International Conference on Gas Hydrates (ICGH) (6th : 2008)


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Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  __________________________ Corresponding author: Phone +1 845 365 8796 E-mail:   STRESS AND GAS HYDRATE-FILLED FRACTURE DISTRIBUTION, KRISHNA-GODAVARI BASIN, INDIA  Ann Cook and David Goldberg Borehole Research Group Lamont-Doherty Earth Observatory of Columbia University 61 Rt. 9W, Palisades, New York USA   ABSTRACT In this research, we study high resistivity fractures found in unconsolidated clay sediments on logging-while-drilling borehole resistivity images from Indian continental margin collected during the National Gas Hydrate Program Expedition 01. These fractures, found at Sites 5, 6, 7, and 10 are likely filled with natural gas hydrate. Gas hydrate is identified on borehole logs and images as high resistivity responses without associated density increases or indications of free gas. The local state of stress at the time of fracturing can be determined by fracture orientations. In Holes 5A, 5B, 6A an 7A the gas hydrate-filled fractures have an aligned, preferred orientation likely associated with a local stress regime. At Site 10, where 130 m of gas hydrate-filled fractures were observed, fracturing is chaotic, likely due to high gas flux.  Keywords: fractures, resistivity images, India  NOMENCLATURE a Tortuosity coefficient m Cementation exponent Ro Predicted water saturated resistivity [ohm*m] Rt Measured resisivity [ohm*m] Rw Resistivity of pore water [ohm*m] ϕ Density porosity [%]  INTRODUCTION The Indian National Gas Hydrate Program (NGHP) Expedition 01 sailed during the summer of 2006 with the goal of characterizing the presence of natural gas hydrate on India’s continental margins.  Natural gas hydrate was identified at a most of the 21 drill sites.  Gas hydrate was discovered in silt, sand, and ash at a few sites, but most of the gas hydrate occurred in clay-dominated sediments.  Pressure cores from several NGHP-01 sites revealed gas hydrates in clay-dominated sediments forms complex fracture plane networks, with both high-angle vertical fractures as well as horizontal features [1]. Logging-while-drilling (LWD) resistivity images from Holes 5A, 5B, 6A, 7A and 10A in the Krishna-Godavari basin also depict gas hydrates residing in fracture planes.   Figure 1.  Map of LWD fracture Sites 5,6,7, and 10 in the Krishna-Godavari basin, eastern Indian continental margin.  In this paper, we analyze and discuss gas hydrate- filled and conductive fractures appearing on LWD resistivity images in Holes 5A, 5B, 6A, 7A and 10. We determine fracture orientation and shallow sediment stress orientations for each hole.  DETERMINING HYDRATE BEARING SEDIMENTS High porosity water saturated clay sediments from the top of the marine sediment column typically exhibit resistivities of ~1 ohm*m.  The addition of gas hydrate, an electrical insulator, increases the measured resistivity.  Thus, to determine which sections of a log are hydrate bearing, we calculate the predicted water saturated resistivity (i.e. the resistivity of the sediments with no hydrate present), Ro, and compare it to the measured in situ resistivity, Rt.  Ro is defined as:  ! R o = aR w "m  The resistivity of the pore water, Rw is calculated using a combination of downhole temperature and salinity. Density porosity, ϕ, is calculated using pore water density, the in situ density log, and grain density measurements from the cores.  The values a and m are determined by selecting sections of the log believed to be water saturated, and matching Ro with Rt.  This method was originally developed by G.E. Archie to determine saturations of oil or gas residing in the pore space of rocks [2].  While Archie’s relationship commonly used to calculate hydrate saturation, it is not applicable in environments where gas hydrate does not primarily occupy the pore space, for example, environments where gas hydrate resides as fill in fractures. However, this technique can still be used at gas hydrate-filled fracture sites to determine zones that are hydrate bearing.   Figure 2.  Log curves from Hole 5A. In the fourth track, the measured resistivty curve is plotted (Rt) and the calculated Ro.  The zone where these curves separate from 56-94 mbsf is believed to be hydrate-bearing.  