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Chen Qiang, Hu Gao-wei, Wu Neng-you, Liu Chang-ling, Meng Qing-guo, Li Cheng-feng, Sun Jian-ye, Li Yan-long. 2020. Evaluation of clayed silt properties on the behavior of hydrate production in South China Sea. China Geology, 3(3), 362‒368. doi: 10.31035/cg2020050.
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After more than 40 years of exploration and investigation, there are huge amounts of gas hydrate prospective resources discovered around the world (Milkov AV, 2004). Now it is a common sense that gas hydrate would become a new energy resource in the next decades, instead of the ordinary oil and petroleum (Boswell R, 2009). Several trial productions were conducted to evaluate the feasibility of the exploitation method and equipment (Uddin M et al., 2014; Moridis G et al., 2011; Guang XJ et al., 2016). Three kinds of methods were mainly used including thermal stimulation, depressurization, and inhibitor injection (Wu NY et al., 2013, 2018). Messoyakha gas hydrate field was the first commercial exploitation reservoir (Makogon YF and Omelchenko RY, 2013). It is located in the permafrost of Siberia and was tested by depressurization, from 1970 to 2011, about 5.4×109 m3. Mallik region located in the northwest of Canada, the production test of the gas hydrate was conducted in 2002, 2007, and 2008, using both thermal stimulation and depressurization method (Boswell R and Collett TS, 2006; Takahashi H et al., 2003). Another test spot is Alaska North Slope, 2007 and 2012, a trial test using depressurization and carbon dioxide injection was conducted respectively (Yamamoto K and Dallimore SR, 2008; Collett TS et al., 2011). After several times of onshore tests, Japan started the first offshore hydrates test in Nankai Trough in 2013 (Fujii T et al., 2008). All of these tests faced similar problems, failure of sand control, and low gas production efficiency (Moridis GJ et al., 2004; Anderson BI et al., 2008; Rose K et al., 2011; Cyranoski D, 2013).
In China, geophysical surveys, in situ drilling, and lab research have been conducted for hydrate exploration and investigation since 2000. Plenty of evidence of geology, geophysics, geochemistry, and coring has been found to support the existence of gas hydrate in Shenhu area, South China Sea (Zhang GX et al., 2014; Liang JQ et al., 2014; Lu ZQ et al., 2013; Su M et al., 2014). The amount of potential energy is about 7×109 t of oil equivalent.
In the year 2017, the China Geological Survey (CGS) conducted the first offshore production test in the Shenhu area, South China Sea. Guided by the “three-phase control” theory, the test lasted for 60 days with 3.09×105 m3 methane gas in total, created a world record with the longest continuous duration of gas production and maximal gas yield (Li JF et al., 2018; Zhang W et al., 2017; Fang YX et al., 2019; Liang JQ et al., 2020; Shi YH et al., 2019; Wu NY et al., 2018; Xu CL et al., 2018; Ye JL et al., 2018; Zhang RW et al., 2018). In the year 2020, CGS successfully accomplished the second offshore gas hydrate production, extracted 8.614×105 m3 methane gas in 30 days, created two new world records (Ye JL et al., 2020; Qin XW et al., 2020; Wang L et al., 2020). However, several abnormal phenomena were observed during the test, including gas flow fluctuation and poor water production. It was suggested that offshore hydrates production still existed a series of scientific problems in geology, engineering, and environment (Li YL et al., 2016; Zhu CQ et al., 2017; Ye JL et al., 2018). The physical property of the reservoir is a branch of geology, it plays an important role to evaluate the reservoir quality, making production strategy, and estimating engineering practicability.
In this paper, clayed silt sediment cored from the Shenhu area was analyzed focused on physical characteristics. The thermal properties and irreducible water of clayed silt reservoirs with gas hydrate were obtained through experimental tests. The behavior of gas, water, sand production, and their control mechanism was explained through these test results.
Naturally occurred gas hydrates consist of two kinds of structure based on the gas molecules captured in water cages. Gas molecules like methane, ethane, and carbon dioxide formed structure I and mixture gas containing propane usually formed structure II (Chen Q et al., 2010, 2013). The gas storage capacity, the thermal stability between two structures also differs a lot, which would directly influence the depressurization and reservoir sand control plan during production. Therefore, it is critical to find out the component and structure of the target hydrate reservoir before conducting any trial production.
