Article pubs.acs.org/EF
Memory Effect Test of Methane Hydrate in Water + Diesel Oil + Sorbitan Monolaurate Dispersed Systems Jun Chen,† Ke-Le Yan,† Meng-Lei Jia,† Chang-Yu Sun,*,† Yan-Qin Zhang,‡ Si Si,† Qing-Lan Ma,† Lan-Ying Yang,† Xiao-Qin Wang,† and Guang-Jin Chen† †
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China China Petroleum Engineering Co., Ltd. Beijing 100085, China
‡
ABSTRACT: Formation and reformation of methane hydrate in (water + diesel oil + sorbitan monolaurate) dispersed systems have been investigated to test the memory effect using particle video microscope and focused beam reflectance measurement probes. The factors which would affect methane hydrate formation, the dosage of sorbitan monolaurate, water cut, and initial experimental temperature, were examined. The results show that there exists obvious memory effect when methane hydrate reformed in water/diesel oil dispersed systems at initial temperature near methane hydrate formation zone, even maintaining at the initial temperature for 168 h after methane hydrate dissociation. The subcooling will increase with prolonging of maintaining time, which suggesting that memory effect would disappear if time is long enough. When initial temperature increases to 5 K higher than the equilibrium value, the subcooling of reformation of methane hydrate is similar to that of hydrate first formation, which implying that memory effect disappears. spectroscopy31,32 have been used to detect hydrate formation in water/oil systems. Similarly, particle video microscope (PVM) and focused beam reflectance measurement (FBRM) probes have been used to in situ measure the morphology and size distribution of water droplet or hydrate particle in dispersed systems.33−37 But this technique has not been used to test memory effect in water in oil dispersed systems in reported literatures. In this work, formation and reformation of methane hydrate in water in diesel oil dispersed systems were conducted in an autoclave with PVM and FBRM probes to test the rule of memory effect. The influences of water cut, initial temperature, and the dosage of additives on the memory effect of hydrate formation were examined.
1. INTRODUCTION Clathrate hydrate is that guest molecules are inserted into the host water molecules and then form a nonstoichiometric solid substance at suitable temperature and pressure. The related technology based on hydrate can be potentially applied to gas mixtures separation, hydrogen storage, refrigeration, etc.1−8 However, it can also cause problems such as blocking the transport pipeline.9 During the research on the hydrate formation process, an interest phenomenon called memory effect often appears, which means that the reformation of hydrates occurs under relatively milder conditions than the initial nucleation. Memory effect can cause fast reformation of gas hydrate and pipeline plug, which is a problem in gas and oil production,9,10 and is also related to the development of technology based on hydrate. The reason that causes memory effect may ascribe to the residual structure of the dissociated hydrate in the aqueous solution or dissolved gas which remains in solution after hydrate dissociation.9,11−14 Memory effect has been reported in many systems which including hydrate formed from hydrocarbon,12,13,15−19 CO2,20 tetra-n-butyl ammonium bromide,21 and tetrahydrofuran.10 Molecular dynamics simulations22,23 were also used to search for the mechanism of memory effect. Most of above memory effect experiments were conducted in cells with windows, where hydrate formation from water,20 aqueous solution,21 or hydrocarbon/water interface15 can be easily observed and detected. Water in oil dispersed systems, especially water in oil emulsions, are very common in gas and oil production and transportation.24−26 However, memory effect for hydrate formation in water/oil dispersed systems was seldom reported. One reason is that hydrate formed from this kind of system is often opaque with or without addition of additives,25−27 which is inconvenient for observing during the experimental process. In recent years, differential scanning calorimetry,26 X-ray diffraction,28,29 nuclear magnetic resonance,30 and dielectric © 2013 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Materials. Analytical grade (99.99%) methane supplied by the Beijing Beifen Gas Industry Corporation was used in this work. Twice distilled water was made in our laboratory. The commercial surfactant sorbitan monolaurate (Span 20, analytically pure), purchased from Beijing Chemical Reagents Company (BCRC), China, was added to form water in diesel oil dispersed system, where the dosage of Span 20 is based on the mass of water. An electronic balance with a precision of ±0.1 mg was used for preparing aqueous solution. The composition of the diesel oil is shown in Table 1. To prepare the water-in-oil emulsion, a certain amount of Span 20 was first dissolved into the diesel oil phase, and then stirred with the appropriate amount of distilled water adequately. For the experimental temperature range, the prepared system can be in stable water-in-oil dispersed conditions. To keep the dispersed system in stable state for a long period of time, stirring is in progress throughout the whole experiments. 