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ANALYZING ELECTRICAL ANISOTROPY IN GAS HYDRATE RESERVOIRS USING LOGGING-WHILE-DRILLING DIRECTIONAL RESISTIVITY DATA FROM THE GULF OF MEXICO GAS HYDRATE

JOINT INDUSTRY PROJECT

Ann Cook* National Research Council/National Energy Technology Laboratory Methane Hydrate

Postdoctoral Fellow at Lamont-Doherty Earth Observatory USA

John Rasmus, Qiming Li and Keli Sun

Schlumberger USA

Barbara Anderson

Consultant USA

Timothy Collett

US Geological Survey USA

Dave Goldberg

Lamont-Doherty Earth Observatory USA

ABSTRACT We present new results on the electrical anisotropy of gas hydrate reservoirs from logging data collected during the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II. We focus on two flat-lying, sand-dominated reservoirs in Walker Ridge Block 313 (WR313) and Green Canyon Block 955 (GC955) of the Gulf of Mexico that contain high saturations of gas hydrate. In addition, we also present preliminary results for a fine-grained interval at Site GC955 where gas hydrate occurs as fill in near-vertical fractures. Using a new logging-while-drilling directional resistivity tool and a one-dimensional inversion developed by Schlumberger, we resolve the resistivity of the current flowing parallel to the bedding, R|| and the resistivity of the current flowing perpendicular to the bedding, R⊥. In the gas-hydrate bearing sands at Sites GC955 and WR313, R|| is between 2 to 30 Ωm, and R⊥ is generally an order of magnitude higher. The resolved R|| and R⊥ suggest the gas hydrate-bearing sand reservoirs are highly anisotropic. The anisotropy is caused by gas hydrate chiefly forming in thin layers, on the order of 10 to 100 cm thick, interbedded with equally thin layers of sediment with little to no gas hydrate present.

Keywords: gas hydrate reservoirs, electrical anisotropy, JIP Leg II, Gulf of Mexico

* Corresponding Author: Phone: 1-845-365-8796; Fax: 1-845-365-3182; Email: acook@ldeo.columbia.edu

Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom, July 17-21, 2011.

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NOMENCLATURE n Saturation exponent [dimensionless] Ro Resistivity of water-saturated sediment [Ωm] Rt True resistivity [Ωm] R|| Resistivity parallel to a bedding plane [Ωm] R⊥ Resistivity perpendicular to a bedding plane

[Ωm] RWL Resistivity of the water-saturated layers [Ωm] RHL Resistivity of the hydrate-filled layers [Ωm] Sh Hydrate saturation, as a fraction of φ [m3/m3] VWL Volume of the water-saturated layers [m3/m3] φ Porosity, as fraction of pore space in the

sediment system [m3/m3] GAS HYDRATES AND ELECTRICAL ANISTROPY Natural gas hydrate is an electrical insulator, and thus resists the flow of electric charge [1]. This characteristic of natural gas hydrate has been frequently used to assess the amount of gas hydrate in the sediment pore space, φ, using well logging tools that measure either resistivity or conductivity [2-7], with the premise that the higher the measured resistivity, the larger the amount of gas hydrate occupying the pore space [8]. This assumption, based on Archie’s formulation, quantifies hydrate saturation, Sh, based on the ratio of the resistivity of water-saturated sediment, Ro, to the measured resistivity, Rt. Here we use the simplified Archie quick look equation:

(1)

which involves estimating Ro [8]. The saturation exponent, n, calibrates the resistivity ratio. Further description of the application of Archie’s equation in gas hydrate environments can be read in [2-7]. The measured resistivity is not solely dependent on the amount of hydrate occupying the pore space. One other significant factor controlling the measured resistivity is the orientation of the resistivity measurement with respect to the formation bedding planes. Consider the thin-layered cube shown in Figure 1, which is similar to layers of flat-lying sediment. For the purpose of this example, we assume the grey, water-saturated layers have a resistivity, RWL, of 1 Ωm, and the white, gas hydrate-filled layers have a resistivity, RHL, of 150 Ωm. Half of the cube volume is filled

with the water-saturated layers, VWL, and the other half of the cube is filled with the hydrate-saturated layers (1-VWL). If electrodes are placed on opposite ends of the cube as marked with the red arrows, the current flows parallel to the layers, R||, and may be represented by the harmonic mean:

