Offshore geological sequestration of CO2 offers a viable approach for reducing greenhouse gas emissions into the atmosphere. Strategies include injection of CO2 into the deep-ocean or ocean-floor ...sediments, whereby depending on pressure–temperature conditions, CO2 can be trapped physically, gravitationally, or converted to CO2 hydrate. Energy-driven research continues to also advance CO2-for-CH4 replacement strategies in the gas hydrate stability zone (GHSZ), producing methane for natural gas needs while sequestering CO2. In all cases, safe storage of CO2 requires reliable monitoring of the targeted CO2 injection sites and the integrity of the repository over time, including possible leakage. Electromagnetic technologies used for oil and gas exploration, sensitive to electrical conductivity, have long been considered an optimal monitoring method, as CO2, similar to hydrocarbons, typically exhibits lower conductivity than the surrounding medium. We apply 3D controlled-source electromagnetic (CSEM) forward modeling code to simulate an evolving CO2 reservoir in deep-ocean sediments, demonstrating sufficient sensitivity and resolution of CSEM data to detect reservoir changes even before sophisticated inversion of data. Laboratory measurements place further constraints on evaluating certain systems within the GHSZ; notably, CO2 hydrate is measurably weaker than methane hydrate, and >1 order of magnitude more conductive, properties that may affect site selection, stability, and modeling considerations.
We report on grain-scale characteristics and gas analyses of gas-hydrate-bearing samples retrieved by NGHP Expedition 01 as part of a large-scale effort to study gas hydrate occurrences off the ...eastern-Indian Peninsula and along the Andaman convergent margin. Using cryogenic scanning electron microscopy, X-ray spectroscopy, and gas chromatography, we investigated gas hydrate grain morphology and distribution within sediments, gas hydrate composition, and methane isotopic composition of samples from Krishna–Godavari (KG) basin and Andaman back-arc basin borehole sites from depths ranging 26 to 525 mbsf. Gas hydrate in KG-basin samples commonly occurs as nodules or coarse veins with typical hydrate grain size of 30–80 μm, as small pods or thin veins 50 to several hundred microns in width, or disseminated in sediment. Nodules contain abundant and commonly isolated macropores, in some places suggesting the original presence of a free gas phase. Gas hydrate also occurs as faceted crystals lining the interiors of cavities. While these vug-like structures constitute a relatively minor mode of gas hydrate occurrence, they were observed in near-seafloor KG-basin samples as well as in those of deeper origin (>100 mbsf) and may be original formation features. Other samples exhibit gas hydrate grains rimmed by NaCl-bearing material, presumably produced by salt exclusion during original hydrate formation. Well-preserved microfossil and other biogenic detritus are also found within several samples, most abundantly in Andaman core material where gas hydrate fills microfossil crevices. The range of gas hydrate modes of occurrence observed in the full suite of samples suggests a range of formation processes were involved, as influenced by local in situ conditions. The hydrate-forming gas is predominantly methane with trace quantities of higher molecular weight hydrocarbons of primarily microbial origin. The composition indicates the gas hydrate is Structure I.
•We report on grain characteristics of gas hydrates from the E. Indian margin.•The hydrate-forming gas is mostly CH4, of microbial origin, and predicts Structure I.•Hydrate forms here as nodules, coarse veins, small pods, layers, or disseminated.•Less commonly it forms cavity-lining crystals, fills fossil crevices, or is salt-rimmed.•The modes of occurrence suggest localized formation from free-phase gas.
Methane hydrate was synthesized from pure water ice and flash frozen seawater, with varying amounts of sand or silt added. Electrical conductivity was determined by impedance spectroscopy, using ...equivalent circuit modeling to separate the effects of electrodes and to gain insight into conduction mechanisms. Silt and sand increase the conductivity of pure hydrate; we infer by contaminant NaCl contributing to conduction in hydrate, to values in agreement with resistivities observed in well logs through hydrate‐saturated sediment. The addition of silt and sand lowers the conductivity of hydrate synthesized from seawater by an amount consistent with Archie's law. All samples were characterized using cryogenic scanning electron microscopy and energy dispersive spectroscopy, which show good connectivity of salt and brine phases. Electrical conductivity measurements of pure hydrate and hydrate mixed with silt during pressure‐induced dissociation support previous conclusions that sediment increases dissociation rate.
Plain Language Summary
Methane hydrate is a frozen mixture of methane gas and water ice and occurs naturally in the seafloor of the continental shelves worldwide. Hydrate is variously considered a source of energy, a natural hazard, or a potential contribution to ocean acidification and climate change. Measurement of seafloor electrical conductivity, either using borehole logs or geophysical prospecting methods, is one of the most reliable ways of estimating hydrate location and abundance, but such methods need to be calibrated using laboratory measurements on hydrate‐sediment mixtures. We have made laboratory conductivity measurements on mixtures of hydrate, sand and silt, and seawater. Our results are in good agreement with borehole logs through seafloor sediments fully saturated with hydrate and will allow other scientists to more reliably estimate hydrate concentration from electrical conductivity.
