Permeability of sediment cores from methane hydrate deposit in the Eastern Nankai Trough

Marine and Petroleum Geology xxx (2015) 1e9

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Research paper

Permeability of sediment cores from methane hydrate deposit in the Eastern Nankai Trough Yoshihiro Konno a, *, Jun Yoneda b, Kosuke Egawa a, 1, Takuma Ito a, 2, Yusuke Jin a, Masato Kida a, Kiyofumi Suzuki c, Tetsuya Fujii c, Jiro Nagao a, ** a

Methane Hydrate Research Center (MHRC), National Institute of Advanced Industrial Science and Technology (AIST), 2-17-2-1 Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan MHRC, AIST, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan c Methane Hydrate Research & Development Division, Japan Oil, Gas and Metals National Corporation (JOGMEC), 1-2-2 Hamada, Mihama-ku, Chiba-city, Chiba 261-0025, Japan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 September 2014 Received in revised form 6 February 2015 Accepted 16 February 2015 Available online xxx

Effective and absolute permeability are among the most important factors affecting the productivity of hydrate-bearing sediments during gas recovery operations. In this study, effective and absolute permeability have been measured using natural sediment cores obtained from a methane hydrate reservoir in the Eastern Nankai Trough off the shore of Japan. The cores were recovered under pressure and shaped cylindrically with liquid nitrogen spray after rapid pressure release. The cylindrical core was inserted into a core holder for flooding tests in order to apply a near in situ effective stress. The effective permeability of water in the hydrate-bearing sandy sediment was 47 millidarcies (md) with a hydrate saturation of 70%. After hydrate dissociation, the absolute permeability was estimated to be 840 md. Other test results showed that the absolute permeability of the hydrate-free sediments was estimated to be tens of microdarcies for clayey sediments, tens of md for silty sediments, and up to 1.5 darcy for sandy sediments. Absolute permeability showed a strong correlation with sediment grain size in logelog plots. In addition, the effective permeability of hydrate-bearing sandy sediments and the absolute permeability of hydrate-free sandy sediments correlated with the effective porosity. We compared measured data to other experimental data using pressure cores recovered from the same well and wireline pressure tests from a well near the coring well. The results are consistent with each other. At this location, we found that the effective permeability for hydrate-bearing sandy sediments was in the range of 1e100 md, which was 2e3 orders of magnitude higher than conventional estimates. Finally, the change of permeability, potentially caused by depressurization-induced gas production, was analyzed. It was found that the high effective stress owing to depressurization and freshwater generation originating from hydrate dissociation caused reduction in absolute permeability. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Gas hydrate Effective permeability Absolute permeability Flooding test Pressure coring Turbidite

1. Introduction

* Corresponding author. Tel.: þ81 11 857 8949. ** Corresponding author. Tel.: þ81 11 857 8948. E-mail addresses: [email protected] (Y. Konno), [email protected] (J. Nagao). 1 Present address: Subsurface Evaluation Unit, Technical Division, INPEX Corporation, Tokyo 107-6332, Japan. 2 Present address: CO2 Storage Research Group, Research Institute of Innovative Technology for the Earth, Kyoto 619-0292, Japan.

Natural gas hydrates are crystalline solids composed of water and guest molecules (Sloan and Koh, 2008). Methane hydrate is a gas hydrate in which predominantly methane molecules are trapped, and it is common in natural environments, such as permafrost regions and shallow sediments on marine continental margins. Recent studies show that hydrate-bearing sands are the most feasible energy resource targets for recovery of gas. The reason for the great resource potential of sand sediments is their greater intrinsic (absolute) permeability (Boswell and Collett, 2011). For economic gas recovery from hydrate-bearing sand sediments, initial effective

http://dx.doi.org/10.1016/j.marpetgeo.2015.02.020 0264-8172/© 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Konno, Y., et al., Permeability of sediment cores from methane hydrate deposit in the Eastern Nankai Trough, Marine and Petroleum Geology (2015), http://dx.doi.org/10.1016/j.marpetgeo.2015.02.020