(2)

Using the prescribed resistivity values above, R|| for the cube is approximately 2 Ωm. In contrast, if the electrodes are placed at opposite ends of the cube as marked with the blue arrows, the current flows perpendicular to the layers, R⊥. R⊥ may be calculated using the arithmetic mean:

(3) R⊥ for the cube is approximately 76 Ωm. Clearly, the direction resistivity is measured across the cube strongly influences the measured resistivity value. When the measured resistivity depends on the direction of the measurement, electrical anisotropy is present. Electrical anisotropy as related to logging measurements, as well as Equations 2 and 3, are well documented in the literature [e.g., 9-12].

Figure 1: An idealized reservoir composed of thin layers of water-saturated sediment (grey) and hydrate-saturated sediment (white). Electrodes placed on the ends of the cube marked with red arrows measure the current flowing parallel to

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layers, R|| (Equation 2). Electrodes placed on the ends of the cube marked with blue arrows measure the current flowing perpendicular to the layers, R⊥ (Equation 3). LOCATION AND MEASUREMENT The Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (JIP Leg II) drilled at three locations in the Gulf of Mexico with logging-while-drilling (LWD) tools, exploring for gas hydrate in reservoir sands [13]. Significant accumulations of gas hydrate in sand sediment were discovered at two of these Gulf of Mexico sites: Green Canyon Block 955 (GC955) [14] and Walker Ridge Block 313 (WR313) [15]. In addition, gas hydrate was discovered as filling within near-vertical fractures in fine-grained sediment at both sites. The PeriScope1 LWD tool was added to the JIP Leg II bottom-hole assembly to assess the electrical anisotropy of these gas hydrate reservoirs [16]. PeriScope collects a number of transverse and axial directional electromagnetic measurements that contain information about R||, R⊥, formation dip and azimuth [17, 18]. Using a 1D inversion developed by Schlumberger that employs a layer-cake formation model, the PeriScope measurements can resolve R|| and R⊥, formation dip and azimuth with a high level of confidence [18]. In this paper, we present new results from this inversion of the PeriScope measurements collected during JIP Leg II, with focus on flat-lying sand reservoirs. INVERSION RESULTS Figures 2 and 3 display the results of the inversion for the gas hydrate-filled reservoir sands for Holes GC955-H and WR313-H. We report values for R||, R⊥ and the inversion confidence in tracks 2 and 3 of each figure. The inversion confidence is presented on a scale from 0 to 1, with 1 being excellent confidence [18]. Although dip and azimuth are resolved through the inversion, they are not particularly informative in the relatively flat-lying sand beds and are not reported in this paper.

1 Mark of Schlumberger.

Figure 2. Periscope inversion results for Hole GC955-H. Depth in meters below seafloor. The first track shows conventional caliper and gamma ray logs. The second track displays the inversion results for R|| and R⊥. The third track shows the inversion confidence level, with 1 being the highest confidence.

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Figure 3. Periscope inversion results for Hole WR313-H, with depth in meters below seafloor. The first track shows the measured caliper and gamma ray logs. The second track displays the inversion results for R|| and R⊥. The third track shows the inversion confidence, with 1 being the highest confidence.