2.2. Apparatus. The schematic diagram of the experimental apparatus used in this work for testing methane hydrate memory effect in water-in-oil dispersed systems is shown in Figure 1. The apparatus is mainly constituted by three parts: a high pressure autoclave with water Received: August 3, 2013 Published: November 25, 2013 7259
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fixed at 2 m/s for all experiments. The chord length of droplets or particles in a small region in front of a sapphire window is counted at a certain time interval. The chord length distribution can then be determined as shown in Figure 3c. Mean chord length, which can stand for the size of the droplet or particle to some extent,37 is obtained from chord length distribution or IC FBRM software. PVM and FBRM probes have been introduced in hydrate community to measure water droplet or hydrate particle size in oil phase in recent years.33−35,37,38 More information on the probe and the technique can refer to the user’s manuals.39,40 2.3. Experimental Procedure. Before the experiment, the reactor and all the connections were flushed with hot distilled water, dried with pure nitrogen, and evacuated. Subsequently, about 220 mL prepared water/diesel oil solution with a known initial water cut and Span 20 concentration was charged through a hand pump into reactor, which has been evacuated again to remove the air dissolved in the solution. The temperature was first set to a value (which is named as the initial temperature in this work) and methane was injected into the reactor at an initial pressure around 7.0 MPa (which is close to methane hydrate formation pressure at 283.2 K). The temperature and pressure were maintained constant for 5 h. The stirrer was started up to maintain the stable of the size of the fluid mean chord length if no phase change happened. Then the temperature in water bath was decreased at the rate of about 0.2 K/min until methane hydrate formed (the first time of hydrate formation) in water in diesel oil dispersed system. The corresponding temperature value was marked as the experimental temperature for hydrate formation, which can be confirmed by both PVM pictures and FBRM chord length distributions. After the formation of methane hydrate, the temperature of the reactor was increased to the initial temperature value to dissociate the formed methane hydrate thoroughly and then maintained at this temperature for 2 h. Afterward, the water bath temperature was decreased again at the rate of 0.2 K/min until methane hydrate reformed (the second time of hydrate formation). The temperature when hydrate reformed was recorded as the hydrate reformation temperature. This procedure was repeated for the third and the fourth time, where the difference was that after the hydrate dissociated thoroughly, the system was maintained at the constant initial temperature for 4 h (the third time) and 12 h (the fourth time), respectively. During the whole experiment, the magnetic stirrer was run at a speed of 350 rpm. The variation of temperature, pressure, and chord length distribution of water droplet/hydrate particle were
Table 1. Composition of the Diesel Oil Used in This Work component
mol /%
wt /%
heptanes octanes nonanes decanes undecanes dodecanes tridecanes tetradecanes pentadecanes hexadecanes heptadecanes octadecanes eicosanes tetracosanes octacosanes plus total
0.22 1.35 3.60 3.70 5.90 5.16 8.34 13.61 11.37 10.08 9.59 8.71 11.42 6.81 0.15 100.00
0.10 0.70 2.09 2.39 4.19 3.99 6.98 12.26 10.97 10.37 10.47 10.07 14.66 10.47 0.30 100.00