(i) Hydrate structure. Hydrate structure measurements were carried out using a confocal microscopic Raman spectrometer (Renishaw, model: inVia) equipped with a Leica microscope and a solid laser at 532 nm (Fig. 1a). The groove of the grating is 2400/mm, with a resolution of about 2 cm–1. The hydrate samples were cold-loaded into a heating-cooling stage at 77 K, and the Raman spectra were recorded in the region of 100–4000 cm–1 and the stretching vibration regions of C-H, C-O, and C-C, respectively (Meng QG et al., 2010).
(ii) Gas composition analysis. The gas compositions were measured with a gas chromatograph, model: Agilent 6890N (Fig. 1b). All hydrocarbons (C1-C5+) were separated, detected, and quantified with a capillary column (HP-Plot Al2O3 S, 0.53 mm ID, 50 m length) which was connected to a Flame Ionization Detector. Abundances of gas compositions were given as a relative portion of the sum of C1-C5+. The confidence limits of the measurement were 0.005% for C1-C2 hydrocarbons and 0.001% for C3-C5+ hydrocarbons (He XL et al., 2012).
Seventeen core samples with gas hydrate from the Shenhu area were tested and the gas composition results were shown in Table 1. It can be seen that methane was the most predominant component rather than ethane and propane, higher to 99.97%. With the increasing depth, the percentage of methane became smaller, but ethane and propane increased a little bit.
| No. | Depth/m | Methane/% | Ethane/% | Propane/% |
| 1-1 | 135 | 99.91 | 0.01 | 0.08 |
| 1-3 | 155 | 99.78 | 0.22 | 0 |
| 1-4 | 160 | 99.30 | 0.64 | 0.06 |
| 1-5 | 160.5 | 99.05 | 0.89 | 0.06 |
| 2-1 | 147 | 99.83 | 0 | 0.16 |
| 2-2 | 148 | 99.97 | 0.01 | 0.02 |
| 2-3 | 161 | 98.55 | 1.40 | 0.06 |
| 2-4 | 162 | 98.62 | 1.31 | 0.07 |
| 2-5 | 163 | 97.80 | 1.24 | 0.95 |
| 3-1 | 143 | 99.72 | 0.04 | 0.23 |
| 3-2 | 144 | 99.78 | 0.18 | 0.04 |
| 3-3 | 158.6 | 98.53 | 1.47 | 0 |
| 3-4 | 169.8 | 97.60 | 2.38 | 0.02 |
| 4-1 | 136 | 99.78 | 0.22 | 0 |
| 5-1 | 128 | 99.94 | 0.06 | 0 |
| 5-2 | 137.8 | 99.76 | 0.11 | 0.13 |
| 5-3 | 143 | 99.81 | 0.19 | 0 |
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As everyone knows, propane usually formed structure II hydrate, and relatively hard to decompose (Liu CL et al., 2013). Fig. 2 showed the calculated data of the structure I and II hydrate phase diagram, using the measured gas composition value. It suggested that under the same temperature, the dissociation pressure of structure II hydrates was much lower than structure I. For example, around 10℃ as Shenhu hydrates formation, there are 3 MPa to 5 MPa more differential pressure to break phase equilibrium. It means that using the same production method, structure II reservoir need a lot more pressure drop, which may cause a critical problem to wellbore stability.
In order to ensure the target hydrate structure, samples were measured by Raman Spectroscopic. And similar Raman spectrums were found (Fig. 3), which indicated that all the hydrates clathrate were structure I dominated.
As a result, though some samples contained propane, there was no significant evidence to ensure the massive presence of structure II hydrate, so the risky to affect reservoir phase equilibrium is low. As production, depressurization strategy should be made with the structure I hydrate. However, the risky of hydrate reformation should be considered as the flow assurance in the wellbore.
Gas hydrate dissociation was always accompanied by a violent endothermal effect, which was significantly different from conventional oil and gas production. Thermal physical properties were key parameters to evaluate the heat transfer and temperature distribution during hydrate production. If the reservoir temperature decreased large enough, the gas production rate would be slow down, and the risky of hydrate reformation around the wellbore would be higher (Chen Q et al., 2010). In this paper, dissociation heat and thermal conductivity of Shenhu samples were measured, to provide some information to analyze the production behavior during the trial test.
(i) Dissociation heat measurement. Dissociation heat measurement was accomplished by the high-pressure differential scanning calorimetry (Fig. 4a), which is a specially designed equipment to analyze hydrate formation and dissociation process. The heat flow of the hydrate phase change can be measured, which is used to calculate the dissociation heat (Chen Q, 2014).
