NS54A-01
Natural Gas Hydrates: Recent Developments on this Potential Future Energy Resource
Gas hydrates, potentially one of the most important energy resources for the future, are naturally occurring ice-like
solids in which water molecules trap gas molecules in a cagelike structure known as a clathrate. Although many
gases form hydrates, methane hydrate is by far the most common. Gas hydrates exist in huge quantities in
marine sediments below the sea floor and are found in association with permafrost in the Arctic. The volume of
carbon contained in methane hydrates worldwide is estimated to be twice the amount contained in all other fossil
fuels on Earth. The demand for natural gas throughout the world makes the immense volumes of methane
hydrates worldwide an extremely attractive research target. However, the contribution of gas hydrate's role in
meeting world energy needs will depend on the availability, producibility, and cost of extracting methane from the
hydrate phase. The overall resource base and producibility of gas hydrates is still very much in question, in part
because gas hydrates are not stable at normal sea-level pressures and temperatures. Despite the obstacles to
the study and development of gas hydrate resources, it is important to remember that research break-throughs
and technological developments have led to the utilization of resources once thought to be unavailable, such as
coalbed gas. This presentation will hiqhlight some of the recent research, production tests, assessments, and
critical studies, including work on the Alaska North Slope and the offshore of India, that may ultimately lead to
facilitating gas hydrate's contribution to the world's energy mix.
http:energy.usgs.gov/other/gashydrates/
NS54A-02
EXPLORATION FOR GAS HYDRATES IN DEEPWATER NORTHERN GULF OF MEXICO
In recent years, gas hydrates have drawn significant attention from scientific community worldwide due to their potential as an alternative energy resource and as a possible agent for both shallow drilling hazard, and global climate change. Gas hydrates have been known to exist extensively in shallow sediments from permafrost regions to deepwater oceans. The vast amount of naturally occurring hydrates is a large potential for an energy resource. While the world demand for fossil fuel is ever increasing and the supply is dwindling, it is imperative to assess whether gas hydrates can provide energy to fill the void. As a principle technology in hydrocarbon exploration, the seismic reflection method becomes a natural choice for exploring gas hydrates. In this paper, we present a petroleum systems approach to exploration of gas hydrates in which seismic data analysis and interpretation techniques play key roles. We developed an integrated, seismic-based, five-step workflow to delineate and quantify gas hydrates in the deepwater Gulf of Mexico (GoM). The method integrated geological interpretation, seismic processing and inversion, and rock physics modeling to ascertain the existence and quantify the naturally occurring gas hydrates. We applied the methodology on two blocks in the northern GoM and estimated hydrate concentration in the pore space, both at selected locations in 1D and a cube in 3D. Due to lack of hard data (well control) for the shallow seismic data, our predictions used analogue models based on geologic interpretation, seismic inversion, and the basic principles of rock physics. Based on model predictions, several wells were drilled recently on two blocks (KC 195 and ATV 14) in the GoM. We collected wireline, LWD/MWD, and core data. The post-drill analysis confirmed our methodology and validated the exploration mode. In this paper, we also present and discuss the drilling results and compare our pre-drill predictions with the drilling data and the lessons learned. The comparison indicates that our pre-drill seismic predictions for gas hydrate saturations are in agreement with the log-based estimates (resistivity and sonic), as well as core data, especially when the hydrate concentrations are high. However, some uncertainties do exist regarding the current seismic detection method. These are mostly due to lack of constraints on shallow rock models and seismic inversion during the pre-drill phase. Despite these uncertainties, our seismic-based methodology did provide valuable pre-drill information for selecting potential drill sites and to further characterize gas hydrate-bearing sediments. This technique could be used elsewhere, both onshore and offshore.
