Methane Transport Research Articles

Impact of vegetation on the methane budget of a temperate forest.

Upland forest soils are known to be the main biological sink for methane, but studies have shown that net methane uptake of a forest ecosystem can be reduced when methane emissions by vegetation are considered. We estimated the methane budget of a young oak plantation by considering tree stems but also the understorey vegetation. Automated chambers connected to a laser-based gas analyser, on tree stems, bare soil and soil covered with understorey vegetation, recorded CH4 fluxes for 7months at 3h intervals. Tree stem emissions were low and equated to only 0.1% of the soil sink. Conversely, the presence of understorey vegetation increased soil methane uptake. This plant-driven enhancement of CH4 uptake occurred when the soil was consuming methane. At the stand level, the methane budget shifted from -1.4±0.4kgCha-1 when we upscaled data obtained only on bare soil, to -2.9±0.6kgCha-1 when we considered soil area that was covered with understorey vegetation. These results indicate that aerenchymatous plant species, which are known to reduce the methane sink in wetlands, actually increase soil methane uptake two-fold in an upland forest by enhancing methane and oxygen transport and/or by promoting growth of methanotrophic populations.

A multiscale model for methane transport mechanisms in shale gas reservoirs

We presented an efficient semi-analytical model-based analysis of methane transport in shale reservoirs including multiple horizontal wells with nanopores and complex hydraulic fractures. Multiple important gas transport mechanisms in nanopores such as gas desorption, gas slippage and diffusion and pressure-dependent fracture conductivity are fully incorporated in the model. Both simple and complex fractures in multiple shale-gas wells can be easily and effectively dealt with in accordance with the discretization approach, which does not require detailed gridding like numerical model. The superposition principle is utilized to capture the interactions between all fracture segments in order to accurately simulate real gas transport from matrix to fractures and finally from fractures to wellbores. We verified the semi-analytical model against a numerical compositional reservoir model for two shale-gas wells with multiple simple planar hydraulic fractures considering the gas desorption effect. The semi-analytical model was used to analyze ultimate gas recovery of different complex hydraulic fracture patterns in a two-well scenario, which were generated from a complex fracture propagation model with and without considering natural fractures. In addition, it was applied to perform field-scale simulation with multiple wells and complex fracture geometries in accordance with microseismic data. The simulation results show that multiple gas transport mechanisms, pressure-dependent fracture conductivity and complex fracture geometries play an important role in controlling well productivity and gas recovery, which should be properly accounted for in the physical model in order to achieve more accurate long-term production forecasting of multiple-fractured shale-gas horizontal wells.

Hindered Gas Transport through an Aqueous Salt Solution Interface

Gas transport through water plays an important role in various natural processes and applications as the most common form in liquid–gas transport. Conventionally, it is expected that the flux of gas through water would be determined by the gas solubility and diffusion in water. However, it is still unclear whether such behavior holds in aqueous salt solutions. In this paper, we find that the methane transport is heavily hindered through the aqueous salt solutions by molecular dynamics simulations. Surprisingly, the hindrance to methane is not caused by the methane solubility or the diffusion change in aqueous salt solutions, but by the gas concentration barrier at the liquid–gas interface. Our simulation results show that the gas concentration barrier originates from the variance of the mass density profile in aqueous salt solutions. Furthermore, it is found that the flux of methane is negatively linearly dependent upon the gas concentration barrier at the liquid–gas interface by investigating various sal...

Methane seepage inferred from pore water geochemistry in shallow sediments in the western slope of the Mid-Okinawa Trough