Hole GC955-H Hole GC955-H was drilled in a late Pleistocene sand channel complex just off the eastern edge of the Sigsbee escarpment in the Green Canyon Block 955 in the Gulf of Mexico [14]. The water depth in this area is about 2 km. In Hole GC955-H, the channel sand appears from 390 to 490 mbsf (meters below seafloor). Gas hydrate occurs in parts of the channel: a thick section between 413.5 and 440.5 mbsf, a small section between 445 and 447 mbsf and a thin bed near 449.5 mbsf. The total gross thickness of hydrate in sand in Hole GC955-H is 29 m [14]. In Figure 2, the sections of the log containing gas hydrate show an in-gauge borehole (caliper log is close to the 8.5-in bit size), suggesting the logging measurements are of good quality in those sections. This improves the likelihood for success in the inversion. Figure 2 also displays the results of the inversion for Hole GC955-H, originally presented in Sun et al [18]. Above and below the hydrate-bearing sand, the water saturated sand has R|| values between 0.7 and 1.2 Ωm. In the water-saturated sand, R⊥ is typically 1 to 2 Ωm greater than R||, as expected because of the slight anisotropy in horizontally deposited layers [18]. In the hydrate-bearing sand, R|| values range between 4 and 20 Ωm. R⊥ is approximately an order of magnitude higher in the hydrate-bearing sand, with values ranging from 20 to 220 Ωm. The inversion confidence for R|| and R⊥ are both high, suggesting accuracy in these inversion results. Hole WR313-H McConnell and Kendall [19] hypothesized that gas hydrate occurs in sand layers at Gulf of Mexico Walker Ridge Block 313 because the seismic sections show a phase reversal in sand layers above and below the base of gas hydrate stability. Drilling at Site WR313 validated this hypothesis. Figure 3 shows the gas hydrate-bearing sand layer and the Periscope inversion results for Hole WR313-H. The gamma ray log indicates two sand layers occurring between 805.5 and 810 mbsf and between 811.5 and 818.5 mbsf, for a total gross thickness of 11.5 m of hydrate-bearing sediment [15]. In the clay-rich sediment surrounding this sand layer, R|| values are slightly less than 2 Ωm, and R⊥ ranges from 2 to 3 Ωm. In the hydrate-bearing sands, R|| ranges from 2 to 30 Ωm, and R⊥

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ranges from 7 to 240 Ωm. The lowest values for R⊥ coincide with the lowest value of confidence (0.5), suggesting that R⊥ may be inaccurate and potentially higher in this interval.

Figure 4. Several different types of measured resistivity from Hole GC955-H: propagation resistivity (Track 1), ring resistivity (Track 2) and button resistivity (Track 3). The inversion R||/R⊥ results are also shown in Track 2. Depth in meters below seafloor.

DISCUSSION Formation of hydrate in thin layers In both holes, the large separation between R|| and R⊥ may seem surprising because they both occur in thick sand layers, which could have relatively isotropic gas hydrate saturations. The PeriScope inversion results, however, show significant separation between R|| and R⊥, suggesting that gas hydrate forms in thin layers within the sand. Ring resistivity and button resistivity measurements in both holes support this interpretation (Figures 4 and 5). The ring resistivity and button resistivity logs recorded thin high-resistivity layers, which vary in thickness from 10 to 100 cm. The vertical resolution of the ring and button resistivity measurements is about 7 cm [20, 21], generally less than the minimum thickness of these observed layers. No thin layers are evident in the gamma ray logs within hydrate-bearing sands in Holes WR313-H and GC955-H (Figures 2 and 3). The term “sand” is used to describe the hydrate-bearing sediments in both Holes WR313-H and GC955-H because the sediment has a lower natural radiation than other sediments in the hole, and thus is considered to have less clay content. The vertical resolution of the gamma ray measurement is 15 cm [16]. Thus, thin layers of sand/clay or sand/silt may be present but remain below the log resolution and cause the significant anisotropy. An alternative interpretation is that these sediments are composed entirely of sand grains, but form in layers with variable grain sizes and grain sorting. Thin layers with different grain size typically cause variations in permeability and capillary pressure which, in turn, would affect both the migration of gas and the formation of gas hydrate within larger pore spaces [22, 23]. Both interpretations—presence of thin clay layers or variable sorting of sand grain size within these gas hydrate reservoirs—could cause the observed R||/R⊥ anisotropy, but cannot be distinguished from these data.

