Geomechanical and Thermal Responses of Hydrate-Bearing

Article pubs.acs.org/EF

Geomechanical and Thermal Responses of Hydrate-Bearing Sediments Subjected to Thermal Stimulation: Physical Modeling Using a Geotechnical Centrifuge Tae-Hyuk Kwon,† Tae-Min Oh,† Yun Wook Choo,† Changho Lee,∥ Kang-Ryel Lee,⊥ and Gye-Chun Cho†,* †

Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea ∥ School of Civil, Environmental, and Architectural Engineering, Korea University, Seoul, Korea ⊥ Korea Electric Power Research Institute, Daejeon, Korea ABSTRACT: The geomechanical and thermal responses of sediments can be significantly affected by the dissociation of gas hydrates via various emergent phenomena such as fluid volume expansion, free gas generation, gas migration, and sediment softening. This study explores the geomechanical and thermal responses of hydrate-bearing sediments subjected to thermal stimulation, using physical modeling with a geotechnical centrifuge that enables the simulation of near-seafloor sediment conditions. A water-saturated and CO2 hydrate-bearing sand column was prepared in a large cylindrical pressure vessel, and a linear stress gradient for a near-seafloor condition was created by increasing centrifugal acceleration. The hydrate-bearing sand column was subjected to thermal stimulation, and changes in temperature, pressure, compressional wave velocity (VP), shear wave velocity (VS), and electrical resistance were monitored at various locations across the column. It was found that VP and electrical resistance were good indicators of the presence of free gas, while VS reflected the reduction in shear stiffness, caused by decementation resulting from hydrate dissociation. The thermal diffusivity of hydrate-bearing sediments significantly decreased as the gas hydrate dissociated and free gas saturation increased. Such a process is expected to retard gas production from hydrate deposits. Temporary accumulation of excess pore pressures >200 kPa was observed even in the fine sandy sediment; this excess pressure resulted from the increased capillary pressure exerted by the hydrate formed at the grain contacts, coupled with the continuous hydrate dissociation against pressure diffusion and resulting gas migration. This suggests a possible sediment volume expansion, an uplifting deformation at the seafloor, or a fracture generation in sediments. By contrast, the vanishing of solid hydrate crystals by hydrate dissociation led to decementation and softening of sediments, indicating a possible postdissociation subsidence at the seafloor and at dissociated regions during gas production from hydrate-bearing sediments.

1. INTRODUCTION The dissociation of gas hydrates in sediments causes various phenomena, such as a fluid volume expansion, multiphase flow of gas and water, a reduction in porosity, an increase in sediment compressibility, and changes in pore fluid pressure and effective stress. Moreover, gas production from gas hydratebearing sediments may involve fines migration, pore clogging, and gas-driven fractures. 1,2 In particular, the thermal stimulation of hydrate-bearing sediments is often associated with excess pore pressure generation, plastic deformation of sediments, and seafloor deformation.3 While these emergent phenomena are expected to occur as a coupled process during unexpected or intentional hydrate dissociation, reliable assessments of the geo-hazards associated with such hydrate dissociation, such as sediment instability and borehole failure, require a better understanding of the geomechanical and thermal responses of the sediments undergoing hydrate dissociation. For example, drilling and operating a wellbore through hydrate deposits presents a significant challenge because of insufficient knowledge of the geomechanical responses of sediments undergoing hydrate dissociation.4 Since oceanic gas hydrates are mostly found in unconsolidated sediments up to a distance of several hundred meters © 2013 American Chemical Society

below the seafloor, effective stress plays a key role in determining the geomechanical properties and behavior of the hydrate-bearing sediments. In fact, it has been proven that deeper sediments show a higher strength and stiffness because of the greater effective stress.2,5−7 It is clear, therefore, that sediments at shallower depths are likely to be more susceptible to instability such as deformation or shear failure because of the lower effective stress (e.g., see simulation results in Kwon et al.3). Therefore, the simulation of an effective stress gradient is critically important to adequately capture the geomechanical responses of hydrate-bearing deposits during hydrate dissociation. This can be achieved by the use of two approaches: numerical or physical modeling. As opposed to the intensive efforts made in the numerical modeling,3,8−11 few attempts have made using large-scale physical modeling of the geomechanical responses of hydrate-bearing sediments, for a variety of reasons, such as the difficulty in attaining the high pressure required for hydrate stability conditions, the inability to ensure safety, and the lack of ability to simulate in situ Received: November 18, 2012 Revised: June 13, 2013 Published: June 17, 2013 4507

dx.doi.org/10.1021/ef3018699 | Energy Fuels 2013, 27, 4507−4522

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Figure 1. Reaction vessel and instrumentation. (a) The vertical cross section; (b) a digital photograph; and (c) the horizontal cross section of the vessel instrumented with pressure transducers (PT), thermocouple (TC), PZT disks (VP), bender elements (VS), and electrodes (ER).

After the preparation of a 765 mm-high sand column, that was hydrate-bearing and water-saturated, we generated linear gradients of stress and fluid pressure across the sand column via centrifugal acceleration. Then, using an embedded electric heater, a one-dimensional thermal stimulation was applied to the hydrate-bearing sand column. During thermal stimulation, temporal and spatial changes in temperature, pressure, shear wave velocity, compressional wave velocity, and electrical resistance were monitored at various locations. Associated analyses and interpretation of the obtained results follow, to provide a clearer insight into the geomechanical and thermal responses of hydrate-bearing sediments subjected to thermal stimulation.

effective stress conditions. For example, Su et al. explored the hydrate formation and accumulation in sands, using a large vessel with an inner diameter of 0.5 m and a height of 1 m, in which the compressional wave velocity, electrical resistivity, and temperature were obtained at various locations inside a vessel with no stress gradient; the results of further hydrate dissociation testing is expected.12 Jung et al. conducted physical modeling experiments on hydrate dissociation in sandy sediments with fine particles, using a large seafloor simulator at Oak Ridge National Laboratory, TN, U.S.A., in which vertical effective stress was applied to the top of sediment samples in a rigid-walled container, with no stress gradient generated.2 In this study, the geomechanical and thermal responses of hydrate-bearing sediments subjected to thermal stimulation are explored using a large pressure vessel and geotechnical centrifuge equipment at the Korea Advanced Institute of Science and Technology. The geotechnical centrifuge imposes a high centrifugal acceleration (G-level) by the swinging of its long arm; thus, it can simulate an in situ effective stress gradient and a hydrostatic pressure gradient in a sediment column model, scaling field stress conditions in the laboratory.13 Despite the benefits of this geotechnical centrifuge equipment, physical modeling of hydrate-bearing sediments using a geotechnical centrifuge has never been attempted.