Abstract Widely distributed cold seeps are crucial sources of carbon to the seawater, whereas anaerobic oxidation of methane (AOM) and precipitation of authigenic carbonate might change the compositions of the methane-rich fluids and thus reduce the outputs of dissolved carbon to seawater. In this work, we analyze the pore water compositions of four gravity cores with high methane concentration abnormalities in pore water or in overlying seawater in the western slope of the Mid-Okinawa Trough. For the northern research area, active weak methane seepage through the seafloor is identified in the vicinity of mud volcanoes (sites C01 and C10), and strong emissions of gas and fluids may occur in the central mud volcanoes. In a submarine canyon at the south, C23 site exhibits rarely methane seepage, while high rate transportation of methane from deep sediments and associated AOM are identified at site C25 where small vertical faults and gas-bearing layers are developed at the surrounding. AOM at site C25 is indicated by (1) quasi-linearly decrease of sulfate concentrations with depths, (2) intense increases of dissolved inorganic carbon (DIC) concentrations and (3) significantly negative δ13C values (vary from −7.2 to −26.3‰) of DIC. Organoclastic degradation in the shallow sediments is not obviously recognized in the study area because of the low concentrations of NH4+, as well as negligible sulfate consumption at three of the four sites. Our data reveal that the cold systems are one of the DIC sources (the fluxes are ca. 11.2 mmol/m2/yr) to the seawater in the Okinawa Trough, which can be tracked by high depletion of 13C (δ13CDIC ranges from −7.2 to −26.3‰ VPDB).

Burial-driven methane recycling in marine gas hydrate systems

Natural gas hydrate may be buried with sediments until it is no longer stable at a given pressure and temperature, resulting in conversion of hydrate into free gas. This gas may migrate upward and recycle back into the hydrate stability zone to form hydrate. As of yet, however, no quantitative description of the methane recycling process has been developed using multiphase flow simulations to model burial-driven gas hydrate recycling. In this study, we present a series of 1D multiphase transport simulations to investigate the methane recycling process in detail. By invoking the effects of capillary phenomena on hydrate and gas formation in pores of varying size, we find that a free gas phase can migrate a significant distance above the bulk base of hydrate stability. Since the top of the free gas occurrence is often identified as the base of the hydrate stability zone from seismic data, our results demonstrate that not only could this assumption mischaracterize a hydrate system, but that under recycling conditions the highest hydrate saturations can occur beneath the top of the free gas occurrence. We show that the presence of pore size distributions requires a replacement zone through which hydrate saturations progressively decrease with depth and are replaced with free gas. This replacement zone works to buffer against significant gas buildup that could lead to fracturing of overlying sediments. This work provides a framework for simulating flow and transport of methane within the 3-phase stability zone from a mass conservation perspective.

Effect of Gas Flow Rate on Hydrate Formation Within the Hydrate Stability Zone

AbstractWe form methane hydrate in brine‐saturated, coarse‐grained samples, under hydrate‐stable conditions, by injecting methane vapor at various flow rates. Decreasing the flow rate results in higher hydrate saturation, lower brine saturation, a smaller affected volume, and larger average pressure differentials across the sample. We interpret that the longer execution times at lower flow rates allow for additional methane transport and hydrate formation at the hydrate‐brine interface. As a result, the hydrate skin is thicker at lower flow rates and thus is capable of sustaining larger pressure differentials. In several experiments, we stop brine flow and supply methane gas to the sample for an additional 800 hrs. During this period, hydrate continues to form, pressure differentials develop, and the bulk density changes within the affected volume. We interpret that there is gas present in the sample that is disconnected from the gas source. Hydrate forms around the disconnected gas due to methane transport through the skin that surrounds it, causing the internal gas pressure to decline and leading to inward collapse and net volume decrease. This lowers the brine pressure and creates a differential pressure across the sample that induces gas flow. This study indicates that lower gas flow rates through the hydrate stability zone can produce very high saturations of hydrate but require a larger differential pressure to sustain flow. Ultimately, this process is an alternative mechanism for sustained upward gas flow and hydrate formation far above the base of the hydrate stability zone.

Emergent Properties of Antiagglomerant Films Control Methane Transport: Implications for Hydrate Management.

