Guan Jinan (1980—), male, associate researcher, mainly engaged in hydrate dynamics research, E-mail:guanja@ms.giec.ac.cn.
1. Key Laboratory of Renewable Energy and Natural Gas Hydrate, China Academy of Sciences/Guangzhou Energy Research Institute, Chinese Academy of Sciences, Guangzhou 5 10640.
2. Key Laboratory of Heat Transfer Enhancement and Process Energy Conservation of South China University of Technology, Ministry of Education, Guangzhou 5 10640.
The formation of leakage hydrate is the result of multiphase flow interaction. In order to accurately evaluate the resource potential of marine hydrates, it is necessary to study the formation process of such hydrates. Methane gas (free) and in-situ pore water (including dissolved gas and dissolved salt) moving upward in the leakage system interact with the solid skeleton to generate hydrate and deposit gel in the pores to form reservoirs; In this process, the migration of free gas changes the geological properties of sedimentary layers, which is one of the controlling factors of lost hydrate accumulation. Based on the flow-transport-reaction mechanism, a multiphase flow model is established, and the linkage relationship between pore capillary pressure, permeability, saturation of each phase and salinity at the beginning and end of the reaction during hydrate formation is deduced. Combined with the geological exploration data of Shenhu sea area in the northern South China Sea, the occurrence state of hydrate is evaluated, and it is speculated that the hydrate saturation in this sea area can reach 75% at the highest.
Keywords: free gas; Methane hydrate; traffic
Methane hydrate formed by migration of free gas in hydrate stability zone in leaking sea area and resource estimation.
Guan Jinan 1, Liang Deqing 1*, Wu Nengyou 1, Fan Shuaishi 2.
1. Guangzhou Institute of Energy Conversion, China Academy of Sciences/Key Laboratory of Renewable Energy and Natural Gas Hydrate, Guangzhou 5 10640.
2. South China University of Technology, Key Laboratory of Heat Transfer Enhancement and Energy Saving, Ministry of Education, Guangzhou 5 10640.
Abstract: Hydrate formation in seeping seabed sediments is a typical multiphase flow process. In order to accurately evaluate the potential of marine hydrate resources, it is necessary to study the formation of hydrate in seabed sediments. Rising free methane, in-situ pore water (including dissolved gas and salt) and solid particles in sediments interact to form methane hydrate and precipitate in the pores. The migration of free gas changes the geological properties of sedimentary layers. It is one of the key factors to control hydrate formation in seepage system. According to this flow-transfer-reaction process, a multiphase flow model including water-gas (free gas)-salt-hydrate is established. According to two different scenarios (beginning and end), the relationship between capillary pressure, permeability, phase saturation and salinity with hydrate formation is deduced. According to the law of simulation and the information obtained from field drilling, the formation of methane hydrate in Shenhu area of South China Sea is predicted. It is speculated that there may be a moderate methane flow below this submarine HSZ. If the flow rate is about 0.5 kg m-2a-1,it will continue to evolve about 2 700 ka until the hydrate saturation in the pores reaches its peak (about 75%).
Keywords: free gas; Methane hydrate; Methane flux
Introduction to 0
Methane hydrate (MH) widely exists in the global ocean and frozen soil [1-2]. It is estimated that about1.2×10/7m3 methane exists in the form of hydrate (STP) in the ocean, and about 10 15mol methane exists in free or dissolved form. Seepage hydrate reservoir has the characteristics of concentrated distribution, high reservoir density and excellent physical and chemical conditions for reservoir formation [5], and its resource significance is very important.