2. EXPERIMENTAL SETUP AND PROGRAM 2.1. DevicesPressure Vessel Specification and Sensor Instrumentation. A large cylindrical and rigid-walled reaction vessel composed of an aluminum alloy (duralumin, AA2024), was built and used in this study. The inner diameter of the vessel was 200 mm; the height of the interior, 1000 mm; the internal volume, 31.4 L; and the maximum allowable working pressure of the vessel, 20 MPa (Figure 1). Various sensors were installed at predetermined positions within the vessel (see Figures 1 and 2). Five pressure transducers 4508


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Table 1. Instrumentation and Location of Sensors

Figure 2. Digital images of installed sensors in the reaction vessel. (a) A thermocouple (TC); (b) a pressure transducer (PT); (c) a bender element (VS) for measuring shear wave velocity; (d) a PZT disk (VP) for measuring compression wave velocity; (e) electrodes (ER) for measuring electrical resistance; and (f) an electrical heater.

layer no.

depth from the soil surface, z [cm]

installed sensor

0 1 2 3 4 5 6 7 8 9

+15 −5 −15 −25 −35 −45 −55 −65 −70 −75 (bottom)

PT0, TC0 PT1, TC1, VS1, VP1, ER1 VS2, ER2 PT3, TC3, VS3, VP3, ER3 VS4, ER4 PT5, TC5, VS5, VP5, ER5 VS6, ER6 PT7, TC7 heating disk

this subsection. In addition, Table 2 summarizes the properties of the soil used and the procedures followed. CO2 Hydrate Formation in Partially Water Saturated Sand. Carbon dioxide (CO2) was used as a hydrate former in this study. CO2 hydrate is presumed to be a suitable analogue for methane (CH4) hydrate, in terms of the mechanical responses of hydrate-bearing sediments, because different guest molecules in the hydrates have a minimal impact on the mechanical properties of gas hydrates and hydrate-bearing sediments,15−17 whereas hydrate growth morphology has a greater impact on seismic velocities, and thus the mechanical properties. In addition, the structural similarity between CH4 hydrate and CO2 hydrate as a structure I hydrate also supports the analogy between CO2 hydrate and CH4 hydrate.18,19 A clean, fine quartz sand (Ottawa F110, with a uniform grain size and a mean particle diameter of 120 μm) was used as the host sediment (see Table 2). Partially water-saturated sand with a water content (w, defined as mass of water divided by mass of dry soil mineral) of 8.68% was packed into the vessel by handtamping. After compaction, the height of the sand column was 765 mm. The resulting average initial porosity (ϕ) of the sand column was 0.42, and the pore saturation of water (Sw, water saturation, defined as water volume divided by total pore volume) was 31.8%. No additional effective stress was applied. Research-grade CO2 gas (purity of ∼99.9%) and distilled water were used to form the gas hydrate inside the sand column. No electrolyte was used in this study. CO2 gas was introduced into the partially water-saturated sand and pressurized to ∼3 MPa at room temperature. Because the initial air saturation before injecting the CO2 was ∼62%, the air phase was presumed to have percolated throughout the specimen, as the water tended to form menisci at the grain contacts.20−22 Accordingly, injected CO2 gas was expected to occupy the rest of the pores and be homogeneously distributed across the column. The sand column in the vessel was then cooled to approximately 3−4 °C, bringing the pressure and temperature condition into a thermodynamically stable region for the formation of CO2 hydrate. During hydrate formation, the CO2 gas pressure was kept constant, until geophysical signatures indicated no further hydrate formation. The hydrate-containing sand column sample was kept for more than 24 h. Figure 3 shows the experiment setup for CO2 hydrate formation. During the phase transformation of water to hydrate, CO2 molecules become trapped in water-bonded lattices, expanding the volume of water; thus, the resulting pore saturation of CO2 hydrate (Sh, hydrate saturation, defined as hydrate volume divided by total pore volume) is generally larger than the initial water saturation. The occupancy of hydrogen-bonded lattices (hydration number or stoichiometric number; χ) determines

(denoted as PT) were instrumented with an interval of 200 mm. Five thermocouples (TC; T-type) were buried in the soil at the same height as the pressure transducers. Pressure and temperature were recorded during the experiments, to monitor heat transfer and pore fluid pressure change. Six pairs of bender elements (VS), made of two layers of lead zirconate titanate (PZT) sheets, were housed to measure the shear wave velocity (S-wave velocity, VS). Six bender elements installed on one side of the wall were used as transmitters (sources) generating shear waves, and the other six bender elements facing the source bender elements were used as receivers to receive the transmitted shear waves. Three pairs of PZT disks (VP) were installed to measure compressional wave velocity (P-wave velocity, VP). In the same manner as the bender elements, three PZT disks were installed on one side of the wall and used as transmitters (sources), while three more PZT disks were used as receivers. Stainless steel high-pressure tubes (SUS314) with an outer diameter of 9.5 mm (3/8 in.), precut to ∼100 mm long pieces, were used to house the bender elements and PZT disks, as shown in Figure 2. With placing a sensor in a tube, the tube was filled with epoxy to secure the electrical wires and to achieve a pressure seal. Six pairs of electrodes (ER) made of stainless steel (SUS314) were instrumented to measure electrical resistance. Each disk-shaped electrode with a diameter 9.5 mm and a thickness of 3 mm was attached to the end of a 100-mm-long polycarbonate tube with 9.5 mm outer diameter. The inside of the polycarbonate tubes were also filled with epoxy to secure the electrical wires. These custom-made sensors for VS, VP, and electrical resistivity proved able to tolerate a pressure of ∼10 MPa in our previous study.14 An electrical heater, with a capacity of 100 W, was embedded 50 mm above the bottom of the vessel and was used to initiate the thermal stimulation of hydrate-bearing sediments, transferring heat upward in a one-dimensional direction. The location of the installed sensors is summarized in Table 1, and the sensors are named and numbered by location. 2.2. Sample Preparation and Test Procedure. The experiment was designed to involve three procedures: (1) the formation of CO2 hydrate in partially water-saturated sand; (2) the preparation of water-saturated and hydrate-containing sand; and (3) the use of a centrifuge test for the thermal dissociation of gas hydrate in sand. These procedures are detailed within 4509


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Table 2. Soil Properties and Testing Procedures

*

Note that the upper limit of the pore saturation of hydrate formed (Sh), 39%, was estimated by assuming a 100% phase transformation of water to hydrate, a hydration number of 7, and a density of CO2 hydrate of 1.1 g/cm3. The lower limit of Sh, 32%, was estimated by considering the CO2 dissolution into pure water in pores.

(Figure 3). When injecting the water, the pressure of the vessel was maintained at ∼3 MPa by using the water pump and a precision metering valve. After water saturation, the pressure inside the vessel was adjusted to approximately 3.5 MPa, leaving a head space filled with CO2 gas. The temperature was kept at 3−4 °C, to maintain the pressure−temperature condition inside the thermodynamic CO2 hydrate stability region. Prior to loading the vessel on the basket of the geotechnical centrifuge equipment and commencing a dissociation test, the valves at the top and bottom of the vessel (see Figure 3) were closed, so that a constant volume condition was made with no mass flux (i.e., isochoric condition). The introduction of water under-saturated with CO2 was presumed to cause the dissolution of CO2 hydrate in the sand column, resulting in a decrease in the hydrate saturation Sh. Assuming the CO2 hydrate dissolved into pure pore water and the CO2 solubility in pure water as ∼0.013 in mole fraction at 3 °C and 3.5 MPa,24,25 the hydrate saturation is calculated to decrease from 38.9% (based on 100% water-to-hydrate conversion) to ∼32.2% (upon the completion of dissolution); this hydrate saturation, estimated considering such CO2 dissolution into pure water can be thought as a lower limit. However, the reduction in porosity by the increased gravitational acceleration during the geotechnical centrifuge test, would again increase hydrate saturation, adding another uncertainty to the estimation of hydrate saturation. Therefore, before commencing thermal stimulation, the initial hydrate

such a volume expansion and the density of formed hydrates. Assuming a 100% phase transformation of water to hydrate, a hydration number of 7.0, and a CO2 hydrate density of 1.1 g/ cm3,23 the average initial hydrate saturation of the sand column (Sh) can be calculated by the following equation: Sh =

