Energy Fuels 2009, 23, 5731–5736 Published on Web 10/28/2009
: DOI:10.1021/ef900634w
Evolution of Compressional Wave Velocity during CO2 Hydrate Formation in Sediments Tae-Hyuk Kwon† and Gye-Chun Cho* Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea † Present address: Earth Science Division, Lawrence Berkeley National Laboratory (LBNL), 1 Cyclotron Rd., Berkeley, CA 94720. Received June 23, 2009. Revised Manuscript Received September 27, 2009
While the acoustic wave-based survey is considered to be one of the most effective and promising means for monitoring the behavior of particulate/discrete geomaterials, the P-wave velocity has scarcely been used to monitor the long-term behavior of CO2 sequestered sediments or to understand the characteristics of CO2 hydrate formation in sediments. Furthermore, there are still only limited reliable laboratory results quantifying the P-wave velocity change in sediments that results from CO2 hydrate formation and accumulation processes. This study presents experimental measurements on the evolution of P-wave velocity as CO2 hydrate saturation increases in unconsolidated sediment. The measured data are compared with the simple yet robust asymptotic Gassmann model to estimate CO2 hydrate saturation (volume fraction of hydrate in pore space) in sediments. Given that in situ techniques to measure acoustic waves are well-established for the exploration of deep oceanic sediment, the methodology presented in this paper for estimating hydrate saturation is of potential significance in the monitoring of the long-term behavior of CO2 reservoirs after sequestration in deep marine sediments.
processes in CO2 sequestered sediments, because these techniques have also been used to evaluate methane hydrate quantities in reservoirs. Regarding the seismic monitoring of the gas hydrate formation process per se, many laboratory studies have been performed to advance our understanding of the hydrate formation process. However, a key problem remains, in that a laboratory technique controls hydrate formation at the preferred loci and therefore leads to innate bias in some physical parameters. Hence, in light of this insight, the determination of a hydrate forming method, as well as the interpretation of the data, should be considered for proper simulations of natural processes. In a scenario of CO2 storage in deep oceanic sediments, CO2 hydrate is likely to be produced from dissolved CO2, which is fairly similar to the natural formation process of most of the methane hydrate in oceanic environments. However, a laboratory technique that consistently produces gas hydrate from a dissolved gas phase in porous samples continues to be a challenge, despite many successful studies that are related to CO2 and methane hydrate.5-7 Furthermore, acoustic data on hydrate-bearing sediments that forms from dissolved gas; particularly that related to CO2 hydrate-bearing sediments; are rare. Thus, the present study attempts to imitate a consequential process to CO2 storage in which CO2 hydrate is
1. Introduction Gas hydrates are solid compounds that favor high pressure and low temperature. As the capture of carbon dioxide (CO2) from point sources and its storage in deep oceanic sediments is one of promising solutions for stabilizing atmospheric greenhouse gas concentrations, interest is also growing in CO2 hydrate and CO2 hydrate-bearing sediments, both of which are potential byproducts of carbon sequestration.1 Because CO2 in the liquid or supercritical phase injected in an oceanic sediment environment is fated to diffuse into its surroundings, some of the CO2 molecules that are transported to shallower sediments will