TOC graphic:
Introduction
Vast quantities of energy that mankind craves hidden at the bottom of ocean, natural gas hydrate (NGH) is a representative one (Huang et al. , 2011). NGH only form under low temperature and high pressure area with sufficient gas and water (Zhong et al. , 2016). Seismic reflection method is one the most effective method to detect marine gas hydrate (Tian and Liu , 2020). There are two chief kinds of seismic waves: body waves and surface waves, body waves are mainly used in geological survey. Body waves are divided into longitudinal waves (P waves) and shear waves (S waves). For hydrate in sediments, S waves are more sensitive than P waves, and seismic surveys take the advantage of this acoustic property to detect NGH (Helgerud et al. , 2011). The bottom simulating reflector (BSR) on seismic profile is considered the mark of interface between gas hydrate area and free gas area. Above the BSR, the natural gas exists in hydrate form, and the natural gas exists in the form of free gas below (Petersen et al. , 2007). Well logging is another important geophysical method in gas hydrate detect besides seismic reflection, mainly include resistivity log, spontaneous potential log, caliper log, density log and so on (Ning et al. , 2013).
According to preliminary statistics, the total amount of stored NGH is approximately 2.1\(\times 10^{16}\) \(m^{3}\), nearly twice of the traditional fossil fuel (oil and natural gas) reserves in the world, and the total NGH stored in deep sea sediment is up to 99% (Makogon , 2010). In the South China Sea, the NGH resourced is up to (64.35–77.22)\(\times 10^{9}\text{\ t}\) of oil equivalent (Liu et al. , 2019), amounting to about half of the total resources of onshore and offshore oil and gas in China (Shi et al. , 2019). So far, two NGH production test were carried out in China. In 2017, the China Geological Survey conducted the first production test in Shenhu area (Li et al. , 2018). From October 2019 to April 2020, the second offshore NGH production test was conducted in 1225 m deep Shenhu Area (Liang et al. , 2020). The success of second production test indicates that safe and effective NGH exploitation is feasible in clayey silt NGH reservoirs (Qiang et al. , 2020). Meanwhile, multiple techniques have been tested in Shenhu drilling area. The amplitude behaviors of gas hydrate from stacked seismic data were analyzed, the result shown that free gas zone was accompanied below the gas hydrate zone (Pibo et al. , 2017). Through detailed logging data and core analysis from 2020 offshore production test, there was a 24.6 m thick layer consisting of hydrate, free gas and water which was below the hydrate layer (Qin et al. , 2020).
Not just in South China Sea, the free gas zone was found stably existed underlain the hydrate reservoir in other areas (Flemings et al. , 2003; Merey and Longinos , 2018), and it was estimated that the free gas zone may contain from 1/6 to 2/3 of the total methane trapped in hydrate (Hornbach et al. , 2004). Moridis et al. (2007) also pointed the spatial structure of hydrate reservoir with the free gas and water below the hydrate zone. Recent research had confirmed that in the Hydrate Ridge in offshore Oregon, the hydrate, free gas and seawater coexisted in the hydrate stability zone (Milkov et al. , 2004). As for as the Green Canyon in the Gulf in Mexico (Zhang. and Mcconnell , 2010) and the Nankai Trough in Japan (Miyakawa et al. , 2014). In 1967, the Messoyakha gas field was discovered in Siberia permafrost, the gas zone was enclosed in an anticlinal tectonic circle, and the top of free gas zone was covered with gas hydrate (Makogon et al. , 2005). The impermeable boundary was the sedimentary layer which between the gas hydrate zone and the free gas zone (Collett and Ginsburg , 1998). Simulation Study shown that the production intervals should be placed far from the impermeable boundary to attain high gas production rates (Graver et al. , 2008).