NS54A-03
Geologic Framework of the 2005 Keathley Canyon Gas Hydrate Research Well, Northern Gulf of Mexico
The Keathley Canyon DOE/JIP drill sites are located along the southeastern edge of an intraslope minibasin, the Casey Basin, in the northern Gulf of Mexico at 1,335 m water depth. A grid of 2D high-resolution multichannel seismic lines around the drill sites, targeted for imaging depths down to at least 1,000 m subbottom, reveals multiple disconformities that bound 7 mappable seismic stratigraphic units. A major disconformity in the middle of the units stands out for its angular baselapping geometry. From the seismic and drilling data, three episodes of sedimentary deposition and deformation are inferred. The oldest episode consists of fine-grained muds deposited during a period of relative stability in the basin (units e, f, and g). A second episode (units c and d) consists of large vertical displacements associated with infilling and ponding of sediment. This second interval corresponds with intercalated fine and coarse-grained material in the drill hole, which sampled the thin edges of much thicker units. The final episode (units a and b) occurred during much subdued vertical displacement. Hemipelagic drape (unit a) characterizes the modern seafloor deposits. The basin is mostly filled. Its sill is part of a subsiding graben that is only 10-20 m shallower than the deepest point in the basin, indicating that gravity- driven transport would mostly bypass the basin. Contemporary faulting along the basin margins has selectively reactivated an older group of faults. The intercalated sand and mud deposits of units c and d are tentatively correlated with late Pleistocene deposition derived from the western shelf-edge delta/depocenter of the Mississippi River, which was probably most active from 320 ka to 70 ka (Winker and Booth, 2000). Gas hydrate occurs within near-vertical fractures in units e and f of the oldest episode. The presence of sand within the gas hydrate stability zone is not sufficient to concentrate gas hydrate even though dispersed gas hydrate occurs deeper in the fractured mud/clay-rich sections of units e and f. Winker, C.D., and Booth, J., 2000, Sedimentary dynamics of the salt-dominated continental slope, Gulf of Mexico: integration of observations from the seafloor, near-surface, and deep subsurface: GCSSEPM Foundation 20th Ann. Res. Conf., Deep-water Reservoirs of the World, p. 1059-1086 (CD-ROM publication).
NS54A-04
Fracture-controlled Gas Hydrate Systems in the Gulf of Mexico
We present new results illustrating that permeable fractures control the presence of natural gas hydrate in low permeability marine muds on continental margins. We analyze logging-while-drilling resistivity images acquired during the Department of Energy/Chevron Joint Industry Project in the Gulf of Mexico. A strong correlation exists between local geologic constraints and natural gas hydrate formation in marine sediments at Keathley Canyon Site 151, drilled during this project. We find that gas hydrate accumulated at Site KC151 in two modes: filling high- angle fractures and sub-horizontal permeable beds. High angle hydrate-filled fractures are the most common depositional mode for gas hydrate at this site, accounting for 93% of fractures. A total of 5 m of hydrate-filled beds were distributed throughout the hole and usually occurred adjacent to hydrate-filled fractures. This suggests natural gas traveled primarily through the fractures and only a small distance through bedding layers. Most of the hydrate-filled fractures found at Site KC151 dip at angles greater than 40 degrees and occur between 220-300 meters below seafloor. This hydrate-filled fracture interval correlates to significant natural gas hydrate saturations between 15% and 40%, as determined by Archie’s saturation equation. All fractures at Site KC151 strike approximately N-S and dip easterly or westerly. This orientation is consistent with the 165SE- 345NW maximum horizontal stress determined from borehole breakouts. The maximum horizontal stress aligns with local topography. In one interval of hydrate-filled fractures porosity increases with hydrate saturation, indicating that high pore pressure may have dilated the sediments and the formation of gas hydrate may have forced fractures apart. Results from Site KC151 will be compared to other fracture-controlled hydrate systems on continental margins, including Cascadia Margin and the Indian continental margin.
NS54A-05
Natural System Mimics: Development and Evaluation of Formation/Decomposition Methods for Methane Hydrate Hosted in Fine Sediments
Marine hydrates constitute a major fraction of the total global hydrate inventory that to date need further characterization. Pristine samples recovered from several marine hydrate sites share a common feature: they invariably show high variability with respect to methane saturation. A fundamental understanding of host sediment characteristics with and without filled hydrates is sought. Previous studies (Winters et al., 2004) show discrepancies between field and laboratory data that was attributed to factors such as different methods used to reform hydrates in the laboratory. Our studies focus on hydrates in fine-grained natural sediments and discerning the effect of methods used to synthesize them under subsurface-mimic conditions. Our integrated approach involves characterization of host sediments by Computed Microtomography (CMT) at a beamline of the National synchrotron Light Source (NSLS) to establish parameters such as porosity and tortuosity on a grain scale of sediments with and without hydrates. The CMT technique is complemented by a recently constructed unit, namely Flexible Integrated Study of Hydrates (FISH) in which one of the pressure vessels is of 200 mL volume and fitted with a 12" sight glass. The vessel mimics hydrate formation under near-surface marine conditions. We have evaluated a sediment sample from the Gulf of Mexico (GOM) recovered during the DOE-JIP cruise. After fractionation, it was classified as Very Fine Silt (< 10 mm) under the Udden-Wentworth particle-size classification scheme. The CMT data collection was followed by charging the sample to the FISH unit and hydrates were formed by the Static method in which gas was periodically charged to maintain the pressure. Pressure dependency was established by conducting three runs at 900, 1200, and 1500 psi for hydrate formation. Results for our CMT and the formation/decomposition kinetics will be discussed. Keywords: Host sediments, sediment-hydrate interaction, methane hydrate, FISH unit, computed microtomography.