Key Points
We quantified the electrical conductivity of laboratory‐formed methane hydrate mixed with sand or silt plus a fluid phase
Conductivity measurements of hydrate and sand agree with borehole logs through highly saturated hydrate
Hydrate saturation in our samples is consistent with a simple version of Archie's law
Ice in both terrestrial and planetary settings often contains rock particles. Here we present an experimental investigation of the influence of intergranular particles on the rheological behavior of ...ice. Experiments were performed on samples fabricated from 10‐μm ice powders +1‐μm graphite or 0.8‐μm alumina particles and subjected to elevated confining pressures. A critical particle fraction, ∼6%, was observed, below which samples behave like pure ice and deform by both grain boundary sliding (GBS) and dislocation creep, and above which GBS creep is impeded. Above this critical fraction, ice grains occur in particle‐free clusters surrounded by bands of particles mixed with fine‐grained ice, resulting in the impedance of GBS in the bands as well as sliding between the ice clusters. Our results imply that South Polar Layered Deposits and midlatitude lobate debris aprons on Mars must contain >94% ice and that the shallow subsurface of Ceres could contain >90% ice.
Plain Language Summary
Ice on Mars, Ceres, and icy satellites often contains rock particles. The presence of particles in ice changes its flow behavior and thus is important for understanding the composition and evolution of planetary ice masses. Based on laboratory experiments on samples made of fine‐grained ice and intergranular particles, we determined a critical quantity of particles, about 6% by volume, below which the ice‐particle samples flow like pure ice, and above which, sliding between grains (so‐called grain boundary sliding, or GBS) is impeded. The impedance of GBS by particles has not previously been observed. At planetary conditions, GBS is often the dominant flow mechanism for pure ice. Our result thus imply that the South Polar Layered Deposits and midlatitude lobate debris aprons on Mars must contain >94% ice and that the shallow subsurface of Ceres could contain more than 90% ice.
Key Points
Below the critical fraction, ∼6%, particle‐bearing ice behaves like pure ice and deforms by both grain boundary sliding and dislocation creep
Above the critical fraction, ∼6%, GBS creep is impeded in particle‐bearing ice, as bands of particles separate ice clusters
Icy masses on Mars, Ceres, and outer planet satellites could contain more ice than previously expected
CO2 and CH4 clathrate hydrates are of keen interest for energy and carbon cycle considerations. While both typically form on Earth as cubic structure I (sI), we find that pure CO2 hydrate exhibits ...over an order of magnitude higher electrical conductivity (σ) than pure CH4 hydrate at geologically relevant temperatures. The conductivity was obtained from frequency‐dependent impedance (Z) measurements made on polycrystalline CO2 hydrate (CO2·6.0 ± 0.2H2O by methods here) with 25% gas‐filled porosity, compared with CH4 hydrate (CH4·5.9H2O) formed and measured in the same apparatus and exhibiting closely matching grain characteristics. The conductivity of CO2 hydrate is 6.5 × 10−4 S/m at 273K with an activation energy (Ea) of 46.5 kJ/mol at 260–281 K, compared with ∼5 × 10−5 S/m and 34.8 kJ/m for CH4 hydrate. Equivalent circuit modeling indicates that different pathways govern conduction in CO2 versus CH4 hydrate. Results show promise for use of electromagnetic methods in monitoring CO2 hydrate formation in certain natural settings or in CO2/CH4 exchange efforts.
Plain Language Summary
Gas hydrates are crystalline solids that resemble snow and consist of frozen water molecules forming cage‐like structures that trap individual gas molecules within. Hydrates form naturally where temperature, pressure, and sufficient gas supply combine to make them stable, such as at depth in continental shelves worldwide and in polar regions. Typically containing methane, gas hydrates are of intense interest for energy considerations as well as for their potential risk as natural hazards or to geotechnical operations, or as contributors to climate issues. CO2 hydrate, on the other hand, is a possible prospect for carbon storage efforts due in part to its greater stability range compared to methane hydrate. Here we report the surprisingly large effect of guest‐molecule composition on the electrical properties of gas hydrate. We show that pure CO2 hydrate exhibits significantly higher electrical conductivity than methane hydrate over the range of temperatures where they can both form on Earth, despite their similarities in crystal structure. Their distinct electrical signatures could aid in the monitoring of CO2 in certain remote settings.
Key Points
Electrical conductivity (σ) of CO2 hydrate is 6.5 × 10−4 S/m, over 1 log unit higher than CH4 hydrate at geologically relevant temperatures
Activation energy of CO2 hydrate is 46.5 kJ/mol over the range −13°C–8°C, ∼33% greater than CH4 hydrate and closely comparable to ice
These first‐ever σ measurements on pure CO2 hydrate show potential for geophysical monitoring of CO2 stored as hydrate in certain settings
Direct measurement of decomposition rates of pure, polycrystalline methane hydrate reveals a thermal regime where methane hydrate metastably “preserves” in bulk by as much as 75 K above its nominal ...equilibrium temperature (193 K at 1 atm). Rapid release of the sample pore pressure at isothermal conditions between 242 and 271 K preserves up to 93% of the hydrate for at least 24 h, reflecting the greatly suppressed rates of dissociation that characterize this regime. Subsequent warming through the H2O ice point then induces rapid and complete dissociation, allowing controlled recovery of the total expected gas yield. This behavior is in marked contrast to that exhibited by methane hydrate at both colder (193−240 K) and warmer (272−290 K) test conditions, where dissociation rates increase monotonically with increasing temperature. Anomalous preservation has potential application for successful retrieval of natural gas hydrate or hydrate-bearing sediments from remote settings, as well as for temporary low-pressure transport and storage of natural gas.