(ii) Thermal conductivity measurement. Hydrate thermal conductivity was measured by independently developed equipment (Fig. 4b). It was composed of a high-pressure vessel and a thermal-time domain reflection (TDR) probe. The probe was used to measure the temperature variation as a heat impulse added into the sample before and after, thermal conductivity can be calculated with these data. Besides, the TDR signal can be used to calculate hydrate saturation in porous media.
The peak area of the heat flow curve represented the total dissociation heat of hydrate, as shown in Fig. 5. Hydrates were formed and dissociated at eight pressure points in these experiments to compare the pressure relevance (Table 2). It can be seen that the average value is about 610 J/g, not significantly affected by pressure.
| P/ MPa | Standard latent heat of ice melting/(J/g) | Latent heat of ice melting/J | Dissociation heat of hydrate/J | Dissociation heat of hydrate per unit mass /(J/g) |
| 5 | 331.36 | 9.75 | 0.25 | 641.45 |
| 7 | 331.36 | 9.51 | 0.41 | 531.94 |
| 10 | 331.36 | 9.29 | 1.05 | 680.99 |
| 13 | 331.36 | 8.99 | 1.57 | 607.27 |
| 15 | 331.36 | 8.35 | 3.01 | 625.56 |
| 20 | 331.36 | 8.09 | 3.47 | 609.49 |
| 22 | 331.36 | 6.82 | 5.92 | 586.41 |
| 25 | 331.36 | 7.33 | 4.97 | 594.92 |
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Hydrate dissociation will cool down the temperature of nearby sediment, when there was a temperature difference between the dissociation surface and nearby sediment, heat would be spread out through conduction and convection. In order to evaluate the relationship between reservoir thermal physical properties and the gas production efficiency, some calculation was done as below: The reservoir parameters of Shenhu area were shown in Table 3, taken cubic sediment as a research unit if there was no thermal exchange with the nearby unit, the temperature drop range would be decided by specific heat and hydrate dissociation heat, as in Eq. (1):
| Q/J | Cpm/ [J/(g·K)] | Cpw/ [J/(g·K)] | ρw/ (g/cm3) | ρh/ (g/cm3) | φ/% | Sh/% |
| 610 | 1.01 | 4.02 | 1.03 | 0.98 | 35 | 53 |
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ΔT=QCpb×ρb |
(1) |
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Cpb×ρb=Cpm×ρm×(1−φ)+Cpw×ρw×[(1−Sh)×φ+ρh×Sh×φρw) |
(2) |
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ρb=ρm×(1−φ)+ρw×(1−Sh)×φ+ρh×Sh×φ |
(3) |
Q: Hydrate dissociation heat; Cp: Specific heat; ρ: Density; Sh: Hydrate saturation; φ: porosity; b: Mixture; m: Sediment; w: Brine water; h: Hydrate.
It turns out that, in the reservoir with 35% porosity and 53% saturation, hydrate dissociation would let the temperature of the single unit drop-down 44K, which means hydrate need extra energy from the nearby unit, otherwise the dissociation process cannot continue. There were two kinds of heat transfer mode, thermal conduction trough particles, and thermal convection trough fluid flow, they affected the reservoir temperature field together, then influenced the hydrate dissociation speed.
Pore water was much easier to be bonded in clayed silt sediment, especially at high hydrate saturation, so the thermal convection was weak, thermal conduction became the most important factor on temperature change. An unstable instant thermal probe was used to measure the thermal conduction of hydrate samples formed in the South China Sea sediment (Chen Q et al., 2013), and a relationship between saturation and conduction was shown in Fig. 6.
The thermal conductivity is around 0.7 W/(m·K), which was bigger than the coarse grain sediment. And the variation trend was increasing at first, and then decreased as hydrate saturation continuously increased. The possible reason was that thermal contact resistance caused by sediment and hydrate particles weaken the thermal transferability. Based on the experiment data, it can be seen that clayed silt with higher thermal conductivity at the beginning, it was propitious to thermal transfer; hydrate filled in pore spaces increased the thermal contact resistance and consumed free water, made heat transfer harder, so the conductivity decreased.
In the South China Sea hydrate production, the gas flow was discontinuous once in a while, the possible reason was hydrate reformation, as the temperature near the well was cooled down enough. In this study, the experiment data can support this hypothesis. A large amount of hydrate dissociation would absorb lots of heat, once the thermal transfer was not quick enough, the dissociation area would probably form new hydrate and block the gas outlet channel.