The relationship between collective properties and performance of antiagglomerants (AAs) used in hydrate management is handled using molecular dynamics simulations and enhanced sampling techniques. A thin film of AAs adsorbed at the interface between one flat sII methane hydrate substrate and a fluid hydrocarbon mixture containing methane and n-dodecane is studied. The AA considered is a surface-active compound with a complex hydrophilic head that contains both amide and tertiary ammonium cation groups and hydrophobic tails. At a sufficiently high AA density, the interplay between the surfactant layer and the liquid hydrocarbon excludes methane from the interfacial region. In this scenario, we combine metadynamics and umbrella sampling frameworks to study accurately the free-energy landscape and the equilibrium rates associated with the transport of one methane molecule across the AA film. We observe that the local configurational changes of the liquid hydrocarbon packed within the AA film are associated with high free-energy barriers for methane transport. The time scales estimated for the transport of methane across the AA film can be, in some cases, comparable to those reported in the literature for the growth of hydrates, suggesting that one possible mechanism by which AAs delay the formation of hydrate plugs could be providing a barrier to methane transport. Considering the interplay between the structural design and collective properties of AAs might be of relevance to improve their performance in flow assurance.

Methane surface diffusion capacity in carbon-based capillary with application to organic-rich shale gas reservoir

Organic matter is widely distributed in organic-rich shale gas reservoir and has a large specific surface area to adsorb a significant quantity of methane molecules. In such nanoscale organic pores, the methane adsorbability and surface chemistries give rise to complex methane transport behaviours. Bulk state methane transports beyond continuum flow regime while surface diffusion for adsorbed methane plays a vital role in contributing to the total methane transport ability. In this study, we establish a methane surface diffusion model that considers the influence of confined pore space on methane adsorption, isosteric sorption heat and adsorbed methane coverage under high pressure. Grand canonical Monte Carlos simulations are carried out to estimate the adsorption isotherms of methane across a range of pore sizes and are applied to predict maximum methane concentration inside the adsorption layer volume. The contribution of surface diffusion on total methane transport ability in nanoscale confined pore space is investigated. Study results show that methane permeability for organic pores first decreases and then increases with the increase of pore size. Methane permeability for organic pore size less than 4 nm in relatively low pressure (<5 MPa) can be comparable to methane permeability for 20–25 nm inorganic pores. This can be attributed to the fact that the surface diffusion effect is enhanced in relatively low pressure and small pore sizes.

Experimental and Modeling Study of Methane Adsorption onto Partially Saturated Shales

AbstractShale gas equilibrates through gas‐liquid‐solid interactions in reservoirs, but the role of moisture is rarely investigated. To determine how adsorbed water influences methane behavior, three carboniferous shale samples from the Qaidam Basin, China, were humidified at five levels up to a relative humidity of 89%, and their methane capacities at pressures up to 12 MPa were studied. The experimental results indicate that two water‐related mechanisms, “water blocking for methane transport” and “surface competition for gas‐solid interaction,” are primarily responsible for the methane capacity variations. A compositional comparison suggests that a high abundance of clay minerals plays a favorable role in methane migration by retaining water in interlayer pores. Based on the experimental data, an optimized method for calculating the adsorption amount based on an approximation of density distribution is proposed. The model predicts the average thickness of the adsorption layer and the adsorbed methane density distribution on the surface at a given pressure. The methane adsorption layer “thins” in a stepped pattern by up to 45% in the presence of water, with little further change observed at relative humidities greater than 75% in the studied samples.

Effects of gas reservoir configuration and pore radius on shale gas nanoflow: A molecular dynamics study.

We calculated methane transport through cylindrical graphite nanopores in cyclical steady-state flows using non-equilibrium molecular dynamics simulations. First, two typical gas reservoir configurations were evaluated: open (OS) and closed (CS) systems in which pores connect to the gas reservoir without/with a graphite wall parallel to the gas flow. We found that the OS configuration, which is commonly used to study nanoflows, exhibited obvious size effects. Smaller gas reservoir cross-sectional areas were associated with faster gas flows. Because Knudsen diffusion and slip flow in pores are interrupted in a gas reservoir that does not have walls as constraints, OSs cannot be relied upon in cyclical nanoflow simulations. Although CSs eliminated size effects, they introduced surface roughness effects that stem from the junction surface between the gas reservoir and the pore. To obtain a convergent nanoflow, the length of a side of the gas reservoir cross-section should be at least 2 nm larger than the pore diameter. Second, we obtained methane flux data for various pore radii (0.5-2.5 nm) in CSs and found that they could be described accurately using the Javadpour formula. This is the first direct molecular simulation evidence to validate this formula. Finally, the radial density and flow-velocity distributions of methane in CS pores were analyzed in detail. We tested pores with a radius between 0.5 nm and 2.5 nm and determined that the maximum ratio (∼34%) of slip flow to overall flow occurred in the pore with a radius of 1.25 nm. This study will aid in the design of gas reservoir configurations for nanoflow simulations and is helpful in understanding shale gas nanoflows.