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permeability (initial water or gas permeability of hydrate-bearing sediments) is the most important factor because it directly affects the gas production rate and recovery factor. Konno et al. (2010a) numerically simulated the depressurization-induced gas production and concluded that the initial effective permeability of hydratebearing sediments is a crucial factor for successful gas production. They suggested that effective permeability higher than the threshold value is absolutely necessary for depressurization. To date, experimental studies have evaluated the effective permeability of hydrate-bearing sediments using cores with synthetic methane hydrate (Kleinberg et al., 2003; Kneafsey et al., 2011b; Liang et al., 2011; Seol and Kneafsey, 2011; Konno et al., 2013) and CO2 hydrate (Kumar et al., 2010). In order to characterize methane hydrate-bearing sediment, proton nuclear magnetic resonance (NMR) is applied to measure pore size distribution of unconsolidated sediment (e.g., Minagawa et al., 2008). Modeling has also been attempted to understand microscale flow mechanisms by accounting for gas invasion and gas nucleation processes (Jang and Santamarina, 2014) and hydrate pore-scale growth habit and meso-scale heterogeneity (Dai and Seol, 2014). Although cores with synthetic hydrate are widely used for permeability measurements, it is important to study naturally occurring hydrate-bearing sediment in actual reservoirs to ensure accurate evaluation of gas productivity. For natural sediments, wireline logging has been widely used to estimate effective permeability. Combinable nuclear magnetic resonance (CMR) logging was conducted in the Eastern Nankai Trough, and the initial effective permeability of hydrate-bearing sediments was estimated to be in the range of 0.01e10 md (Uchida and Tsuji, 2004). At the Mount Elbert site on the Alaska North Slope, a formation pressure test was conducted to estimate initial effective permeability through numerical simulation (Anderson et al., 2011; Kurihara et al., 2011). Estimated initial effective permeability was in the range of 0.12e0.17 md. At the Mallik field on Mackenzie Delta, initial effective permeability in a highly hydrate-saturated sand layer was estimated to be lower than 1 md based on CMR logging (Fujii et al., 2012). In contrast, a few experimental studies estimated the permeability of natural sediments in gas hydrate deposits. Jin et al. (2007) analyzed frozen hydrate-bearing sediments recovered from the Eastern Nankai Trough offshore of Japan using microfocus X-ray computed-tomography to investigate the flow channel structure. For the sediment core recovered from the same area, Konno et al. (2010b) conducted a numerical simulation analysis as part of a dissociation experiment and reported that initial effective water permeability was 3e4.8 md at a hydrate saturation of 52.0% for naturally occurring hydrate-bearing sediment. Sediment cores used in these studies were recovered using a Pressure Temperature Core Sampler (PTCS) in 2004 (Takahashi and Tsuji, 2005). In the permafrost region of Mount Elbert, Winters et al. (2011) measured intrinsic (absolute) permeability of sediment cores. However, the initial effective permeability could not be determined because the gas hydrate was already dissociated. To estimate effective and relative permeability (defined as the ratio of effective permeability and absolute permeability) of hydrate-bearing conditions, Johnson et al. (2011) artificially formed gas hydrate in sediment cores recovered from the Mount Elbert's permafrost region. To estimate effective permeability, Li et al. (2014) recently conducted NMR measurements in the laboratory using samples of hydrate-bearing sandstone recovered from the Shenhu area of the South China Sea. Pressure coring systems, such as PTCS, are expected to be the most effective methods of core recovery for hydrate-bearing sediments. However, it is difficult to preserve hydrates from disturbance during the core handling process. This is because a pressure release process must be conducted in order to shape the cores and place them in a core holder for effective permeability studies. To