S Mh ρw Vh = w χ M w ρh Vpore

(1)

where Mh and Mw are the molecular masses of the CO2 hydrate and water, respectively, and ρh and ρw are the densities of CO2 hydrate and water, respectively. Therefore, the average hydrate saturation of the column (S h ) was calculated to be approximately 38.9%. However, it was possible that water could be trapped and remaining in isolated pores where CO2 gas could not invade; thus, the estimated hydrate saturation with the assumption of complete conversion of water into hydrate can be considered as an upper limit. Preparation of the Water Saturated Hydrate-Containing Sand Column. Oceanic methane hydrate-bearing sediments have mostly been found in the absence of free methane gas (or vapor phase) in pores, because any excess methane that exceeds the methane solubility of seawater would be converted into gas hydrate, and is often referred to as the excess water condition. To achieve an excess water condition, degassed and distilled water with no dissolved CO2 was injected at the same temperature as that of the vessel (∼4 °C) through the fluid port at the bottom, thus removing the free gas phase in the pores 4510


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Figure 3. Experimental setup for CO2 hydrate formation in a sand column. (Note that the figure is not drawn to scale.)

After the sample was prepared, the vessel was loaded on the basket of the geotechnical centrifuge (Figure 4b). The cooling system was turned off because the refrigerating circulator does not operate under elevated G-levels. To avoid unwanted heat transfer from the surroundings into the vessel during the swinging, a number of plastic bags containing ice cubes were wrapped around the vessel. The G-level was brought up to 30 G during the first 40 min by swinging the flight arm (centrifuge spinning). The electrical heater was then powered on by the supply of 100 W (20 V and 5 A) DC power, 50 min after the spinning started. While maintaining the G-level at 30 G, the heat was transferred to the overlying hydrate-containing sands, resulting in dissociation of CO2 hydrate. After 193 min, the Glevel was decreased to 1 G and the experiment was concluded. The time stamp and the relevant events that occurred during the centrifuge test are summarized in Table 3. During hydrate dissociation and consequential gas release and sediment destabilization, 1-D spatial and temporal changes in pressure, temperature, S-wave velocity (VS), P-wave velocity (VP), and electrical resistance (ER) were recorded. The natural formation history of hydrate-bearing deposits, determined by the sedimentation rate and the hydrate formation rate, can have a profound effect on the physical properties of such hydrate-bearing sediments, and gas hydrate may form during or after sedimentation (i.e., equivalent to cementation before or after loading). Thus, our procedure, where the effective stress was applied after hydrate formation, represents a condition where the sedimentation rate is much slower than the hydrate formation rate or where rapid sedimentation occurs after hydrate formation.29,30 2.3. Measurement and Control Systems for the Centrifuge Test. While it is important to monitor the changes in S-wave velocity (VS), P-wave velocity (VP), electrical resistance (ER), pressure (P), and temperature (T), over the

saturation was assumed to be approximately 32−39%, which took into account the limited estimation of hydrate saturation. Despite the post-water saturation, the hydrate-forming method used in this study (where gas hydrate was formed in a partially water-saturated sand) is expected to have caused preferential hydrate formation at the grain contacts, resulting in cementation (i.e., grain-cementing hydrate).14,26,27 This condition is expected where active gas venting occurs in oceanic hydrate stability regions. However, most of the gas hydrates found in natural marine settings are presumed to form from dissolved gases in an aqueous phase,28−30 and such gas hydrates formed from dissolved gas in sandy sediments are known to exhibit a pore-filling or load-bearing habit when hydrate saturation is less than ∼20−40%.5,7,27,31−37 Thus, it is worth noting that the physical properties of hydrate-bearing sediments are strongly influenced by the hydrate-forming methods, and by the resulting hydrate loci in sediment pores.31,37,38 Therefore, the prepared hydrate-bearing sand in this study might have a limited applicability to certain conditions with gas hydrate formed from dissolved gas. Geotechnical Centrifuge Test for Thermal Dissociation of CO2 Hydrate. The geotechnical centrifuge used in this study is able to increase the level of the centrifugal acceleration (Glevel) to a maximum of 100 G, with a 2.4 ton payload, by the swinging of its 5-m-long arm (Figure 4a; details of the equipment are presented in Kim et al.).39 In this study, the Glevel was increased to 30 G, in order to produce a linear stress gradient across the sand column (Figure 5). This increased gravity replicates the effect of the body forces of the sand grains and fluids.13 Thus, there will be zero vertical effective stress (0 kPa) at the top of the sand column, while a vertical effective stress of ∼225 kPa will be applied at the bottom of the column when the centrifugal acceleration (G-level) is at 30 G. 4511


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for the transmitters, ±120 V, using an analog linear power amplifier (Model SVR bip 350/1, Piezomechanik GmbH). The time delay caused by the amplifier was corrected when determining the travel times of transmitted waves. The output channel of the voltage amplifier was connected to a 32-channel multiplexer switch (PXI-2527, NI) that switched the connections among nine output channels connected to each of six bender elements and the three PZT disks on the side of the transmitter. On the receiver side, two multichannel data acquisition modules with a high sampling rate (8 channels and 500 kS/s; PXI-6123, NI) were used: one for the six bender elements and the other for the three PZT disks. The analog output signal from PXI-6251 was also recorded at the same time. Electrical Resistance Measurement. A low-frequency LCR meter (4263B, Agilent) was used to measure electrical resistance. The high potential and high current channels at the LCR meter were shared while the low potential and low current channels were grouped. Six electrodes on one side of the wall were connected to a high potential/current port through a multiplexer switch (PXI-2527, NI). The other six electrodes were then connected to a low potential/current channel through the same multiplexer switch. Via the control of the multiplexer switch and the pairing of two electrodes located at the same elevation (or layer), the electrical resistance from each pair at each layer was acquired. An AC current of 1 V at a frequency of 1 kHz was used for measuring the electrical resistance values, avoiding electrode polarization and ion exclusion. Measuring Pressure and Temperature and Controlling the Electrical Heater. A multifunction data acquisition module (PXI-6251, NI) was used to record pressures and temperatures every 0.1 s during the experiment. All the pressure transducers were powered by a signal-conditioning module (SCXI-1520, NI), and their outputs were conditioned by the same module, before being recorded by the multifunction data acquisition module (PXI-6251, NI). A signal-conditioning module (SCXI1102c, NI) was also used for all the thermocouples, and the temperature outputs were recorded by the multifunction data acquisition module (PXI-6251, NI). A solenoid switch with an ability to connect or disconnect electrical wires was used to manipulate the electrical heater and was controlled by the multiplexer switch module (PXI-2527, NI). While a DC power supplier provided continuous power (20 V and 5 A, thus 100 W) to the electrical heater, the solenoid switch, located between the power supplier and the heater, was used to connect or disconnect power to the heater. A thermocouple was embedded inside the electrical heater, and thus, the temperature of the heater was also monitored.