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Figure 5. Several different types of measured resistivity from Hole WR313-H: propagation resistivity (Track 1), ring resistivity (Track 2) and button resistivity (Track 3). The inversion R||/R⊥ results are also shown in Track 2. Depth in meters below seafloor. Which is the “best” resistivity measurement? Considerable discussion persists about values for Archie parameters a, m and n, which are used to calculate the resistivity of water-saturated

sediment and hydrate saturation [e.g. 1, 6, 24, 25]. These parameters can greatly affect the calculated hydrate saturation. In this analysis, however, we consider the significance of “true resistivity” or Rt in Equation 1. In practical terms, Rt does not necessarily equal the measured formation resistivity because different resistivity measurements using different sensors record significantly different resistivity values. In addition, environmental factors can significantly affect the measured resistivity. Figures 4 and 5 display a selection of measured resistivity logs from Holes WR313-H and GC955-H. The first track in these figures shows a selection of propagation resistivity logs that represent the maximum and minimum measured values. Propagation resistivity tools measure the change in phase (P) and attenuation (A) of an electromagnetic wave to determine formation resistivity. Measurements are made at 2 MHz (H) and 400kHz (L) frequencies and at a variety of receiver spacings ranging 16 to 40 in. All of the variables that describe the particular log measurement are used in its heading in each figure track. For example, the A40H log is a 2 MHz attenuation log with source and receiver spacing of 40 in. For petroleum exploration, low-frequency attenuation resistivity is often used for Rt, such as the A16L log, for example, because it penetrates deep into the formation (up to 1.5 m) and is minimally affected by borehole rugosity and/or the invasion of drilling fluids. Invasion of conductive or resistive drilling fluids into a permeable formation can dramatically alter the resistivity measurements, but neither Holes WR313-H or GC955-H show the characteristic separation of propagation resistivity and/or button resistivity curves that would be suggestive of this effect [e.g., 11, 12]. Furthermore, the intervals in these wells that contain hydrate-bearing sands exhibit smooth and in-gauge boreholes, nearly equal to the 8.5-in. bit size (see Figures 2 and 3). Therefore, it is unlikely that the measured resistivity logs are affected by either invasion or borehole shape for these sections in Holes WR313-H and GC955-H. In these sands, the presence of hydrate may indeed have restricted permeability and drilling fluid invasion as well as strengthened the formation and reduced spalling of the borehole walls.

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Figure 6. Gas hydrate saturation calculated using Archie’s quick look method (Equation 1) in Hole GC955-H using the A16L resistivity log (blue curve) and the ring resistivity log (red curve) for Rt. Depth in meters below seafloor. In Figures 4 and 5, we observe that the average of the propagation resistivity measurements is effectively equal to R||. That result is expected in an environment where layers are at or below the resolution of the propagation resistivity

measurement [18]. The vertical resolution of the propagation resistivity measurements is from 50 to 180 cm for attenuation resistivity measurements and from 20 to 30 cm for phase resistivity measurements. Thus, the propagation resistivity logs in the sand sections are dominated by the resistivity of the more conductive layers, similar to the R|| measurement in the cube model (Figure 1). The vertical resolution of propagation resistivity measurements is poor in comparison to the ring resistivity or the button resistivity measurements. These electrode-type LWD resistivity measurements can resolve beds as thin as 7 cm. The ring measurement is often used to represent Rt in gas hydrate environments because it has the finest vertical resolution for its depth of investigation (~18 cm) [5, 6, 16]. The deep, medium and shallow button resistivity measurements have depths of investigation of 13 cm, 8 cm and 3 cm, respectively [21]. In Holes WR313-H and GC955-H, the ring resistivity is nearly equal to the button measurements and resolves highly resistive layers with thicknesses, on average, between 10 and 100 cm. The ring and button resistivity logs fall between R⊥ on the high end and R|| on the low end, as expected because of the thin layering in Holes GC955-H and WR313-H [18]. Figures 6 and 7 show calculated gas hydrate saturation from the Archie quick look method (Equation 1) using ring resistivity and the A16L resistivity log for Rt in both holes. We use a constant value of Ro=0.9 Ωm in Hole GC955-H and Ro=1.5 Ωm in Hole WR313-H, based on the resistivity of water-saturated sediment in each hole. The value n = 2 was used for the saturation exponent in both holes. Within the hydrate-bearing sand sections in Holes GC955-H and WR313-H, using the ring resistivity for Rt best resolves the thin hydrate-filled layers. Outside these sections, however, the ring resistivity log may not be the ideal choice, such as in the sections of each hole containing gas hydrate-filled fractures.