estimate nearly in situ properties from these cores, it is important to understand the degree of disturbance. In this study, we investigated the effect of the pressure release process on the sample texture and the amount of remaining hydrates in the sediment cores recovered using the pressure coring system. By introducing an effective pressure release process, the effective water permeability of undisturbed hydrate-bearing sediments was determined. In addition, the results were compared to recent pressure core analyses and field logging data. This was possible because new cutting-edge analysis and transfer systems of pressure cores allow nondestructive (conducted without pressure release) analyses of hydrate-bearing sediments (Schultheiss et al., 2011; Santamarina et al., 2012). On the basis of a comparison with these studies, the effective permeability of the hydrate reservoir at this location is discussed in this paper. In addition, absolute permeability, which is the intrinsic permeability of the hydrate-free sediment, was analyzed for various sediment lithology in the hydrate deposit. Finally, the change of absolute permeability resulting from depressurization-induced gas production was analyzed. 2. Methods 2.1. General core information Sediment cores were recovered from well AT1-C in the Eastern Nankai Trough off the shore of Japan. The coring operation was conducted during JuneeJuly 2012 using the Hybrid Pressure Coring System developed by the Japan Agency for Marine-Earth Science and Technology, Aumann and Associate Inc., and JOGMEC (Kubo et al., 2014; Inada and Yamamoto, in this issue). During the coring operation, most of the cores were maintained within the hydrate phase stability PeT conditions. However, some of the cores were unexpectedly depressurized during core retrieval owing to mechanical problems. In this study, both of pressure-preserved and unpreserved cores were used for permeability measurements. The diameter of the recovered cores was 50.8 mm (2 in). The length of the cores, which depends on the recovery ratio, was up to 3 m. The exterior of the cores was covered by using a 3-mm-thick plastic liner with an inner diameter of 53.6 mm. The lithology of recovered cores indicated clayey, silty, and sandy layers of a turbidite reservoir. On the basis of the P-wave velocity measurement conducted under pressure, the pressure-preserved silty and sandy cores were determined to be methane-hydrate-bearing sediments (Suzuki et al., in this issue). The pressure-preserved cores were stored in pressure vessels (storage chambers) with freshwater. 2.2. Sample preparation procedure Before the permeability measurements, all cores were subjected to a liquid nitrogen (LN2) treatment for the purpose of cylindrically shaping the test samples. The procedure was performed at atmospheric pressure after the pressure release of the pressurepreserved cores. The storage chamber of the pressure-preserved core was immersed in a cooling bath to minimize hydrate dissociation during the pressure release process. In this study, we followed two different cooling procedures: cooling in seawater and freezing in gas atmosphere. Most of the core storage chambers were immersed for 1 h in a cooling bath with salt and ice at 2  C, which is the point at which seawater freezes. In this case, the pore space seawater was not frozen. One storage chamber was instead immersed in a cooling bath with brine solution overnight at approximately 4  C to freeze the pore space seawater. Thus, the seawater and freshwater mixture in the storage chamber was replaced by methane gas at 8 MPa prior to freezing because the frozen water prevented the core recovery from the storage chamber

Please cite this article in press as: Konno, Y., et al., Permeability of sediment cores from methane hydrate deposit in the Eastern Nankai Trough, Marine and Petroleum Geology (2015), http://dx.doi.org/10.1016/j.marpetgeo.2015.02.020

Y. Konno et al. / Marine and Petroleum Geology xxx (2015) 1e9

after the pressure release. After the cooling/freezing processes, the cores were depressurized and quickly quenched with LN2 to prevent hydrate dissociation. For each core, the elapsed time from depressurization below the equilibrium condition to quenching with LN2 was short, up to a few minutes. To observe inner structure and determine the sediment lithology and hydrate-saturated sections, the quenched core (LN2 core) was scanned at approximately 100  C using an X-ray CT scanner (Hitachi Medico Technology, MCT-130 CBHS). On the basis of CT scanning, sediment sections with the same lithology and no large cracks were used in the permeability measurements. The test samples were separated from the bulk cores and shaped cylindrically using a saw and a lathe with LN2 spray. The diameter of cylindrical test samples was 29 mm and the length was 32e70 mm. Except for the cooling/freezing and pressure release processes, pressure-unpreserved cores were treated in the same way as pressure-preserved cores. Residual cuttings from the core were used for the analysis of fundamental sediment properties such as grain density, grain size, and mineral composition (Ito et al., in this issue; Egawa et al., in this issue).

(2)

(3)

(4)

2.3. Permeability measurement procedures Flooding tests were conducted to measure effective water permeability of hydrate-bearing sediment and absolute permeability of hydrate-free sediment. Figure 1 shows schematic drawings of the core holders used in the study. To prevent hydrate dissociation, the permeability of originally pressure-preserved cores was measured using a pressure-temperature-controlled core holder (PTCH), which was modified for this study on the basis of a previous study by Konno et al. (2010b) (Fig. 1, left). However, the permeability of the unpreserved cores was measured at atmospheric pressure and room temperature (approximately 20  C) using a simplified core holder (SCH) (Fig. 1, right) because it was not necessary to maintain the hydrate phase stable PeT conditions. The function of both core holders was to apply triaxial effective stress on the test sample. Tests were conducted along the length of each core. Therefore, the measured permeability represents the sediment permeability along the vertical direction. Test procedures varied according to the pressure history of core. For originally pressure-preserved and potentially hydrate-bearing cores, the following procedure was applied. (1) The test sample immersed in LN2 was placed in the PTCH at approximately 2  C. The pore and confining pressures were

