The hydrate saturation in hydrate reservoir is usually very high, according to the geological exploration results (Lpa et al. , 2019). Thus, when the \(S_{h}\) reached about 35%, opened the outlet value of the reactor immediately, so the pressure in the reactor was controlled by the back pressure pump, this was the preparation for the gas–seawater flow. At 274.15 K, the phase equilibrium pressure of methane hydrate in seawater was 3.18 MPa (Lafond et al. , 2012), the back pressure in this study is 3.5 MPa or 4.0 MPa, which are all above the methane hydrate phase equilibrium pressure. The hydrate reservoir was usually in the state of water saturation (Almenningen et al. , 2018; Chong et al. , 2017). Therefore, to better simulate the academician state of hydrate in marine environment, the seawater was injected into the reactor using high–precision water injection pump at the flow rate of 0.5 ml/min. The seawater flow rate was low enough so that it can be considered that there was no hydrate decomposition during this process (Chen et al. , 2019c). After that, the gas and seawater were injected into the reactor at the same time using pumps. The temperature of injected methane gas and seawater was 273.95 K, which was slightly below the reservoir temperature, for the purpose to avoid the influence of heat injection on hydrate dissociation. The hydrate formation datas are showns in Table 1, it was obviously that all experimental conditions were inside the hydrate thermodynamic stable area to avoid the dissociation of hydrate due to other reasons (depressurization, temperature rise).

Methane gas and seawater were injected into the hydrate–bearing sediments to simulate the gas and seawater ascending in the seafloor sediments. Thus, gas–water two phase flow is a method to verify the sealing effect of hydrate reservoir. The water–saturated hydrate samples were made by injecting water into hydrate samples, as shown in Fig. 2. Furthermore, the gas–seawater flow rate ratio was constant at 4 for all Cases. The real–time characteristics of hydrate reservoir state was investigated using MRI visualization technology.