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.