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Figure 7. Gas hydrate saturation calculated using Archie’s quick look method (Equation 1) in Hole WR313-H using the A16L resistivity log (blue curve) and the ring resistivity log (red curve) for Rt. Depth in meters below seafloor. Water-saturated sand layers? In Figure 1, we introduced an idealized cube with thin water-saturated layers and thin hydrate-saturated layers. In Holes WR313-H and GC955-

H, gas hydrate may not be segregated within specific sand layers. In Figures 6 and 7, many of the thin sand layers have significant hydrate saturation, ranging from 60 to 85%. Interbedded with the layers with high hydrate saturation are thin layers (generally 7- to 25-cm thick) that have resistivities between 2 and 6 Ωm and corresponding hydrate saturations of 30 to 50%. These layers may either contain a moderate saturation of gas hydrate or remain fully water saturated with slightly higher bulk resistivity due to the averaging effects across adjacent high-resistivity beds. This effect is similar to the idealized cube model presented in Figure 1 as the difference between the “measured” R|| (2 Ω) and the “actual” RWL (1 Ωm). Alternatively, these layers do contain gas hydrate, but at saturation lower than that calculated using Archie’s formulation. Because some thin layers may have thicknesses close to (or below) the vertical resolution of the ring resistivity tool, it is likely that at least some of them measure greater-than-actual resistivity values, and thus would contain lower hydrate saturation than estimated using Equation 1. Gas hydrate in near-vertical fractures Gas hydrate was identified within fine-grained sediment in both Holes WR313-H and GC955-H. On LWD resistivity images, the gas hydrate appears to form primarily as filling in thin, near-vertical fractures [26, 27]. In Hole WR313-H, a near-vertical fracture set is evident between 175 and 320 mbsf; in Hole GC955-H, fractures occur between 190 and 355 mbsf. The fracture set in each hole has similar azimuthal orientation with respect to true north. An example of a small interval showing gas hydrate-filled fractures appears in Figure 8. Electrical anisotropy significantly influences the measured resistivity in near-vertical gas hydrate-filled fractures, and correspondingly affects the calculated hydrate saturation [28, 29]. In a vertical borehole with flat-lying sediment, most resistivity logging tools effectively measure R||. If the dip of the sediment increases with respect to the borehole, however, the measured resistivity becomes a combination of R|| and R⊥. The greater the dip angle, the more significant the contribution of R⊥ is to the measured resistivity.

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Figure 8. An LWD resistivity image recorded from 341.5 to 346.5 mbsf in Hole GC955-H. The thin, white sinusoid peaking near 344 mbsf is a gas hydrate-filled fracture with a dip of ~75 degrees and a dip direction of southeast. Archie’s formulation was developed at a time when boreholes were vertical and geology was typically layer-cake. Thus the resistivity measured using logging tools was effectively R||. If Archie’s formulation is directly applied to estimate hydrate saturation in the presence of near-vertical fractures, it typically overestimates the amount of gas in place [28, 29]. Resolving R|| and R⊥ in gas hydrate-filled fractures is essential to improving gas hydrate saturation estimates. The original inversion as presented by Sun et al [18] was applied to Holes WR313-H and GC955-H. However the inversion did not resolve the near-vertical fractures as seen on the LWD resistivity images, which have dip angles from 60 to nearly 90 degrees. This is likely caused by the

