Mathematical models of gas hydrates dissociation in porous media

In strata hydrates may exist in different forms: as hydrates alone, hydrates in conjunction with free gas, or with gas and water in state of thermodynamic equilibrium, hydrates with excess ice or water, and other forms. A series of new mathematical models of gas production from gas hydrate reservoirs are proposed. The purpose of this work is to investigate dissociation problem for various initial states of gas hydrates fields under thermal stimulation and depressurization. The simplest case of a stratum in which the gas hydrate completely fills the pore space Is proposed. It is shown that there are two different regimes of hydrate dissociation. In the strata with a positive Initial temperature, the calculated dissociation temperature at the front may fall below the,vater crystallization temperature. This means that the hydrate does not satisfy the assumed decomposition into gas and water and the mathematical model has internal thermodynamic contradiction. In this case the hydrate dissociates with formation of an ice-gas mixture zone. The gas hydrate dissociation problem coexisting with gas Is considered. It is shown that for high permeability the front solution does not apply, because the local temperature of the hydrate in the zone ahead of the dissociation front Is higher than the local phase transition temperature calculated from the pressure distribution. In this case there are three zones: gas-water, gas-hydrate-water, and gas-hydrate. If the stratum initially contains gas, water, and hydrate in thermodynamic equilibrium the frontal model always gives a contradiction. It is shown that hydrate dissociation takes place in the extended gas-hydrate-water zone. For media with high permeability the amount of gas hydrate dissociated in this zone exceeds the amount of hydrate decomposed in the total dissociation zone by several orders.