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In Situ Methane Hydrate Morphology Investigation: Natural Gas Hydrate-Bearing Sediment Recovered from the Eastern Nankai Trough Area Yusuke Jin,*,† Yoshihiro Konno,† Jun Yoneda,‡ Masato Kida,† and Jiro Nagao† †

Methane Hydrate Production Technology Research Group, Research Institute of Energy Frontier, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan ‡ Methane Hydrate Geo-mechanics Research Group, Research Institute of Energy Frontier, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan S Supporting Information *

ABSTRACT: The hydrate morphology of natural gas hydrate-bearing (GH) sediments recovered from the eastern Nankai trough area was investigated under hydrostatic pressurized conditions that prevent dissociation of gas hydrates in a sediment. We developed a novel X-ray computed tomography system and an attenuated total reflection infrared (ATR-IR) probe for use in the Instrumented Pressure Testing Chamber for our set of Pressure-Core Nondestructive Analysis Tools (PNATs), which can measure the sediment structure, primary wave velocity (PWV), density, and shear strength under pressurized conditions. The hydrate saturation values estimated using the ATR-IR absorption bands of H2O molecules strongly correlate with PWV. Assuming homogeneity of hydrate distribution in the planes perpendicular to the sample depth direction, the hydrate morphology of natural GH sediments in the eastern Nankai trough area demonstrated a load-bearing morphology type. The predicted hydrate morphology results are in good agreement with data reported in the literature. The combination of PNATs including ATR-IR spectroscopy can be used to estimate the properties of GH sediments without the release of pressure to atmospheric conditions in order to model gas hydrate reservoirs for natural gas production.

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INTRODUCTION Gas clathrate hydrates (gas hydrates) are clathrate compounds that crystallize by enclosing guest molecules in H2O structures.1 Because gas hydrates are formed under high-pressure, lowtemperature conditions, natural gas hydrates are found in oceanic and permafrost sediments.1,2 Guest molecules in natural gas hydrates are almost exclusively methane (CH4); CH4 obtained from natural gas hydrates is considered to be a potential new unconventional energy resource. Gas production behavior from gas hydrates has already been studied under laboratory conditions. Some methods of gas production from natural gas hydrate reservoirs have recently been reported using large-scale apparatuses.3−6 Several gas production tests on natural gas hydrate reservoirs have been performed. The first test was conducted at the Messoyakha sites (Russia) in 1970,7,8 following which other tests were conducted at the permafrost fields of the Mallik sites (Canada) and the North Slope of Alaska (United States) in 2002 and 2007 and 2011/2012, respectively.9−11 The latest test was performed in 2013 at an offshore field in the eastern Nankai area (Japan).12 The knowledge obtained from natural gas hydrate-bearing (GH) sediments is crucial for understanding natural GH deposit layers and for modeling gas hydrate reservoirs. The coring of natural GH sediments has generally been conducted using a core sampler that maintains in situ pressure conditions in order to avoid dissociation of gas hydrates in the recovered cores.13−16 Until 2004, after the release of the core sampler pressure to atmospheric conditions, © 2016 American Chemical Society

the natural GH sediments were treated with liquid nitrogen (LN2) and were analyzed in onshore laboratories. However, the sediment structure of natural GH sediments is damaged by this pressure release and the LN2 treatment.17−20 Natural GH sediments damaged in such a manner can lead to over/underestimation and misunderstanding of sediment properties.21−23 More recently, pressurized-core analysis systems that can measure pressurized core samples without pressure release and LN2 treatment have been developed.24−26 The Pressure Core Characterization Tools (PCCTs) system, which was established by the Georgia Institute of Technology (Georgia Tech), is an innovative system for measuring the properties of natural GH sediments under pressurized conditions.26 The Instrumented Pressure Testing Chamber (IPTC) included in the PCCTs is a pressure vessel equipped with multiple sensor probe ports that can measure the primary wave (P-wave) velocity (PWV), secondary wave (S-wave) velocity (SWV), shear strength, and resistivity of pressurized GH sediments using various sensors through the probe ports.25,27,28 The properties obtained from the IPTC are crucial for a better understanding of natural GH sediments and the characterization of gas hydrate reservoirs. The Research Consortium for Methane Hydrate Resources in Japan (MH21),29 which initiated Japan’s Methane Hydrate R&D Program [which is now managed by the Ministry of Received: April 1, 2016 Revised: June 21, 2016 Published: June 26, 2016 5547