(5)

(6)

3

immediately increased while preventing the effective stress from exceeding the in situ level. After reaching the equilibrium condition, the pore pressure, effective stress, and temperature were set at 10 MPa, the in situ value (1.5e1.7 MPa, calculated using the sediment density and depth), and 10  C, respectively. The pore pressure was applied by using artificial seawater (3.5 wt % solution of sea salts, SigmaeAldrich). Effective water permeability was analyzed by measuring the differential pressure during injection of artificial seawater at constant rate. Pore pressure was decreased to dissociate the hydrate. The confining pressure was also decreased according to the reduction in pore pressure to maintain constant effective stress, the goal of which was to prevent additional consolidation during depressurization. After hydrate dissociation, vacuum resaturation with artificial seawater was undertaken. The effective stress was set at the in situ value. Absolute permeability was analyzed by measuring differential pressure during the injection of artificial seawater at constant rate. The pressure boundary condition at the end of the core was maintained at atmospheric pressure. The confining pressure was increased to 10 MPa to simulate the high effective stress owing to the depressurization-induced gas production. Absolute permeability was analyzed by measuring differential pressure during the injection of artificial seawater at constant rate. The pressure boundary condition at the end of the core was maintained at atmospheric pressure; thus, the effective stress of this step was 10 MPa. The injection fluid was switched from artificial seawater to distilled water in order to simulate water flow after hydrate dissociation. Absolute permeability was analyzed by measuring differential pressure during the injection of distilled water at constant rate. The pressure boundary condition at the end of the core and the effective stress were maintained at atmospheric pressure and 10 MPa, respectively.

For hydrate-free clayey sediments, processes (1) and (2) were applied. For pressure-unpreserved cores, the procedure was as follows. (1) The test sample immersed in LN2 was placed in the SCH. The effective stress was set at in situ value (1.4e1.5 MPa). Vacuum re-saturation with artificial seawater was conducted. Pore

Figure 1. Core holder schematics. On the left is a pressureetemperature-controlled core holder or PTCH, which was modified for this study on the basis of a previous study by Konno et al. (2010b). On the right is a simplified core holder or SCH. The function of both core holders was to apply tri-axial effective stress on the test sample. The glass beads were used as a spacer and a filter to prevent loose sediment grains to flow out.

Please cite this article in press as: Konno, Y., et al., Permeability of sediment cores from methane hydrate deposit in the Eastern Nankai Trough, Marine and Petroleum Geology (2015), http://dx.doi.org/10.1016/j.marpetgeo.2015.02.020

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pressure and temperature were atmospheric pressure and room temperature, respectively. (2) Absolute permeability was analyzed by measuring differential pressure during the injection of artificial seawater at constant rate. (3) The injection fluid was changed from artificial seawater to distilled water in order to simulate water flow after hydrate dissociation. Absolute permeability was analyzed by measuring differential pressure during the injection of distilled water at constant rate. After permeability measurements, the test sample was removed from the core holder and dried in order to obtain the weight of sediment. Porosity was derived on the basis of the weight of the sediment, the grain density, and the bulk volume of the core. Hydrate saturation was estimated from the derived porosity and collected gas volume during the depressurization process. The method for estimating saturation is presented in Konno et al. (in this issue). Core properties and experimental conditions are summarized in Table 1. The flow rates in all measurements were controlled in the range of 0.001e5 ml/min. Most of the hydraulic gradient was maintained below 20 kPa; however, in the measurements of AT1-C-3P (78.5e83.5), AT1-C-5P (275e282), and AT1-C13P (133e138), the hydraulic gradient increased to approximately 250 kPa, 190 kPa, and 280 kPa, respectively. Throughout this paper, numbers in parentheses after the core designations refer to intervals measured in centimeters from the top of the core.