Variations in Gas and Water Pulses at an Arctic Seep: Fluid Sources and Methane Transport

AbstractMethane fluxes into the oceans are largely dependent on the methane phase as it migrates upward through the sediments. Here we document decoupled methane transport by gaseous and aqueous phases in Storfjordrenna (offshore Svalbard) and propose a three‐stage evolution model for active seepage in the region where gas hydrates are present in the shallow subsurface. In a preactive seepage stage, solute diffusion is the primary transport mechanism for methane in the dissolved phase. Fluids containing dissolved methane have high 87Sr/86Sr ratios due to silicate weathering in the microbial methanogenesis zone. During the active seepage stage, migration of gaseous methane results in near‐seafloor gas hydrate formation and vigorous seafloor gas discharge with a thermogenic fingerprint. In the postactive seepage stage, the high concentration of dissolved lithium points to the contribution of a deeper‐sourced aqueous fluid, which we postulate advects upward following cessation of gas discharge.

Year-round simulated methane emissions from a permafrost ecosystem in Northeast Siberia

Abstract. Wetlands of northern high latitudes are ecosystems highly vulnerable to climate change. Some degradation effects include soil hydrologic changes due to permafrost thaw, formation of deeper active layers, and rising topsoil temperatures that accelerate the degradation of permafrost carbon and increase in CO2 and CH4 emissions. In this work we present 2 years of modeled year-round CH4 emissions into the atmosphere from a Northeast Siberian region in the Russian Far East. We use a revisited version of the process-based JSBACH-methane model that includes four CH4 transport pathways: plant-mediated transport, ebullition and molecular diffusion in the presence or absence of snow. The gas is emitted through wetlands represented by grid cell inundated areas simulated with a TOPMODEL approach. The magnitude of the summertime modeled CH4 emissions is comparable to ground-based CH4 fluxes measured with the eddy covariance technique and flux chambers in the same area of study, whereas wintertime modeled values are underestimated by 1 order of magnitude. In an annual balance, the most important mechanism for transport of methane into the atmosphere is through plants (61 %). This is followed by ebullition ( ∼ 35 %), while summertime molecular diffusion is negligible (0.02 %) compared to the diffusion through the snow during winter ( ∼ 4 %). We investigate the relationship between temporal changes in the CH4 fluxes, soil temperature, and soil moisture content. Our results highlight the heterogeneity in CH4 emissions at landscape scale and suggest that further improvements to the representation of large-scale hydrological conditions in the model will facilitate a more process-oriented land surface scheme and better simulate CH4 emissions under climate change. This is especially necessary at regional scales in Arctic ecosystems influenced by permafrost thaw.

An accurate computational method for the diffusion regime verification

The diffusion regime (sub-diffusive, standard, or super-diffusive) is defined by the order of the derivative in the corresponding transport equation. We develop an accurate computational method for the direct estimation of the diffusion regime. The method is based on the derivative order estimation using the asymptotic analytic solutions of the diffusion equation with the integer order and the time-fractional derivatives. The robustness and the computational cheapness of the proposed method are verified using the experimental methane and methyl alcohol transport kinetics through the catalyst pellet.

