SS - Gas Hydrate:Numerical Analysis on the Rate-Determining Factors of Depressurization-Induced Gas Production from Methane Hydrate Cores
Authors: Konno Yoshihiro , Masuda Yoshihiro , Oyama Hiroyuki , Kurihara Masanori , Ouchi Hisanao 
Published: July 2010

Abstract
Depressurization is a gas recovery method to dissociate methane hydrate (MH) by lowering wellbore pressure below the hydrate stability pressure. Depressurization method is considered to be the most promising method because the highest energy profit ratio could be achieved among proposed methods. However, the gas productivity of MH production wells is much different depending reservoir properties such as permeability and temperature. In order to understand the key factors for depressurization method, we performed laboratory scale depressurization experiments using artificial MH cores and analyzed the rate-determining factors of gas production by numerical simulations.
There are three major factors to determine the depressurization-induced gas production rate: kinetic of MH dissociation, gas flow through the reservoir, and heat transfer to the dissociating zone. The gas production behavior significantly changes depending on these rate-determining factors. Thus, the analysis on rate-determining factors is crucial to understanding the gas productivity and is applicable for developing the strategy of depressurization-induced gas production.
In order to analyze the rate-determining factors, we calculated the potential methane fluxes generated by kinetic of MH dissociation, gas flow and heat transfer, and compared these methane fluxes. We conducted numerical simulations using MH21-HYDRES (MH21 Hydrate Reservoir Simulator) to calculate the potential methane fluxes during MH dissociation. From the calculation and comparison of the potential methane fluxes in the experiments, we concluded that gas productions in the laboratory scale experiments were mainly limited by heat transfer. For a low permeability core, the fluid flow dominated gas production behavior in a very early stage. However, the rate-determining factor transited from the fluid flow to the heat transfer immediately because the permeability increased with time by hydrate dissociation. The kinetic of hydrate dissociation had a limited role in determining the gas production rate from MH hydrate cores. We also discussed the experimental condition required to simulate the gas production behaviors in field scale reservoirs.
Analysis on rate-determining factors in field-scale hydrate reservoirs will be an issue in the future; it should give us a clue to 1) understanding the applicable reservoir conditions of depressurization method, and 2) developing the suitable Enhanced Methane Hydrate Recovery (EMHR) method for hydrate deposits expected poor productivity by depressurization.

Introduction

Background.
Gas hydrates are crystalline solids composed of gas and water. Methane hydrate (MH) is a natural gas hydrate found in oceanic and permafrost environments. Currently, MH deposits have been found at high saturations within reservoir-quality sands from the Eastern Nankai Trough and the Gulf of Mexico (Fujii et al., 2008, NETL, 2009). The " sweet spots?? of high concentrations in the ocean are expected to be the first targets for oceanic hydrate recovery (Sloan et al., 2008).
The three major methods of hydrate recovery are proposed:depressurization, in which the pressure is lowered below the equilibrium pressure;thermal stimulation, in which the temperature is raised above the equilibrium temperature; andinhibitor (such as methanol) injection, in which the equilibrium condition is shifted to lower temperature and higher pressure region (Sloan et al., 2008).
The depressurization method is considered to be the most promising method because the highest energy profit ration could be achieved among these methods.