Effect of Gas Hydrate Saturation on Hydraulic Conductivity of Marine Sediments

*Chul-Whan Kang1), Ah-Ram Kim2), Hak-Sung Kim3), Gye-Chun Cho4) and Joo-Yong Lee5)

1), 2), 3), 4) Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea

5) Petroleum and Marine Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Korea

3) gyechun@kaist.edu

ABSTRACT Methane gas, one of the unconventional energy sources, is stored under the sea-bed or permafrost as a form of gas hydrate. Numerous countries are researching gas hydrate production. One of the production methods is depressurization which induces the dissociation of gas hydrates by pumping pore water through the production well. The water pumping reduces the pore pressure lower than the equilibrium pressure of the gas hydrates. Thus, the dissociation rate and gas productivity are significantly dependent on the water permeability (i.e., hydraulic conductivity) in hydrate-bearing sediments. The hydraulic conductivity of hydrate-bearing sediments is considerably affected by gas hydrate saturation because the gas hydrates generally exist as a pore-filling and fracture-filling solid in the sediment pores. Therefore, the purpose of this research was to determine the correlation between gas hydrate saturation and hydraulic conductivity in sediments through various laboratory experiments. In the experiment, gas hydrates were synthesized in a high pressure oedometer cell, with an artificial specimen which has a similar grain size distribution with a recovered core sample from the Ulleung basin of Korea. Gas hydrate formation and dissociation were monitored by geophysical sensors including P-and S-wave and electrical resistivity sensors. The experimental results suggest there is an empirical relationship between gas hydrate saturation and water permeability in gas hydrate-bearing sediments. 1. INTRODUCTION

1)

Graduate Student 2), 3), 5)

Ph. D 4)

Professor

Gas hydrates exist as crystalline structures, and water molecules enclose smaller gas molecules such as methane (Sloan et al., 2008). Natural gas hydrates are stored in permafrost sediments and offshore sea beds because gas hydrates are stable in low temperature and high pressure conditions (Sloan, 2003). Natural gas hydrates, mostly methane hydrates, are considered an unconventional energy source because of the large amount of deposits on the Earth (Mikov, 2004). Gas hydrate production projects are ongoing world-widely, mainly in the US, Canada and Japan. The stable condition of natural gas hydrates is determined by the pressure and temperature conditions seen in the phase diagram of Fig. 1. The depressurization method reduces the pressure below the stable pressure of the gas hydrates causing the hydrates to dissociate into water and methane gas. The pressure reduction is caused by pore water pumping through the production well in which dissociated methane gas and water are produced. During the pumping, the hydraulic conductivity of gas hydrate-bearing sediments has an important role in the productivity and dissociation rate. Gas hydrates obstruct the flow of liquid and gas phase materials because gas hydrates fill the voids between soil particles. Therefore, gas hydrate saturation has an effect on the water and gas permeability.

Fig. 1 CO2 gas hydrate stability diagram

High gas hydrate saturation (over 60%) reduces water permeability almost 100 times lower than zero gas hydrate saturation (Mohana, 2014). Therefore, the relation between gas hydrate saturation and hydraulic conductivity is important for the productivity and dissociation rate during gas hydrate production. 2. EXPERIMENTAL PROGRAM 2.1 Specimen During deep drilling expeditions in 2007 and 2010, 13 drill sites were explored. Among the drilled sites, UBGH2-6 was chosen as one of the potential production test sites taking into consideration the presence of gas hydrates and the composition of the sediment layers. A comparison of the grain size distribution between the artificial specimen and the core sample from UBGH2-6 is shown in Fig. 2. The specimen is silty sand with a specific gravity of 2.62 and mean particle size of 130 um.

Bulk

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Fig. 2 Grain size distribution

2.2 Experimental setup

Fig. 3 shows a schematic diagram of the experiment apparatus for gas hydrate formation and permeability measurements. CO2 gas was injected from the top of the specimen, and water was injected from both sides of the specimen. P- and S-wave velocities and electrical resistance were measured at the lateral side of the specimen with a signal generator and oscilloscope. Two syringe pumps were located at the upper and lower sides of the specimen to control the pressure.

