Analytical solution and numerical modeling study of gas hydrate saturation effects on porosity and permeability of porous media, An

Abstract

A gas hydrate is a type of crystallized compound formed by small gas molecules and water under high pressure and low temperature. Natural gas hydrate reservoirs exist mostly in offshore areas of outer continental margins, and some also occur in permafrost areas. Worldwide methane content in gas hydrate accumulations has an estimated volume ranging from 500 Tcf to 1.2 million Tcf. Economic values of these gas hydrate reservoirs are tremendous. A better understanding of the properties of gas hydrate-bearing reservoirs could lead to the development of novel safe and economic production methods. There are two major deposition types of gas hydrate in porous media: pore filling and grain contact cementing-not including those related to geomechanics effect during research of hydrate- bearing reservoirs. The difference between these two distribution types is related to a nucleation condition at the beginning of hydrate cluster formation. Previous work from Verma and Pruess, 1988, shows that hydrate distribution in the pore volume and pore throat depends on the length difference for correlation between the porosity and permeability change in porous media. The previous correlation only considers the contact cementing deposition type. This thesis focuses on the study of correlation models between permeability and porosity changes during formation and dissociation of gas hydrates in porous media, called the permeability adjustment factor. A series of equations has been developed based on consideration of different parameters in the correlation, such as the power factor and critical porosity. In previous permeability adjustment factor equations, permeability is calculated with the permeability equation based on a tubes-in-series model-only considering the contact cementing deposition type. This research first derives the permeability equation for the pore filling deposition type. To give a more comprehensive equation considering both types of deposition, MATLAB has been used to correlate permeability and porosity. This resulted in equations that involve a combination of pore filling and contact cementing deposition types. Finally, the TOUGH+Hydrate, T+H, numerical simulator was modified to include these equations to show the different results of production and saturation results for a depressurization process.