Figure 2, for example, displays a section of the log curves from Hole 5A from 30-160 meters below seafloor (mbsf). Before calculating Ro, the logs are analyzed. The caliper log records the geometry of the borehole and in Hole 5A is relatively smooth. This yields high quality data. The gamma ray log, which indicates if clay sediments are present, is high, suggesting the lithology is clay-dominated. Cores from Site 5 confirm lithology was composed of one clay-dominated unit from 0-200 mbsf [1]. Hole 5A exhibits high resistivity values from ~54- 115 mbsf, which may be due to gas hydrate.  For Hole 5A, intervals from 40-50 and 120-180 mbsf were selected as water-saturated.  The matching of Ro to Rt in these intervals yields a=1.44 and m=1.83.  The intervals where Rt clearly separates from Ro are assumed to be gas hydrate-bearing. Thus, the hydrate bearing zone (HBZ) for Hole 5A extends from 54 to 94 mbsf.   The higher resistivity values from 94-115 mbsf are a result of decreased porosity (increased density), likely caused by cementation or compaction of the sediments. The HBZ differs from the thermodynamically calculated base of the gas hydrate stability zone (BGHSZ), which extends to 130 mbsf at Site 5. The calculated values of a and m as well as the interval of occurrence of the HBZ and the BGHSZ are listed for each hole in Table 1.  In every hole, the HBZs appear above the BGHSZ.  Hole a m HBZ (mbsf) BGHSZ1 (mbsf) 5A 1.4 1.8 56-94 130 5B 1.1 2.2 56-90 130 6A 3.3 0.7 115-195 N/A2 7A 1.2 1.9 73-95, 137-184 198 10A 1.63 1.63 28-158 160 Table 1. Values for a, m, HBZ, and the BGHSZ for each hole. 1Values from [3]. 2Temperature measurements not recorded at Site 6. 3a & m values are from neighboring Hole 2A, because water saturated intervals were not selected.  FRACTURES Resistive, gas hydrate-filled fractures and conductive fractures are clearly visible on the LWD resistive images from Holes 5A, 5B, 6A, 7A, and 10A. The LWD tool records geographic orientation as the resistivity is measured, producing an oriented resistivity image.  Using Schlumberger’s log analysis software Geoframe, each visible fracture is fit with a sinusoid, which produces a dip and dip direction for each fracture (Fig. 3). Dip direction is perpendicular to strike. Image resolution increases the uncertainty of the dip direction measurement by ±6 degrees.  For the dip measurement, uncertainty decreases the higher the dip angle.  Dips greater than 70 degrees have less than ±1degree uncertainty. In contrast, orientations for features dipping at angles lower than 10 degrees are impossible to accurately resolve due to the image resolution. Thus, we do not discuss the orientation of any fractures that are lower than 10 degrees.  To be clearly visible on the images, the fractures must exhibit a higher or lower resistivity than the surrounding sediment. Sometimes, a single fracture is difficult to resolve because multiple fractures intersect at a certain depth.  The presence of gas hydrate-filled veins or nodules or gas hydrate disseminated in the pore space also increases the difficulty of identifying gas hydrate-filled fractures. Alternatively, conductive water filled fractures or resealed fractures may be impossible to resolve in similarly conductive, high porosity sediment. In this paper, we make every effort to correctly report the distribution of gas hydrate- filled fractures and conductive fractures at the Krishna-Godavari sites.   Figure 3.  A section of the deep resistivity image from Hole 10A, from 75-91 mbsf, with selected sinusoids overlaid on the gas hydrate-filled fractures.  On the image, darker colors are more conductive, while lighter colors are more resistive.  In the second track the dot indicates the dip angle in degrees and the tail represents the dip direction of each fracture.  STRESS Fracturing in each hole has different characteristics.  On Figure 4, a pole represents each gas hydrate-filled fracture plane on a lower hemisphere Equal Area stereonet for each hole. An example the relationship between a fracture plane and a pole on an Equal Area stereonet appears on Figure 5. In Holes 5A, 5B, 6A, 7A, the poles are clustered, which indicates a well-ordered fracture system.  Additionally, all of the poles appear near the circumference of the stereonet, indicating the fracture planes are high-angle. The direction of σ2 and σ3 (the maximum horizontal stress direction and the minimum horizontal stress direction, respectively) can be resolved quite accurately in holes that exhibit well-ordered, high- angle fracture orientations.   Figure 4.  The poles to the fracture plane of each gas hydrate-filled fracture from Holes 5A, 5B, 6A, 7A, and 10A.  