DOI: 10.1021/acs.energyfuels.6b00762 Energy Fuels 2016, 30, 5547−5554

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The sample used in this study was recovered from 286.65−286.77 mbsf in AT1-C. We called the sample “AT1-C-10P, ” following the Daini-Atsumi Knoll drilling campaign.42 In the Daini-Atsumi Knoll area, instead of nodule- and massive-type hydrates, fine hydrate particles exist in the pore spaces of sandy sediments, the so-called pore-filling type.44 AT1-C-10P (286.65−286.77 mbsf) was recovered from a sand layer in an alternation of sand and mud layers. The grain size of AT1-C-10P was 84.5 μm (median). The index properties of the AT1-C-10P core are listed in Table 1. The grain size distribution of AT1-C-10P is shown in Figure S1 of the Supporting Information.

Economy, Trade, and Industry (METI)] is currently planning a second offshore gas production test in the eastern Nankai area. To aid in this effort, MH21 is developing its own IPTC-based pressurized-core analysis system in strong collaboration with Georgia Tech and the U.S. Geological Survey (USGS). The permeability, stiffness, and thermal conductivity of GH sediments are important characteristics determining the gas production from a gas hydrate reservoir. These properties strongly relate to the gas hydrate morphology in sandy sediments, which is mainly classified into three models: cementing, grain-coating, and load-bearing.30 The P-wave traveling through the GH sediment is one indicator of the stiffness of sediment samples. PWV is determined in part by the hydrate saturation, Sh (%), which is the volume ratio of hydrates in the pores.31−34 As gas hydrate morphology can affect the PWV behavior through alterations in the Sh,35 the morphology can be predicted through the correlation between PWV and Sh. At present, PWV can be measured under pressurized conditions using the IPTC system, whereas Sh in natural GH sediments is estimated directly from the gas volume released by hydrate dissociation and by the hydrate number of the gas hydrate.18,36,37 Because there are few methods for Sh measurement under pressurized conditions, PWV−Sh correlations in pressurized GH sediments have not yet been established. Although the absorption of the O−H stretching infrared (IR) bands of H2O are too strong to measure via a normal IR system, the O−H stretching IR band of H2O molecules measured by attenuated total reflection−IR (ATR-IR) is suitable for monitoring solid−liquid water mixture. Jin et al.38,39 observed hydrate formation behavior and ice formation behavior in pore spaces during hydrate dissociation from the variation of the O− H stretching bands using an ATR-IR system. ATR-IR measurements could be productive for in situ observation of gas hydrates coexisting with liquid H2O, i.e., the pore spaces in GH sediments. In this study, we developed an ATR-IR probe designed for use in the IPTC system to measure the Sh of GH sediments under pressurized conditions. We also developed a large-scale X-ray computed tomography (CT) system for longlength core imaging and a bulk density measurement system for use under pressurized conditions. Furthermore, we demonstrated the nondestructive evaluation of gas hydrate morphology in pressurized natural GH sediments recovered from the eastern Nankai area using this pressurized-core analysis system.

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Table 1. Indexes of the Natural GH Sediment Sample AT1C-10P Used in This Study depth (mbsf) top

bottom

core length (cm)

median grain size (μm)

