Analytical Investigation of Gas Production from Methane Hydrates upon Thermal Stimulation

Abstract

In this study, analytical models are developed to investigate methane hydrate dissociation in porous media upon thermal stimulation employing wellbore heating. This work investigates how the wellbore’s external and internal structure affect the dissociation process. The models are based on both Radial (2D) and Cartesian coordinates (1D) to couple the wellbore heating process and the associated methane response in the hydrate dissociation in the reservoir. Different types of heat-sources are studied: i) a flat heat-source in the 1D cases with a constant temperature; ii) line heat-source in radial cases with a constant temperature; iii) wellbore heat-source in radial cases, employing both a constant temperature and a coaxial wellbore. Wellbore’s external layers consist of casing, gravel, and cement. In the coaxial wellbore heat-source, both conduction and convection heat transfers are considered. It consists of an inner tube and an outer structure (casing, gravel, and cement layers). The analytical solution employed a similarity solution, in which a moving boundary to separate the dissociated (containing produced gas and water) and undissociated (containing only methane hydrate) zones is assumed, to model the dissociation in the reservoir. Two different operating schemes have been studied for water inlet of the coaxial wellbore heat-source: i) inner tube; and ii) annulus section of the wellbore. The results of temperature distribution along the wellbore (for the coaxial heat-source), temperature and pressure distributions in the reservoir, hydrate dissociation rate, and energy efficiency considering various initial and boundary conditions and reservoir properties are presented and compared with those of the previous studies. Direct heat transfer from the heat source to the reservoir in the case with a line heat-source higher dissociation rate and gas production compared to those of the wellbore-heat-source models, considering the heat conduction in the wellbore thickness causes. Decreasing the heat-source pressure and increasing its temperature increases the dissociation rate and gas production. Employing them simultaneously also increases gas production but reduces energy efficiency. The wellbore thickness affects the energy efficiency of the process negatively. The two different operating schemes affect the process in almost the same way with slightly higher gas production in the case with annulus hot-water inlet because the annulus is in direct contact with the reservoir. Increasing the inlet water temperature and decreasing the wellbore pressure simultaneously results in a greater gas production and energy efficiency. Porosity, thermal diffusivity, thermal conductivity, and thickness of the reservoir have direct relation with the dissociation rate, but the permeability and gas viscosity of reservoir almost do not impact the process. The wellbore parameters, such as water flow rate, inlet temperature, and wellbore radius except the inner tube radius, directly impact the wellbore mean temperature and the dissociation process. The findings of this study make a major contribution to research on methane hydrate dissociation upon thermal stimulation with wellbore heating by analytically demonstrating, for the first time, how the wellbore structure affect the process.