be combined with water molecules and produce carbon dioxide hydrate (CO2 hydrate), as a byproduct of CO2 sequestration.2 Either when CO2 is injected or when CO2 is produced by natural geochemical and microbial activities in deep sea sediments, CO2 hydrate-containing formations will preferentially occur in shallow sediments within 200 m of the seafloor, typically at geothermal temperatures of less than ∼283 K.3,4 Acoustic wave-based techniques (e.g., seismic surveys) are the most promising means of characterizing natural pore-scale *Author to whom correspondence should be addressed. Tel.: þ82-42350-3622. Fax: þ82-42-350-3610. E-mail:
[email protected]. (1) Koide, H.; Shindo, Y.; Tazaki, Y.; Iijima, M.; Ito, K.; Kimura, N.; Omata, K. Deep sub-seabed disposal of CO2;The most protective storage. Energy Convers. Manage. 1997, 38, S253–S258. (2) House, K. Z.; Schrag, D. P.; Harvey, C. F.; Lackner, K. S. Permanent carbon dioxide storage in deep-sea sediments. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 14255–14255. (3) Inagaki, F.; Kuypers, M. M. M.; Tsunogai, U.; Ishibashi, J.; Nakamura, K.; Treude, T.; Ohkubo, S.; Nakaseama, M.; Gena, K.; Chiba, H.; Hirayama, H.; Nunoura, T.; Takai, K.; Jorgensen, B. B.; Horikoshi, K.; Boetius, A. Microbial community in a sediment-hosted CO2 lake of the southern Okinawa Trough hydrothermal system. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 14164–14169. (4) Kwon, T. H.; Kim, H. S.; Cho, G. C. Dissociation Behavior of CO2 Hydrate in Sediments during Isochoric Heating. Environ. Sci. Technol. 2008, 42, 8571–8577. r 2009 American Chemical Society
(5) Tohidi, B.; Anderson, R.; Clennell, M. B.; Burgass, R. W.; Biderkab, A. B. Visual observation of gas-hydrate formation and dissociation in synthetic porous media by means of glass micromodels. Geology 2001, 29 (9), 867–870. (6) Spangenberg, E.; Kulenkampff, J.; Naumann, R.; Erzinger, J. Pore space hydrate formation in a glass bead sample from methane dissolved in water. Geophys. Res. Lett. 2005, 32, L24301 (doi:10.1029/ 2005GL024107). (7) Spangenberg, E.; Beeskow-Strauch, B.; Luzi, M.; Naumann, R.; Schicks, J. M.; Rydzy, M. The process of hydrate formation in clastic sediments and its impact on their physical properties. In The 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, Canada, 2008.
5731
pubs.acs.org/EF
Energy Fuels 2009, 23, 5731–5736
: DOI:10.1021/ef900634w
Kwon and Cho
Figure 1. Experimental setup for CO2 hydrate formation in sediments. Pressure is monitored with a pressure transducer (denoted as PT in the figure) and pressure gauges (denoted as P). The sample’s internal temperature is monitored by the internal thermocouple (denoted as TCin). The P-wave velocity is measured across the sample using the P-wave transducers, denoted as VP (source) and VP (receiver) in this figure.
nucleated from dissolved CO2 and accumulated from diffused CO2, increasing hydrate saturation in the sediment sample. This study presents experimental measurements on the evolution of P-wave velocity with increasing gas hydrate saturation in unconsolidated sediment and compares the experimental data with a proper theoretical model that considers solid gas hydrate saturation in sediment pores. The nucleation and accumulation of gas hydrate in the sediment sample are experimentally observed, and the P-wave velocity is monitored during hydrate formation. The saturation of CO2 hydrate in pores (i.e., the volume fraction of the hydrate in the pore space) is then estimated using the P-wave velocity of hydrate-containing sediments and compared to prior published models. The important physical observations and the use of theoretical models are discussed.