bath and a magnetic stirrer, PVM and FBRM probes, and data acquisition system. The effective internal volume of reactor is 535 mL (51.84 mm in diameter and 320 mm in depth). A secondary platinum resistance thermometer (type: Pt100) and a differential pressure transducer (type: Trafag 8251) were installed in the reactor to detect temperature and pressure, in which the uncertainties are 0.1 K and 0.02 MPa, respectively. Both PVM probe and FBRM D600X probe were purchased from Mettler-Toledo Lasentec. The PVM probe consists of six lasers which illuminating a small area in front of the probe face as shown in Figure 2. The probe creates digital images of the illuminated area with a field of view of 1680 μm × 1261 μm. The image provides clear resolution to approximately 5 μm. The FBRM probe is inserted into the system containing droplets or particles, and the schematic is illustrated in Figure 3. There is a rotating optical lens at the probe tip which can deflect the laser as shown in Figure 3a. When it starts working, the emitted laser is reflected if it scans across the surface of a particle as shown in Figure 3b. The distance between the point a at time t1 and point b at time t2 that laser passes through was named as the chord length with an uncertainty of 0.5 μm. It can be determined by the product of the measured reflectance time (t2-t1) and the laser scan speed (vb), which can be adjusted from 2 to 16 m/s in model D600X according to the experimental need. In this work, it is
Figure 1. Schematic diagram of the experimental apparatus for measuring water droplet/hydrate particle distribution in (water + diesel oil) dispersed system. 1, gas cylinder; 2, autoclave reactor; 3, water bath; 4, pressure transducer; 5, FBRM probe; 6, vent; 7, magnetic stirrer; 8, computer; 9, PVM probe; 10, platinum resistance thermometer; 11, hand pump. 7260
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Figure 2. (a) Schematic of PVM probe; (b) Typical picture captured by PVM.
Figure 3. (a) FBRM probe; (b) Measurement of a particle chord length; (c) Chord length distribution obtained from FBRM software. recorded on line. The morphology of the fluid was also captured from PVM software. Typical experimental procedure for (10 vol% water +90 vol% diesel oil +1.0 wt % Span 20) dispersed system is shown in Figure 4. In Figure 4, points a, e, f, and g are almost at the same pressure, which implying that there exists no nonmelted hydrate in the fluid. Subcooling, the difference between the hydrate equilibrium temperature and the experimental temperature, is a convenient parameter to evaluate the potential of the additive for inhibiting hydrate formation.17 It will change with hydrate reformation and then was used to describe the difficulty degree and memory effect of methane hydrate formation in water/diesel oil dispersed systems.
3. RESULTS AND DISCUSSION 3.1. Effect of Temperature and Pressure on Water Droplet Size. The system pressure and temperature will change during the memory effect test process. Therefore, the influences of temperature and pressure on water droplet size were first examined. Two groups of experiments were performed using a (20 vol% water +80 vol% diesel oil) dispersed system with 3.0 wt % Span 20 as additive, where one group examining the effect of temperature (274.2, 278.2, and 283.2 K) on water droplet size under 0.1 MPa, the other group examining the effect of pressure (0.1, 2.20, 4.53, and 7.14 MPa)
Figure 4. Typical experimental procedure for (10 vol % water +90 vol %diesel oil +1.0 wt % Span 20) dispersed system. (a) constant initial temperature; (b) slow cooling; (c) hydrate formation or reformation; (d) rapid heating up; (e) maintaining temperature stable for 2 h; (f) maintaining temperature stable for 4 h; (g) maintaining temperature stable for 12 h.
under 283.2 K. The mean chord length at different temperatures and pressures are showed in Table 2. The variation of 7261
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the change of mean chord length is so little by the effect of temperature and pressure that it can be ignored in the following methane hydrate formation and reformation experiments. 3.2. Determination of Hydrate Formation by FBRM and PVM. To examine the memory effect, it is important to determine the formation of hydrate timely and effective. Hydrate formation is an exothermic process and should be detected from the temperature change by using a thermocouple inserted in the solution. However, for a middle size autoclave, when only a few hydrate initially formed, the temperature of the solution may not increase obviously. There exists the hysteresis effect of temperature due to the large volume of the solution and the refrigeration of the water bath during the experimental procedure. Therefore, PVM and FBRM probes adopted in this work are more suitable to confirm methane hydrate formation in water/diesel oil dispersed systems. Figure 7 shows the
typical chord length distributions with temperature and pressure are also shown in Figures 5 and 6, respectively. Table 2. Variation of Droplet Size with Temperature and Pressure for (20 vol% Water +80 vol% Diesel Oil +3.0 wt % Span 20) System 0.1 MPa
283.2 K a
temperature/K
mean chord length / μm
pressure/ MPa
mean chord length /μm
274.2 278.2 283.2
5.4 5.5 5.9
2.20 4.53 7.14
5.3 5.2 5.2
a
The uncertainty of the mean chord length (MCL) in this work is ±0.5 μm.