On a passive margin, the maximum principal stress (σ1) on a sediment column is assumed to be up, due to the weight of the ocean water.  Fractures primarily propagate in the direction of σ1, and open in the direction of minimum stress, σ3 [4]. On an equal area stereonet, the maximum horizontal stress direction (σ2) is parallel to the strike, or perpendicular to the pole of the fracture plane (Figure 4).   Figure 5. The relationship between a fracture plane and a pole on a stereonet.  On passive continental margins, σ2 usually parallels the edge of the continental slope, i.e. σ2 parallels the contour lines (5, 6). However, this trend is documented in holes deeper than NGHP- 01. Sites 5, 6, and 7 all contradict this trend, and σ2 orients downslope. Since fractures are very shallow in the sediment column (<160 mbsf), this contradiction my be the result of finer bathymetric changes due to local slumps and slides which are not resolvable on available bathymetry data. Stress orientations at NGHP-01 sites are likely not the result of deeper tectonic stresses.  In comparison with other NGHP-01 holes, Hole 10A exhibits a more chaotic fracturing environment, with a wider range of dip and strike angles. Poles tend to cluster on the western side of the stereonet, suggesting more easterly dipping fractures as the most dominant fracture orientation. Clearly, many fractures contradict the dominant trend. The fracture distribution in Hole 10A significantly reduces the accuracy of resolved stress directions.  In the following sections we present the fracture distribution with depth in each hole and discuss the fracturing in each hole in detail.  SITE 5 Two LWD holes were drilled at Site 5, Hole 5A and Hole 5B (Fig. 6 & 7), approximately 11 m apart. Gas hydrate-filled fractures were identified in both holes from 60 to 91 mbsf on the resistivity images. No conductive fractures were identified in either hole. Fractures in both holes are quite similar; occurring at the same depth intervals, with the same strike orientation (NW-SE) and similar high angle dips. Twenty-six fractures were identified in Hole 5A, and eighteen fractures were identified in Hole 5B. One fracture appeared outside the HBZ at 54 mbsf in Hole 5B; while it appears resistive in the image, the resistivity increase is not significant enough to cause a noticeable increase on the one-dimensional resitivity log at that depth.   Figure 6. Gas hydrate-filled fractures in Hole 5A.  The dot represents the dip angle and the tail represents the dip direction.   Figure 7. Gas hydrate-filled fractures in Hole 5B.  At Site 5, fracture strikes change from more N-S to more E-W with depth (Fig.8).  On average, fracture strike changes 1.2 degrees per meter from 60-91 mbsf. This suggests either a stress change over time, with the fractures occurring slowly as the stress is changing. Alternatively, a stress change may occur over the interval depth, perhaps due to some slow sediment creep not visible at the resolution of available bathymetry or on the seismic sections.  No other holes in NGHP-01 exhibit any correlation between depth and strike angle or depth and dip angle.  The fracture occurrence in Hole 5A seems to suggest that larger separations between Ro and Rt may correlate with a gas hydrate-filled fracture density (Fig.6).  However, Hole 5B (Fig.7) contradicts this observation, as largest separation between Ro and Rt, from 73-83 mbsf contains only four fractures, where as smaller separations between Ro and Rt contain higher numbers of fractures.   Figure 8.  SITE 6   Figure 9. Gas hydrate-filled and conductive fractures in Hole 6A.  Site 6 contains the fewest identified gas hydrate- filled fractures.  Additionally, Site 6 also has the lowest separation between Ro and Rt, usually less than 1 ohm*m, suggesting very low gas hydrate occurrence. Like Site 5, the gas hydrates filled fractures identified are well-ordered and high- angle.  Many conductive fractures were visible on the Hole 6A image.  These fractures appear to have a random orientation, and are generally much lower angle than the gas hydrate-filled fractures.  The thermodynamic base of the GHSZ was not determined for this site, but the bottom-simulating reflector (BSR), which can represent the base of gas hydrate stability on seismic sections, was estimated at 210 mbsf [1].  Thus, it is possible that conductive fractures below 210 mbsf are relics of fractures that were once gas hydrate-filled. Conductive fractures below the BSR may also transport gas into the gas hydrate stability zone.  