the bedding could be observed, which was consistent with the image of pressure cores obtained from an onboard X-ray analysis (Suzuki et al., in this issue). The hydrate saturation of the frozen core in a gas atmosphere, obtained by mass balance calculations after dissociation, was comparable to that of the logging and pressure core data (Fujii et al., in this issue; Konno et al., in this issue). However, the hydrate saturation of cracked cores cooled in the ice bath was zero or smaller than that of the logging and pressure core data. It was determined that a large part of the hydrate in the ice-cooled core was dissociated during the pressure release process. For clayey sediments, there was no obvious disturbance in the unpressurized core (Fig. 2d). The results indicate that gas expansion was not obvious during core retrieval because the sediment was hydrate-free. In contrast, many small cracks were observed in the pressure-preserved core that was frozen in a gas atmosphere (Fig. 2e). We infer that the freezing process affected the cracking phenomena. Generally, small cracks appear during the ice formation process, especially in sediments with low permeability (Kneafsey et al., 2011a). These observations indicate that pressure and cooling/freezing history has a significant impact on the core texture and the amount of residual hydrate. For hydrate-bearing sediments, freezing in a pressurized gas atmosphere is a suitable procedure for preventing hydrate dissociation when the pressure has to be released. In contrast, the freezing process may cause small cracks in clayey cores with low permeability.

3. Results and discussion

3.2. Permeability measurements

3.1. Condition of test samples

The results of permeability measurements are summarized in Table 2. In this section, the results of measurements are discussed in detail.

X-ray images and photos of test samples are shown in Figure 2. Large-scale X-ray images and photos are shown in the Appendix. The features differed substantially according to core pressure and cooling/freezing history. The pressure-unpreserved (hereinafter called “unpressurized”) core was characterized by cavities in the silty sediment that had probably been hydrate-bearing in situ (Fig. 2a). The cavities were considered to be formed by gas bubbles decomposed from methane hydrate during core retrieval. This is a common structure observed in hydrate-bearing sediments that are recovered by conventional (unpressured) coring systems. For pressure-preserved cores, the cavity structure was not observed for presumed hydrate-bearing sediments. However, many obliquely parallel cracks were detected in the cores cooled in the ice bath (Fig. 2b). The cracks seemed to be formed along the sediment bedding (dip angle approximately 20 ). The cracks were likely formed as a consequence of gas expansion during the pressure release process, because the onboard X-ray analysis showed the original recovered cores were not cracked along the bedding dip (Suzuki et al., in this issue). In contrast, the freezing process in a gas atmosphere resulted in an unbroken mass of likely hydrate-bearing sediment (Fig. 2c). The evidence indicated that

3.2.1. Effective water permeability of hydrate-bearing sediment There were two test samples containing methane hydrate in this study. One was the core cooled in the ice bath, and the other was the core frozen in a gas atmosphere. In the former case (AT1-C-8P (135.5e139.5)), hydrate saturation was 24% with a porosity of 51%. The effective water permeability was 200 md (1 md ¼ 9.869233  104 mm2). In the latter case (AT1-C-13P (123.5e128.5)), hydrate saturation was 70% with a porosity of 42%. The effective water permeability was 47 md. The P-wave velocity obtained by onboard analysis showed equally high values (over 2800 m/s) in both sections. This indicates that the level of original hydrate saturation was the same in both cores (Suzuki et al., in this issue; Konno et al., in this issue). From this result, it can be inferred that the effective water permeability of AT1-C-8P (135.5e139.5) was not an original value but the transition value during hydrate dissociation. In contrast, it was determined that the sample condition of AT1-C-13P (123.5e128.5) was sufficient to estimate the in situ effective water permeability.

Table 1 Core properties and experimental conditions. Core# AT1-C-

Pressure history

Cooling/freezing process

In-situ effective Sample stress (MPa) size (mm)

3P (78.5e83.5) 5P (275e282) 8P (135.5e139.5) 12P (1e6) 13P (123.5e128.5) 13P (133e138) 20P (180e183)

unpreserved unpreserved preserved preserved preserved preserved preserved

Cooling in sw. Cooling in sw. Cooling in sw. Cooling in sw. Freezing in methane Freezing in methane Cooling in sw.

1.4 1.5 1.5 1.6 1.6 1.6 1.7

F: 29, L: 51 F: 29, L: 70 F: 29, L: 38 F: 29, L: 51 F: 29, L: 48 F: 29, L: 49 F: 29, L: 32

Porosity Hydrate Clay Silt (%) Sand Grain diameter Grain density (%) saturation (%) (%) (%) (median, mm) (g/cm3) 51 50 51 43 42 42 46

NA NA 24 0 70 0 0

17.2 33.2 3.5 1.7 2.8 35.9 5.3

58.2 62.6 35.8 24.5 24.9 61.6 38.2

24.5 4.2 60.7 73.8 72.3 2.4 56.5

28.0 6.1 106.8 132.2 133.2 5.2 84.7

2.67 2.62 2.77 2.63 2.73 2.68 2.70

Please cite this article in press as: Konno, Y., et al., Permeability of sediment cores from methane hydrate deposit in the Eastern Nankai Trough, Marine and Petroleum Geology (2015), http://dx.doi.org/10.1016/j.marpetgeo.2015.02.020

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Figure 2. X-ray images and photos of test samples.

