2-2-2 尿素共存下におけるメタンハイドレート生成条件

抄録

Methane hydrates are widely deposited under sea around Japan. There are two types of methane hydrate deposition under the sea: Shallow-type methane hydrates and sandy-bed type methane hydrates. The former methane hydrates are on the seafloor at a relatively shallow depth. The latter sandy-bed type methane hydrates are at a relatively deep depth. The methane hydrates are stable only at high pressure and low temperature conditions. Water-head pressures at such depths stabilize the methane hydrates at subsea temperatures. In order to dissociate the subsea methane hydrates and to produce methane gas, it is necessary to destabilize the methane hydrates by depressurization of heating. Under unstable conditions, methane hydrates dissociate to be methane gas and water. When the subsea methane hydrates are dissociated, the gas and water flow in the two phase, i.e., gas and liquid phases, up to a sea level. In the case such as operation stop, the pressure and temperature conditions return to be stable conditions for methane hydrate, and the methane hydrates reform in the flow line. The methane hydrates would cause plugging of the flow line. To prevent the plugging by methane hydrates, inhibition techniques are necessary. Thermodynamic inhibitors are chemicals that can melt methane hydrates by freezing point depression. To avoid severe effects on environments, spill of the harmful chemicals should be avoided. Urea is one of the thermodynamic inhibitors which are environmentally-friendly. In the previous paper [Muromachi S., ....Fluid Phase Equilib., ], we report experimentally measured formation conditions of methane hydrates coexisting with urea at pressures below 5 MPa. Adding urea shifted the equilibrium temperatures toward lower region. In this study, we measure the formation conditions of methane hydrates in a higher pressure region, i.e., 5–13 MPa.