Enormous amount of latent heat generates/absorbs at the formation/dissociation process of gas hydrates and controlls their thermal condition themselves. In this paper we investigated the effect of ethane concentration on dissociation heat of mixed-gas (methane and ethane) hydrate. It has been reported by researchers that a structure II gas hydrate appears in appropriate gas composition of methane and ethane. We confirmed by using Raman spectroscopy that our samples had the following three patterns: structure I only, structure II only and mixture of structures I and II. Dissociation heats of the mixed-gas hydrates were within the range between those of pure methane and ethane hydrates and increased with ethane concentration. In most cases two peaks of heat flow appeared and the dissociation process was divided into two parts. This can be understood in the following explanation that (1) the sample contained both crystal structures, and/or (2) ethane-rich gas hydrate formed simultaneously from dissociated gas and showed the second peak of heat flow.

Dissociation and specific heats of synthetic methane and ethane hydrates were measured under high-pressure condition by using a heat-flow type calorimeter to understand thermodynamic properties of gas hydrates under submarine/sublacustrine environments. Ice powder was put into the sample cell and pressurized by methane and ethane up to 5MPa and 2MPa, respectively. After the completion of gas hydrate formation, samples were heated from 263K to 288K at the rate of 0.01 K min-1. Large negative peaks of heat flow corresponded to the dissociation of gas hydrates were detected in a temperature range 279-282K at a pressure of 5MPa for methane hydrate and 283-286K at 2MPa for ethane hydrate, respectively. We also obtained the specific heats of gas hydrates in the range 264-276K for methane and 264-282K for ethane under pressure.

Reservoir testing and analysis are fundamental tools in understanding reservoir hydraulics and hence forecasting reservoir responses. The quality of the analysis is very dependent on the conceptual model used in investigating the responses under different flowing conditions. The use of reservoir testing in the characterization and derivation of reservoir parameters is widely established, especially in conventional oil and gas reservoirs. However, with depleting conventional reserves, the quest for unconventional reservoirs to secure the increasing demand for energy is increasing; which has triggered intensive research in the fields of reservoir characterization. Gas hydrate reservoirs, being one of the unconventional gas reservoirs with huge energy potential, is still in the juvenile stage with reservoir testing as compared to the other unconventional reservoirs. The endothermic dissociation hydrates to gas and water requires addressing multiphase flow and heat energy balance, which has made efforts to develop reservoir testing models in this field difficult. As of now, analytically quantifying the effect on hydrate dissociation on rate and pressure transient responses are till date a huge challenge. During depressurization, the heat energy stored in the reservoir is used up and due to the endothermic nature of the dissociation; heat flux begins from the confining layers. For Class 3 gas hydrates, just heat conduction would be responsible for the heat influx and further hydrate dissociation; however, the moving boundary problem could also be an issue to address in this reservoir, depending on the equilibrium pressure. To address heat flux problem, a proper definition of the inner boundary condition for temperature propagation using a Clausius-Clapeyron type hydrate equilibrium model is required. In Class 1 and 2, crossflow problems would occur and depending on the layer of production, convective heat influx from the free fluid layer and heat conduction from the cap rock of the hydrate layer would be further issues to address. All these phenomena make the derivation of a suitable reservoir testing model very complex. However, with a strong combination of heat energy and mass balance techniques, a representative diffusivity equation can be derived. Reservoir testing models have been developed and responses investigated for different boundary conditions in normally pressured Class 3 gas hydrates, over-pressured Class 3 gas hydrates (moving boundary problem) and Class 1 and 2 gas hydrates (crossflow problem). The effects of heat flux on the reservoir responses have been addressed in detail.