Figure 5. Typical chord length distribution for (20 vol% water +80 vol % diesel oil +3.0 wt % Span 20) system at three different temperatures under 0.1 MPa.
Figure 7. Determination of hydrate formation by PVM and FBRM for (methane +10 vol% water +90 vol% diesel oil) system with addition of 3.0 wt % Span 20. (a) variation of chord length distribution with time; (b) typical PVM pictures before hydrate formation; (c) PVM pictures during hydrate formation.
variation of chord length distribution of droplet and/or hydrate particle with time for (methane +10 vol% water +90 vol% diesel oil) system with addition of 3.0 wt % Span 20. Two typical PVM pictures before and during methane hydrate formation in this system is also shown in Figure 7. One can see that chord length distribution of the fluid can maintain stable before the formation of hydrate. However, when methane hydrate forms, the chord length distribution changes obviously, which can confirm the formation of hydrate in water in diesel oil dispersed system, as the red curve shown in Figure 7a. From PVM pictures as shown in Figure 7b, water droplets are dispersed in diesel oil phase before hydrate forms. When hydrate forms, large particles appear which can be obviously seen in PVM picture as shown in Figure 7c. Combined PVM pictures with FBRM chord length distribution data, methane hydrate formation in water in diesel oil dispersed systems can be easily confirmed. This method is then used to evaluate the memory effect for water in oil dispersed systems.
Figure 6. Typical chord length distribution for (20 vol% water +80 vol % diesel oil +3.0 wt % Span 20) system at four different pressures under 283.2 K.
From Table 2, it can be seen that the mean chord length increases from 5.4 to 5.9 μm when the temperature increases from 274.2 to 283.2 K under 0.1 MPa. For the investigated system and temperature range, the typical chord length distribution only changes a little with the variation of temperature as shown in Figure 5. When the system pressure increases from 0.1 to 7.14 MPa under the temperature of 283.2 K, the mean chord length decreases from 5.9 to 5.2 μm. This may be due to that the compressed methane gas can make water droplet size of water in oil dispersed solutions decrease.41 However, compared with the influence of hydrate formation, 7262
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Table 3. Mean Chord Length (MCL) and Subcooling (ΔTa) of Eight Groups of Dispersed Systems for Methane Hydrate Formation and Reformation under the Similar Initial Pressure (P0) and the Same Initial Temperature (283.2 K) first time
a
second time
third time
fourth time
systems
P0 /MPa
MCL /μm
ΔT /K
MCL /μm
ΔT /K
MCL /μm
ΔT /K
MCL /μm
ΔT /K
5 vol% water +95 vol% diesel oil +0.5 wt % Span 20 5 vol% water +95 vol% diesel oil +1.0 wt % Span 20 5 vol% water +95 vol% diesel oil +3.0 wt % Span 20 10 vol% water +90 vol% diesel oil +0.5 wt % Span 20 10 vol% water +90 vol% diesel oil +1.0 wt % Span 20 10 vol% water +90 vol% diesel oil +3.0 wt % Span 20 20 vol% water +80 vol% diesel oil +0.5 wt % Span 20 30 vol% water +70 vol% diesel oil +0.5 wt % Span 20
7.08 6.93 7.24 7.10 6.97 7.03 7.12 7.02
23.3 17.0 7.7 11.5 8.2 7.3 8.0 5.4
8.0 7.8 7.5 8.3 7.8 7.1 6.2 7.0
24.2 18.7 9.0 14.8 11.5 7.3 8.0 5.6
3.2 2.4 3.7 3.4 2.4 3.6 3.0 3.2
32.4 25.3 9.3 15.4 13.3 7.4 8.1 5.8
2.9 2.5 3.6 3.2 2.9 3.5 2.9 3.4
36.8 29.9 10.0 15.6 14.0 7.4 8.5 5.9
3.1 2.5 3.7 3.4 3.1 3.7 3.1 3.6
The uncertainty of ΔT is ±0.2 K for all the measurement.