3.2.2. Absolute permeability of hydrate-free sediment Absolute permeability was obtained for all test samples and indicated a trend related to sediment lithology and porosity. For both clayey cores (AT1-C-5P (275e282) and AT1-C-13P (133e138)), absolute permeability was estimated to be on the order of tens of md. The grain diameter and clay/silt/sand ratio in both cores showed almost the same features. Although the pressure and cooling/ freezing history of these test samples were different, the small cracks observed in the frozen sample did not affect the measurement. By applying effective stress equal to the approximate in-situ values, it was determined that the cracks were closed. These cores were originally hydrate-free sediments. Thus, their absolute permeability was Table 2 Summary of permeability measurements. Core# AT1-C3P (78.5e83.5) 5P (275e282) 8P (135.5e139.5) 12P (1e6) 13P (123.5e128.5) 13P (133e138) 20P (180e183)

Effective water Absolute Absolute perm., perm. (md) perm. (md) under 10 MPa confining pressure/after swelling (md) NA NA 200 NA 47 NA NA

2.6 0.027 620 1500 840 0.027 83

NA/0.26 NA/0.019 390/320 710/800 NA NA 34/29

considered to be the initial effective water permeability in the reservoir because the specimens were initially fully saturated with water. For silty cores (AT1-C-3P (78.5e83.5) and AT1-C-20P (180e183)), absolute permeability was estimated to be from md to tens of md and decreased with increasing ratios of clay and silt. For sandy sediments (AT1-C-8P (135.5e139.5), AT1-C-12P (1e6), and AT1-C-13P (123.5e128.5)), absolute permeability increased to a maximum of 1.5 d. The grain diameter and sand ratio were over 100 mm and 60%, respectively. These cores were originally hydratebearing sediments. Thus, in the tests, the absolute permeability was measured by fully saturating in water after hydrate dissociation. Figure 3 shows the logelog plot between absolute permeability and median grain diameter (d50) for all test samples. Absolute permeability was closely correlated with median grain diameter. The fitted curve is

  k ¼ 7:55  105 d3:31 R2 ¼ 0:987 ;

(1)

where k and d are absolute permeability (md) and the grain diameter (mm), respectively. Generally, the relation between permeability and grain size suggests that permeability scales with the square of the grain size (e.g. Shepherd, 1989). However, the exponent in (1) is higher than that in previous studies. Although we are unsure of the exact cause, the absolute permeability of the hydrate-dissociated sediments may be higher than that of common sandy sediments.

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The analysis of the relation between P-wave velocity and hydrate saturation shows that in this deposit, the pore space hydrate morphology is load-bearing (Konno et al., in this issue). This suggests that the hydrate crystals behave as sediment grains and share the sediment load. This causes uncommonly high porosity in the host sediment compared to the porosity commonly expected in sandy sediments. In the adopted test procedure, the effective stress was constant during hydrate dissociation. Thus, the porosity of hydratebearing sediments was high even after hydrate dissociation. Consequently, the absolute permeability of the hydrate-dissociated sediments would be higher than the general trend. Figure 4 shows the relation between absolute permeability and porosity. For comparison, the effective water permeability of the hydrate-bearing sediments was plotted against the effective porosity determined by multiplying the porosity by the proportion of pore space not occupied by gas hydrate (1Sh, where Sh is hydrate saturation). It was found that the absolute permeability of the hydrate-free sandy sediments and the effective water permeability of hydrate-bearing sediments plotted along the same positive trend against porosity and effective porosity. The comparison suggests that the permeability of sandy sediments can be explained by the effective porosity in spite of the presence or absence of hydrate. The results strongly suggest that the hydrate is load-bearing and behaves as sediment grains.