The Transpolar Drift conveys methane from the Siberian Shelf to the central Arctic Ocean

Methane sources and sinks in the Arctic are poorly quantified. In particular, methane emissions from the Arctic Ocean and the potential sink capacity are still under debate. In this context sea ice impact on and the intense cycling of methane between sea ice and Polar surface water (PSW) becomes pivotal. We report on methane super- and under-saturation in PSW in the Eurasian Basin (EB), strongly linked to sea ice-ocean interactions. In the southern EB under-saturation in PSW is caused by both inflow of warm Atlantic water and short-time contact with sea ice. By comparison in the northern EB long-time sea ice-PSW contact triggered by freezing and melting events induces a methane excess. We reveal the Ttranspolar Drift Stream as crucial for methane transport and show that inter-annual shifts in sea ice drift patterns generate inter-annually patchy methane excess in PSW. Using backward trajectories combined with δ18O signatures of sea ice cores we determine the sea ice source regions to be in the Laptev Sea Polynyas and the off shelf regime in 2011 and 2015, respectively. We denote the Transpolar Drift regime as decisive for the fate of methane released on the Siberian shelves.

Methane Bubble Growth and Migration in Aquatic Sediments Observed by X-ray μCT.

Methane bubble formation and transport is an important component of biogeochemical carbon cycling in aquatic sediments. To improve understanding of how sediment mechanical properties influence bubble growth and transport in freshwater sediments, a 20-day laboratory incubation experiment using homogenized natural clay and sand was performed. Methane bubble development at high resolution was characterized by μCT. Initially, capillary invasion by microbubbles (<0.1 mm) dominated bubble formation, with continued gas production (4 days for clay; 8 days for sand), large bubbles formed by deforming the surrounding sediment, leading to enhanced of macropore connectivity in both sediments. Growth of large bubbles (>1 mm) was possible in low shear yield strength sediments (<100 Pa), where excess gas pressure was sufficient to displace the sediment. Lower within the sand, higher shear yield strength (>360 Pa) resulted in a predominance of microbubbles where the required capillary entry pressure was low. Enhanced bubble migration, triggered by a controlled reduction in hydrostatic head, was observed throughout the clay column, while in sand mobile bubbles were restricted to the upper 6 cm. The observed macropore network was the dominant path for bubble movement and release in both sediments.

Comparative study of the methane and methanol mass transfer in the mesoporous H-ZSM-5/alumina extruded pellet

H-ZSM-5/alumina catalyst pellet was prepared using extrusion method. The as-prepared mesoporous material was characterized using nitrogen adsorption, IR, XRD, and TEM methods. Transport of methane and methanol in the obtained H-ZSM-5/alumina extruded grain was studied. We demonstrate that the methanol transport may be described by the time-fractional diffusion equation in a fairly good manner. The measured value of the fractional order of the time-fractional derivative reveals the fast super-diffusive regime of the methanol transport in the mesoporous solid. Contrary, the methane transport has been found to follow a standard diffusion and described by the second Fick’s law. These findings show that mass transfer kinetics is characterized by the order of the temporal derivative. The latter is a unique property of the individual porous media and the diffusing agent.

Methane Transport during a Controlled Release in the Vadose Zone

Core Ideas Vadose zone CH4 migration was densely monitored during a controlled‐release scenario. Soil moisture strongly influenced vadose zone CH4 fate and CH4 oxidation. Soil CH4 fate appears primarily controlled by biological CH4 oxidation. Shallow, small‐rate releases of ethanol‐blended fuels from underground storage tanks (USTs) may be quite common and result in subsurface CH4 generation. However, vadose zone transport of CH4 generated from these fuel releases is poorly understood, despite the potential to promote vapor intrusion or create explosion hazards. In this study, we simulated shallow CH4 generation with a controlled subsurface CH4 release from July 2014 to February 2015 to characterize subsurface CH4 migration and surface emissions and to determine environmental controls on CH4 fate and transport. July 2014 through November 2014 was an extended period of drought followed by precipitation during December 2014. Throughout the experiment, under varied CH4 injection rates, CH4 formed a radially symmetrical plume around the injection point. Surface efflux during the drought period of the experiment was relatively high and stable, with approximately 10 to 11 and 34 to 52% of injected CH4 reaching the ground surface during the low‐ and high‐rate injections, respectively. Following the period of precipitation and increased soil moisture, efflux dropped and stabilized at approximately 1% of injected CH4, even as soil moisture began to decrease again. Tracer and inhibitor experiments and estimates of soil diffusivity suggest that microbial CH4 oxidation was responsible for the observed drop in efflux. The decrease in efflux only after soil moisture increased suggests a strong environmental control over the transport and oxidation of vadose zone CH4.

