Mathematical modeling of gas production from gas hydrate reservoirs
MetadataShow full item record
AbstractEnormous quantities of methane gas are trapped in the form of hydrate in permafrost and offshore environments. As a result of increasing world energy demand, gas hydrates, which are natural gas molecules trapped in the structure of solid water molecule are being considered as a potential resource for clean energy. Much interest and research have been devoted in the last two decades towards the mathematical modeling of hydrate decomposition and gas production from hydrate reservoirs. Large resources of hydrate have been explored worldwide, including in the Northwest Territories of Canada, Siberia, Alaska and Japan. The main mechanisms involved in the process of hydrate decomposition and gas production are thermodynamic and kinetics of decomposition, heat transfer by convection and conduction, and gas-water two-phase flow. In addition to finding an appropriate mathematical method to solve the system of nonlinear equations, the treatment of the multi-scale physics that exists in the process of decomposition is one of the challenging aspects of the mathematical modeling. This dissertation has two main components: first, a 3-dimensional four-phase four-component numerical model for simulation of gas production from hydrate reservoirs is developed. The numerical model uses a control volume method to solve the system of nonlinear equations. Capillary pressure along with the relative permeability model is modified to account for the presence of the solid phase (hydrate and ice). Two kinetics models are used to represent reactions that turn hydrate to gas and water and also ice to water and vice versa. The validity of the numerical model is investigated, based on existing analytical and numerical solutions. The multi-scale nature of the problem is studied in both space and time. A new discretization formulation is used to overcome grid dependency of the problem. An operator splitting technique is suggested to separate selection of time steps for each mechanism and speed up the computation operation. In the second part of the thesis, the numerical simulator is used for engineering studies. The issue of ice formation, its effect on gas production and selection of criteria to prevent it are studied. The results suggest that a simple criterion can be defined to avoid freezing in the depressurization technique. The Joule-Thompson cooling effect may reduce the temperature, but it does not violate the ice formation criterion. The Joule-Thompson effect may prevent hydrates from decomposition unless the bottom-hole pressure is reduced to a certain point. Behaviour of type III hydrate reservoirs is investigated with the use of the developed simulator. Two similarity behaviours are identified that may aid in the development of analytical solutions.
Bibliography: p. 292-303