3.3. Memory Effect Test when Initial Temperature near Equilibrium Zone. The experiments of methane hydrate formation and reformation in (water + diesel oil) dispersed systems were performed according to the experimental method introduced above. Table 3 shows the experimental results of eight groups of dispersed systems, where the water cut is 5, 10, 20, and 30 vol%, the additive concentration is 0.5, 1.0, and 3.0 wt %, and the initial pressure is about 7.0 MPa. The initial temperature is set at 283.2 K, which is near hydrate equilibrium temperature corresponding to the initial pressure. Four times of methane hydrate formation and reformation experiments were performed for each group of dispersed systems. The mean chord length of water droplet before hydrate formation and the subcooling for each group of memory effect experiment are listed in Table 3. Since there are no measurable differences between the bulk solutions and emulsions from the thermodynamic point of view,42 the equilibrium temperature was calculated using Chen-Guo hydrate model43 for describing the subcooling. From Table 3, one can find that the subcoolings of methane hydrate formation at the second time are obviously lower than those at the first time for every system with different ratio of Span 20 and water cut, which means that the experimental conditions to reform methane hydrate is milder than those in the first time. Compared with hydrate formation at the first time, the subcooling of methane hydrate reformation in the second time can be reduced with a minimum value of 3.2 K for (20 vol% water +80 vol% diesel oil +0.5 wt % Span 20) dispersed system, while a maximum value of 5.4 K for (10 vol% water +90 vol% diesel oil +1.0 wt % Span 20) dispersed system. These results strongly imply that there exists memory effect for (methane + water + diesel oil + Span 20) dispersed systems. From Table 3, one can see that the subcooling decreases with increasing dosage of Span 20 when under the same water cut (5 or 10 vol%) for the first time of methane hydrate formation, which suggesting that Span 20 may promote the formation of hydrate although they are usually acted as an antiaggomerant.27 In comparison, water cut seems to have no regular effect on methane hydrate formation in water-in-oil dispersed systems. In addition, it can be found that the mean chord length of water droplet before methane hydrate formation decreases with the increase of the dosage of Span 20 for (5 vol% water +95 vol% diesel oil) and (10 vol% water +90 vol% diesel oil) systems. Similarly, the mean chord length decreases with the increase of water cut for different water cut systems with the same 0.5 wt % Span 20. Since the adding ratio of Span 20 is based on water mass fraction, the mean chord length may be affected by both the amount of water cut and Span 20.
Figure 8 shows the variation of the mean chord length of water droplet at different water cuts and doses of Span 20
Figure 8. Variation of mean chord length with run time at different conditions: (a) 5 vol% water +95 vol% diesel oil +0.5 wt % Span 20; (b) 5 vol% water +95 vol% diesel oil +1.0 wt % Span 20; (c) 5 vol% water +95 vol% diesel oil +3 wt % Span 20; (d) 10 vol% water +90 vol % diesel oil +0.5 wt % Span 20; (e) 10 vol% water +90 vol% diesel oil +1.0 wt % Span 20; (f) 10 vol% water +90 vol% diesel oil +3.0 wt % Span 20; (g) 20 vol% water +80 vol% diesel oil +0.5 wt % Span 20; (h) 30 vol% water +70 vol% diesel oil +0.5 wt % Span 20.
