Protocol for Measuring the Thermal Properties of a Supercooled Synthetic Sand-water-gas-methane Hydrate Sample

Podsumowanie

We present a protocol for measuring the thermal properties of synthetic hydrate-bearing sediment samples comprising sand, water, methane, and methane hydrate.

Streszczenie

Methane hydrates (MHs) are present in large amounts in the ocean floor and permafrost regions. Methane and hydrogen hydrates are being studied as future energy resources and energy storage media. To develop a method for gas production from natural MH-bearing sediments and hydrate-based technologies, it is imperative to understand the thermal properties of gas hydrates.

The thermal properties' measurements of samples comprising sand, water, methane, and MH are difficult because the melting heat of MH may affect the measurements. To solve this problem, we performed thermal properties' measurements at supercooled conditions during MH formation. The measurement protocol, calculation method of the saturation change, and tips for thermal constants' analysis of the sample using transient plane source techniques are described here.

The effect of the formation heat of MH on measurement is very small because the gas hydrate formation rate is very slow. This measurement method can be applied to the thermal properties of the gas hydrate-water-guest gas system, which contains hydrogen, CO₂, and ozone hydrates, because the characteristic low formation rate of gas hydrate is not unique to MH. The key point of this method is the low rate of phase transition of the target material. Hence, this method may be applied to other materials having low phase-transition rates.

Wprowadzenie

Gas hydrates are crystalline compounds that comprise cage structures of hydrogen-bonded water molecules containing guest molecules in the cage¹. Large amounts of methane hydrates (MHs) in the ocean floor and permafrost regions are interesting future energy resources but may affect global climate conditions².

In March 2013, the Japan Oil, Gas, and Metals National Corporation conducted the world's first offshore production test to extract gas from natural MH-bearing sediments in the eastern Nankai Trough using the "depressurization method" ^3,4.

Gas hydrates can store gases such as methane¹, hydrogen⁵, CO₂^1,6, and ozone⁷. Hence, methane and hydrogen hydrates are studied as potential energy storage and transportation media. To reduce the CO₂ emissions released into the atmosphere, CO₂ sequestration using CO₂ hydrates in deep-ocean sediments have been studied⁶. Ozone is currently used in water purification and food sterilization. Studies of ozone preservation technology have been conducted because it is chemically unstable⁷. The ozone concentration in hydrates is much higher than that in ozonized water or ice⁷.

To develop gas production from natural MH-bearing sediments and hydrate-based technologies, it is imperative to understand the thermal properties of gas hydrates. However, the thermal properties data and model studies of gas hydrate-bearing sediments are scarce⁸.

The "depressurization method" can be used to dissociate MH in the sediment pore space by decreasing the pore pressure below the hydrate stability. In this process, the sediment pore space components change from water and from MH to water, MH, and gas. The thermal properties' measurement of the latter condition is difficult because the melting heat of MH may affect the measurements. To solve this problem, Muraoka et al. performed the thermal properties' measurement at supercooled conditions during MH formation⁹.

With this video protocol, we explain the measuring method of supercooled synthetic sand-water-gas-MH sample.

Figure 1 shows the experimental setup for measuring the thermal properties of the artificial methane hydrate-bearing sediment. The setup is the same as shown in reference⁹. The system mainly comprises a high-pressure vessel, pressure and temperature control, and thermal properties of the measurement system. The high-pressure vessel is composed of cylindrical stainless steel with an internal diameter of 140 mm and a height of 140 mm; its inner volume with the dead volume removed is 2,110 cm³, and its pressure limit is 15 MPa. The transient plane source (TPS) technique is used to measure the thermal properties¹⁰. Nine TPS probes with individual radii of 2.001 mm are placed inside the vessel. The layout of the nine probes⁹ is shown in Figure 2 in reference⁹. The TPS probes are connected to the thermal properties' analyzer with a cable and switched manually during the experiment. The details of the TPS sensor, connection diagram, and setup in the vessel are shown in Figures S1, 2, and 3 of the supporting information in reference⁹.