Table 1. Properties of the Sediment F110 sand
property
value
mean particle diameter specific gravity specific surface area nmax/nmina porosityb pore sizec sphericity roundness
120 μm 2.65 =0.019 m2/g 0.46/0.35 0.397 190-233 μm 0.7 0.7
a
Note: nmax and nmin are the maximum and the minimum porosity, respectively. b Porosity represents the porosity of the sample sediment used for the experiment. c The pore size of the sand sample is calculated by assuming simple cubic packing and tetrahedral packing of the spherical particles with the mean particle diameter. Data taken from Cho et al.26 and Yun et al.13
inside the sediments. No electrolyte was considered in this study. 2.2. Experimental Procedures: Hydrate Formation in Sediments. The fine-grained sand sample was compacted in the reaction cell by hand-tamping, to achieve a final and uniform porosity of 0.397. The sample was partially saturated with distilled water, and no effective stress was applied (i.e., only a self-weight stress without external confining stress). CO2 gas was injected and pressurized to ∼3 MPa. Upon the closing of a valve on the top of the reaction cell (refer to Figure 1), the distilled water was injected into the sediment sample, to dissolve the CO2 gas and saturate the sediment. After the fluid pressure was adjusted to ∼4.8 MPa, the sample was left for two days until the pressure remained stable. However, CO2 bubbles were not completely eliminated in sediment pores, because of the limited supply of distilled water. A constant-volume condition was made by closing the valves around the reaction cell (i.e., isochoric conditions with no mass flux) before cooling. The temperature and pressure inside the reaction cell were continuously monitored during the experiment. P-waves were also measured by the pinducers. Under an initial pressure of ∼4.8 MPa, the sample was cooled to 0.8 °C. The temperature of the sample was then held constant. In addition, a Peltier cooler was used to create
2. Experimental Program 2.1. Laboratory Setup and Materials Used. An experimental setup was designed to explore the evolution of the compressional wave velocity (i.e., P-wave velocity) during CO2 hydrate formation in sediments (refer to Figure 1). We made a transparent, cylindrical, and rigid-walled reaction cell (i.e., polycarbonate material, with a volume of 3.17 cm3, internal diameter of 6.35 mm, and height of 100 mm) with a Peltier cooler, where CO2 hydrate can be formed in sediments. The reaction cell was instrumented with one thermocouple (TMTSS; grounded sheath T-type; OMEGA), one pressure transducer (PX302; OMEGA), and one pair of pinducers (Valpey Fisher, VP1093). The cell was submerged in a bath, the temperature of which was controlled between 20 °C and -5 °C by circulating temperature-controlled fluids from a refrigerating circulator (Fisher Scientific). The pressure of the pore fluid in the sediment sample was controlled by a water pump (up to 10 MPa) and the compressed gas pressure (up to 4.7 MPa) of a CO2 gas cylinder. Fine-grained sand (Ottawa F110, with a uniform grain size and a mean particle diameter of 0.12 mm) was used as a host sediment. The physical properties of the sediment are summarized in Table 1. A scientific-purpose CO2 gas (purity of ∼99.9%) and distilled water were used to form gas hydrate 5732
Energy Fuels 2009, 23, 5731–5736
: DOI:10.1021/ef900634w
Kwon and Cho
preferential conditions for CO2 hydrate to form in the P-wave monitoring area (i.e., at middle height of the sample) over the peripheral area. Because the P-wave only represents the local area where the P-wave transmits, we intended to form hydrate in the area where the pinducers were installed. By supplying DC voltage to the Peltier cooler, the temperature in the area of interest was kept ∼0.5 °C lower than the surrounding area (see the temperature difference in Figure 3a, presented later in this paper). We used a pulser (Panametrics, 500PR) to apply pulse signals to the source pinducer, and the transmitted and received signals were periodically recorded at the receiver pinducer using a digital oscilloscope (Agilent, 54622D). The frequency contents of the received P-wave signals ranged from 5 kHz to 1 MHz, depending on the sediment stiffness and the pore fluid composition. Because the heads of the pinducers were instrumented at the outer wall of the reaction cell, the P-wave was expected to propagate through the system in the order of source, cell wall, sediment sample, cell wall, and receiver. Therefore, the P-wave velocity of the sediment sample contained in the reaction cell was estimated as follows: Vwall ð1Þ Vsed ¼ Lsed Δtmea Vwall -Lwall
Figure 2. The PT trace during the cooling process of the CO2containing sediment in a closed system. Figure legend: H, hydrate phase; LW, liquid water; LCO2, CO2 in the liquid phase; and VCO2, CO2 in the vapor phase. The points are plotted with a time interval of 4 min. The temperature was measured by the internal thermocouple (TCin).
observed on the sample surface in the monitoring area. In addition, the initial P-wave velocity, which was >1600 m/s before hydrate nucleation, also confirmed that a watersaturated condition was created in the area; otherwise, the P-wave velocity would have been
