AN EXPERIMENTAL STUDY OF DISSOCIATION OF ARTIFICIAL METHANE HYDRATE SEDIMENT

Takao Ebinuma, Yasushi Kamata, Hideki Minagawa, Ryo Ohmura, Jiro Nagao, Hideo Narita
National Institute of Advanced Industrial Science and Technology (AIST)

We made a sandy core sample containing methane hydrate, hereafter an artificial core sample, and then studied the dissociation of the methane hydrate experimentally. The artificial core sample was made by exposing a sample of wet, packed sand to methane at the overburden pressure. The properties of the sample were the followings: average sand grain diameter 0.2 mm, porosity 40 %, and methane hydrate saturation equal to 60%. The core sample was 50 mm in diameter and 100 to 150 mm in length. We used a syringe pump to saturate the pore spaces in the sample with water and then began the dissociation experiment.

In the experiments, a core holder was used to apply a confining pressure to the core sample through a rubber sleeve. The confining pressure was kept constant by using the syringe pump. The design pressure of the core holder was 25 MPa. To accurately control the temperature of the dissociation experiment, a jacket was installed around the core holder for circulating brine from a temperature-controlled bath. The pore water pressure of the core sample was controlled independently of the confining pressure by using the syringe pump and a back pressure regulating valve. The two–phase gas–liquid flow produced by the dissociation of the methane hydrate was separated with a trap, and the flow rates of the methane gas and the water were measured with a gas flow meter and a balance. Pressures at both ends of the core sample and temperatures were continuously measured with pressure transducers and thermocouples. The gas recovery ratio was obtained by dividing the amount of methane gas that escaped from the core sample during dissociation to the total amount of gas produced by sample dissociation. We used two dissociation methods, decompression and thermal stimulation.

In the decompression method, the temperature around the core sample was kept at 285.7 K or 278.2 K. Pressure was decreased by adjusting the back pressure regulating valve, from 10.0 MPa at which the methane hydrate is stable in both temperatures, to a pressure that was 2.0-8.5 MPa lower, hereafter the pressure difference. Temperature in the core sample was decreased sequentially from the end of the core sample that connected to the back pressure regulating valve. The temperature and pressure changes indicated that the dissociation zone in the core sample progressed in one direction. The production rate of the methane gas was in the range of 6.0x10^-4-3.0x10^-1 m³/hr, a rate that increased exponentially with an increase of the pressure difference. The gas production rate was larger at higher boundary temperature of the core sample. The gas recovery ratio increased in proportion to the pressure difference, and had a small dependency on the boundary temperature.

The thermal stimulation experiments were done at a constant pore water pressure of 8 MPa by heating the core sample. The initial temperature was 281.2 K or 283.2 K, temperatures at which the methane hydrate was stable. The dissociation zone of the methane hydrate was made to progress in one direction by heating one end of the core sample using a heating plug in which a hot brine was circulated. The boundary temperature of the core sample was kept constant at 281.2 K or 283.2 K. The temperature difference between the boundary and heating surfaces ranged from 40.0 K to 80.0 K. The temperatures in the core sample rose sequentially from the heated side. At each point, the temperature rose again after it had become constant at 284.7 K, the phase equilibrium temperature of the methane hydrate at the pore water pressure of 8 MPa. The gas production rate was ranged from 3.6x10^-4 to 2.2x10^-3 m³/hr, a rate that increased in proportion to the temperature difference. The gas production rate by the thermal stimulation method was two orders smaller than that using the decompression method. The gas recovery ratio increased slightly with an increase of the temperature difference, but remained at 38% or less. The gas recovery ratio was also lower than that using the decompression method. Thus, according to the present experiments, the depressurization method is a more effective method to retrieve gas from methane hydrate.