Figure 4. (a) Geotechnical centrifuge facility used in this study and (b) the reaction vessel loaded on the basket of the geotechnical centrifuge.

course of the thermal dissociation of gas hydrate, a simultaneous acquisition of analog signals from the multiple transmitters and receivers in such an experiment is challenging. Thus, we developed a multiplexing system that allowed for the selection of a single transmitter-receiver pair by switching the connections among the multiple transmitters and receivers. Our multiplexing system consisted of (1) a measurement system for VS and VP; (2) a measurement system for electrical resistance; and (3) a system for measuring pressure and temperature and for controlling the electrical heater, as shown in Figure 6. VS and VP Measurements. For the measurements of VS and VP, we improved the multiplexing system on the basis of the Swave velocity tomography system that was previously developed for centrifuge testing.40 A multifunction data acquisition module (PXI-6251, National Instruments, NI), which included an arbitrary function generator was used to generate a ±10 V step function signal at a frequency of 10 kHz, and to excite bender element transmitters and PZT disk transmitters. Since the distance between transmitters and receivers (i.e., 130 mm) was relatively far, and the testing environment generated ambient vibration noises, the ±10 V driving signal was amplified to the maximum operation voltage

3. EXPERIMENTAL RESULTS 3.1. Hydrate Formation. Temperature traces and changes in S-wave signatures, during the processes of the hydrate formation and the water injection, were obtained. CO2 hydrate started forming in the partially water-saturated sand after approximately 3−4 h of cooling, when the temperature and pressure were inside the stability region (i.e., less than 7 °C under 3 MPa of CO2). CO2 hydrate nucleation and formation was inferred by exothermal temperature jumps recorded at the thermocouples located at each layer (Figure 7a) and VS obtained at each layer (Figure 7b). In particular, VS was observed to increase throughout the entire layers of the sand column, which suggests that CO2 hydrate was formed across 4512


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Figure 5. Basic concept of centrifuge modeling. Note that the vertical effective stress of the column model under an acceleration of N-G is equivalent to the vertical effective stress at field. σv, uw, and σ′v are the vertical total stress, pore water pressure, and vertical effective stress, respectively. ρsat and ρw are the densities of saturated soil and water, respectively. ρ′ is the submerged density of soil and ρ′ = ρsat − ρw.

Table 3. Centrifuge TestTime Progressiona time [min]

gravity [G-level]

heater

0 11 27 35 48b 191c 193 199 207 216 235

1 (G increased) 10 20 30 30 30 30 (G decreased) 20 10 1 1

off off off off on off on on on on off

The pressure at PT7, where the water column height was ∼76.5 cm, was measured as ∼3.7 MPa at 30 G. The pore fluid pressure at each depth can also be estimated by the height of the water column and the G-level, in addition to the gas pressure in the head space. The measured pressure is in agreement with the calculated pressure, with consideration of a water column height of 76.5 cm, the pressure in the gas-filled head space (∼3.5 MPa), and 30 G (i.e., 3.5 MPa + 1000 kg/m3 × 9.8 m/s2 × 0.765 m × 30 G = 3.72 MPa). This confirms that the installed pressure transducers operated properly at a level of 30 G, resulting in reliable measurements. It also confirms that a linear gradient of the pore fluid pressure from the top of the water surface (∼3.5 MPa) to the bottom (∼3.75 MPa) was successfully produced at 30 G during the centrifuge test. Meanwhile, a slow and gradual decrease in the pressure of the gas-filled head space was observed. This was expected because of the dissolution of CO2 gas into water. The pressure drop during the first 50 min was found to be less than ∼0.05 MPa, showing asymptotical convergence; and the pressure drop in the head space over the course of the centrifuge test (i.e., 240 min for a level of 30 G) was less than ∼0.2 MPa. It is presumed that this pressure drop was too small to affect the hydrate stability, compared to the change in the pressure range of the test (i.e., from 3.5 to 5 MPa). As the G-level increased, a linear gradient of the vertical effective stress was produced across the sand column, and the vertical effective stress (σ′v) can thus be determined by the product of the overlying weight of a sand column and the Glevel. Assuming the homogeneous sand column with a saturated density of 1957 kg/m3, the vertical effective stress at the bottom is estimated to be ∼7.2 kPa at 1 G and ∼215 kPa at 30 G (i.e., σ′v = (ρsat − ρw)gz × G-level = (1957 kg/m3 − 1000 kg/m3) × 9.8 m/s2 × 0.767 m × 30 G = 215 kPa). Therefore, a linear effective stress gradient would be created from the soil surface (i.e., σ′v = ∼0 kPa) to the bottom of the sand column and would be proportional to the G-level during the centrifuge test. The shear stiffness, and hence the shear wave velocity of soils, are directly governed by the effective

a

Note that between 48 and 191 min, thermal dissociation at a level of 30G was conducted. bHeat started transferring from the electrical heater at a level of 30G. cThe electrical heater was turned off accidentally at 191 min.

the sand column though the local hydrate saturation may have varied spatially. Such an increase in VS was caused by the cementation due to the hydrate formation at the grain contacts. Approximately 12 h after hydrate formation, no change in VS was observed, indicating no further hydrate formation. After water saturation, approximately 18 h was allowed for the vessel to stabilize at 3−4 °C and 3.5 MPa with no mass flux, prior to the centrifuge-dissociation test. Owing to the dissolution of CO2 hydrate into the injected fresh water, the S-wave velocity decreased. 3.2. Increase in G-level. The centrifugal acceleration level (G-level) gradually increased to 30 G over a period of 40 min from the commencement of the centrifuge test and the swinging of the arm. Figure 8a shows the pressure measured by PT0 in the gas-filled head space and the pore fluid pressure measured by PT7, located 10 cm above the bottom of the column (see Figure 1). While the gas pressure in the head space was not affected by the G-level, the pore fluid pressure increased in response to the stepwise increments in the G-level. 4513


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Figure 6. Configuration of the measurements and control system used for the geotechnical centrifuge test.

Figure 8. (a) Changes in pore fluid pressures; and (b) VS, with increasing G-level. Note that the pressure at the head space (PT0) is fairly constant regardless of the G-level.

At 10 G, VS at VS6 (i.e., 55 cm belowthe soil surface) was measured as ∼95 m/s, and VS at VS1 (i.e., 5 cm below the soil surface) was ∼80 m/s. At 30 G, VS at VS6 was measured as ∼120 m/s while VS at VS1 was ∼100 m/s. This confirmed the increase in the stress gradient with an increase in G-level. It is concluded that a linear effective stress gradient from 0 kPa at the soil surface to ∼215 kPa at the bottom of the sand column was created during the centrifuge test at a level of 30 G; therefore, the 765-mm-thick sand column at 30 G represents a

Figure 7. (a) Temperatures and (b) S-wave velocities recorded during hydrate formation.

stress. Accordingly, the increase in G-level and the resulting increase in effective stress caused the increase in shear wave velocity at all depths of the sand column, as shown in Figure 8b. 4514


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near-surface layer from the seafloor to ∼23 m below the seafloor. Owing to the increased centrifugal acceleration, some of the electric wires that were connected to sensors such as VP3, VS5, and ER2 were damaged and disconnected, and no data was acquired for these sensors thereafter. 3.3. Thermal Stimulation Test at 30 G-level. Temperature Responses. Figure 9 shows the temperature and pressure

which was placed in the gas-filled head space, occurred during the increase and decrease of the G-level, because the needle of the thermocouple TC0 was bent and vibrated during acceleration and deceleration, resulting in uncontrollable noise at TC0. Pressure Responses. Increases in pore fluid pressure across the sand column were observed with increasing temperature, as shown in Figure 9b. Figure 10 shows the pressure−temperature