Figure 1: The experimental setup for measuring the thermal properties of the artificial methane hydrate-bearing sediment. The figure is modified from reference⁹. Please click here to view a larger version of this figure.

The TPS method was used to measure the thermal properties of each sample. The method principles are described in reference¹⁰. In this method, the time-dependent temperature increase, ΔT_ave, is

where

In Equation 1, W₀ is the output power from the sensor, r is the radius of the sensor probe, λ is the thermal conductivity of the sample, α is the thermal diffusivity, and t is the time from the start of the power supply to the sensor probe. D(τ) is a dimensionless time dependent function. τ is given by (αt/r)^1/2. In Equation 2, m is the number of concentric rings of the TPS probe and I₀ is a modified Bessel function. The thermal conductivity, thermal diffusivity, and specific heat of the sample are simultaneously determined by inversion analysis applied to the temperature increase as power is supplied to the sensor probe.

Protokół

Note: Please consult all relevant material safety data sheets as this study uses high-pressure flammable methane gas and a large high-pressure vessel. Wear a helmet, safety glasses, and safety boots. If the temperature control system stops, the pressure in the vessel increases with MH dissociation. To prevent accidents, the use of a safety valve system is strongly recommended to automatically release the methane gas to the atmosphere. The safety valve system can work without electrical power supply.

1. Preparation of the Sand-water-methane Gas Samples⁹

1. Place the high-pressure vessel on the vibrating table.
2. Pour 1.5 L of pure water in a water bottle and 4,000 g silica sand in a sand bottle. Accurately weigh the masses of sand and water in the sand and water bottles, respectively.
3. Pour 1 L of pure water in the high-pressure vessel with an inner volume of 2,110 cm³ from a water bottle until the water fills half the inner vessel.
4. Turn on the vibrating table to vibrate the entire vessel. Set the vibration rate and the power supply to 50 Hz and 220 W, respectively. Apply the vibration until the completion of step 1.5. Remove the residual air in the drain line and sintered metallic filter at the bottom of the vessel by vibrating the vessel.
5. Pour 3,300 g silica sand from a sand bottle into the vessel at a constant rate of approximately 1 g sec⁻¹ using a funnel held near the water surface while the entire vessel is vibrated to ensure uniform packing.
6. Stop the vibration when the water reaches the rim of the vessel.
7. Place a ring as a temporary wall on the rim of the vessel to prevent water from spilling.
8. Vibrate the vessel again at 50 Hz and 220 W.
9. When the sand reaches the rim of the vessel (height 140 mm), turn off the vibration.
10. Remove the temporary wall and excess pore water using the drain line. Pour the excess pore water back into the water bottle.
11. Pack the sand by vibrating the vessel once or twice at 50 Hz and 300 W for 1 sec and add more sand if necessary.
12. Weigh the masses of sand and water in the sand and water bottles. Calculate the sand and water masses in the vessel from the mass differences in the sand and water bottles. In this experiment, the masses of sand and water in the vessel were 3,385 g and 823.6 g, respectively. The mass of water in the vessel is denoted as w_total.
13. Cover the high-pressure vessel with a stainless steel lid and tighten the bolts of diagonally opposite pairs in sequence.
14. Move the high-pressure vessel from the vibrating table to the table intended for the experiment.
15. Cover the high-pressure vessel with the heat insulator for controlling the temperature.
16. Connect the high-pressure pipelines and the cooling water flow lines to the high-pressure vessel.
17. Open the valves of the input and output gas pipelines. Ventilate 10 L methane at a rate of 800 ml min⁻¹ until no excess water discharges into the trap under atmospheric pressure. The sand discharge is prevented by a sintered metallic filter fixed on the bottom of the vessel. The residual water remains on the sand surface because the hydrophilic silica sand adsorbs the water molecules.
18. Weigh the mass of water in the trap, w_trap, to determine the gas volume in the vessel. Determine the mass of residual water, w_res, in the vessel using the equation w_res = w_total − w_trap. In this case, w_res and w_trap was 360.6 g and 463.0 g, respectively.
19. Determine the sample porosity using the formula Ѱ = 1 - V_sand / V_cell, where V_sand is the volume of the sand determined by the ratio of sand mass to sand density (i.e., ρ_s = 2,630 kg m⁻³), and V_cell is the inner volume of the vessel. The porosity Ѱ of the sample was 0.39.
20. Close the valve of the output gas line. Inject methane to increase the pore pressure of methane in the vessel to approximately 12.1 MPa at room temperature (i.e., 31.6 °C).
21. Close the valve of the input gas line.
22. Start recording the pressure and temperature in the vessel during the experiment using the data logger. The data sampling interval is 5 sec. The total experimental time is approximately 3,000 min.