3.2.3. Effective water permeability of the Eastern Nankai Trough The effective water permeability of the Eastern Nankai Trough is critical in analyzing the gas production behavior of the world's first offshore experiment in this location in 2013. In this section, we compare the measured effective water permeability to experimental results and wireline logging data from the Eastern Nankai Trough. Figure 5 shows the relation between the effective water permeability and hydrate saturation. Laboratory experiments were a pressure relaxation test using the pressure-preserved cores with LN2 treatment (Yoneda et al., in this issue, using the same sample preparation procedure as outlined here) and two different flooding tests using pressure-preserved cores without pressure release and LN2 treatment (Priest et al., in this issue; Santamarina et al., in this issue). The experiments were conducted independently using different test apparati, although all samples were recovered from the same well (AT1-C). In addition, in situ estimates by wireline pressure testing using a XPT PressureXpress tool (Schlumberger Ltd.) were compared (Fujii et al., in this issue). The logging test was conducted at well AT1-MC located approximately 20 m southwest of well AT1-C. It was confirmed that the sediments around wells AT1-C and AT1-MC were correlated (Fujii et al., in this issue).

Figure 3. Relationship between absolute permeability of hydrate-free sediments and particle diameter of sediments.

Figure 4. Relation between absolute permeability and porosity. Circles, squares, and triangles are for clayey, silty, and sandy cores, respectively. For hydrate-bearing cores shown by black triangles, the effective permeability was plotted against the effective porosity determined by porosity  (1  Hydrate saturation).

The comparison suggests that the laboratory experiments agree with the data from hydrate-bearing sediments. The effective water permeability of the hydrate-bearing sediments was in the range of 1e100 md and showed weak negative correlation with hydrate saturation. Wireline pressure tests showed a similar range of values for hydrate-bearing sediments; however, a negative correlation between effective water permeability and hydrate saturation was not observed. In the hydrate-free clayey sediments, all laboratory experiments produced data in the range of 0.01e0.1 md. In contrast, the wireline pressure data were higher than the laboratory experimental data and ranged between 0.1 and 10 md. In principle, in situ estimates are easily affected by the conditions of the borehole walls and represent average values owing to spatial resolution problems. The wireline pressure test data probably represent the average effective permeability of the hydrate-bearing and hydrate-free sediments because the hydrate reservoir at this location consists of relatively thin alternating sand- and clay layers. Hence, it is reasonable to conclude that the wireline pressure test data are within the range of the laboratory experiments.

Figure 5. Relation between initial effective water permeability and hydrate saturation at the Eastern Nankai Trough. Laboratory experiments include a pressure relaxation test using pressure-preserved cores treated with LN2 (Yoneda et al., 2015, same sample preparation) and two different flooding tests using pressure-preserved cores without pressure release and LN2 treatment (Priest et al., 2014; Santamarina et al., 2015). All samples were recovered from the same well (AT1-C). Wireline pressure testing using a XPT PressureXpress tool (Schlumberger Ltd.) was conducted at well AT1-MC located approximately 20 m southwest of well AT1-C (Fujii et al., 2015). It was confirmed that the lithology of sediments around wells AT1-C and AT1-MC correlated.

Please cite this article in press as: Konno, Y., et al., Permeability of sediment cores from methane hydrate deposit in the Eastern Nankai Trough, Marine and Petroleum Geology (2015), http://dx.doi.org/10.1016/j.marpetgeo.2015.02.020