Figure 10. Pressure−temperature traces at Layers 1, 3, 5, and 7 during thermal stimulation. The phase equilibrium boundary of CO2 hydrate is calculated by CSMHYD software (available at http://hydrates. mines.edu/CHR/Software.html).

traces during thermal stimulation, superposed with the CO2 hydrate equilibrium boundary. After 191 min had elapsed, the pressures had elevated by more than ∼1 MPa (from 3.5−3.7 MPa to 4.6−4.8 MPa at 30 G). In particular, a remarkable increase in pore fluid pressure was noted, as measured by PT5 and PT1, possibly because of the hydrate dissociation and accumulation of overpressurized gas. As the local excess pore pressure at both regions of PT5 and PT1 was dissipated, the pressure was equilibrated. As can be seen during the time period from 120 to 190 min after the commencement of centrifuge spinning, the pressure difference (ΔP) between the two locations was kept constant (e.g., ΔP between PT7 and PT5, ΔP between PT5 and PT3, and ΔP between PT3 and PT0) and stabilized at hydrostatic pressure in accordance with the depth of the water column under the 30 G-level. Measured pressure at PT1 and PT3 was the same after 110 min and showed no dependency on G-level and the depth of the water column. One plausible explanation for this phenomenon is that these two pressure transducers (PT1 and PT3) were in contact with the percolated gas phase and were influenced by the gas pressure not by the pore water pressure. PT5 and PT7 showed the pressure gradient generated by the G-level. Noticeably, the composition of pore fluids in contact with the diaphragm of the pressure transducer and an associated undergoing process at the local measuring area can result in a bias on the pressure measurement, which imposes a challenge on the interpretation of the pore fluid pressure of a multiphase soil system during a centrifuge test. VS results. The S-wave velocity (VS) of sands can be correlated with the vertical effective stress by a power law relationship (e.g., VS ∝ (σ′v)β). The vertical effective stress across the sand column varies, increasing linearly with depth. Hence, the S-wave velocity (VS) is proportional to the G-level and depth (Figure 11). When the G-level increased to 30 G, VS

Figure 9. Evolution of (a) temperature and (b) pressure, over the course of the centrifuge test. Note that the heater was on and transferring heat, 48 min after centrifuge spinning commenced.

changes over the course of the centrifuge test, where the hydrate-containing sand was thermally stimulated. The thermal stimulation began 48 min after the centrifuge commenced spinning (Figure 9a), when the electrical heater installed at 5 cm above the bottom was turned on (see Figure 1). The heater transferred heat continuously for approximately 200 min, until the G-level decreased to 1 G and the centrifuge test was ended. As heat diffused upward from the heating disk, a significant temperature increase was observed at TC7, located 5 cm away from the heater. An intermediate temperature increase at TC5, located 25 cm away, and a small increase in temperature at TC3, located 45 cm away, were also observed. However, the temperature increases at TC0 and TC1 were presumably the result of insufficient insulation and a lack of temperature control for the vessel and not caused by the thermal stimulation from the heater. This therefore implies that a thermal front from the heat source only just arrived at the TC3 layer (45 cm above the heat source). While the thermal front propagated ∼45 cm during 160 min, no endothermic indication was observed. The rate of thermal front propagation can be analyzed using the temporal temperature changes measured at those three locations (i.e., the heater, TC7, and TC5) and can be used to determine the thermal diffusivity of the hydratecontaining sand. Note that the heater was accidentally turned off during the test (at 191 min); however, it is presumed that the halt of heat transfer for 2 min was negligible to the heat diffusion process, as indicated by the thermal responses of other thermocouples. The severe temperature fluctuations at TC0, 4515


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saturated and contains free gas bubbles. Therefore, the velocity and amplitude of P-waves are good indicators of the presence of free gas in unconsolidated soils. As can be seen in Figure 11b, VP at VP5 dropped significantly from ∼2000 m/s to ∼700 m/s, because of the free gas released from the hydrate dissociation, during the period of time between 50 and 120 min after the commencement of centrifuge spinning. This is consistent with the concurrent overpressure measured at PT5 (Figure 11b) and the concurrent VS decrease at VS6 (Figure 11a), placed 10 cm below the VP5. After a VP drop at VP5, a decrease of VP at VP1 was observed, occurring during the period of time between 80 and 150 min. While there was no noticeable temperature increase in Layer 1 (5 cm below the soil surface), the VP drop observed at VP1 was not caused by the hydrate dissociation. It is believed to have been caused by gas bubbles that migrated from the dissociated regions below Layer 1. As mentioned previously, the pressure at PT1 and PT3 was measured as being identical, presumably because of the percolated gas that was in contact with the local pressure transducers. Thus, it is reasonable to hypothesize that the free CO2 gas that was detected by VP1 was produced by hydrate dissociation in the lower regions and which migrated to Layer 1. Electrical Resistance Results. Electrical resistances measured were normalized by the initial values, to highlight changes reflecting pore-scale processes (Figure 11c). The dissociation of gas hydrate in an excess water condition mostly causes an increase in electrical resistivity of hydrate-bearing porous media, because free gas bubbles produced from gas hydrate reduce the charge passage within the porous media. Expectedly, the electrical resistances at ER4, ER5, and ER6 increased as the heat transfer commenced and the hydrate was dissociated. The electrical resistance at ER6 (i.e., located 15 cm above the heater) showed a sudden increase of ∼180% at a time of ∼50 min; the values at ER5 (i.e., located 25 cm above the heater) jumped at a time of ∼60 min; and the values at ER4 (i.e., located 35 cm above the heater) gradually increased and then suddenly jumped at a time of ∼190 min. Such timely ordered jumps of electrical resistance values at ER6 and ER5 were caused by the sequential hydrate dissociation, as heat transferred upward. However, a gradual increase at ER4 during the period of time between 50 and 170 min is considered to have been caused by the migration of gas from the deeper dissociation region, while the sudden jump at ∼190 min was due to the combined effects of hydrate dissociation and gas migration. Owing to the gas migration caused by fluid flow from an overly pressurized region of the hydrate dissociation, it is extremely difficult to identify the relative contributions from the hydrate dissociation and gas migration to the electrical resistance values. Meanwhile, the electrical resistances at ER1 and ER3 decreased because of the temperature increase of the pore fluids in Layer 1 and Layer 3, respectively. No effect of migrating free gas on the electrical resistance was detected in Layer 1 and Layer 3. In contrast, the pressure change of PT1 and the VP trace of VP1 proved the presence of free gas in Layer 1, suggesting the heterogeneous distribution of gas bubbles in Layer 1. The electrode in Layer 2 (ER2) was disconnected from the multiplexing system during the centrifuge test, and therefore, no measurement from ER2 was obtained.