2. MH Synthesis and Thermal Properties' Measurement of the Supercooled Sample⁹

1. Turn on the chiller for cooling the vessel from room temperature to 2.0 °C by circulating the coolant. Let the coolant circulate from the chiller to the bottom of the vessel, from there to the lid of the vessel, and finally back to the chiller. The temperature change rate in the vessel was approximately 0.001 °C sec⁻¹.
2. Set the measurement parameters using the TPS analyzer software. Set the sensor type to sensor design #7577. Set the output power W₀ to 30 mW and the measuring time to 5 sec. Note that the appropriate parameters should be changed if the sensor type or sample conditions change. Set the parameters to increase the temperature from 1 °C to 1.5 °C.
3. Calculate the degree of supercooling, ΔT_sup, with the following equation:
   ΔT_sup = T_eq(P) − T. (3)
   T_eq(P) is the equilibrium temperature of MH as a function of pressure P. T_eq(P) is calculated using the CSMGem software¹.  P and T are the pressure and temperature in the vessel measured by using pressure and temperature gauges, respectively.
4. Simultaneously measure the thermal conductivity, thermal diffusivity, and volumetric specific heat using the TPS analyzer after ΔT_sup is greater than 2 °C.
5. Switch the TPS probe connected to the thermal properties analyzer after each measurement. Switch the cables between the TPS probes and the analyzer manually during the experiment⁹. The connection diagram is shown in Figure S2 in reference⁹. The switching sequence for each sensor is no. 6 → 2 → 7 → 5 → 1 → 9 → 4 → 3 → 8 → 6…. The sequence is based on the distance between the sensors, which is set as far as possible to prevent the residual heat from affecting the measurements. Collect data every 3-5 min.
6. Repeat the measurements until ΔT_sup reaches 2 °C again. In this experiment, ΔT_sup initially increases with time. After ΔT_sup reaches the maximum value, ΔT_sup gradually decreases to 0 °C because the pressure decreases with the formation of MH. Check if ΔT_sup is greater than 2 °C prior to the TPS measurements using equation 3.
7. Ensure that the temperature profile is not affected by MH melting. If MH melts during the measurements, the temperature will not increase because melting of MH is an endothermic reaction. Check the temperature profile during the measurements, and is discussed in the results section.
8. Perform the thermal properties' analysis for all temperature profile data using the TPS technique.

3. Calculation of the Saturation Change of the Sample^9,11

Note: The degree of saturation for MH, water, and gas in the sample as a function of time t is calculated using the equation of state of the gas. The calculation details and equations used are previously described¹¹.

1. Calculate the methane gas volume V_gas,_t at time t
   
   where Q is the initial volume of gas in the vessel, V_{MH,t − 1} is the volume of MH at time t − 1, and R_Vhw is the volume ratio of water and MH.
   