We have carried out a small-scale deep-sea field test of the hypothesis that CH4 gas can be spontaneously produced from CH4 hydrate by injection of a CO2/N2 gas mixture, thereby inducing release of ...the encaged molecules with sequestration of the injected gas. Pressure cell studies have shown that, under some pressure and temperature conditions, this gas mixture can induce formation of a solid N2/CO2 hydrate with no associated liquid water production. We transported a cylinder of pure CH4 hydrate, contained within a pressure vessel, to the sea floor at 690 m depth off shore Monterey, CA, using the remotely operated vehicle (ROV) Ventana. Upon opening the pressure vessel with the vehicle robotic arm, we emplaced the hydrate specimen on a metal stand and covered this with a glass cylinder full of a 25% CO2/75% N2 gas mixture, thereby fully displacing the surrounding seawater (T = 4.92 °C). We observed complete and rapid dissociation of the CH4 hydrate with release of liquid water and creation of a mixed gas phase. This gas composition will undergo transition over time because of the high solubility of CO2 in the displaced water phase. We show that the experimental outcome is critically controlled by the injected gas/hydrate/water ratio.
Using cryogenic scanning electron microscopy (CSEM), powder X-ray diffraction, and gas chromatography methods, we investigated the physical states, grain characteristics, gas composition, and methane ...isotopic composition of two gas-hydrate-bearing sections of core recovered from the BPXA–DOE–USGS Mount Elbert Gas Hydrate Stratigraphic Test Well situated on the Alaska North Slope. The well was continuously cored from 606.5
m to 760.1
m depth, and sections investigated here were retrieved from 619.9
m and 661.0
m depth. X-ray analysis and imaging of the sediment phase in both sections shows it consists of a predominantly fine-grained and well-sorted quartz sand with lesser amounts of feldspar, muscovite, and minor clays. Cryogenic SEM shows the gas-hydrate phase forming primarily as a pore-filling material between the sediment grains at approximately 70–75% saturation, and more sporadically as thin veins typically several tens of microns in diameter. Pore throat diameters vary, but commonly range 20–120 microns. Gas chromatography analyses of the hydrate-forming gas show that it is comprised of mainly methane (>99.9%), indicating that the gas hydrate is structure I. Here we report on the distribution and articulation of the gas-hydrate phase within the cores, the grain morphology of the hydrate, the composition of the sediment host, and the composition of the hydrate-forming gas.
Electromagnetic (EM) remote‐sensing techniques are demonstrated to be sensitive to gas hydrate concentration and distribution and complement other resource assessment techniques, particularly seismic ...methods. To fully utilize EM results requires knowledge of the electrical properties of individual phases and mixing relations, yet little is known about the electrical properties of gas hydrates. We developed a pressure cell to synthesize gas hydrate while simultaneously measuring in situ frequency‐dependent electrical conductivity (σ). Synthesis of methane (CH4) hydrate was verified by thermal monitoring and by post run cryogenic scanning electron microscope imaging. Impedance spectra (20 Hz to 2 MHz) were collected before and after synthesis of polycrystalline CH4 hydrate from polycrystalline ice and used to calculate σ. We determined the σ of CH4 hydrate to be 5 × 10−5 S/m at 0°C with activation energy (Ea) of 30.6 kJ/mol (−15 to 15°C). After dissociation back into ice, σ measurements of samples increased by a factor of ∼4 and Ea increased by ∼50%, similar to the starting ice samples.
Key Points
We developed a pressure cell to synthesize and measure sigma of gas hydrate
We determined the sigma of CH4 hydrate to be 5 × 10−5 S/m at 0° C
Sigma measurements are a factor of ∼4 and Ea is ∼50% lower for CH4 hydrate than ice
The composition of methane hydrate, namely n w for CH4·n wH2O, was directly measured along the hydrate equilibrium boundary under conditions of excess methane gas. Pressure and temperature conditions ...ranged from 1.9 to 9.7 MPa and 263 to 285 K. Within experimental error, there is no change in hydrate composition with increasing pressure along the equilibrium boundary, but n w may show a slight systematic decrease away from this boundary. A hydrate stoichiometry of n w = 5.81−6.10 H2O describes the entire range of measured values, with an average composition of CH4·5.99(±0.07)H2O along the equilibrium boundary. These results, consistent with previously measured values, are discussed with respect to the widely ranging values obtained by thermodynamic analysis. The relatively constant composition of methane hydrate over the geologically relevant pressure and temperature range investigated suggests that in situ methane hydrate compositions may be estimated with some confidence.
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