Figure 11. Evolution of (a) S-wave velocity VS; (b) P-wave velocity VP; and (c) electrical resistance change, over the course of the centrifuge test. Resistance change, (R−Ro)/Ro, is calculated relative to the initial resistance values at 0 min.

at VS6 (i.e., 55 cm below the soil surface) was measured to be approximately 120 m/s while VS at VS1 (i.e., 5 cm below the soil surface) was approximately 100 m/s. After beginning thermal stimulation at 30 G, VS at VS6 showed a decrease from ∼120 m/s to ∼100 m/s during hydrate dissociation (50 to 120 min in Figure 11a), and VS at VS4 showed a similar tendency. However, VS at VS1, VS2, and VS3 did not show observable decreases, indicating that the gas hydrate above the VS3 layer (45 cm above from the heat source) was not dissociated, because such regions were not thermally stimulated because of the limited rate of heat diffusion. This observation is supported by the pressure−temperature traces in Figure 10 and the minimal temperature changes at TC3 and TC1. Given the complete dissipation of excess pore pressure after 130 min, the decrease in VS was attributable to the loss of cementation during hydrate dissociation. Note that the bender elements installed in Layer 5 (VS5) and the PZT disks in Layer 3 (VP3) were short-circuited because of elevated water pressure and the poor insulation of the sensors and actuators. It was also found that the pressurized CO2 gas and CO2-saturated water were highly corrosive to the polyurethane coatings on the bender elements and PZT disks. VP Results. P-wave propagation in porous media is greatly affected by the presence of free gas bubbles in pores. Since the stiffness of free gas is much less than that of water, an increase in the size of the gas bubbles causes a decrease in velocity and an increase in attenuation.41−43 For instance, VP of an unconsolidated soil of less than 1500 m/s, (the typical VP value of pure water) indicates that the soil is not fully water4516


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Figure 12. Changes in (a) temperature distribution; (b) pressure distribution; and (c), excess pore pressure, with time. In Figure 11b, the dashed lines indicate the theoretical water pressure gradients calculated based on pressure at the head space and the water depth. The gradient is governed by the G-level (i.e., 30 G). (d) Observed marks of fractures and gas seep on the soil surface after the disassembly of the vessel.

4. ANALYSES AND DISCUSSION 4.1. Spatial Distributions of Parameters during Thermal Stimulation: Evidence of Hydrate Dissociation, Gas Migration, and Fracture Generation. Excess Pore Pressure Generation. Figure 12 shows the one-dimensional spatial distributions of temperature, pressure, and excess pore pressure with time elapsed, during the centrifuge test. The dotted lines in Figure 12b represent the theoretical pressure profile estimated by the pressure gradients according to water column height and gas pressure in the head space. Profiles of the excess pore pressure (overpressure) are calculated by subtracting the theoretical pressure from the measured pressure, as shown in Figure 12c. During the period of time between 45 and 104 min after the commencement of the centrifuge spinning, there were pronounced excess pore pressures generated in Layer 5 (45 cm below the surface and 25 cm above the heat source) and in Layer 1 (5 cm below the surface), as shown in Figure 12b and c. In Layer 5, the maximum excess pore pressure of ∼240 kPa was generated near the heat source at 69 min, owing to the thermal dissociation of

gas hydrate. Thereafter, the overpressure dissipated with time. Despite the high permeability of the host sediment (i.e., 5 × 10−12 m2 or ∼5 D), hydrate dissociation resulted in an observable increase in pore fluid pressure by ∼6% relative to the hydrostatic fluid pressure. The observed overpressures were peculiarly large considering the low vertical effective stress of ∼127 kPa in Layer 5 (−45 cm), while the capillary pressure of the host sand (Pc = 4Ts/d, where Ts is the surface tension between water and gas) was ∼24 kPa, assuming the pore throat size (d) as being ∼12 μm (10% of mean particle size) and the surface tension as 0.072 N/m. This could be attributed to two mechanisms. Because CO2 hydrate was synthesized in partially water-saturated sand, the hydrate preferentially formed at grain contacts. Therefore, it is first assumed that the hydrate formation at grain contacts significantly reduced the pore throats, increased the capillarity pressure, and thus blocked gas invasion; second, that the hydrate continued dissociating, thus bringing up the pore pressure against pressure diffusion. Meanwhile, the overpressure in Layer 1 was presumed to have been caused by the migrated and overpressurized fluids 4517


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Figure 13. Spatial distribution of (a) VS changes; (b) VP changes; and (c) electrical resistance changes with time. Each change is calculated relative to the value at 45 min (at which time heat transfer had not commenced).

centrifuge was spinning (Figure 12d). In addition, an uplift deformation at the top of the sand column (∼10 cm) was observed, implying a possible uplift deformation at the seafloor during thermal dissociation of gas hydrates. VS, VP, and Electrical Resistance. Figure 13 shows the onedimensional spatial distribution of changes in VS, VP, and electrical resistance with time elapsed, during the centrifuge test. As temperature increased, sequential reductions of VS from the bottom to the top of the column (from Layer 6 to Layer 3) were observed. Decreases in VS of ∼25% were observed in Layer 6 (i.e., 15 cm above the heater) and ∼15% in Layer 4 (i.e., 35 cm above the heater), which was attributed to the decementation effect by hydrate dissociation. In contrast, the VS values in the upper layers showed small variations (of less than 10%), with no clear trend. Likewise, VP dropped in Layer 5 (i.e., 25 cm above the heater) at 69 min because of the released gas and decementation effect. However, VP in Layer 1 (i.e., ∼5 cm below the soil surface) decreased at ∼80 min, indicating the presence of free gas in the layer. Electrical resistance showed more drastic changes, increasing by more than 100% at the

from dissociation regions, noting that the temperature in Layer 1 (TC1) did not increase enough to dissociate the hydrate. The maximum excess pore pressure of ∼230 kPa in Layer 1 occurred at a time of 69 min. Because of the increased capillary pressure due to the hydrate formed at the grain contacts, it can be hypothesized that the hydrate-containing sand in Layer 1 acted as a sealing layer, and the free gas could therefore not escape, causing the pore pressure to accumulate in Layer 1. Note that the magnitude of excess pore pressure generation can be less if pore-filling or load-bearing hydrates were formed from dissolved gas. It is also of note that the observed overpressures were large enough to cause fractures in the media, accelerating gas venting. Although the fracture generation was not captured from our measurements, the severe fractures and gas seep marks were observed at the surface of the column when the vessel was disassembled after completion of the test, as shown in Figure 12d. Assuming depressurization only caused the minor gas seep marks, the severe fractures are presumed to be generated by the overpressure during thermal hydrate dissociation while the 4518


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gradient, the heat transport in homogeneous soils can be described by Fourier’s heat conduction law as follows:

