E-mail: Wang Feifei110 Wang Feifei, 1@foxmail.com (1987-), female, master's degree candidate, mainly engaged in experimental simulation of natural gas hydrate.
Lu (1972-), male, professor, mainly engaged in the study of gas hydrate accumulation mechanism and distribution law. E-mail: Luwanjunug @126.com.
1. School of Resources, China Geo University, Wuhan 430074.
2. Key Laboratory of Marine Oil and Gas Resources and Environmental Geology, Ministry of Land and Resources, Qingdao, Shandong 26607 1.
3. Qingdao Institute of Marine Geology, Qingdao, Shandong 26607 1.
Abstract: Hydrate is formed in a relatively low temperature and high pressure environment. In this case, it is difficult to directly observe the concentration of residual methane in the fluid after hydrate formation. In this paper, the dissolution and growth kinetics of methane hydrate in different concentrations of salt solutions and sediments in two boreholes SH2 and SH5-2 in Shenhu drilling area of South China Sea were studied by Raman spectroscopy and microscopic observation. In 5% Na C 1 solution, the temperature is 274. 15 ~ 293. 15 K, and the pressure is 10. 1 ~ 40.2 MPa at10% NAC/. Solubility data of methane in sediments from Shenhu and SH52 boreholes in the South China Sea under the conditions of 12.6 ~ 39.0 MPa pressure, 277. 15 ~ 293. 15 K temperature and 10 ~ 30.0 MPa pressure. The growth-dissolution process of hydrate and the change of hydrate particle size with time were observed. Comparing the dissolution and growth of hydrate in pure water system and brine system, it is found that the equilibrium concentration of methane in the fluid decreases with the increase of salinity. Generally speaking, at the same temperature, the saturated concentration of methane in the sediments of Well SH5 is higher than that of Well SH2. The formation of hydrate in the sediments of well SH5 is not as easy as that in the sediments of well SH2, which may be one of the reasons why hydrate has not been drilled yet.
Keywords: methane hydrate; Saline solution; Shenhu sediment; Raman spectrum; experimental research
1 Introduction
Gas hydrate is an ice-like, flammable, non-stoichiometric caged crystalline compound, which is formed by natural gas and water under specific conditions. Widely distributed in the permafrost zone of the earth and deep seabed sediments, it has huge reserves, high heat release during combustion and little environmental pollution.
The method of obtaining the concentration of dissolved methane in the fluid by calculating the peak area ratio of Raman spectrum is very mature, because under the same test conditions, the peak area ratio of Raman spectrum of each component in the fluid has a good linear relationship with the molar fraction of these components, and the error of obtaining the molar concentration by using the experimentally corrected proportional coefficient is about 5%. Therefore, this paper intends to synthesize methane hydrate in a high-pressure transparent cavity, and use Raman spectroscopy to observe the dissolution and growth of methane hydrate in salt solutions and sediments with different concentrations in Shenhu drilling area of the South China Sea at low temperature and high pressure. The temperature in 5% sodium chloride solution is 274. 15 ~ 293.438+05 K, the pressure is 10. 1 ~ 40.2 MPa, and the temperature in 10% sodium chloride solution is 274.1.
Experiment on solubility of table 1 methane in salt solution
1 experimental equipment and methods
1. 1 brine solution-preparation of methane sample
This experiment was carried out in a small tubular transparent high pressure chamber. The main part of the high pressure chamber is a transparent fused capillary quartz tube with a length of about 10 cm (0.36 mm×0.36 mm outside and 0.05 mm×0.05 mm inside). One end of the quartz tube is sealed with oxyhydrogen flame, and the other end (open end) is connected with the sampling pressurization system through high-pressure sealant and valve (Figure 1). Because the cavity is small, the control of pressure and temperature is very easy. In the experiment, the purity of methane is 99.99%, the pressure is read by a digital pressure gauge, and the temperature is controlled by the TMS94 temperature controller of the hot and cold table and automatically monitored [24-25].
During the experiment, the following steps were adopted: 1) repeatedly flushing the high-pressure cavity and pipeline with methane gas (opening all valve switches except V 14, V7 and V2, and flushing by repeatedly switching V 14 and V7), and then closing V 14, V7 and V8; 2) Filling a certain amount of methane into the high-pressure cavity (the increase of methane amount can be obtained by pressurizing with a pressurizing pump), and closing V 1 to temporarily seal the methane in the high-pressure cavity; 3) Water filling (first close V6, V4 and V8, and open V2 to release the pressure between V8 and V2; Then turn on V8, push the syringe to fill the pipeline between V8 and V2 with secondary deionized water, and turn off V2 and V3); After the exhaust pipe connecting V2 comes out; 4) Pressurization: open V4, V 1, pressurize with a pressurizing pump, push the water column of V4-V2, and compress the methane gas in the high-pressure chamber. When the water-gas ratio in the high-pressure chamber is appropriate, write down the pressure (such as 24 MPa and 35 MPa). ) and close VL; 5) After 5)2 ~ 3d days, the system basically reached the dissolution equilibrium; The tubular transparent high-pressure cavity is cooled by nitrogen cooled by liquid nitrogen. At about -40℃, hydrate first appeared near the gas-liquid interface and grew rapidly. After a period of time, the gas phase is completely transformed into hydrate, and the crystal grows gradually. 6) Place the tubular transparent high-pressure cavity horizontally in the constant-temperature air bath on the X-Y-Z console, and keep it for a certain time before Raman observation.