One of the key factors limited to hydrate production was the migration efficiency of gas and liquid in the sediment layer. The effect such as mineral composition and grain size would induce wettability and capillarity different in clayed silt layer (Li HB et al., 2015; Sun JC et al., 2011; Deng YE et al., 2004), and bounded the pore water tightly, then prevented the liquid and gas flow. Therefore, it was important to understand the characteristics of reservoir irreducible water. In this paper, a high-speed centrifuge was used to test the Shenhu sediment pore water. First, a relationship between centrifuge speed and the pressure was established and then analyzing the trend of water outlet with sample pressure, which can be used to evaluate the free and irreducible water of the reservoir at different pressure during hydrate production.
The samples were tested by Hatachi CR22GIII rotors centrifuge at 15℃. Sediment samples were loaded on the metal rotor, which was divided into two parts (Fig. 7), M1 contained sediment and M0 was used to collect pore water. 2000 rpm, 5000 rpm, 8000 rpm, and 10000 rpm were selected to centrifuge the sample, and weighing the M0 to get the water mass. The total water content can be calculated by the comparison of original weight and dried weight. Table 4 showed the information on bound water under different centrifugal speed.
| Rotor 1 | Rotor 2 | |||||||
| M0/g | M1/g | Free water/% | Bound water/% | M0/g | M1/g | Free water/% | Bound water/% | |
| Empty | 100.305 | 220.603 | 100.519 | 220.653 | ||||
| Loaded | 301.774 | 303.106 | ||||||
| 2000 rpm | 100.316 | 0.04 | 99.96 | 100.528 | 0.03 | 99.97 | ||
| 5000 rpm | 101.906 | 5.88 | 94.12 | 101.977 | 5.48 | 94.52 | ||
| 8000 rpm | 109.699 | 292.411 | 34.51 | 65.49 | 109.848 | 293.82 | 35.04 | 64.96 |
| 10000 rpm | 111.343 | 290.722 | 40.55 | 59.45 | 111.501 | 292.088 | 41.25 | 58.75 |
| Dried | 274.554 | 276.485 | ||||||
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If the force is strong enough, irreducible water can be converted to free water. The centrifugal force taken as sample pressure increased with rotate speed, so the water production feature can be evaluated by the experiment data, described as the equation below:
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RCF=1.118×10−5×n2×r |
(4) |
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P=FS=ρb×RCF×g×h×10−6 |
(5) |
Where RCF: Relative centrifugal force; n: Rotor speed (rpm); r: Radius (cm); P: Pressure (MPa); F: Absolute centrifugal force (N); S: Rotor bottom area (m2); ρb: Sample density (kg/m3); g: Gravity (m/S2); h: Height of the sample (m).
Fig.8 showed free water and irreducible water variation trend under different pressure. It can be seen that at 0.3 MPa the ratio of free water was very low, more than 99.96% pore water was bounded by sediment; as the pressure increased, irreducible water was released step by step. From 1.9 MPa to 4.9 MPa, the water releasing speed was fast; after that, the speed was slow down, irreducible water became stable in the sediment. In this experiment, at 7.9 MPa, there was still 59.45% of pore water was bounded with sediment.
According to these experiments, clayed sediment from the South China Sea has a strong ability to bound pore water, but free water production would be increased by ΔP, and the most sensitive ΔP range was 1.9–4.9 MPa. It can be inferred that at low ΔP the driving force was not enough to absorb water and sand into the wellhole, so it is recommended to use low ΔP during hydrate production.
In this study, the thermophysical characteristics and pore water binding of hydrate reservoirs in the Shenhu area of the South China Sea were studied through simulation experiments and theoretical derivation. Combined with the actual situation of the hydrate trial production, the experimental data were analyzed to discuss the influence of reservoir physical parameters. The preliminary conclusions were obtained:
(i) It was confirmed that the use of phase equilibrium data based on the structure I methane hydrate was the right choice for the trial production scheme in the South China Sea.
(ii) The experimental results confirmed that gas fluctuation during hydrate production was due to the hydrate reformation in the reservoir. The temperature drop caused by the hydrate production was related to the hydrate dissociation rate and the heat transfer capacity of the formation. Compared with the sand hydrate reservoir, the thermal conductivity of the clayed silt reservoir was superior, but thermal convection was insufficient, and the high hydrate saturation will further weaken the thermal conductivity of the reservoir. Therefore, as the hydrate dissociation was too fast to transfer the heat, the temperature will be greatly reduced, leading to the reformation of the hydrate.