classification

286.65

286.77

12

84.5

silty sand

Pressure-Core Nondestructive Analysis Tools. Our developed pressure-core nondestructive analysis tools (PNATs) were designed for analyzing hydrostatic pressurized natural GH sediments. The PNATs essentially comprised a manipulator for core-cutting and transferring (PNATs-MAN) and the IPTC (PNATs-AIST-IPTC) systems. The PNATs-MAN and PNATs-AIST-IPTC were installed in a cold room kept at a controlled temperature of 278−283 K. The basic PNATs components were developed in collaboration with Georgia Tech and USGS; these components and the original IPTC/PCCTs are described in the literature.17,18,25,26 We supplemented the alreadydeveloped PNATs components by developing a set of nondestructive analysis tools for pressurized natural GH sediments; these are described below, and their functions are summarized in Table 2. Nondestructive Imaging of Sediment Structure (PNATs-X). The sediment structure of the pressurized natural GH sediment sample was nondestructively measured using an X-ray CT system (PNATs-X, TOSCANER-32300 μFD, Toshiba IT control systems corporation). The PNATs-X was equipped with a flat panel detector (1024 × 1024 pixels) and a microfocus X-ray tube (maximum source voltage and current, 230 kV and 1000 μA, respectively; minimum focus size, 4 μm; white-beam X-rays). To obtain CT images of the pressurized natural GH sediment, the samples were stored in a highpressure vessel made of duralumin (Al alloy containing Cu, Mn, and Mg), using PNATs-MAN. The vessel was contained in a cold box in which the temperature was maintained at approximately 276 K by circulating cold air during CT collection. The inner pressure of the vessel was maintained at 10 MPa using a syringe pump. Collection of the raw CT data of the sample was performed every 0.15° (2400 views) at 180 kV and 600 μA. A schematic of PNATs-X is shown in Figure S2 of the Supporting Information. P-Wave Velocity Profiles (PNATs-AIST-IPTC-PWV). P-wave profiles of the sediment sample were measured using a contact pulse transmission system with piezoelectric PZT ceramic sensors (OYO Co., Japan) through probe ports in the PNATs-AIST-IPTC. A schematic of the P-wave measurement is shown in Figure S3 of the Supporting Information. Details of the P-wave measurements via the IPTC system are described in the literature.25,26 The PWV can be estimated from the arrival time of a P-wave transmitted through the object from an oscillator to a receiver. We estimated the PWV, VP (m/ s), of the core sample using the following equation:

EXPERIMENTAL SECTION

Natural Gas Hydrate-Bearing Sediment. The natural GH sediment sample used in this study was recovered from a coring well (hereafter AT1-C) in the eastern Nankai area during the “DainiAtsumi Knoll” drilling campaign of June−July 2012.40 Core recovery was carried out by the deep sea drilling vessel CHIKYU (owned by the Center for Deep Earth Exploration, CDEX, of the Japan Agency for Marine Earth Science and Technology, JAMSTEC). To avoid gas hydrate dissociation and maintain the sediment structure, the core samples were recovered using a Hybrid PCS core sampler.16 The natural GH sediment samples were transferred to our laboratory (Sapporo, Japan) and stored at 278 K and 15 MPa (hydrostatic pressure). From the logging wired drilling (LWD) and seismic reflection results in the AT1 region,41,42 the methane hydrate concentrated zone (MHCZ) is approximately 277−318 m below seafloor (mbsf). The MHCZ comprises two zones: alternations of sand and mud layers (upper-depth zone) and sand-dominant sequences (lower-depth zone). Further details on the Daini-Atsumi Knoll drilling campaign have been described in the literature.40,42,43

VP = L /(tArrival − t Delay )

(1)

where L, tArrival, and tDelay are the path length of the transmitting Pwave, the arrival time of the transmitted P-wave, and the signal delay in the measurement system, respectively. L was estimated using the installed distance between the P-wave sensors. In our measurement system, tDelay was 1.8 μs. The P-wave profiles were collected from the core at intervals of 1 cm. In general, pressurized natural GH sediments were stored in a plastic liner that protects the recovered sediment (Figure S3); therefore, the core sample was cut to a designated length 5548

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Energy & Fuels Table 2. Function Summary of the Developed PNATs Family PNATs family (PNATs-) MAN X AISTIPTC

PG

-PWV -IR -Gamma -PWV

TACTT SUB

description

note

Manipulating sediment samples for sample cutting in a designated length, sample transfer to high-pressure vessel/PNATs apparatus, and sample moving to change point of measurement by using PNATs apparatus. Microfocus X-ray CT system for measuring 3-dimensional sediment structure. Core analysis system developed with the strong cooperation of Georgia Tech and the USGS. Core properties can be measured using several measurement probes. Use with PNATs-MAN. Probe designed for IPTC. P-wave velocity measurements for estimating hydrate saturation. AIST original probe designed for IPTC. ATR-IR spectra measurement for estimating hydrate saturation. Gamma-ray density system for measuring bulk density of sediment samples. Use with PNATs-MAN Noncontact P-wave velocity measurement system. Use with PNATs-MAN Triaxial testing system for measuring strength, deformation behavior, and/or permeability. Resampling of small core from sediment samples.

ref 18 New development. Basic concept is described in ref 25. New development. New development. New development. Details in the Supporting Information. Under development. ref 18 ref 17