Figure 1 Schematic diagram of experimental device
The shaded part in the figure is a three-way valve with two control switches, and v1-v14 is a valve switch.
1.2 preparation of shenhu sediment-seawater sample
A certain amount of Shenhu sediment is filled in the timely circular tube, and the total length of the sediment is usually1~ 2 mm. After vacuumizing, a certain amount of South China Sea water is filled, which is connected to the high-pressure pipeline after connecting the valve. After vacuumizing again, methane gas is pumped into the tube, so that the liquid is pressed into the tube by the gas under high pressure, thus forming the distribution of sediment-seawater-methane gas in the tube (Figure 2). The sample is an open sample, which is connected with a section of pipeline and valve filled with methane gas to ensure the pressure to remain stable during the experiment, so that the methane saturation in the sediment-seawater system under various pressures can be determined.
The detection of dissolved methane in fluid is mainly near the growth front of hydrate. When measuring, the laser is focused on the center height of the horizontally placed high-pressure cavity to obtain the best effect. The spectrum was collected in the range of 2 700 ~ 4 000 cm- 1 for 40 s each time, with a total of 3 times. The peak area of methane is integrated with GRAMS32/AI software (Galactic Industries). A(H2O+CH4) is the total peak area of 2754 ~ 3850, A(CH4) is the peak area of 2890 ~ 2930, A(H2O) is the difference between them, representing the peak area of water, A(CH4)/A(H2O) is the peak area ratio of methane to water, and X(CH4) is the corrected methane.
Fig. 2 schematic diagram of sediment and bubble distribution in transparent high-pressure cavity.
2 Results and discussion
Growth-dissolution equilibrium experiment of methane hydrate in 2. 1 salt solution
It was found that with the growth of hydrate, the methane in the solution was gradually consumed, and the intensity of methane peak gradually decreased, indicating that the concentration of dissolved methane gradually decreased, and the decreasing range gradually decreased with time, and remained basically unchanged after 2 ~ 3 hours. At this time, the hydrate and dissolved methane in the system basically reach equilibrium, and the corresponding concentration can be regarded as the saturated concentration of methane in the fluid after hydrate formation. Using this method, the saturated concentrations of two kinds of salinity (5%, 10%), temperature (1 ~ 20℃) and pressure (10 ~ 40 MPa) were measured respectively (Figures 3 and 4).
Fig. 3 Raman spectra of hydrate crystals in 5% NaCl solution.
Fig. 4 Experimental determination of micro hydrate crystals synthesized in brine system.
2. Calibration of dissolved methane content in1.1brine
There are two technical problems to be solved in the determination of hydrate dissolution-growth equilibrium in salt solution: first, salt ions affect the interaction between methane and water in fluid and Raman quantitative factor; Secondly, the growth-dissolution process of hydrate will lead to the change and heterogeneity of salinity.
In CH4-H2O-Na Cl system, the Raman active substances are CH4 and H2O. As the most common single anion in natural fluids, Cl will form a stable complex group with water. There is no Raman display in the measured temperature range, and Cl ions will hinder the formation of hydrogen bonds in methane hydrate, which means that salt can inhibit the formation of hydrate. In CH4-H2O-Na Cl system, the concentration ratio of CH4 and H2O is linear with its corresponding Raman peak area ratio, that is:
Enrichment Law and Exploitation Basis of Natural Gas Hydrate in South China Sea
Where: a is the Raman peak area; C is molar concentration; η, φ and S are the scattering coefficient, instrumental influence coefficient and salinity influence coefficient of the spectral cross section, respectively; F is Raman quantitative factor; That is to say, Raman quantitative factor is the correlation coefficient between peak area ratio and molar concentration ratio.
Studies show that the Raman quantitative factor increases with the increase of salinity in salt solution (Figures 5 and 6). The influence of salinity on Raman quantitative factor is approximately linear (Figure 7).
Relationship between molar fraction of methane and peak-to-height ratio under different salinities.
Fig. 6 Relationship between molar fraction of methane and peak area ratio (4) at different salinities.
Fig. 7 The influence of salinity measured experimentally on Raman quantitative factor is approximately linear.
PAR is the peak area ratio and HR is the peak height ratio.
2. 1.2 hydrate growth-dissolution equilibrium concentration in brine
The peak area ratio of hydrate dissolution-growth equilibrium in 5% and 10%Na C 1 solution was determined experimentally. In the range of 1 ~ 100℃, the linear relationship between the peak area ratio and the molar fraction of methane in L-V equilibrium and L-H equilibrium has nothing to do with the change of temperature, so the equilibrium concentration of dissolved methane in hydrate and Na Cl solution can be calculated by using the correlation coefficient in gas-liquid equilibrium. The results are shown in Table 2, Table 3, Figure 8 and Figure 9.
