Predictions of Mixed Hydrate Phase Equilibria and the Swapping of CH4 Hydrate with CO2 and CO2+N2 Mixtures.

摘要

Natural gas hydrates are likely to contain more carbon than in all other fossil fuels reserves combined worldwide1. They can be formed either by bacterial activity at shallow depths which contains mostly CH4 with trace amounts of C2H6 or by thermal pyrolysis of fossil organic matter which contains CH4 and significant amount of higher hydrocarbons (C2-C5) and non- hydrocarbon gases like CO2 . Thus, if CH4 stored in the natural gas hydrates can be recovered, 1 the hydrates could potentially become energy resource for the future2. Different techniques that are currently being proposed for the production of CH4 from gas hydrates include depressurization, thermal stimulation, and injection of inhibitors, all of which involve dissociation of gas hydrate and the release of significant amount of water causing geomechanical stress on the reservoir leading to subsidence. A newly-proposed method is to replace CH4 in the hydrate by pure CO2 and CO2+N2 mixture, which serves dual purpose of long-term storage of a greenhouse gas (CO2) and the production of natural gas (CH4)3,4 and this process will enable the ocean floor to remain stable because the hydrate maintain same crystalline structure even after replacement. To understand the production of CH4 gas from the hydrate reservoirs it is necessary to have knowledge of the phase equilibria of the mixed hydrates like CH4-C2H6, CH4-CO2, CH4- N2 etc. The phase equilibrium conditions of a pure hydrate changes significantly and the structure of the hydrate formed may also change due to the presence of other hydrocarbons. The structure of the mixed gas hydrate depends on the relative gas composition.