Figure 1. ATR-IR probe developed for IPTC: (a) schematic of the measurement using the ATR-IR probe and (b) image of the edge of the ATR-IR probe. and transferred from the original liner to a dedicated IPTC liner with holes every 1 cm using the PNATs-MAN after the PNATs-X measurement. The P-wave measurement probes were brought into direct contact with the core surface. Infrared Spectral Profiles (PNATs-AIST-IPTC-IR). IR spectra of sediment samples were measured using a Fourier transform IR spectrometer (VIR-200, Jasco Co., Japan) with an indium antimonide (InSb) detector and an ATR-IR system with a probe custom-designed for the IPTC system. A schematic of the ATR-IR measurement process used by the PNATs-AIST-IPTC and an image of the edge of an ATR-IR probe are shown in Figure 1. The ATR-IR probe is a coneshaped diamond crystal (incidence angle θ = 45°) with a surface area of approximately 10 mm2. As done for the P-wave measurement, the ATR-IR probe was brought into direct contact with the core surfaces. The ATR-IR spectra of the samples were measured using 32 scans at the same core positions as the P-wave measurements and at a resolution of 4 cm−1 over an observed range of 2220−5000 cm−1. A background spectrum was collected at atmospheric pressure.

high X-ray attenuation; such regions represent the natural GH sediment. The low gray region outside the core shows the original plastic liner (0.3 cm thickness). The cross-sectional CT image shows that the core sample used in this study comprises three regions: the first (bottom) is 0−3 cm in core length (286.77−286.74 mbsf), the second (middle) 3−6 cm (286.74− 286.71 mbsf) in length, and the third (top) 6−12 cm (286.71− 286.65 mbsf) in length. This separation into three regions was created during the core recovery operation.42 Using the PNATs-MAN, the AT1-C-10P sediment was divided into two pieces (286.77−286.695 mbsf and 286.695−286.65 mbsf). In this study, one piece from 286.77−286.695 mbsf (0−7.5 cm as indicated by the square in Figure 2) was used for measuring the P-wave profiles and ATR-IR spectra. Hereafter, the measured points are described using the core length (0−7.5 cm), as indicated in Figure 2. P-Wave Velocity. Figure 3 shows the measured P-wave profiles of AT1-C-10P (286.77−286.695 mbsf) at 283 K and 10 MPa. The x- and y-axes of Figure 3 show the elapsed time from the P-wave oscillation starting point and the amplitude of the Pwave signal, respectively. Closed circles indicate the arrivals of the first P-wave signals through each measurement point. The arrival times of the P-waves are delayed by the stif f ness of the sample; in general, soft materials show longer delays than hard materials. The first P-waves arriving at 2 cm were much faster

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RESULTS AND DISCUSSION Sediment Structure. Figure 2 shows a cross-sectional Xray CT image of AT1-C-10P (286.65−286.77 mbsf). The spatial resolution in the obtained CT image is approximately 65 μm/pixel. The core length and diameter are approximately 12 and 5.36 cm, respectively. The gray scale in Figure 2 reflects the relative linear X-ray mass attenuation coefficients of each component, with the bright gray regions showing material with 5549

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Table 3. P-Wave and ATR-IR Measurement Results for AT1C-10P (286.77−286.695 mbsf) depth (mbsf)

core position (cm)

actual arrival time (μs)a

wave path length (mm)

P-wave velocity (m/s)

first moment (cm−1)b

hydrate saturation (%)

286.77 286.76 286.75 286.74 286.73 286.72 286.71 286.70

0 1 2 3 4 5 6 7

30.0 25.5 16.4 20.6 24.6 21.0 23.2 24.0

44.6 49.6 51.6 49.6 52.1 53.6 52.6 52.6

1486.7 1945.1 3146.3 2407.8 2117.9 2481.0 2267.2 2191.7

3258.1 3263.6 3275.0 3266.8 3263.7 3270.4 3262.8 3266.5

(2.0) 33.3 98.2 51.5 33.8 72.0 28.7 49.8

a

Actual arrival time was estimated by subtracting the system delay (1.8 μs). bThe first moment values of water and ice Ih are 3257.9 and 3275.6 cm−1, respectively.