bottom (Layer 6 and Layer 5). In particular, the gradually increasing electrical resistance in Layer 4 is expected to be caused by the combined effects of released gas from hydrate dissociation and the migrated gas bubbles. Therefore, the measured changes in pressure, VS, VP, and electrical resistance indicate that the hydrate dissociation front moved upward by approximately 350−450 mm (i.e., from the region near the electrical heater to Layer 4), from the start of the thermal stimulation that lasted ∼150 min during the centrifuge spinning. The propagation of the hydrate dissociation front is governed by the rate of heat transfer, and this thermal diffusion is analyzed in the following subsection. In addition, the increase in pore pressure and the decrease in VP at the top of the sand column demonstrate the pressure diffusion and upward gas migration from the hydrate dissociation region. 4.2. Effect of Dissociation on Thermal Properties of Hydrate-Bearing Sediments. Thermal Conductivity. Dissociation of hydrate releases free gas and water, and the released free gas in water-saturated pores is likely to first displace the water phase before creating the gas flow. The thermal conductivity of the free gas phase is in the order of 0.01 W m−1 K−1, which is significantly smaller than the values of water and hydrate (e.g., 0.024 W m−1 K−1 for air at 0 °C,44 0.56 W m−1 K−1 for water at 0 °C,44,45 and 0.56−0.62 W m−1 K−1 for methane hydrate46,47). Thus, it is presumed that the thermal conductivity of a hydrate-bearing sediment subjected to thermal stimulation progressively decreases as the hydrate dissociates and gas saturation consequentially increases. The thermal conductivity of dry soils is less than 0.5 W m−1 K−1 in most cases, which is smaller than the value of water, because the main heat transfer path in dry soils is through solid particle contacts.48 The thermal conductivity ranges from 1.2−4 W m−1 K−1 for water-saturated sands and approximately 2−3 W m−1 K−1 for water-saturated clays.49−51 Thus, it is thought that hydrate dissociation can reduce the thermal conductivity of hydrate-bearing soils by a maximum of 1 order of magnitude. Thermal Diffusivity. The thermal diffusivity of hydratebearing sediments can be quantified from thermal conductivity, specific heat, and the density of sediments (i.e., κ = λ/(Cpρsed)). Specific heat of hydrate-bearing sediments depends simply on the mass fractions of mineral grain, hydrate, water, and gas. Therefore, hydrate dissociation, releasing free gas and displacing pore water, results in decreases in sediment density and specific heat, while at the same time decreasing thermal conductivity. In the absence of systematic studies on the effects of free gas generation on the thermal diffusivity of hydratebearing sediments, decreases in density and specific heat may or may not compensate for the reduction of thermal conductivity with increasing free gas saturation during hydrate dissociation, raising an important questionwill the hydrate dissociation decrease or increase the thermal diffusivity of the hydratebearing sediments? Therefore, to explore this question, the one-dimensional heat transfer in hydrate-bearing sands was examined using the temperature records obtained during the thermal stimulation of the gas hydrate-bearing sand column. In this study, the conduction of heat is assumed to be the most relevant process in heat transport, because the host sediment used (F110 Ottawa quartz sand) is a fine sand with a mean diameter of ∼120 μm. Further, the permeability of the same quartz sand, having a similar porosity, is reported as approximately 5 × 10−12 m2 (5 D).52 Thus, if the thermal properties of a medium are kept constant and if no fluid flow is forced by the pressure

∂T ∂ 2T =κ 2 ∂t ∂x

where

κ=

λ Cpρsed

(2)

where T is temperature, κ is the thermal diffusivity of the medium (m2 s−1), λ is the thermal conductivity (W m−1 K−1), Cp is the specific heat (J kg−1 K−1), and ρsed is the density of the sediment. While thermal diffusivity κ is a measure of the rate of the temperature of a body subjected to an external heat flux, thermal conductivity λ quantifies the efficiency of heat transport. However, as thermal stimulation causes hydrate dissociation, the thermal properties of sediments (e.g., thermal conductivity, specific heat and thermal diffusivity) change with time. Thus, thermal diffusivity was chosen to represent heat transfer in sediments undergoing phase transformation and changes in pore fluid compositions. Herein, the temperatures measured at TC3 (45 cm above the heater), TC5 (25 cm above the heater), TC7 (5 cm above the heater), and at the heater were selected, and the net temperature changes (ΔT) were calculated for a time period of 100 min from the beginning of thermal stimulation. The temperature change in the heater was used as a heat source, and the temporal and spatial changes in temperatures at other locations (TC7, TC5, and TC3) were estimated using Fourier’s heat conduction theory (eq 2) with different thermal diffusivities. The estimated results were superimposed with the measured temperature records, as shown in Figure 14. Based on the best

Figure 14. Temperature changes measured and estimated in each layer during thermal stimulation.

fit with experimental data, it was found that the hydrate-bearing sand column prepared in this study had a thermal diffusivity of ∼10−5 m2/s. However, the measured temperature records started deviating from, and became lower than, the projected temperatures by conduction theory (i.e., TC7 at 50 min and TC5 at 80 min). This observation suggests that the thermal diffusivity of hydrate-bearing and water-saturated sediments be most likely to decrease as hydrate dissociation occurs and free gas saturation increases. On the contrary, the temperatures measured at TC3, higher than the predicted values, are more likely to have been caused by fluid transport from the dissociated region, where heat was transported by the fluid flow to the upper soil column. 4519


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The upper bound for the thermal conductivity λ of hydratebearing sands is the value of a water-saturated and hydratecontaining sand. The water-saturated F110 sands are reported to have thermal conductivities ranging from 3.7 to 4.5 W m−1 K−1.51 Water and methane hydrate have similar thermal conductivities, with less than 10% difference between the two; and thus, the presence of hydrate does not increase the thermal conductivity of the sediments significantly (e.g., 10% methane hydrate saturation increases the thermal conductivity of a water-saturated sediment by less than 1%).47 The F110 sand column with hydrate prepared in this study can be assumed to have a thermal conductivity of ∼4 W m−1 K−1 before commencement of thermal dissociation. The lower bound for thermal conductivity λ would be the value of dry sands. This condition is expected when dissociated gas displaces pore water because of the high capillary pressure resulting from small pore throats. For air-dry F110 sands, thermal conductivity is reported at 0.28−0.48 W m−1 K−1, depending on the effective stress and porosity.51 Thus, upon the completion of the hydrate dissociation, the minimum thermal conductivity of the sand (which might be partially water-saturated or, at worst, dry) that can be achieved is ∼0.5 W m−1 K−1. Given a porosity of 0.42, the densities, ρsed, of the water-saturated sand with 40% hydrate saturation and the dry sand are estimated to be 1945 kg/m3 and 1540 kg/m3, respectively. With the specific heats, Cp, of quartz (730 J kg−1 K−1),44 water (4218 J kg−1 K−1),45 hydrate (2100 J kg−1 K−1),53 and gas (2170 J kg−1 K−1),37 a water-saturated sand with 40% hydrate saturation is calculated to have a specific heat of 1292 J kg−1 K−1 and a dry sand to have a specific heat of 733 J kg−1 K−1. Using the equation for calculating thermal diffusivity κ = λ/(Cpρsed), the thermal diffusivity of a water-saturated sand with 40% hydrate saturation is estimated to be 1.6 × 10−6 m2 s−1 while a dry sand has a thermal diffusivity of 4 × 10−7 m2 s−1. Thus, this calculation shows that there can be a decrease in thermal diffusivity of ∼70% throughout the process of hydrate dissociation. This calculation supports the obtained experimental results (Figure 14), showing that the thermal diffusivity of the hydrate-bearing sand decreases during hydrate dissociation. Therefore, a reduction in thermal conductivity and thermal diffusivity during dissociation is presumed to retard methane production from natural hydrate deposits. However, for depressurization, advection of fluids from warmer surrounding sediments is expected to be a main heat source during endothermic dissociation; thus, methane production via depressurization may not be significantly affected by the reduction of such thermal properties in dissociating regions. 4.3. Implications for Seafloor Instability. Instability Due to Excess Pore Pressure. Gas hydrate dissociation produces a pronounced volume expansion, owing to the release of a large amount of gas.1,54,55 Specifically, if hydrate dissociates in clayey sediments with a permeability of ∼1 mD, the released gas cannot escape, and hence, the pore fluid pressure in sediments can increase by several megapascals, or fractures in the medium can be generated when the gas pressure exceeds the effective stress of the skeleton.1,3 However, if hydrate dissociation occurs in sandy sediments with a permeability larger than 1 D, it has been demonstrated that there is only a slight development of excess pore fluid pressure because the overpressure dissipates rapidly to the surrounding regions.3 Thus, when thermally stimulating hydrate-bearing sediments, the magnitude of the excess pore pressure generation is governed by the relative rate of the pressure diffusion to the hydrate dissociation or thermal

