the endothermic dissociation of hydrates during pressure drawdown. This implies a continuously changing bottom-hole flowing temperature for every bottom-hole flowing pressure, unlike what is seen in conventional gas reservoirs. This paper presents new analytical models for well testing in class 2 gas hydrates. The heat consumption during hydrate dissociation, activated by a pressure depression, requires heat transport models to be incorporated in the diffusivity equation. By combining mass balance and energy balance techniques, a representative diffusivity equation for the reservoir behavior is derived. Constantly changing reservoir temperature, relative permeability and reservoir fluid properties make it imperative to use pseudo-pressure integrals to describe flow. The analytical solutions to the model are represented both for constant rate and constant pressure conditions for pressure transient and rate transient analyses, respectively. By expressing the analytical model in terms of dimensionless pseudo-parameters and considering the presence of free mobile fluid, for the constant terminal rate case, solutions provided van Everdingen et.al, Hantush et al., Jaeger et al. are implemented. Due to the dependence of hydrate dissociation on the bottom-hole flowing pressure, constant pressure solutions provide a good tool for investigating the reservoir behavior through rate transient analysis. With further simplifications of the numerical approximations of the constant pressure solution provided by Erdwardson et al., semilog and diagnostic plots could be made. Solutions to the constant rate and pressure are presented in this paper. A more general equation equation for rate decline analysis has been developed, with applicability in conventional gas reservoirs. A dimensionless compressibility-mobility function is incorporated in the model which is then related to the Arps decline curve models. Different methods of rate transient analysis are addressed in this paper to identify different reservoir parameters.