The mechanism of sealing effect for gas hydrate layer is the capillary force between water and hydrate (Dillon et al. , 1980; Su et al. , 2009). Early in 1999, Clennell et al. (1999) systematically analyzed the effect of capillary pressure on the pore pressure, they explained the pressure of underlying free gas was higher than the theoretical pressure of the formation fluid due to capillary pressure. In recent years, the research of gas hydrates has been gradually developing towards microscopic direction, and the influence of capillary force on gas hydrate has been paid more attention by scholars at home and abroad. Touil et al. (Touil et al. , 2019) studied the formation of carbon dioxide hydrate in thin glass tube, the capillary force constrained the direction of hydrate growth, and hydrate grow along the front of the glass wall. Buleiko et al (2017) found the change of propane hydrate formation pressure in porous media using micro calorimeter, because the thermodynamic properties of propane was changed by the capillary effect of pores, which affected propane hydrate.
The large amount of free gas underling the hydrate–containing sealing layer cannot migrate upward because of the sealing effect of hydrate–containing sealing layer (Dillon et al. , 1980). Early, natural gas in the deep ocean floor moved upward mainly through diffusion effect (Ming et al. , 2017),and solid hydrate is formed in the sedimentary layer that meets the conditions of hydrate formation during this process. Thus, the porosity of the sediments is significantly reduced. The residual water in the narrow pore space of the sedimentary layer creates capillary forces, which pointed down the free gas layer with larger pores volume. Thereby, the hydrate–containing sealing layer is formed to inhibit the upward migration of underlying gas (Xu and Ruppel , 1999).
However, the hydrate–containing sealing layer is not unbreakable. There have been cases of gas leakage under the seabed, such as along the Cascadia continental margin (Heeschen et al. , 2005) and offshore Vancouver Island (He et al. , 2009). The cold fluids consisting water and hydrocarbon (mostly methane) below the seafloor deposit interface migrate to the seabed by leakage, gushing, or diffusion, this is the cold vent (Logan et al. , 2010). The appearance of cold vent is a typical breakthrough of hydrate–containing sealing layer. The gas and seawater generated by breakthrough of hydrate–containing sealing layer will lead to sedimentary layer deformation above the hydrate layer, even cause marine geological disaster (Yang et al. , 2020). Therefore, the destruction of hydrate–containing sealing layer must be considered during the hydrate exploitation process.
Although the Marine geology theory has long proposed that hydrate–containing sealing layer is the key for hydrate accumulation, there is no direct experimental evidence of hydrate–containing sealing layer. In this study, the formation process and the existence mode of the hydrate–containing sealing layer were investigated by simulating the process of gas–water migration, the effect of different gas–seawater flow rate and different reservoir pressure on the formation of the hydrate–containing sealing layer was also investigated by MRI. MRI tecnique was widely used in hydrate investigations because it can distinguish the solid hydrate and liquid water (Song et al. , 2015). The research considerition was quit novel, the findings has the significant practical application value to ungerstand the characteristics of hydrate reservoir.
Experimental apparatus and procedure
Apparatus
The experimental apparatus is shown in Fig.1. The core device was the MRI system (Varian, Inc Palo Alto, CA, USA), which was used for visualizing methane hydrate formation and dissociation process. And the MRI was operated at 400 MHZ with a magnetic field strength of 9.4 Tesla, the standard spin echo pulse to obtain the two–dimensional proton density weighted image. MRI images were constructed by a spin echo multi–slice pulse sequence (SEMS) (Wang et al. , 2017), and the experimental parameters were: echo time (TE) 4.39 ms, repetition time (TR) 1000 ms, image data matrix (RO \(\times\) RE) 128 \(\times\) 128, field of view (FOV) 30 mm \(\times\) 30 mm with 2.0 mm thickness. The sequence acquisition time was 2.14 min for per vertical section image and 3.2 min for per cross section image. A high–pressure polyimide tube with a maximum pressure limitation of 13.0 MPa as the reactor had an effective height of 200 mm and diameter of 16 mm. And the reactor was surrounded by a heat preservation jacket in which the coolant circulates continually to keep the tube at required temperature.