cm. There are no such waves at 2−7 cm. The high-amplitude wave at 1 cm is considered to be a surface wave that travels near the sample surface; at this position, there may have been a slight poor contact between the PWV probe and the core sample. In the case of poor probe contact, a surface wave traveling along the water layer near the core surface would be observed in addition to the P-wave throughout the sample. The PWVs estimated using eq 1 are also summarized in Table 3. The estimated PWV at 0 cm is very close to that of water (approximately 1480 m/s).33 The PWVs at 2−7 cm were distributed from approximately 2000 to 3150 m/s. The PWV value of the pure methane hydrate is approximately 3800 m/s,45 and the PWVs of sandy GH sediments depend on the volume of gas hydrates in the pores.46,47 Variation of the PWV would reveal differences in the Sh at each point. Because the PWV at 2 cm has the highest value at approximately 3150 m/s among those in the 1−7 cm region, the sample region at around 2 cm is considered to contain the heaviest hydrate volume in the pores, i.e., the highest Sh. Based on the literature,36 the values of Sh at 2 cm can be predicted to be approximately 30%, 50%, and 95% in the cementing, grain-coating, and load-bearing cases, respectively. ATR-IR Spectra. Figure 4 shows the ATR-IR spectra in AT1-C-10P (286.77−286.695 mbsf) at 283 K and 10 MPa. Considering the ATR window area and the grain size in AT1C-10P, the ATR-IR spectra in Figure 4 would show the average state of H2O molecules over a large amount of pore spaces. For spectral comparison, the ATR-IR spectra of liquid water (283 K and 10 MPa) and ice (atmospheric conditions) are also displayed as blue and light blue lines, respectively, in Figure 4. To measure the ATR-IR spectra of f resh sample surfaces, prior to spectral measurements, we bored a hole (of a depth of approximately 5 mm) through the silt layers covering the core surfaces using a drill included in the PNATs-AIST-IPTC. The ATR-IR bands in the range 2800−3600 cm−1 are designated as the O−H stretching vibration bands of H2O molecules; a change in the shape of these can be seen in Figure 4. In particular, the band shape at 2 cm differs from those at other points. Furthermore, the C−H stretching peak of the CH3group (at approximately 2910 cm−1) can be observed at 2 cm. A weak C−H stretching peak was also detected at 5 cm. The peak observed at around 2910 cm−1 can be assigned to CH4 molecules encaged in the gas hydrate structure. The O−H stretching vibration bands of H2O molecules comprise hydrogen bonded (HBd)/non-HBd symmetric and

Figure 2. Cross-sectional X-ray CT image of AT1-C-10P (286.65− 286.77 mbsf). The core length and diameter are 12 and 5.36 cm, respectively. The core top and bottom are 286.64 and 286.77 mbsf in depth, respectively. The core sample used for the PWV and ATR-IR measurements is shown within the square.

Figure 3. P-wave profiles for AT1-C-10P (286.77−286.695 mbsf). The closed circles at each measurement point indicate the determined arrival times. P-wave measurements were carried out at 278 K and 10 MPa.

than those at other points, as shown in Figure 3. The actual arrival time at 2 cm can be estimated to be 16.5 μs by subtracting tDelay = 1.8 μs from tArrival = 18.2 μs. The actual arrival times at each point are listed in Table 3. The 0 cm in the cross-sectional CT image is considered to be the measurement point for the P-wave of water, as shown in Figure 2. In the Pwave profile at 1 cm, P-waves with high amplitude are observed at 35 μs after the first P-wave arrivals at 27.3 μs. These highamplitude waves are similar to the P-wave profile of water at 0 5550

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Energy & Fuels ν̅ =

∫ ν ln(I0/I )dν/∫ ln(I0/I )dν

(2)

where v, I0, and I are the wavenumber, the intensities of the blank IR spectrum, and the sample IR spectrum, respectively. The first moments were estimated in the wavenumber range 2500−4000 cm−1. The first moments of water and ice can be estimated as approximately 3257.9 and 3275.6 cm −1 , respectively, and a high saturation point is expected to show a high first moment value. Using the first moments of liquid (water) and solid (ice) H2O molecules, we could then establish a correlation between the first moment and the hydrate saturation via ATR-IR, ShIR: ShIR = 0.056941ν ̅ − 185.5

(3)