inversion function and resolution, because the original code was optimized for horizontal beds with low dips, and does not resolve beds thinner than ~ 1 m [18]. Gas hydrate-filled fractures in Holes WR313-H and GC955-H are likely on the order of millimeters to centimeters in thickness, similar to gas hydrate-filled fractures observed during the Indian National Gas Hydrate Program Expedition 01 [5]. To accommodate these thin, near-vertical fractures in the inversion, we amended the inversion process to account of high dip angles and thinner beds. In Figure 9, we show preliminary results from this amended inversion process over a small section of Hole GC955-H. Track 1 displays the measured propagation resistivity logs, which show separation between 440 and 448 mbsf. When propagation curve separation occurs in fine-grained sediments in a vertical borehole, near-vertical hydrate-filled fractures are likely to be present [29]. Track 2 shows the results of the amended inversion. Track 3 displays the orientation of fractures that were handpicked from the LWD resistivity images. These observed fractures have an average dip close to 75 degrees (dots) and generally dip in the southeast dip direction (tails), which is very close to the resolved inverted dips. In Figure 9, R|| has values between 1 and 4 Ωm and R⊥ is between 2 and 40 Ωm. R⊥ remains consistently lower than in the P40H log, however, which peaks just over 500 Ωm at 345 mbsf. The low values for R|| in this section suggest that gas hydrate likely forms mostly within the fractures. A mismatch based on the difference between the measured propagation logs and inversion results is calculated in lieu of the inversion confidence value for the amended process. This mismatch is reported on a percent scale (0 to 100), with 100 being the worst match, and is presented in Figure 9, Track 3. While the mismatch is low, the results should be treated as preliminary and conditional, especially because the gas hydrate-filled fracture environment is not one-dimensional, and inversion code is based on one-dimensional layered model.

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Figure 9. Preliminary inversion results for Hole GC955-H over a small interval containing gas hydrate-filled fractures. Depth in meters below seafloor. Track 1: Measured propagation resistivity logs. Track 2: Preliminary inversion results, with the ring resistivity for comparison. Track 3: Data/inversion mismatch (black line), fracture orientations picked from LWD resistivity images (blue dots and tails), and the orientation used to constrain the inversion (green dots and tails). Dots indicate dip angle (0 to 90 degrees) and

tails indicate azimuth (0 to 360 degrees), with north towards the top of the page.

Figure 10. Selected measured propagation resistivity logs compared with propagation logs reconstructed from R||, R⊥, dip and azimuth from the results of the preliminary inversion (Figure 9). The measured and reconstructed logs closely match. Depth in meters below seafloor.

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We were nevertheless able to verify the preliminary inversion results by reconstructing the propagation resistivity logs based on the resolved model parameters. The reconstructed propagation logs appear in Figure 10, along with the measured propagation resistivity logs. Clearly, there is a close match between the measured and reconstructed logs suggesting the preliminary inversion is resolving realistic values for R||, R⊥, dip and azimuth. SUMMARY In this paper we compute R|| and R⊥ for two hydrate-bearing sand reservoirs in JIP Leg II Holes WR313-H and GC955-H. R|| and R⊥ were obtained through a one-dimensional inversion of directional LWD resistivity data. In both holes, the gas hydrate-bearing sand reservoirs are electrically anisotropic, with values of R|| and R⊥ differing by an order of magnitude. In both holes, the sand reservoirs are composed of thin layers of high-saturation gas hydrate interbedded with thin layers of higher water saturation. We also model R|| and R⊥ values for a short interval containing near-vertical gas hydrate-filled fractures and demonstrate that these features significantly affect the measured resistivity. REFERENCES [1] Pearson, C. F., P. M. Halleck, P. L. McGuire, R. Hermes, and M. Mathews (1983), Natural gas hydrate deposits: a review of in situ properties, The Journal of Physical Chemistry, 87(21), 4180-4185. [2] Paull, C. K., R. Matsumoto, and P. J. Wallace (1996), Gas hydrate sampling on Blake Ridge and Carolina Rise. Proceedings of the Ocean Drilling Program, Intial Reports Leg 164, College Station, TX. [3] Hyndman, R. D., T. Yuan, and K. Moran (1999), The concentration of deep sea gas hydrates from downhole electrical resistivity logs and laboratory data, Earth and Planetary Science Letters, 172(1-2), 167-177. [4] Collett, T. S. (2001), A review of well-log techniques used to assess gas-hydrate-bearing reserviors, in Natural Gas Hydrates: Occurrence, Distribution and Detection, edited by C. K. Paull and W. P. Dillion, pp. 189-210, American Geophysical Union.

