Fig. 1 . Schematic of the MRI apparatus.
Three high–precision syringe pumps (260D, Teledyne ISCO Inc, Lincoln, NE, USA) were used in the experiment to inject methane gas and deionized/seawater into the reactor and control the back–pressure of the reactor. Two thermostatic baths (FL300 and FL 25, JULABO, Seelbach, Germany) were used to control the temperature of high precision pump and the reactor. The pressure sensors (3510 CF, Emerson Electric Co., Ltd., St. Louis, USA) with an accuracy of ± 0.05% were connected at the inlet and outlet of the reactor to measure the pressure. According to the results from the core analysis of SHSC–4 (Shenhu area of South China Sea), the mean effective porosity within interval “a” is 35% (Li et al. , 2018). Therefore, the glass beads (As–One Co., Ltd., Japan) with a porosity of 35.4% (BZ02) was used to simulate the porous media of marine environment, giving a diameter of 0.177~0.250 mm, so that the maximum of hydrate saturation in theory is higher than 35%. The seawater (deionized water with a salt mass fraction of 3.5%) was used during the flow process, which was more closed to the actual marine environment. In addition, the methane gas with a purity of 99% used in the experiments was produced by Dalian Special Gases Company.
2.2Procedures
Hydrate sample was made through sand filling, vacuum pumping, water saturation, water displacement and pressurization. Firstly, the glass beads were compacted into the reactor, and the reactor was vacuumed for an hour. Secondly, the outlet valve of the reactor was closed, and the reactor inlet was connected with the water injection pump, the pressure and temperature were constant at 6.0 MPa, 274.15 K, respectively. This was the water saturation process and maintained for about 30 min. Thirdly, closed the water injection pump and opened the reactor outlet value, the reactor pressure dropped to atmosphere. The water in the reactor was displaced to the initial water saturation around 39% using gas injection pump, the initial pressure of gas injection pump is 2.5 MPa. Finally, closed the reactor outlet value and maintained the pressure at 6.0 MPa using gas injection pump, the hydrate formation process began at this point. The entire process was recorded by MRI system and the hydrate saturation (\(S_{h}\)) could be calculated by mean intensity (MI) value through follow equation (Chen et al. , 2021):
\begin{equation} \text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }S_{h}=1.25\times\frac{(I_{0}-I_{i})}{I_{0}}\times S_{w0}\times 100\%\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (1)\nonumber \\ \end{equation}\begin{equation} \text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }S_{w0}=\frac{I_{0}}{I_{\text{full}}}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (2)\nonumber \\ \end{equation}
where \(I_{0}\), \(I_{\text{full}}\) and \(I_{i}\) were MI values of initial water saturation, complete water saturation and water saturation at time \(i\), respectively. \(S_{w0}\) represented initial water saturation and can be calculated by equation (2). Additionally, the water saturation at time \(i\) (\(S_{\text{wi}}\)) can be calculated as follow:
\begin{equation} \text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }S_{\text{wi}}=\frac{I_{i}}{I_{\text{full}}}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (3)\nonumber \\ \end{equation}
Table 1 . Conditions and resultes of hydrate sample in all cases.