Equation 3 is valid only for our ATR-IR system; systems with different combinations of ATR windows and detectors will show different correlations between the first moment and the Sh.39 The first moment and the ShIR values at each point were estimated using eqs 2 and 3 and are listed in Table 3. The value of ShIR was widely distributed from 30 to over 90% in the sample, except at 0 cm. It is clear that the measurement points at 2 and 5 cm have high ShIR (or first moment) values. The intensity of the C−H stretching peaks seems to correlate with the ShIR value, as shown in Figure 4. Because the measurement point at 0 cm, a nearly completely water region as described above, the value of ShIR at 0 cm is reasonably close to 0% (estimated value, 2%). Considering the ShIR value at 0 cm, the uncertainty of the ShIR estimation is expected to be ±5%. Hydrate Morphology in Nankai GH Sediment. Figure 5 shows the profiles of the PWV, ShIR (derived from the first moment), density, and porosity in AT1-C-10P (286.77− 286.695 mbsf). Here, the density data were measured using a γ-ray density meter (PNATs-PG-Gamma, PM-1000, Nanogray Inc., Japan). Details of the PNATs-PG-Gamma system are provided in the Supporting Information. The PWV and the ShIR were obtained by line (plane) and spot (surface) measurements, respectively. As shown in Figure 5a,b, there is a strong correlation between the PWV and the ShIR values. The mechanical behavior of the pressurized AT1-C core sample can be well understood by assuming a homogeneous hydrate distribution in the planes perpendicular

Figure 4. O−H stretching ATR-IR bands in AT1-C-10P (286.77− 286.70 mbsf). ATR-IR spectroscopic measurements were carried out at 283 K and 10 MPa.

HBd/non-HBd asymmetric O−H stretching bands. It is generally difficult to decompose the complex O−H stretching band to liquid and solid H2O bands in order to directly estimate the ratio of liquid/solid H2O in the mixture. However, an O−H stretching vibration band observed in a liquid−solid H2O mixture shows an absorption band that is a composite of liquid and solid H2O bands. The shape of the composite absorption band depends on the ratio of liquid−solid H2O areas contacted by the ATR-IR window. As in the ice state, H2O molecules in a hydrate state are bonded to four neighboring H2O molecules by four hydrogen bonds. Therefore, the shapes of the O−H stretching bands of GH sediments depend on the water−hydrate ratio in the pore spaces, i.e., they depend on the hydrate saturation.38,39 We estimated the hydrate saturation by evaluating the band shapes of the ATR-IR spectra, which were evaluated as a first moment (gravity center) of the observed O−H stretching band. The first moment is obtained using

Figure 5. Property profiles for AT1-C-10P (286.77−286.695 mbsf): (a) P-wave velocity, (b) hydrate saturation estimated from the ATR-IR spectra, (c) density obtained using a γ-ray density system, and (d) porosity estimated from the hydrate saturation and density. Each property at 0 cm was obtained from liquid water, not from the sediment. 5551

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Energy & Fuels to the depth direction (hereafter the horizontal planes).18 The AT1-C-10P (286.77−286.695 mbsf) sample is considered to be relatively homogeneous in the horizontal planes (PWV measurement planes), while the PWV and ShIR profiles vary along the depth direction. The bulk density appears to be approximately 1.93−2.01 g/ cm3, except at 0 cm, as shown in Figure 5c. In the AT1-C core samples, the bulk densities of cores taken from the sand and mud layer alternation zones are in the range of 1.8−2.1 g/ cm3.42 The porosity distribution in the core sample was estimated as shown in Figure 5d. The porosity, Φ, can be estimated using the relation ρ = ρs (1 − ϕ) + (ρh ·ShIR + ρw (1 − ShIR )) ·ϕ

the same position.42 Because there have been few reports of sandy natural core samples with high Sh (>90%), this maximum ShIR value might be slightly overestimated, and it will be necessary to obtain a more accurate ShIR estimate to improve eq 3. Sediments with a low Sh value in a load-bearing type may present insignificant inhomogeneity of hydrate distribution in the pores (patchy type). In this case, a slight gap between the prediction and the experimental data for a ShIR value of 30− 50% would be observed owing to the inhomogeneity of the Sh value. The properties of the pressurized natural GH sediment can be deduced using a combination of our PNATs system. The Sh values are widely distributed in the range 30% to over 90% in the AT1-C-10P (286.77−286.695 mbsf), as shown in Figure 5b. Furthermore, our analysis reveals that the pressurized AT1C-10P sample corresponds to a load-bearing GH sediment.