Comparing gas transport in three polymers via molecular dynamics simulation.

People use polymers as materials for preparing separation media or containers. It is necessary to have a molecular level profound knowledge of gas transport in e bulk and interface regimes of different polymers, but few researchers have done a molecular level study of the bulk and interface behavior of gases in several types of non-homologen polymers thoroughly or developed expressions to correlate gas transport properties with cavity size distribution and chain oscillation flexibility. Therefore, in this work, molecular dynamics (MD) simulation was employed to study the transport of methane and n-butane molecules in the bulk and interface region of polyethylene (PE), poly(4-methyl-2-pentyne) (PMP) and polydimethylsiloxane (PDMS). Penetrant diffusivity, solubility and permeability in the bulk were studied first. The subdiffusion behavior of gas molecules is explored to obtain the mechanisms behind penetrant transport. Both penetrants have much smaller diffusivities in PE than in PMP and PDMS, and they have larger diffusivities in PDMS than in PMP. PE has lower accessible cavity fraction (ACF) and average oscillation amplitudes (AOAs) of the chains than PDMS and PMP. PE also has much smaller solubilities and permeabilities of both penetrants than PDMS and PMP. Though the permeabilities of both penetrants in PDMS are higher than the corresponding values in PMP, PMP has a higher selectivity of n-butane over methane than PDMS. Nonequilibrium MD simulation was performed to study the interface property and gas transport in the interface region. Equations to predict penetrant diffusivity and permeability from the accessible cavity fraction (ACF) and average amplitude of chain oscillation were developed successfully. Penetrant diffusivity and permeability are proportional to the value of ACF to the power of one third and that of ACF to the power of four thirds, respectively.

Experimental Study on Suppression Effect of Inert Gas Two Fluid Water Mist System on Methane Explosion

Abstract At present, there have been large numbers of methane transport pipelines in coal mine in china. Explosion accident is most likely to be triggered if affected by leakage or external fire. In order to improve the suppression effect of single fluid water spray technology, this paper studied the suppression performance of two fluid water mist system on methane/air explosion. A transparent organic glass pipeline was designed for gas explosion, with the dimensions of 0.84 × 0.12 × 0.12 m. A two fluid nozzle was used to suppress methane/air explosion. Then testing was performed on the explosion suppression properties of two fluid water mist system by adjusting the spray time and N2 and CO2 pressure. The explosion suppression effect of the inert gas two fluid water mist was examined by analyzing the methane explosion overpressure and flame propagation speed. The experimental results showed that: when using N2 or CO2 for the driving force, the explosion suppression effect of the inert gas two fluid water mist was significant, and the peak explosion overpressure, average rise rate of pressure rise and peak speed value of the explosion flame were all gradually decreased by extending the spray time. Such as when the spray time was 3 s and the CO2 pressure was 0.4 MPa, the average velocity of the flame decreased by 81.32%, peak explosion overpressure decreased by 51.44%, and average pressure rise rate decreased by 72%.

Supercritical Methane Diffusion in Shale Nanopores: Effects of Pressure, Mineral Types, and Moisture Content

Using molecular dynamics, we simulated the diffusion behavior of supercritical methane in shale nanopores composed of different matrix mineral types (organic matter, clay, and calcite). We studied the effects of pore size, pore pressure, and moisture content on the diffusion process. Our results show that confined methane molecules diffuse more rapidly with increases in pore size and temperature but diffuse slowly with an increase in pressure. Anisotropic diffusion behavior is also observed in directions parallel and perpendicular to the basal surfaces of nanoslits. We also found that mineral types composing the pore walls have a prominent effect on gas diffusion. The perfectly ordered structure and ultrasmooth surface of organic matter facilitate the transport of methane in dry pores, even though its adsorption capability is much stronger than that of inorganic minerals. Moisture inhibits methane diffusion, but this adverse effect is more evident in organic pores because water migrates in the form of clu...