diffusion. Owing to the high permeability of the sand column used in this study, the pressure diffusivity (i.e., c = KBsk/(μwϕ) ≈ 10−1 m2 s−1, where K = 5 × 10−12 m2 is the permeability of the medium, Bsk = 10 MPa is the bulk stiffness of the skeleton of the medium, ϕ = 0.4 is the porosity, and μw = 0.001 Pa s−1 is the viscosity of water) is apparently much larger than the thermal diffusivity (i.e., κ = λ/(Cpρsed) ≈ 10−6 m2 s−1 from Figure 14). Therefore, it is expected that the pressure diffusion will be much faster than thermal diffusion and the hydrate dissociation front movement. However, it appears that an excess pore pressure larger than ∼200 kPa could be generated even in sandy sediments, because of the increased capillary pressure by hydrate formed at the grain contacts and by the continuous hydrate dissociation, which tends to increase pore pressure against pressure diffusion until there is a complete dissociation of hydrate in that region (as discussed in section 4.1). A hydrate-bearing layer can also act as a seal, hindering further gas invasion, because of the high capillary pressure induced by hydrate crystals blocking the pore throats. As a result, overpressurized fluids can be accumulated underneath the sealing layer and can render excess pore pressure. An excess pore pressure in the order of several hundreds of kilopascal, whether it is caused by hydrate dissociation or by fluid migration from far fields, will lead to either sediment volume expansion, uplifting deformation at the seafloor, or fracture generation in sediments (see Figure 12d). Vanishing Solid Hydrates and Decementation. When gas hydrates are formed from partially water-saturated sediments, in spite of post-water saturation, the following possible pore morphologies of hydrates are presumed: (1) that solid hydrates cement the sand grains; and/or (2) that hydrate crystals act as solid particles that build skeletal frames, carry the overburden stress and provide lateral supports against the buckling of skeletal frames.26,27,37 Therefore, hydrate formation increases the mechanical stiffness and thus the VS of soils. In opposition to this, the dissociation of gas hydrate in sediments relieves the cementation (i.e., decementation) and decreases the stiffness. Moreover, as solid hydrate crystals vanish, the soil stiffness further decreases. A reduction in VS of ∼25% was observed in Layer 6 (from 130 to 98 m/s) during thermal stimulation and consequential hydrate dissociation (Figure 13a). In Layer 4, VS decreased from 114 to 100 m/s; a reduction of ∼14%. Thus, decreases in shear modulus G of ∼40% and 23% in Layers 6 and 4, respectively, can be calculated based on the relation between shear modulus and shear wave velocity: G = ρVS2. This implies possible postdissociation subsidence where the sediments in the hydrate dissociation region will eventually undergo downward deformation (settlement) by weight of overlying sediments owing to the reduced stiffness (or increased compressibility). As the hydrate particle vanishes, the decementation occurs, and overpressurized fluids diffuse out. Such postdissociation subsidence will appear at the seafloor, when depressurization or thermal stimulation are applied as a hydrate dissociation strategy. If thermal stimulation is used, it is expected that this postdissociation subsidence will take place after the sediment volume expansion at the hydrate dissociation region and after the uplifting deformation at the seafloor. In our test, the hydrate dissociation occurred from the bottom of the sand column to Layer 4 in the sand column (∼40-cm-thick sand column), which is equivalent to a 12-mthick sediment layer. It was observed that the shear stiffness of the hydrate-bearing sediments, with a hydrate saturation of 4520


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∼30−40%, decreased by more than 20% when the hydrate dissociated. Assuming a 20% decrease in elastic constraint modulus in that region, vertical strain can be 20%; thus, the possible settlement is predicted to be as significant as 2.4 m (i.e., 12 m × 20% = 2.4 m). Furthermore, the postdissociation subsidence that would appear at the seafloor increases with the increasing thickness of the hydrate dissociation layer. In addition to this, such postdissociation subsidence may apply a downward force to well structures, but lateral skeletal stress in a yielded zone is significantly reduced because of plastic deformation. As a result of postdissociation subsidence and lack of lateral stress, a slender well structure could be damaged, particularly at a region where a large volume expansion occurs and a yield zone is developed (e.g., a hydrate dissociation region).

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stiffness of sediments, when the initial Sh was ∼32−39%, because of the vanishing of solid hydrate crystals that were cementing the mineral grains or bearing load. This implies the potential of postdissociation subsidence at the seafloor, of the order of several meters, during gas production from hydrate-bearing deposits.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-42-350-3622. Fax: +82-42-350-3610. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to anonymous reviewers for their valuable comments and suggestions. This research was supported by a grant from the Korea Electric Power Corporation (KEPCO) and by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of Knowledge Economy of Korea. The authors are grateful to the Director, Prof. Dong-Soo Kim, and to the lab personnel at KOCED Geotechnical Centrifuge Center at KAIST for their support for this study.

5. CONCLUSIONS In this paper, geomechanical and thermal responses of a hydrate-bearing sand column subjected to thermal stimulation were presented via physical modeling using centrifuge testing. Various coupled phenomena associated with hydrate dissociation were observed, including decementation, softening of soil skeletal frames, pressure diffusion, gas migration, and heat transfer. The main findings are as follows: • A near-seafloor hydrate-occurring condition, from the seafloor to ∼23 m below the seafloor, was created using an increased G-level, where the linear gradients of stress and pore fluid pressure were confirmed by measured VS and fluid pressure, respectively. • VP, VS, and electrical resistance were all good indicators of hydrate dissociation. In paticular, VP and electrical resistance were sensitive to the presence of free gas, which could occur because of hydrate dissociation or gas migration. VS efficiently captured the cementation and decementation effects from hydrate formation and dissociation. • We observed that gas hydrate dissociated from the region near the heat source. The heat diffusion front propagated upward to ∼450 mm from the heat source within 160 min, and the propagation of the hydrate dissociation front was limited by the rate of heat diffusion. • Our analysis on the obtained spatially and temporally varying temperature records revealed that thermal diffusivity (i.e., initially 0.5−1 × 10−5 m2/s) of hydratebearing and water-saturated sediments significantly decreased as hydrate dissociation occurred and free gas saturation increased. Therefore, the production of methane from natural hydrate deposits particularly well as by the thermal stimulation method, is presumed to be mainly limited by the decreasing thermal diffusivity during hydrate dissociation. • In spite of the high permeability of the tested sediments, ∼240 kPa of excess pore pressure was caused by hydrate dissociation near the heat source, owing to continued hydrate dissociation during pressure diffusion, implying a possible volume expansion at the hydrate dissociation region. Furthermore, ∼230 kPa of excess pore pressure very near the sediment surface was produced because of pressure diffusion, indicating a possible uplifting deformation at the seafloor. • Hydrate dissociation caused at a maximum ∼25% decrease in VS, or an ∼40% decrease in the shear

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Geomechanical and Thermal Responses of Hydrate-Bearing

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