The formation of hydrate in sediment pores in the area where multiphase fluid flows on the seabed is a typical multiphase flow and transport process [6]. Besides gas (free gas and dissolved gas), pore water and salt, the generated hydrate also affects the further formation of hydrate. For this multiphase flow process, Clennell et al. [7] explained the influence of methane gas migration and accumulation on hydrate formation; Milkov et al [8] analyzed the hydrate core of ODP 1249 in ODP station, and pointed out that hydrate, gas and salt can exist in this system; The numerical simulation results also show the evolution relationship among temperature, pressure, methane volume fraction and salinity [1 1]. The formation of hydrate will absorb water and gas in pores, which will directly lead to the change of multiphase flow components; At the same time, the framework of hydrate cemented sedimentary strata has changed the structure and properties of hydrate-bearing sedimentary strata, which must be studied and determined.
In the leakage area, the free gas not only accumulates under the bottom boundary of the hydrate stability zone (HSZ), but also moves rapidly in the HSZ, which increases the salinity in the pores of the sedimentary layer, thus changing the geological stratification under this system, resulting in the existence of hydrate-gas-salt three-phase [12] and the phenomenon of submarine gas "flame" [12]. On the one hand, it shows that free gas participates in the formation of hydrate, on the other hand, it also becomes indirect evidence to detect hydrate accumulation. Geological exploration, field drilling sampling and core analysis in Shenhu sea area of South China Sea show that this sea area is likely to be a leaky hydrate reservoir [14- 18]. This paper will analyze the storage of methane hydrate in Shenhu sea area according to the formation law of leakage hydrate.
1 Free gas migration and phase diagram
When the pore water rich in saturated dissolved methane moves upward from the free gas layer (FGZ) with the free gas, it penetrates the bottom of HSZ and enters HSZ. Dissolved methane first generates MH in pores and precipitates, which reduces the methane concentration in pore water, and at the same time, some free methane gas dissolves in pore water to generate MH until the MH concentration in local pores reaches saturation, and the new fluid and free methane at the bottom continue to move up and generate MH repeatedly. Because salt ions are repelled by hydrate crystals, the concentration of salt ions around MH in pores increases, which changes the thermodynamic phase equilibrium conditions of MH formation, inhibits the formation of MH and reduces the thickness of HSZ. The continuous supply of free methane gas enhances this inhibitory effect of salt ions. When MH in the pores of in-situ sediments reaches the maximum saturation, the system tends to be stable and no MH is generated. The free gas successfully crossed the HSZ to reach the seabed, leaked into the upper seawater and dissolved (Figure 1). Generally speaking, the formation of MH in sedimentary pores is a transformation process between two phases and three phases: gas-water phase becomes gas-water-hydrate, and finally becomes gas-water phase.
Figure 1 methane hydrate occurrence area in the northern South China Sea, (a) temperature-depth phase diagram, and (b) concentration-pressure phase diagram.
After the free gas enters the HSZ area, it forms hydrate, which changes the salinity in the pores and then affects the formation and morphology of hydrate. (a) When the salinity increases from 3.4% to 13.6% (four times), the hydrate area decreases from EBC to EAD, and the bottom depth decreases from 403 m to 384 m, which is about 9.85% of the original area, corresponding to the bottom in Figure B. When the free gas rises from the bottom and passes through BH line (3.4% salinity w(Na Cl), or line AI13.6% salinity), hydrate is formed on the right side of the line.
2 multiphase flow analysis
2. 1 model building
For the multiphase flow system consisting of gas-liquid-solid three-phase and methane-water-salt-hydrate, the following assumptions are made:
1) regardless of the molecular diffusion of components, the pores of sediments are always completely filled by components and are isotropic.
2) The leaked gas is monocomponent methane, which exists in the form of free methane and dissolved methane, and the initial dissolved methane is saturated.
3) Salt only dissolves and always exists in pore water, regardless of salt crystallization caused by salt saturation change.
