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.