(4)

where ρ, ρs, ρh, and ρw are the bulk, grain (2.65 g/cm ), hydrate (0.911 g/cm3),2 and liquid water densities (1.005 g/cm3),48 respectively. The porosity varies in the range of 38−43%, and the average porosity is ∼40%. The estimated porosity is in close agreement with the porosities of other pressurized AT1-C cores taken from sand and mud layer alternating zones, which are in the range of 35−46%.18,36,49 Unlike the PWV and ShIR variations, the porosity is distributed through a narrow range in the core sample. It is unclear what causes the relationship between the PWV−density (porosity) and the ShIR−density (porosity) in the sample. Figure 6 shows the relationship between the PWV and the ShIR in AT1-C-10P (286.77−286.695 mbsf). Other pressurized 3

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CONCLUSIONS

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ASSOCIATED CONTENT

We report an in situ hydrate morphology estimation of a natural gas hydrate-bearing sediment recovered from the eastern Nankai trough area, with the first gas production test from the hydrate layer carried out on March 12, 2013. As an addition to our Pressure-Core Nondestructive Analysis Tools that can measure the properties of GH sediments without releasing the sample pressure, we developed an X-ray computed tomography system. PNATs-X can deduce sediment structural profiles in natural gas hydrate-bearing sediment cores (maximum length ∼ 1.2 m). We also developed an attenuated total reflection IR probe for the Instrumented Pressure Testing Chamber (PNATs-AIST-IPTC). The hydrate saturation, i.e., the Sh value at a measured point, can be evaluated using ATRIR measurements through PNATs-AIST-IPTC. In addition to ATR-IR measurements, we measured the P-wave velocity of the natural GH sediment using the PNATs-AIST-IPTC system. We found a strong correlation between the Sh estimated by the ATR-IR band and the PWV, in which points with higher PWVs showed higher Sh values. This correlation is in close agreement with the literature data. Assuming that hydrates are homogeneously distributed in planes perpendicular to the sample depth direction, the hydrate morphology of natural GH sediments in the eastern Nankai trough area can be predicted to be of a load-bearing type. The combination of PNATs including ATR-IR spectroscopy could be used to nondestructively evaluate important properties of gas hydratebearing sediments to model gas hydrate reservoirs and predict their gas production behavior.

Figure 6. Correlation between P-wave velocity and hydrate saturation in AT1-C-10P (286.77−286.695 mbsf). Closed circles, this study; open circles, Konno et al.;36 open squares, Santamarina et al.;49 open diamond, Yoneda et al.18 The dashed lines are PWV−Sh correlations predicted from the properties of the AT1-C samples and the hydrate morphology types (red, cementing; black, grain-coating; blue, loadbearing).36 The PWV−Sh correlation at 0 cm is not shown.

AT1-C sample results are also plotted in Figure 6.18,36,49 It is clear that the PWV increases with the Sh value. The PWV−ShIR relationship in this study is consistent with that found in the literature for the ShIR range up to 80%. Konno et al.36 calculated the PWV−Sh correlation for the three hydrate morphology models (cementing, grain coating, and loadbearing) in AT1-C sediments. The predicted PWV−Sh curves are shown as dashed lines in Figure 6. The PWV−ShIR correlation, including the highest ShIR point, is in close agreement with the predicted load-bearing curve. If the Sh in a load-bearing GH sediment approaches 100%, the PWV approaches values corresponding to the cementing and grain coating models. The PWV at the highest ShIR point is approximately 3150 m/s, and tentative P-wave data from CHIKYU have shown a value of approximately 3000 m/s near

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b00762. Grain-size distribution of the AT1-C-10P sample used in this paper and schematics of PNATs-X (X-ray CT system), PWV measurement system through PNATsAIST-IPTC, and PNATs-PG-Gamma (a γ density measurement system) (PDF) 5552

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Address: 2-17-2-1, Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan. Tel: +81-11-8578526. Fax: +81-11-857-8417. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by funding from the Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium), planned by the Ministry of Economy, Trade, and Industry (METI), Japan. The authors thank Dr. K. Egawa of INPEX Corporation, Dr. T. Ito of Research institute of Innovative Technology for the Earth (RITE) and Dr. H. Haneda, Mr. K. Shinjo, Mr. T. Uchiumi, and Me. H. Kaneko of AIST for their experimental support. The authors also thank all the members of the shipboard team of the drilling campaign “Daini-Atsumi Knoll” in June−July 2012.

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