(515a) Prediction of the Production Condition of Natural Gas from Hydrate Deposits

Gas hydrates are crystalline molecular complexes formed by the physical combination of water and low molecular weight gases such as methane, ethane, and propane. They have the general formula Mn(H₂O)p, where one or more hydrates forming molecules M are called ?guest? associated with p ?host? water molecules. Recently, the importance of natural gas hydrates became emphasized because of its potential to be used as a new energy resource. To harvest natural gas from hydrate deposit, three different methods have been proposed; 1) heat stimulation 2) depressurization 3) hydrate inhibitor injection. Among these method depressurization method has been studied in this research since heat stimulation method and inhibitor injection method need high gas production cost.

This work focuses on the development of a new engineering tool to predict the possible production pressure of methane gas from gas hydrate deposits by applying the percolation theory. Percolation theory deals with the effects of varying the connectivity of elements in a random system. Two virtual sediments have been generated using Box Muller transform technique ? One having sediments porosity of 45% with average pore size of 70Å and standard deviation of 20Å and the other having sediment porosity of 35% with average pore size of 50Å and standard deviation of 30Å. Because of the activity change of water in small pores, the gas hydrate equilibrium pressure at constant temperature in pores would be a function of pore size. During depressurizing the sediments at constant temperature, some of the hydrates in smaller pores would dissociate while hydrates in larger pores would be stable. The possible production quantity of natural gas from hydrate deposit at given temperature and pressure would be calculated from the total amount of dissociated pores. However, even dissociated gas cannot be harvested without having proper connection between pores. The Hosen-Kopelman algorithm is applied to calculate the total number of clusters and average size of clusters consisting of dissociated hydrate pores, and also percolating point. The new tool will be possibly used to calculate the operation pressure for gas production from hydrae reservoir.