RECOVERY OF GAS FROM IN SITU HYDRATE USING MICROWAVE HEATING

Fan Shuanshi^1,2, Li Dongliang^1,2, Liang Deqing^1,2
¹ Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, 510640, China;
² Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, 510640, China

Microwaves belong to the portion of the electromagnetic spectrum with wavelengths from 1 mm to 1 m with corresponding frequencies between 300 MHz and 300,000 MHz. The two most commonly used frequencies are 0.915 GHz and 2.45 GHz. Microwave heating is volumetric heating, energy transfer at a molecular level can have some additional advantages. In addition, the effect of microwave heating is correlated with the dielectric properties of the materials. Water is better medium for microwave heating and the ice is worse, the dielectric constant of hydrate is 58, and it is a litter lower than ice (is 94). So hydrate can absorb a certain microwave power.

Gas hydrate, is an ice-like substance composed of small molecule (e.g. CH₄, C₂H₆, CO₂) trapped inside cages of water molecules. It often forms under higher pressure and lower temperature, with deposits found underneath permafrost in Arctic regions and beneath deep ocean floors. Because as little as 10% of the recovered energy is required for dissociation, hydrate reservoirs have been considered as a substantial future energy resource. There is agreement that the total amount of gas in this solid form may surpass the energy content of the total fossil fuel reserves by as much as a factor of two. With the recognized reservoirs of hydrates in continental margins and in permafrost regions, the question arises about the feasibility and economics of recovery of gas from these substances.

There are many propose methods for gas recovery from in situ hydrate, one of the models is thermal stimulation such as injection of hot water or vapor. Some scientists put forward the use electromagnetic heating to obtain the energy required to dissociate hydrate. In 1985, Malone put forward the use microwave.

This work investigated the hydrate dissociation rule of microwave heating. A visual experimental setup for carrying out experimental studies of the dissociation of natural gas hydrate with microwave has constructed and tested. The frequency of microwave was 2450MHz and the microwave power can be adjusted up to 750 watts, and the dimension of test cell was 100ml. The natural gas hydrate was formed under the condition of the temperature range of 273.15K-275.15K and the pressure range of 4.5-5.5MPa before the microwave tests. The hydrate had slowly grown over a period of about 4 days in order that little free water existed. One of the gas samples was synthetic natural gas (91.95%mol methane, 5.07%mol ethane and 2.98% mol propane), and all microwave experiments were carried out with constant volume.

With experimental investigations, this paper clarifies the dissociation rule of different microwave power and different storage capacity of gas in hydrate. The result show that free water is trapped between solid hydrate particles. This free water may represent a large percentage of the hydrate volume. In dissociation procedure, the lower storage capacity of gas was more rapid than higher storage capacity of gas, and the speed of dissociation increase by microwave power, the total time of the hydrate can write (w is the power of microwave, and the hydrate was formed by water of 50ml, Figure 1).

Based on the results of these case studies, this paper also put forward the scheme for gas recovery from in situ hydrate use microwave heating (Figure 2). The equipment of microwave placed on platform, and the receiving antenna placed under the well. The antenna receives of the microwave output heat to increase the temperature of hydrate reservoir. The wellhead device gathered the gas from the hydrate dissociation.

Acknowledgments

This work was supported by Knowledge Innovation Program of Chinese Academy of Sciences (KGCX2-SW-304).

Figure 1. Hydrate dissociation speed with microwave power.