before hydrate formation and reformation. It can be seen that the mean chord length changes a little except for the systems of (5 vol% water +95 vol% diesel oil +0.5 wt % Span 20) and (5 vol% water +95 vol% diesel oil +1.0 wt % Span 20), in which the mean chord length of water droplet at the first time and the fourth time changes from 23.3 to 36.8 μm, and 17.0 to 29.9 μm, respectively. Since the temperature and pressure in this work have small effect on mean chord length as discussed earlier, the main reason of the large fluctuation of the mean chord length for these two systems may ascribe to the continuous hydrate formation and dissociation process. Lachance et al.44 also showed that gas hydrate formation and dissociation has a destabilizing effect on water-in-oil (W/O) emulsions by differential scanning calorimetry. However, from the experimental results, it can be found that the effect of continuous hydrate formation and dissociation on the mean chord length size decreases with the increase of the amount of Span 20 in water in diesel oil dispersed systems with higher Span 20 dosage or higher water cut. From Table 3, one can also see that the subcooling of hydrate formation in the second time, the third time, and the 7263
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memory effect is very long. Memory effect would not disappear even maintaining the temperature at 283.2 K for 168 h after methane hydrate dissociation. It is known that the residual structure and methane solubility would result in the memory effect.11−14 Guo and Rodger14 found that lowing the temperature is more effective in promoting hydrate nucleation than increasing pressure although both lowing the temperature and increasing pressure can increase methane solubility. Therefore, two higher initial temperatures, 288.2 and 293.2 K under the initial pressure of 7.58 and 7.94 MPa, respectively, which are much higher than methane hydrate formation conditions, were performed for (10 vol% water +90 vol% diesel oil +3.0 wt % Span 20) dispersed system to examine the influence of temperature on memory effect, and the experimental results are shown in Table 4. From Table 4, one can see that when initial temperature is set to 288.2 K, the subcooling of methane hydrate formation keeps unchanged or only decreases a little for maintaining for 2, 4, and 12 h after hydrate dissociation. Memory effect is obviously weakened when the temperature increases to 288.2 K. When initial temperature reaches 293.2 K, there shows no memory effect when methane hydrates reformed in water in diesel oil dispersed systems. Only mean chord length changes a little because of hydrate formation and dissociation. Another four groups of experiments were also performed to study the conditions of disappearance of memory effect and the results are listed in Table 5. For these four groups of experiments, the initial pressure is close to the equilibrium value at the initial temperature. There exist some different for the third time of hydrate formation with the experimental procedure described in Section 2.3. After methane hydrate reformed (the second time in Table 5), the temperature of the reactor was increased to 5 K higher than the initial temperature and maintained for 2 h after hydrate thoroughly dissociated. The water bath temperature was then decreased at the rate of 0.2 K/min until methane hydrate reformed (the third time in Table 5). Compared methane hydrate formation in the first time with the second time as listed in Table 5, it can be found that memory effect also exists for (10 vol% water +90 vol% diesel oil) systems whatever for initial temperature of 280.2 K, 283.2 K, and 286.2 K with/without different dose of Span 20. However, when initial temperature was adjusted to 5 K higher than hydrate equilibrium conditions, the subcooling value of methane hydrate reformation (the third time in Table 5) tends to return to that in the first time. The memory effect was almost eliminated through increasing the subcooling value 5 K higher than equilibrium conditions. No memory effect was also reported for hydrate formation and reformation when the melt temperature exceeded 6.5 K higher than equilibrium conditions for THF/water mixtures.45 Therefore, the increase of temperature is a more useful method to eliminate memory effect than maintaining the systems near hydrate formation zone for a longer time for water/oil dispersed systems.
fourth time do not change so much for the same group of (water + diesel oil + Span 20) systems. The systems maintained at 283.2 K for 4 or 12 h after methane hydrate dissociation cannot eliminate memory effect. However, there is a trend that the subcooling of methane hydrate formation increases after maintaining a longer time. The maximum subcooling increases by 0.3 K for (30 vol% water +70 vol% diesel oil +0.5 wt % Span 20) dispersed system when maintained the systems at 283.2 K for 12 h after methane hydrate dissociation. To test the change of subcooling in a longer period of maintaining time, other three groups of experiments were performed, in which 24 h, 72 h, and 168 h were maintained at the temperature of 283.2 K after methane hydrate dissociation for (10 vol% water +90 vol% diesel oil +3.0 wt % Span 20) dispersed system. Figure 9 shows
Figure 9. Subcooling of (10 vol% water +90 vol% diesel oil +3.0 wt % Span 20) system when maintained at 283.2 K for different time after methane hydrate dissociation.