[5] Collett, T. S., M. Riedel, R. Cochran, R. Boswell, J. Presley, P. Kumar, A. Sathe, A. Sethi, M. Lall, and V. Sibal (2008), Indian National Gas Hydrate Program Expedition 01 Initial Reports. Avalilable on DVD. [6]Malinverno, A., M. Kastner, M. E. Torres, and U. G. Wortmann (2008), Gas hydrate occurrence from pore water chlorinity and downhole logs in a transect across the northern Cascadia margin (Integrated Ocean Drilling Program Expedition 311), Journal of Geophysical Research, 113(B8), B08103. [7] Boswell, R., D. Shelander, M. Lee, T. Latham, T. Collett, G. Guerin, G. Moridis, M. Reagan, and D. Goldberg (2009), Occurrence of gas hydrate in Oligocene Frio sand: Alaminos Canyon Block 818: Northern Gulf of Mexico, Marine and Petroleum Geology, 26(8), 1499-1512. [8] Archie, G. E. (1942), The electrical resistivity log as an aid in determining some reservoir characteristics, Petroleum Transactions AIME, 146, 54-62. [9] Anderson, B., I. Bryant, M. Luling, B. Spies, and K. Helbig (1994), Oilfield ansistropy: Its origins and electrical characteristics, Oilfield Review, 6(4), 48-56. [10] Klein, J. D., P. R. Martin, and D. F. Allen (1997), The petrophysics of electrically anistropic reserviors, Petrophysics, 38(3), 25-36. [11] Anderson, B. I. (2001), Modeling and inversion methods for the interpretation of resistivity logging tool response. Ph.D. Thesis. 375 pp, Delft University, The Netherlands. [12] Ellis, D. V., and J. M. Singer (2007), Well Logging for Earth Scientists, Springer; 2nd Edition, Dordrecht, The Netherlands. [13] Boswell, R., T. Collett, M. Frye, D. McConnell, W. Shedd, R. Dufrene, P. Godfriaux, S. Mrozewski, G. Guerin, and A. Cook (2010), Gulf of Mexico Gas Hydrate Joint Industry Project Leg II: Technical Summary. Available online at: http://www.netl.doe.gov/technologies/oil-gas/FutureSupply/MethaneHydrates/JIPLegII-IR/ [14] McConnell, D., R. Boswell, T. Collett, M. Frye, W. Shedd, G. Guerin, A. Cook, S. Mrozewski, R. Dufrene, and P. Godfriaux (2010), Gulf of Mexico Gas Hydrate Joint Industry Project Leg II: Green Canyon 955 Site Summary. Available online at: http://www.netl.doe.gov/technologies/oil-gas/FutureSupply/MethaneHydrates/JIPLegII-IR/ [15] McConnell, D., R. Boswell, T. Collett, M. Frye, W. Shedd, G. Guerin, A. Cook, S.