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The self-consistency of the laboratory experiments supports the measured effective water permeability. As mentioned in the Introduction, the effective water permeability of the hydrate-bearing sediments was in the range of 0.01e10 md in past studies. However, the comparison suggests that the hydrate-bearing sediments in this location are more permeable than previously thought. The range of 1e100 md for the hydrate-bearing sandy sediments is promising for gas production. The results can be used to adjust modeling parameters used in logging estimations (e.g., NMR logging). 3.2.4. Expected effects on permeability by depressurization-induced gas production Depressurization is considered to be a promising method for the production of gas from hydrate-bearing sandy sediments. It was applied in the world's first offshore experiment conducted in the Eastern Nankai Trough and has been important in permafrost production tests (Yamamoto et al., 2014; Dallimore et al., 2012). Depressurization will also be used for future long-term production tests in gas hydrate provinces. However, there is a concern that reduction in permeability could be caused by changes to the properties of reservoir during production. The most significant potential side effect is sediment consolidation. A host reservoir of methane hydrate generally consists of high-porosity, unconsolidated sediments because methane hydrate exists in shallow sediments on the sea bottom. Owing to the increase in effective stress by applying depressurization, these sediments are easily consolidated after depressurization-induced hydrate dissociation. It is well known that reduction in porosity caused by sediment consolidation results in a reduction of permeability. The second problem is the generation of fresh water by hydrate dissociation. The initial salinity of pore water in hydrate-bearing sediments is similar to that of seawater when the fluid conduit is an open system. However, once hydrate dissociation begins, the salinity of pore water decreases as a result of fresh water that originates in the hydrate crystal. The decrease in salinity causes swelling of the clay minerals and increases the potential for reduced permeability (e.g., Mohan et al., 1993). To evaluate the potential effects on permeability during depressurization-induced gas production, changes in absolute permeability were analyzed after increasing the effective stress and during flooding with distilled water. The results indicated that both increasing the effective stress and fresh water generation reduced permeability (Table 2). However, the effects were different for sediment lithology. The absolute permeability of sandy sediments was approximately half the original values. In contrast, flooding with distilled water had no significant effect on sandy sediments. However, it caused large reductions in permeability for silty and clayey sediments. In particular, the absolute permeability of the silty core (AT1C-3P (78.5e83.5)), decreased by one order of magnitude. The analysis of mineral composition in recovered cores showed that smectite clay minerals, which are expanding clay, were observed in the range from 3 to over 10 weight percent (Egawa et al., in this issue). Their concentration increased with decreasing grain diameter. The results indicate the potential for permeability reduction due to the swelling effect of expanding clay minerals in fine grained sediments. 4. Conclusions Using core-flooding tests, we measured the effective water permeability and absolute permeability in natural sediment cores obtained from a methane hydrate reservoir at the Eastern Nankai Trough. The findings of this study are summarized as follows:

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pressurized gas atmosphere, the level of remaining hydrate saturation was comparable to that of the pressure cores. This process is a valid procedure for handling hydrate-bearing cores when necessary to release pressure. (2) The effective water permeability for a highly-saturated cores of hydrate-bearing sandy sediments was 47 md (hydrate saturation 70%). The result was consistent with other data obtained from pressure-relaxation tests of LN2 cores (Yoneda et al., in this issue) and flooding tests of pressure cores conducted without pressure release (Priest et al., in this issue; Santamarina et al., in this issue). Furthermore, experimental data were comparable to the values estimated from pressure testing conducted using a wireline-logging tool (Fujii et al., in this issue). These analyses showed that the effective water permeability in hydrate-bearing sandy sediments was in the range of 1e100 md, which is 2e3 orders of magnitude higher than conventional estimates. The results indicate that hydrate-bearing sandy sediments at this location appear to be more permeable than previously thought. (3) Absolute permeability was estimated to be tens of md for clayey sediments, tens of md for silty sediments, and up to 1.5 d for sandy sediments. The data in logelog plots exhibited a strong correlation between absolute permeability and the median grain diameter of sediments. The absolute permeability of the hydrate-free sandy sediments and the effective water permeability of the hydrate-bearing sediments plot along the same positive trend with respect to porosity and effective porosity. This suggests that the permeability of the sandy sediments can be explained by its effective porosity in spite of the presence or absence of hydrates. The results also strongly suggest that the hydrate is load-bearing and behaves as sediment grains. (4) Depressurization-induced gas production has the potential to reduce absolute permeability as a result of high effective stress and fresh water generation. Absolute permeability of sandy sediments decreased to approximately half of the original level due to high effective stress of 10 MPa. Fresh water originating in hydrate crystals has the potential to reduce permeability, especially for silty and clayey sediments that contain swelling clay minerals, such as smectite. The data for permeability can be utilized for future reservoir modeling and simulation. The results of this study indicate that the permeability condition seems promising for achieving depressurization-induced gas production at this location. However, there are concerns about potential long-term damage to the reservoir resulting from production tests. Acknowledgments This study was financially supported by the Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium) to carry out Japan's Methane Hydrate R&D Program conducted by the Ministry of Economy, Trade and Industry (METI). The authors gratefully acknowledge them for the financial support and permission to present this paper. The authors thank all members of the shipboard team during the drilling campaign in 2012. The authors also thank W.F. Waite for fruitful discussions and T. Uchiumi, I. Ikeda, K. Shinjou, H. Haneda, E. Fukami, S. Izumi, O. Nishimura, H. Kaneko, S. Kimura, H. Minagawa, and S. Narukama for their technical support. Appendix

(1) The pressure and cooling/freezing history of cores has a significant impact on the sample texture and the amount of remaining hydrate. After applying freezing under a

Large scale X-ray images and photos of test samples are shown below.

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