NS54A-06
Evidence for Focused Accumulations of Methane Hydrate with Possible Resource Implications for the Deep Water Bering Sea Basins
Velocity-amplitude anomalies (VAMPs), comprising coincident seismic travel time anomalies and gas bright spots, are features widely identified in seismic reflection images from the deep-water Aleutian and Bowers Basins in the Bering Sea. The structures are interpreted as acoustic images of large deposits of natural methane hydrate directly overlying columns of ascending fluids that deliver methane gas to the hydrate stability zone. Because these chimneys extend downward to depths where thermocatalytic processes are underway, methane involved in Bering Sea VAMP structures is presumed to be petroleum generated. Interval travel time anomalies have been used to objectively detect these features and to quantify implied resource content in several example cases. Relative travel time variation in the sedimentary intervals above and below a gas hydrate bottom simulating reflection (BSR) are a selected diagnostic for VAMP detection, measuring velocity pull-up in the hydrate stability zone and push-down in the underlying gas zone. The largest VAMP anomalies studied, including all of those associated with hydrate indicators, are located above prominent basement highs. This association suggests that long-lived fluid migration patterns in these undeformed deep- water basins were originally established in response to sedimentation and compaction over the oceanic basement topography, perhaps augmented by hydrothermal circulation involving the crust itself. Thousands of VAMP anomalies occur in the Bering Sea. Individual large VAMP structures appear to involve a volume of methane (0.6-0.9 TCF) equivalent to that of a large gas field. If the hydrate interpretation is correct, then a genuinely vast number of targets for concentrated accumulations of methane (either as gas or hydrate) occur in the Bering Sea Basin.
NS54A-07
Analysis of mesoscopic attenuation in gas-hydrate bearing sediments
Several authors have shown that seismic wave attenuation combined with seismic velocities constitute a useful geophysical tool to infer the presence and amounts of gas hydrates lying in the pore space of the sediments. However, it is still not fully understood the loss mechanism associated to the presence of the hydrates, and most of the works dealing with this problem focuse on macroscopic fluid flow, friction between hydrates and sediment matrix and squirt flow. It is well known that an important cause of the attenuation levels observed in seismic data from some sedimentary regions is the mesoscopic loss mechanism, caused by heterogeneities in the rock and fluid properties greater than the pore size but much smaller than the wavelengths. In order to analyze this effect in heterogeneous gas-hydrate bearing sediments, we developed a finite-element procedure to obtain the effective complex modulus of an heterogeneous porous material containing gas hydrates in its pore space using compressibility tests at different oscillatory frequencies in the seismic range. The complex modulus were obtained by solving Biot's equations of motion in the space-frequency domain with appropriate boundary conditions representing a gedanken laboratory experiment measuring the complex volume change of a representative sample of heterogeneous bulk material. This complex modulus in turn allowed us to obtain the corresponding effective phase velocity and quality factor for each frequency and spatial gas hydrate distribution. Physical parameters taken from the Mallik 5L-38 Gas Hydrate Research well (Mackenzie Delta, Canada) were used to analyze the mesoscopic effects in realistic hydrated sediments.
NS54A-08
Methane Hydrate Saturation in Marine Sediment: Basic Relationships to Methane Flux and Depth of the Sulfate-Methane Transition
A one-dimensional numerical model that simulates accumulation of methane hydrate in marine sediment over geological time scales was developed. Average gas hydrate saturation maps that are valid over a wide range of transport parameters are also computed from our numerical simulations. Two saturation maps explain gas hydrate distributions resulting from methane generated either via in-situ methanogenic reactions or transported through upward fluxes from deeper sources. These two dimensionless maps can explain multiple gas hydrate systems such as Blake Ridge (offshore southeastern USA), Cascadia Margin (offshore northwestern USA), Peru Margin (offshore Peru), Costa Rica Margin and Nankai Trough (offshore Japan). Change in the model parameters over geologic time scales can also be tracked on these maps, so that the temporal evolution of any hydrate province can be easily studied. For gas hydrate settings dominated by upward methane fluxes from deeper sources, average gas hydrate saturation is related to the depth to the sulfate-methane transition (SMT). This scaling provides a quick and inexpensive means to estimate average hydrate saturation from the SMT depth and depth to the base of hydrate stability; no other site specific parameters are required. This new method is tested against hydrate saturations from our model and site data from IODP Leg 311 (Cascadia Margin).