(iii) The clayed silt reservoir has an obvious binding effect on pore water, resulting in a small amount of water and sand production during the trial test. Increasing the differential pressure can free some pore water. It is found that a pressure range of 1.9–4.9 MPa was the most sensitive to the exchange of free and bond water.
Qiang Chen conceived of the presented idea. Qiang Chen, Neng-you Wu and Chang-ling Liu developed the theory. Gao-wei Hu and Yan-long Li verified the analytical methods. Qiang Chen carried out the experiments and analysed the data. Cheng-feng Li, Jian-ye Sun and Qing-guo Meng contributed to sample preparation. All authors discussed the results and contributed to the final manuscript.
The authors declare no conflict of interest.
This study was funded by the National Key Research and Development Program of China (2017YFC0307600) and the China Geological Survey Program (DD20190231).
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Figures(8) / Tables(4)
No. 45 Fuwai Street, Xicheng District, Beijing 100037, P. R. China
Supported by Beijing Renhe Information Technology Co. Ltd Technical support: info@rhhz.net
| No. | Depth/m | Methane/% | Ethane/% | Propane/% |
| 1-1 | 135 | 99.91 | 0.01 | 0.08 |
| 1-3 | 155 | 99.78 | 0.22 | 0 |
| 1-4 | 160 | 99.30 | 0.64 | 0.06 |
| 1-5 | 160.5 | 99.05 | 0.89 | 0.06 |
| 2-1 | 147 | 99.83 | 0 | 0.16 |
| 2-2 | 148 | 99.97 | 0.01 | 0.02 |
| 2-3 | 161 | 98.55 | 1.40 | 0.06 |
| 2-4 | 162 | 98.62 | 1.31 | 0.07 |
| 2-5 | 163 | 97.80 | 1.24 | 0.95 |
| 3-1 | 143 | 99.72 | 0.04 | 0.23 |
| 3-2 | 144 | 99.78 | 0.18 | 0.04 |
| 3-3 | 158.6 | 98.53 | 1.47 | 0 |
| 3-4 | 169.8 | 97.60 | 2.38 | 0.02 |
| 4-1 | 136 | 99.78 | 0.22 | 0 |
| 5-1 | 128 | 99.94 | 0.06 | 0 |
| 5-2 | 137.8 | 99.76 | 0.11 | 0.13 |
| 5-3 | 143 | 99.81 | 0.19 | 0 |
DownLoad:
CSV
| P/ MPa | Standard latent heat of ice melting/(J/g) | Latent heat of ice melting/J | Dissociation heat of hydrate/J | Dissociation heat of hydrate per unit mass /(J/g) |
| 5 | 331.36 | 9.75 | 0.25 | 641.45 |
| 7 | 331.36 | 9.51 | 0.41 | 531.94 |
| 10 | 331.36 | 9.29 | 1.05 | 680.99 |
| 13 | 331.36 | 8.99 | 1.57 | 607.27 |
| 15 | 331.36 | 8.35 | 3.01 | 625.56 |
| 20 | 331.36 | 8.09 | 3.47 | 609.49 |
| 22 | 331.36 | 6.82 | 5.92 | 586.41 |
| 25 | 331.36 | 7.33 | 4.97 | 594.92 |
DownLoad:
CSV
| Q/J | Cpm/ [J/(g·K)] | Cpw/ [J/(g·K)] | ρw/ (g/cm3) | ρh/ (g/cm3) | φ/% | Sh/% |
| 610 | 1.01 | 4.02 | 1.03 | 0.98 | 35 | 53 |
DownLoad:
CSV
| Rotor 1 | Rotor 2 | |||||||
| M0/g | M1/g | Free water/% | Bound water/% | M0/g | M1/g | Free water/% | Bound water/% | |
| Empty | 100.305 | 220.603 | 100.519 | 220.653 | ||||
| Loaded | 301.774 | 303.106 | ||||||
| 2000 rpm | 100.316 | 0.04 | 99.96 | 100.528 | 0.03 | 99.97 | ||
| 5000 rpm | 101.906 | 5.88 | 94.12 | 101.977 | 5.48 | 94.52 | ||
| 8000 rpm | 109.699 | 292.411 | 34.51 | 65.49 | 109.848 | 293.82 | 35.04 | 64.96 |
| 10000 rpm | 111.343 | 290.722 | 40.55 | 59.45 | 111.501 | 292.088 | 41.25 | 58.75 |
| Dried | 274.554 | 276.485 | ||||||
DownLoad:
CSV
DownLoad: