Abstract
Natural Gas Hydrate (NGH) has attracted increasing attention for its great potential as clean energy in the future. The main heat transfer mode that controls the thermal front movement in the process of NGH exploitation by heat injection was discussed through NGH thermal stimulation experiments, and whether it is reliable that most analytical models only consider the heat conduction but neglect the effect of thermal convection was determined by the comparison results between experiments and Selim’s thermal model. And the following findings were obtained. First, the movement rate of thermal front increases with the rise of hot water injection rate but changes little with the rise of the temperature of the injected hot water, which indicated that thermal convection is the key factor promoting the movement of thermal front. Second, the thermal front movement rates measured in the experiments are about 10 times that by the Selim’s thermal model, the reason for which is that the Selim’s thermal model only takes the heat conduction into account. And third, theoretical calculation shows that heat flux transferred by thermal convection is 15.56 times that by heat conduction. It is concluded that thermal convection is the main heat transfer mode that controls the thermal front movement in the process of NGH thermal stimulation, and its influence should never be neglected in those analytical models.
Highlights
Experimental materials and methodsThe Natural Gas Hydrate (NGH) synthesis and exploitation experimental simulation system includes air feed module, feed flow module, synthesis and exploitation module, environmental simulation module, back pressure control module, metering module, data acquisition and process module [22]
1 Introduction v The movement rate of the thermal front r The position of measuring point t Time T Temperature (°C) u The flow velocity of fluid (m/s) x Location along the porous medium (m) u Porosity dimensionless q Density (g/m2) C Heat capacity (kJ/(kg K)) k Thermal conductivity (W/(m K)) Q Heat flux transferred by convection heat transfer (W) Q1 Heat flux transferred by heat conduction (W) Q2 Heat flux transferred by thermal convection (W) h Convective heat transfer coefficient (W/(m2 K)) A Surface area (m2) d Thickness (m) tf Temperature of the hot water (°C) tw Temperature of initial Natural Gas Hydrate (NGH) reservoir (°C)
To gain insight into the effects of heat conduction and thermal convection on the thermal front movement rate, the heat flux transferred by heat conduction and thermal convection on the hydrate dissociation process under thermal stimulation is calculated theoretically
Summary
The NGH synthesis and exploitation experimental simulation system includes air feed module, feed flow module, synthesis and exploitation module, environmental simulation module, back pressure control module, metering module, data acquisition and process module [22]. Experimental procedures are as follows: (1) NGH isovolumetric formation: First, inject methane and water into the porous medium tube with a constant rate to the scheduled pressure 8–10 MPa. close the inlet and outlet valves and keep the temperature of the thermostat at 1.0 °C for methane hydrate formation. (2) NGH dissociation by thermal stimulation: set the temperature of preheat vessel at a certain value to heat the water, when it is steady, inject hot water into the porous medium tube at a constant rate. The temperature changes at each measuring point are recorded during the NGH thermal stimulation experiment [22,23,24]
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