Fig. 3 Experimental setup

2.3 Experimental procedure

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The initial gas hydrate saturation is determined by the partial water saturation of the specimen. CO2 gas injection and system cooling are carried out using the appropriate pressure and temperature for CO2 gas hydrate formation. After the formation of gas hydrates in the specimen, water is injected until fully saturated. The permeability measurement is controlled by the syringe pumps at the upper and lower sides of the specimen. In order to saturate specimen with water, injection pressure was 2.8 MPa. The permeability measurement was conducted for several hours with a 50 kPa difference because low permeability in gas hydrate-bearing sediments is expected.

Gas hydrate saturation is calculated by capturing CO2 gas during the dissociation of the gas hydrates. Gas hydrate dissociation is performed by step-wise depressurization. During the gas hydrate dissociation and capturing of CO2 gas, the measured volume of the CO2 gas represents the decrease in the gas hydrate saturation of the specimen. Gas hydrate dissociation and capturing of CO2 gas continue until dissociation is completion. The temperature, pressure and wave velocities are measured during the entire experiment to monitor gas hydrate saturation. 3. EXPERIMENTAL RESULTS 3.1 Gas hydrate formation

The initial porosity was 0.58, and the void ratio was 1.39. From the calculation, the initial gas hydrate saturation was 0.49. Gas hydrate formation and water saturation were monitored by the temperature and P- and S-wave velocities shown in Fig. 4.

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As shown in Fig. 4(a), a sudden temperature enhancement of about 4°C occurs

during gas hydrate formation. The exothermic reaction of gas hydrate formation increases the temperature of the specimen instantly. The stiffening effect of the gas hydrates result in a wave velocity increase in Fig. 4(b) and Fig. 4(c). After water injection, the P-wave velocity is increased because of the compressional wave characteristics of the P-wave. Otherwise, the S-wave velocity decreases after water injection. The S-wave velocity is only affected by the soil skeleton which implies a decrease in gas hydrate saturation. A partial pressure difference between the injected water and gas hydrates causes the dissociation of gas hydrates as pure water is injected.

3.2 Permeability The experimental result for water permeability with gas hydrate saturation is shown in Fig. 5. Measurement of the permeability was done at a gas hydrate saturation of 37%, 22% and 0%. Water permeability at a gas hydrate saturation of 37% was about 2.1 times lower than that at 22% gas hydrate saturation. Otherwise, the water permeability without any gas hydrates present was almost 102 times bigger than result for the 37% gas hydrate saturation. This large difference implies that the existence of

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gas hydrates obstructs fluid flow along the pores. Permeability coefficient was about 10-

9 m/s in gas hydrate-bearing sediments in the Ulleung basin of Korea.

Fig. 5 Water permeability with gas hydrate saturation

4. CONCLUSIONS Water permeability measurements were performed with 0%, 22% and 37% gas hydrate saturation within an artificial specimen of the core sample from gas hydrate-bearing sediments. The permeability of the gas hydrate-bearing specimen was 102 times lower than the specimen without gas hydrates. This result shows the remarkable effect of gas hydrate saturation on the permeability of sediments. Precise empirical model for permeability with gas hydrate saturation can be established by additional permeability experiments with various gas hydrate saturation condition. ACKNOWLEDGEMENT This research was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of Knowledge Economy of Korea and the “U-City Master and Doctor Course Grant (Education) Program” under the Korea Ministry of Land, Infrastructure and Transport (MOLIT). REFERENCES Milkov, A. V. (2004), “Global estimates of hydrate-bound gas in marine sediments: how

much is really out there?”, Earth Science Reviews, 66(3), 183-197. Mohana, L. D. and Jocelyn L. H. Grozic (2014), “Experimental determination of

permeability of porous media in presence of gas hydrates”, J. of Petroleum Science and Engineering, 120, 1-9.

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Sloan, E. D. and Koh, C. A. (2008), “Clathrate Hydrates of Natural Gases 3rd ed.”, CRC Press, Boca Raton, Florida.

Sloan, E. D. (2003), “Fundamental principles and applications of natural gas hydrates”, Nature, 426, 353-363.

Zhao, J., Zhu, Z., Song, Y., Liu, W., Zhang, Y. and Wang, D. (2014), “Analyzing the process of gas production for natural gas hydrate using depressurization”, Applied Energy, 142, 125-134.