Table 2 Equilibrium concentration of hydrate in 5% NaCl solution (m(CH4)= 3.0875 hours)
Table 3 Equilibrium concentration of hydrate in 10% NaCl solution (m(CH4)= 3.50 1225 hours)
sequential
Fig. 8 Equilibrium concentration of hydrate in 5% NaCl solution.
Fig. 9 Equilibrium concentration of hydrate in 10% NaCl solution.
2.2 Growth-dissolution equilibrium experiment of methane hydrate in sediments-seawater of South China Sea
After the sample was packaged, it was left at room temperature for about 12 hours, so that the dissolved methane was evenly distributed in the sediment and solution. In the experiment, the sample was put into a hot and cold table, and the Raman spectrum of dissolved methane was recorded at room temperature, and then the temperature was reduced until the solution was frozen, and the quartz tube showed weak hydrate information. Then slowly raise the temperature to the temperature where the hydrate will not decompose but the ice in the solution will melt, and then lower the temperature until the gas-liquid interface changes slightly, and the hydrate quickly nucleates and grows. It can be seen that bubbles push the sediments to grow forward, and small hydrate particles begin to appear between the adjacent sediments and the pipe wall. Under the microscope, it can be seen that the hydrate between the sediment and the free gas grows in a needle shape, a hydrate film is formed on the surface of the free bubble, and there are filamentous hydrate crystals on the pipe wall (Figure 10).
After hydrate synthesis, the concentration of saturated methane in pore fluid of sediments was determined. Firstly, the temperature is adjusted to stabilize at the target temperature to be observed. After the target temperature is stabilized for about 0.5 h each time, the Raman spectrum of dissolved methane is drawn, and the Raman responses at different temperatures and pressures are obtained (figure 1 1, figure 12).
Figure 10 Synthetic Hydrates in the South China Sea Sediment-Seawater System
Figure 1 1 SH2 Well Sediment-Seawater System Changes with Temperature at 25.6MPa
Figure12 Variation of liquid Raman spectrum of sediment-seawater system in Well SH5 with temperature at 29.7 MPa
2.2. 1 calibration of dissolved methane content in sediments
In the equilibrium state of methane-Shenhu sediment-seawater, Raman responses at different temperatures and pressures were measured, and the corresponding peak area ratio (PAR) was calculated. The saturated concentration of methane at the corresponding temperature and pressure is calculated by using the gas-liquid equilibrium equation, and the corresponding relationship between the molar fraction of dissolved methane in aqueous solution [m(CH4)] and the Raman peak area ratio is established (Figure 13).
Figure 13 Relationship between molar fraction of methane and peak area ratio in seawater system and pure water system.
2.3.2 Equilibrium concentration of hydrate growth-dissolution in sediment-seawater system
In-situ observation of liquid methane concentration in Shenhu borehole sediments in the South China Sea was carried out at low temperature and high pressure, and the molar fraction of saturated methane in "seawater (LW)- hydrate (H)" two-phase system was obtained at 4 ~ 65438 08℃ and 65438 05.3, 25.6 and 29.7±0.4 MPa.
Table 4 Experimental data of the relationship between the molar fraction of dissolved methane [m(CH4)] in the sediment-seawater system aqueous solution and the peak area ratio (PAR) of Raman spectrum in the liquid phase at different temperatures and pressures.
sequential
The experimental observation shows that the saturated concentration of methane in sediments of Well SH2 and Well SH5 is positively correlated with temperature. The saturated concentration of methane increases with the increase of temperature and decreases with the decrease of temperature (Figure 14). The reason may be related to the different mineral compositions of the two kinds of pore sediments. The calcite content in the sediments of Well SH5 is much higher than that of Well SH2, so whether the solubility is affected by it needs further study.
Figure 14 methane equilibrium molar fraction of hydrate dissolution-growth in South China Sea Shenhu sediment-seawater system
3 Conclusion
By observing the formation and decomposition process of methane hydrate in artificial capillary, the following conclusions can be drawn:
1) From the measured equilibrium concentration of hydrate dissolution-growth in brine solution, the salinity has certain influence, and the increase of salinity reduces the equilibrium concentration of methane in the fluid. At present, there are few solubility data of gas needed to form natural gas hydrate, mainly theoretical values, and few experimental data. Handa calculated the influence of salinity and pressure on methane solubility when methane hydrate-water two-phase system was in equilibrium. The experiment confirmed that methane solubility was more affected by temperature than pressure in this two-phase system, and the same conclusion was obtained in this experiment.
2) Generally speaking, at the same temperature, the saturated concentration of methane in the sediments of Well SH5 is higher than that of Well SH2. It seems that the formation of hydrate in the sediments of Well SH5 needs higher methane content than that of Well SH2, and the formation of hydrate in the sediments of Well SH5 is not as easy as that in Well SH2, which may be one of the reasons why hydrate has not been drilled yet.
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