the results of the subcooling at seven groups of maintaining time. It can be found that the subcooling rises to 4.4 K when maintaining at 283.2 K after hydrate dissociation for 24 h, which is 0.7 K higher than that of the fourth time of methane hydrate formation. The subcooling rises to 5.1 and 5.4 K after maintaining the systems at 283.2 K for 72 and 168 h, respectively. From the subcooling trend line shown in Figure 9, the lowest subcooling appeared in the experiment with maintaining time of 4 h. Afterward, the subcooling increases with the increase of the maintaining time. The time for disappearing the memory effect is very long (at least greater than 168 h) when the temperature is near hydrate formation zone. Therefore, in actual situation, the operation near hydrate formation region should be avoided for preventing hydrate from easily reformation. 3.4. Memory Effect Test when Initial Temperature Higher than Equilibrium Zone. For methane hydrate formation experiments where the dissociation temperature maintained at 283.2 K, which is near methane hydrate formation zone, it was found that the time to eliminate the
Table 4. Experimental Results of Methane Hydrate Formation and Reformation When Initial Temperature Higher than Formation Conditions for (10 vol% Water +90 vol% Diesel Oil +3.0 wt % Span 20) Dispersed System first time
second time
third time
fourth time
initial temperature/K
initial Pressure/MPa
MCL /μm
ΔT /K
MCL /μm
ΔT /K
MCL /μm
ΔT /K
MCL /μm
ΔT /K
288.2 293.2
7.58 7.94
7.4 7.5
5.9 5.5
7.2 8.9
5.8 5.7
6.6 7.6
5.6 5.5
5.1 4.9
5.5 5.4
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Table 5. Experimental Results of Methane Hydrate Formation and Reformation to Test the Conditions of Disappearance of Memory Effect first time
second time
third time
systems
initial temperature/K
initial pressure/ MPa
MCL /μm
ΔT /K
MCL /μm
ΔT /K
MCL /μm
ΔT/K
10 vol% water +90 vol% diesel oil 10 vol% water +90 vol% diesel oil +1.0 wt % Span 20 10 vol% water +90 vol% diesel oil +3.0 wt % Span 20 10 vol% water +90 vol% diesel oil +3.0 wt % Span 20
283.2 280.2 280.2 286.2
7.26 5.23 5.19 10.15
40.6 8.0 7.5 7.2
8.5 7.8 6.6 7.5
42.3 8.2 7.6 7.4
5.8 2.8 3.8 4.2
45.5 8.7 7.9 7.7
8.9 7.9 6.8 7.3
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4. CONCLUSIONS Memory effect of methane hydrate has been tested in the autoclave from the variation of chord length distribution of fluid and its morphology with PVM and FBRM probes for (water + diesel oil + Span 20) dispersed systems. The experimental results show that there exists obvious memory effect in (water + diesel oil + Span 20) dispersed systems when initial temperature is near methane hydrate formation zone, no matter the ratio of water cut and the dosage of Span 20. The memory effect cannot be eliminated even maintaining for 168 h near hydrate formation temperature, while it will be easily eliminated when maintaining the temperature 5 K higher than the equilibrium value after hydrate dissociation. The results suggest that gas hydrate reformation in water in oil dispersed system could be retarded by rising temperature for assuring gas and oil production safety.
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AUTHOR INFORMATION
Corresponding Author
*(C.-Y.S.) Fax: +86 10 89733156. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial support received from National 973 Project of China (No. 2012CB215005), National Science & Technology Major Project (No. 2011ZX05026-004), National Natural Science Foundation of China (Nos. 20925623, U1162205, 51376195), and Science Foundation of China University of Petroleum, Beijing (No. 2462013YXBS004, 01JB0171), are gratefully acknowledged.
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