SITE 7  Figure 10.  Gas hydrate-filled fractures in Hole 7A.  In Hole 7A, fracturing occurs in two distinct intervals from 76-90 mbsf and 140-152 mbsf, each within the HBZ intervals. However, fractures in each interval are almost identical; all fractures in hole strike E-W and dip south.  This suggests fractures formed at the same time or the stress state over these intervals did not change during fracture formation.  SITE 10 Resistivity measurements in Hole 10A are an order of magnitude higher than any other hydrate- bearing site on the Indian continental margin. Two- dimensional seismic sections shot over the area reveal a highly faulted area with both shallow and deep gas occurrence [1]. Many members of the scientific party have speculated that Site 10 may be a vent site that once appeared on the ocean floor and was later buried.  One hundred twelve fractures were identified on the resistivity image.  Only four were categorized as conductive. However, these conductive fractures were identified in the most resistive section of the hole, and may, in fact, be gas hydrate-filled at a lower saturation than surrounding sediments. Alternatively, these fractures may be conduits for water movement through a system highly saturated with gas hydrate.  Gas hydrate-filled fracturing in Hole 10A is significantly more chaotic than in the other holes from NGHP-01.  Fractures tend to dip at angles greater than 60 degrees and strike in a NE-SW direction, but many fractures contradict the general trend. It is likely that many waves of significant gas flux caused the current composition of fractures seen in hole.  Hole 10A clearly illustrates that no broad statements can be made about fracture density and the magnitude of separation between Ro and Rt. Here Ro hovers about 1 ohm*m, but is difficult to see on Fig. 11, so simply consider the measured resistivity. For instance, the lowest measured resistivity occurs from ~100-120 mbsf but the fracture density in this interval is not significantly different from the fracture density the highest resistivity interval from ~50-90 mbsf. Additionally, the lowest fracture density interval from 130-160 mbsf, has higher resistivities than the higher fracture density lowest resistivity interval from 100-120 mbsf.   Figure 11.  Gas hydrate-filled fractures and conductive fractures in Hole 10A.  SUMMARY LWD resistivity images were collected during NGHP Expedition 01.  Resistivity images from holes with gas hydrate bearing zones, Holes 5A, 5B, 6A, 7A, and 10A, show gas hydrate as fill in fractures.  Holes 5A, 5B, 6A and 7A contain well- ordered, high-angle fractures, from which horizontal stress directions can be accurately resolved.  These stress directions, however, contradict the orientations commonly seen on a passive margin, and may be the result of local bathymetry variations. Fracturing in Hole 10A is chaotic, which is likely due to high gas flux.  We observe no link between the magnitude of the measured resistivity and the number of fractures identified.  ACKNOWLEDGEMENTS This research used data from the National Gas Hydrate Program Expedition 01 (NGHP-01). We greatly appreciate the efforts of the crew, staff, and science party of the JOIDES Resolution during NGHP-01. We thank the Directorate General of Hydrocarbons in India and the Ministry of Petroleum & Natural Gas in India for allowing the use of this data by the international scientific community.  REFERENCES [1] Collett, T.S., et al., Indian National Gas Hydrate Program Expedition 01 Initial Reports Directorate General Hydrocarbons, Ministry of Petroleum and Natural Gas, India, 2008. [2] Archie, G.E., The electical resistivity log as an aid in determining some reservior characteristics Transactions AIME 1942; 146: 54-63. [3] Tréhu, A.M., Villinger, H., Fisher, A.T., In Situ Temperatures in the Indian National Gas Hydrate Program (NGHP) Expedition 01 Drill Sites - Implications for Gas Hydrate Stability. From: International Conference on "Gas Hydrates", Noida, India, 2008 [4] Lawn, B. R., and T. T. Wilshaw, Fracture of Brittle Solids, Cambridge University Press, Cambridge, 1975. [5] Tingay, M. R. P., et al., Present-day stress orientation in Brunei: a snapshot of ‘prograding tectonics’ in a Tertiary delta, Journal of the Geological Society 2005, 162(1), 39-49. [6] Yassir, N. A., and A. Zerwer, Stress regimes in the Gulf Coast, offshore Louisiana; data from well- bore breakout analysis, AAPG Bulletin 1997, 81(2), 293-307.   


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