According to the above assumptions, the transport equations of gas and water are established respectively:
For water, it exists in hydrate and liquid water:
Enrichment Law and Exploitation Basis of Natural Gas Hydrate in South China Sea
For methane, it exists in gas phase, liquid phase and hydrate:
Enrichment Law and Exploitation Basis of Natural Gas Hydrate in South China Sea
Where (1) and (2); φ is the formation porosity; Sκ is the component volume saturation; ρK is the component density (kg/m3); The mass fraction of methane in water is its solubility; ηK represents the mass fraction of each component in the hydrate; K and krβ are the inherent permeability (m2) of the formation and the relative permeability of the components, respectively; μK is the component viscosity (pa s); PK is the phase pressure (MPa); G is the acceleration of gravity (kg/m3); Cg is the compressibility of methane gas, which is defined by Formula (3), and Bg is the methane volume coefficient:
Enrichment Law and Exploitation Basis of Natural Gas Hydrate in South China Sea
In addition, combining the limiting condition of saturation and the capillary pressure formula, the equation can be formed:
Enrichment Law and Exploitation Basis of Natural Gas Hydrate in South China Sea
Free gas saturation can be determined by linear regression:
Enrichment Law and Exploitation Basis of Natural Gas Hydrate in South China Sea
See table 1 for the relevant parameters adopted.
2.2 discussion of results
Methane gas (free state) and water (including dissolved gas) enter HSZ and form hydrate under suitable temperature, pressure and salinity conditions; At the same time, various attributes and fluid properties of sedimentary strata also change with the formation of hydrate.
2.2. 1 HSZ MH formation process
According to the critical state that the gas phase methane saturation is always 2% and the methane leakage flux is 0.5 kg/m2 a, the formation process of hydrate is deduced (Figure 2).
Fig. 2 shows (a) the capillary pressure between gas and liquid in pores, (b) the permeability of hydrate-containing deposits, (c) the gas, water and hydrate saturation, and (d) the change of pore salinity from 36d to 3 100ka, as free methane continuously enters HSZ. The bottom boundary of HSZ in the figure is 403 meters (bsf), and the boundary conditions are shown in figure 1.
When t=36 d, methane from the deep seabed enters the bottom of HSZ to form hydrate, and the influence of methane introduced from the bottom on the formation of hydrate in HSZ has reached the seabed (Figure 2a c); Due to the formation of hydrate, the contact area and tension of gas-liquid interface have changed, which leads to the increase of capillary pressure along this distance (Figure 2a a). At the same time, due to the cementation of hydrate, the permeability of sedimentary layer began to decrease (Figure 2a b); The exclusion of salt ions increases the local pore water salinity (Figure 2a-D).
When t=3 100 ka, the reaction ends and MH in HSZ is no longer generated. It can be seen from Figure 2b C that the hydrate saturation gradient here is very large, because the hydrate saturation of the seabed is limited to 0, which also shows that if the HSZ in seawater is considered in the model, hydrate will continue to be produced in this area. At this time, the maximum Pc on the seabed can reach about 30 k Pa (Figure 2b a); However, the permeability of the sand layer cemented by hydrate is about 10-20m2, which has reached a fairly dense level (Figure 2b). Salinity reaches the maximum at the bottom boundary of HSZ, which is about 16% (Figure 2b D).
Table 1 Relevant parameter data used to simulate MH formation in Shenhu sea area of South China Sea.
Methane leakage flux
Another important parameter affecting hydrate formation is methane permeation flux qm(kg/m2). Different seepage flux makes the formation speed of hydrate, reservoir-forming resources and even the reservoir-forming morphology and occurrence of hydrate-bearing sediments vary greatly. Roberts et al. [19] described the geological morphology of the seabed in slow, medium and fast seepage areas in the Gulf of Mexico, and Chen Duofu [20]; The seepage model of 1 is established, and the standard of 1 is put forward to divide these three seepage types. On this basis, under the same free gas saturation (Sg entering the bottom of HSZ is 0. 1), six different seepage fluxes are calculated respectively, and the influence of seepage flux on the formation process and accumulation of MH in the leakage system is discussed. Table 2 gives two different qm values calculated for slow, medium and fast leakage flux intervals, ranging from 0.05 kg/(m2 a) to100 kg/(m2 a), which can basically cover the flux range of hydrate leakage areas found in global waters.