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Mrozewski, R. Dufrene, and P. Godfriaux (2010), Gulf of Mexico Gas Hydrate Joint Industry Project Leg II: Walker Ridge 313 Site Summary. Available online at: http://www.netl.doe.gov/technologies/oil-gas/FutureSupply/MethaneHydrates/JIPLegII-IR/ [16] Mrozewski, S., G. Guerin, A. Cook, T. Collett, and R. Boswell (2010), Gulf of Mexico Joint Industry Project Leg II: LWD Methods. Available online at: http://www.netl.doe.gov/technologies/oil-gas/FutureSupply/MethaneHydrates/JIPLegII-IR/ [17] Li, Q., D. Omeragic, L. Chou, L. Yang, K. Duong, J. Smits, J. Yang, T. Lau, C.B. Liu, R. Dworak, V. Dreuillault, and H. Ye (2005), New directional electromagnetic tool for the proactive geosteering and accurate formation evaluation while drilling, paper presented at SWPLA 46th Annual Logging Symposium, Paper UU, New Orleans, USA, June 26-29, 2005. [18] Sun, K., D. Omeragic, C. Cao Minh, J. Rasmus, J. Yang, A. Davydychev, T. Habashy, R. Griffiths, G. Reaper, and Q. Li (2010), Evaluation of resistivity anisotropy and formation dip from directional electromagnetic tools while drilling, paper presented at SPWLA 51st Annual Logging Symposium, Paper I, Perth, Australia, June 19-23, 2010. [19] McConnell, D., and B. Kendall (2002), Images of the base of gas hydrate stability, northwestern Walker Ridge, Gulf of Mexico, paper presented at Offshore Technology Conference, Houston, USA Paper: OTC 14103. [20] Bonner, S. D., A. Bagersh, B. Clark, G. Dajee, M. Dennison, J. S. Hall, J. Jundt, J. Lovell, R. Rosthal, and D. F. Allen (1994), A new generation of electrode resistivity measurements for formation evaluation while drilling, paper presented at SWPLA 35th Annual Logging Symposium, Paper OO, Tulsa, Oklahoma. [21] Schlumberger (2006), geoVISION: Resistivity image-while-drilling service. Brochure. [22] Clennell, M. B., M. Hovland, J. S. Booth, P. Henry, and W. J. Winters (1999), Formation of natural gas hydrates in marine sediments 1. Conceptual model of gas hydrate growth conditioned by host sediment properties, Journal of Geophysical Research, 104(B10), 22985-23004. [23] Kleinberg, R. L., C. Flaum, D. D. Griffin, P. G. Brewer, G. E. Malby, E. T. Peltzer, and J. P. Yesinowski (2003), Deep sea NMR: Methane hydrate growth habit in porous media and its relationship to hydraulic permeability, deposit

accumulation, and submarine slope stability, Journal of Geophysical Research, 108. [24] Spangenberg, E. (2001), Modeling of the influence of gas hydrate content on the electrical properties of porous sediments, Journal of Geophysical Research, 106(B4), 6535-6548. [25] Santamarina, J. C., and C. Ruppel (2008), The impact of hydrate saturation on the mechanical, electrical, and thermal properties of hydrate bearing sands, silts and clay, paper presented at 6th International Conference of Gas Hydrates, Vancouver, Canada. [26] Cook, A., G. Guerin, S. Mrozewski, T. Collett, and R. Boswell (2010), Gulf of Mexico Gas Hydrate Joint Industry Project Leg II – Walker Ridge Logging While Drilling (LWD) Operations and Results. Available online at: http://www.netl.doe.gov/technologies/oil-gas/FutureSupply/MethaneHydrates/JIPLegII-IR/ [27] Guerin, G., A. Cook, S. Mrozewski, T. Collett, and R. Boswell (2010), Gulf of Mexico Gas Hydrate Joint Industry Project Leg II – Green Canyon Logging While Drilling (LWD) Operations and Results. Available online at: http://www.netl.doe.gov/technologies/oil-gas/FutureSupply/MethaneHydrates/JIPLegII-IR/ [28] Lee, M.W. and T. Collett, Gas hydrate saturations estimated from fractured reservior at Site NGHP-01-10, Krishna Godavari Basin, India. Journal of Geophysical Research, 114(B07102). [29] Cook, A.E., B. I. Anderson, A. Malinverno, S. Mrozewski, and D. S. Goldberg (2010) Electrical anisotropy due to gas hydrate-filled fractures, Geophysics 75(6): 173-185. ACKNOWLEDGEMENTS We would like to thank the members, planners, co chiefs and participants of the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II as well as Chevron and the US Department of Energy. This material is based upon work supported by the US Department of Energy, National Energy Technology Laboratory, while holding a National Research Council Research Associateship Award, under Award Number DE-FC26-05NT42248. DISCLAIMER This manuscript was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or

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responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.