   In Equation 5, n is the hydration number of MH (~6), ρ_MH and ρ_water correspond to the density of MH and water, respectively, and w_MH and w_water denote the molecular mass of MH and water, respectively.
2. Calculate the amount ΔM_t (mol) of MH formed from t − 1 to t
   
   where R is the gas constant, P is the pressure of the methane gas, and Z_t (T_gas,_t, P_gas,_t) is the compression coefficient of methane at time t. We⁹ and Sakamoto et al.¹¹ have used the Benedict–Webb–Rubin (BWR) equation, as modified by Lee and Kesler, for calculating Z_t¹²,¹³. For this calculation, the formulas (3–7.1)–(3–7.4) of the BWR equation¹³ and the Lee–Kesler constants are used in Tables 3–7 of reference¹³.
3. Calculate the volume change ΔV_MH,_t of MH from t − 1 to t
   
   where P_s is the reference pressure of 101325 Pa, T_s is the reference temperature of 273.15 K, Z_s is the compression coefficient at P_s and T_s (Z_s ~1), and V_CH4 is the ratio of the methane gas volume in the unit volume of MH [Nm³ m⁻³]. Use a V_CH4 value of 165.99 [Nm³ m⁻³].
4. Calculate the volume V_MH,_t of MH at time t
   
5. Calculate the volume of water V_water,_t in the pressure vessel at time t
   
   where V_water,1 is the initial volume of water.
6. Repeat the calculations using Equations. 4–9 at time t = 2, 3, … to determine the change in the saturation of water, methane, and MH¹¹. The initial condition is t = 1, i.e., V_gas,₁ = Q. The P and T at time t are taken from the data logs⁹. The calculation results are shown in the following section.

Wyniki

Figure 2a shows the temperature profile that is not affected by MH melting. ΔT_c is the temperature change due to thermal constants' measurement. Figure 2b shows the temperature profile that is affected by MH melting. The profile in Figure 2b cannot be analyzed through Equations 1 and 2 because these equations are derived by assuming stable sample conditions.

Dyskusje

The effect of the formation heat of MH on measurement was estimated. The formation heat of MH was estimated from products of change rate of S_h as shown in Figure 3b and the enthalpy of formation H = 52.9 kJ mol⁻¹ for MH¹⁴. Consequently, the maximum temperature change was 0.00081 °C sec⁻¹. This was much lower than the temperature increase ΔT_c of the TPS sensor between 1 °C and 1.5 °C during th...

Materiały

Name	Company	Catalog Number	Comments
TPS thermal probe, Hot disk sensor	Hot Disk AB Co., Sweden	#7577	Kapton sensor type, sensor radius 2.001 mm
Hot disk thermal properties analyzer	Hot Disk AB Co., Sweden	TPS 2500
Toyoura standard silica sand	Toyoura Keiseki Kogyo Co., Ltd., Japan	N/A
Methane gas, 99.9999%	Tokyo Gas Chemicals Co., Ltd., Japan	N/A	Grade 6 N, Volume 47 L, Charging pressure 14.7 MPa
Water Purification System, Elix Advantage 3	Merck Millipore., U.S.	N/A	5 MΩ cm (at 25 °C) resistivity
Vibrating table, Vivratory packer	Sinfonia Technology Co. Ltd., Japan	VGP-60
Chiller, Thermostatic Bath Circulator	THOMAS KAGAKU Co., Ltd., Japan	TRL-40SP
Coorant, Aurora brine	Tokyo Fine Chemical Co.,Ltd., Japan	N/A	ethylene glycol 71 wt%
Temparature gage	Nitto Kouatsu., Japan	N/A	Pt 100, sheath-type platinum resistance temperature detector
Pressure gage	Kyowa Electronic Instruments., Japan	PG-200 KU
Data logger	KEYENCE., Japan	NR-500
Mass flow controller	OVAL Co., Japan	F-221S-A-11-11A	Maximum flow 2,000 N ml⁠/⁠M, maximum design pressure 19.6 MPa