Table 2 Evolution stages of methane leakage system determined by different methane leakage flux ranges
The calculation shows that under each leakage flux, the hydrate production increases sharply in the initial short time, and then increases slowly in a long time. At slow and near slow and medium speed (0.05 kg/(m2 a), 0.5 kg/(m2 a),1kg/(m2 a)), the time for hydrate formation to reach the maximum value is almost the same, but the higher the qm value, the shorter the time. The Qm is from 0.05 kg/(m2 a) to100 kg/(m2 a), and the time for the hydrate to reach the maximum value is reduced from 2,400 ka to 227 ka, and the time for the maximum qm value is 9.5% of the minimum qm value. However, the output per unit area of hydrate also changed from 38.35 kg/m2 to 36. 15 kg/m2, which decreased by about 5.7%. Judging from this, it is also because the porosity and permeability of the sedimentary layer drop sharply after the acceleration of hydrate formation, which hinders the further full formation of hydrate.
Inversion of hydrate accumulation in Shenhu sea area
The temperature and pressure environment in Shenhu sea area is calculated according to five basic parameters: seabed depth 1 250 m, seabed temperature 3.4℃, geothermal gradient 45℃/km, sedimentary pressure gradient 10 MPa/km and average pore salinity 3.5%. At present, there is no estimation of free gas saturation in HSZ or BSR area in this area, but because the geological structure of the whole slope area of Nanhai North Road is similar to that of the Gulf of Mexico, it can be roughly calculated as 5% of the free gas saturation entering HSZ at the bottom [20]; At the same time, we don't know the flux of methane leakage to the bottom of HSZ in Shenhu area. However, in the SH2 drilling site, the maximum saturation of hydrate reaches 48% in the thickness range of about 80 meters. Compared with typical leakage points, the maximum saturation of Cascadia hydrate ridges in the thickness ranges of about 65,438+0249 and 65,438+0250 can reach about 70%, and its leakage can reach about 70%. Accordingly, with these basic physical parameters, combined with the detected data such as hydrate saturation and pore salinity, we can make reasonable speculation.
Under the above given physical parameters, the general situation of detected hydrate saturation distribution is simulated and calculated. The methane saturation distribution and pore salinity calculated with qm of 0.5 kg/(m2 a) are shown in Figure 3, which is in good agreement with the drilling results.
Fig. 3 Distribution of Hydrate Saturation at Shenhu SH2 Station
The black dot indicates the distribution of hydrate saturation measured in the field, and the red dotted line indicates the curve obtained by simulating the distribution of hydrate in depth, and the maximum saturation is about 48%.
In this model, the hydrate saturation on the seabed surface is limited to 0, and the reaction time for continuous evolution to reach the maximum saturation (75%) is calculated to be about 5 500 ka (Figure 4), which indicates that this area was formed at the initial stage of gas-liquid flow entering the hydrate stability zone and accumulated in large quantities. According to the area of about 16 km2, methane hydrate reservoirs exist, with the area probability of 10% and the depth probability of 20%, assuming that the pore methane conversion rate is 10%. According to the calculation of about 5,500 ka, the maximum resource in this area can reach about10.25 billion m3 of methane gas, which is about 7 ~ 8 times of the current estimate.
Fig.4 Hydrate distribution nephogram after SH2 is fully developed in Shenhu area.
4 conclusion
The submarine leakage area is rich in free gas and moves rapidly, so it is considered as the most potential hydrate reservoir. According to the flow-transport-reaction mechanism, a multiphase flow model is established to simulate the formation and accumulation of leaked hydrate. The linkage relationship between capillary pressure, permeability, saturation of each phase and salinity in sedimentary pores at the beginning and end of free gas migration is deduced, the formation process of hydrate is analyzed, and the influence of leakage flux on hydrate accumulation is discussed. Finally, combined with the geological data of Shenhu sea area, the accumulation types and resource potential of hydrates in this area are inferred.
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