Methane Diffusion Research Articles

Quantifying methane release and dispersion estimations for buried natural gas pipeline leakages

The methane into the soil from buried natural gas pipelines due to small leakages, changes the soil properties, posing potential risks to humans and the environment. It is essential to estimate the leakage rate and monitor the methane diffusion range outside the pipeline, which is challengeable due to the presence of soil. The main contribution of this work is to bridge the gap between estimating the leakage rate of underground pipelines and predicting the diffusion behaviors through calculating the gas concentration in the soil. The quantified leakage rate estimation model for air was firstly established by experimental results and validated by the numerical results, which was further modified by the methane with the numerical simulations. The methane diffusion model in the soil was then performed, through which, the influencing factors were explained and validated. In addition, the methane release and dispersion results in the soil could be used as the boundary conditions of gas diffusion model in the air. The results show that the quantifying estimation correlations can predict the leakage rate and dispersion range in the soil accurately, with errors less than 7.2 % and 15 %, respectively. Moreover, the quantified relations have been validated by the full-field experiments. And, the dispersion behaviors in the air could be portrayed instead of being regarded as a jet flow.

Effect of moisture content on methane diffusion in kaolinite by molecular dynamics simulations

The diffusion behaviours of methane in kaolinite with water contents ranging from 0-5 wt% are analysed by molecular dynamics (MD) simulations. The results indicate that methane molecules can jump between adjacent holes in the kaolinite matrix. The methane diffusion coefficient is very low (3.28 × 10−9 m2/s) and linearly increases with the increasing moisture content. As the moisture content decreases, the value of the first RDF peak between methane and oxygen is larger, indicating that with a lower moisture content, the interaction energy between methane and oxygen in kaolinite is stronger. The interaction between methane and water linearly increases with the moisture content; by contrast, the interaction energy between kaolinite and water and between kaolinite and methane linearly decreases with the increasing moisture content. The diffusion of methane molecules adsorbed on the surfaces can also be accelerated by the fast diffusion of water molecules in the middle interlayer nanopores of the kaolinite. [Received: May 12, 2019; Accepted: September 17, 2019]

Effect of hydrogen addition on the structure and stabilization of a micro-jet methane diffusion flame

The micro-jet diffusion flame can act as the heat source for the micro power generation systems due to some advantages. The present work investigates the effect of hydrogen addition on the structure and stabilization of micro-jet methane diffusion flame by numerical simulation. The results show that the oval flame becomes more and more circular with the increase of hydrogen addition fraction. The addition of hydrogen remarkably suppresses the increase of the flame height with the inlet velocity. The methane sharply decreases around the outlet of the micro-jet tube due to the high fresh fuel temperature. The intermediate species (e.g., H2 and CO) increase sharply before the flame front, and they are consumed sharply within the flame front. With the increase of hydrogen addition fraction, the concentration gradients of reactive species increase before the flame front, while the flame temperature decreases. In addition, with the increase of hydrogen addition fraction, the micro-jet flame root shifts toward the tube-wall and downstream direction at the radial and axial directions, respectively, and the addition of hydrogen decreases the anchoring temperature of the micro-jet flame root, which is conductive to improve the flame stabilization. Meanwhile, a large hydrogen addition fraction is detrimental for the flame stabilization in terms of the thermal interaction between the micro-jet flame and tube-wall. However, the positive effects brought by a large hydrogen addition fraction are noticeably larger than the adjunctive negative effects. This study not only provides the guideline for further expanding the operating range of the micro-jet methane diffusion flame but also helps us to gain insights into the mechanism of hydrogen addition on improving the flame stabilization.

Methane Adsorption Influence and Diffusion Behavior of Coking Coal Macromolecules under Different Moisture Contents

To study the adsorption and diffusion behavior of methane on coal macromolecules under different moisture contents, the macromolecular model of coking coal was established through elemental analysi...

Experimental investigation on methane advection and diffusion in geosynthetic clay liners

Geosynthetic clay liners (GCLs) are widely used in landfill and heap-leach facility cover system for mitigating rainfall infiltration and gas migration into atmosphere. Laboratory tests were conducted to investigate methane diffusion and advection through GCLs. Gas permeability coefficient of GCL for the case with moisture content = 47.5% is one and two orders of magnitude greater than the cases with moisture content = 68.5% and 80.9%, respectively, when 20 kPa vertical stress was applied. The batch adsorption tests indicated that adsorption of methane onto bentonite is negligible. The concentration variation for the adsorption of methane onto bentonite can be neglected. However, methane concentration decreased by 14.2% for the test of methane adsorption onto GCL during the first 2–3 days. This is because methane was adsorbed by the geotextiles rather than by the bentonite in GCL. The large porosity and surface area of geotextiles provide lots of micropores for methane adsorption. Analytical model was then developed to analyze the performance of GCL-based liners system with respect to methane transport. The results indicate that methane emission fluxes for the case with SL + GCL are 7.8 and 5.1 times less than the cases with SL + CCL when the moisture contents were 25.9% and 35.1%, respectively. The methane emission fluxes for both of the SL + GCL and SL + CCL can be neglected when they are fully saturated. GCL is recommended to be used in arid and semi-arid regions rather than CCL. GCL is recommended to be used in arid and semi-arid areas rather than CCL. Advection plays a more important role in methane migration through SL + GCL and SL + CCL than that of diffusion. With moisture contents = 25.9% and 32%, methane emission flux attributed to advection accounts for more than 90% of the total emission flux for both cases of SL + GCL and SL + CCL. With the increase of moisture content of SL, the effectiveness of SL in reducing methane emission increases. The saved space for using GCL + SL composite cover compared with using a single SL cover is 0.7 m when the moisture content equals 25.9%, which is 0.5 m greater than the case when moisture content equals 32%. GMB plays a dominant role in inhibiting methane migration and reducing methane emission flux. When moisture content equals 25.9%, the methane emission fluxes for SL + GMB + GCL and SL + GMB + CCL are 343 times and 2643 times less than the cases with SL + GCL and SL + CCL, respectively.

Alternative methodology to determine effective coefficient of methane diffusion in coal

The article presents the results of research in to the development of an alternative method of determining the effective coefficient of methane diffusion in coal, based on a mathematical model following Fick's second law. The research was conducted based on the recorded courses of methane sorption kinetics in coal samples obtained in laboratory conditions with the precise gravimetric sorption system IGA-001. The value of the coefficient was treated as an element tuning the model in such a way that the average relative error of the ex post was as small as possible. Model verification indicated the correctness of its assumptions. The development of a more accurate methodology to determine the effective coefficient of methane diffusion in coal will enable the verification of the models applied in order to describe the physical and chemical mechanisms of the methane desorption natural phenomenon. It will also pave the way for further research aimed at the determination of: the gas bearing capacity of coal seams, the degree of degassing of seamswhich results from the mining operations being conducted; or to what extent it is possible to degas the deposits.

Effects of gas preheat temperature on soot formation in co-flow methane and ethylene diffusion flames

This study reports the effects of gas preheat temperature on soot formation in a series of methane (CH4), ethylene (C2H4) and nitrogen-diluted C2H4 co-flow diffusion flames with a constant mass flow rate of carbon element. The flames were established in a Santoro-type burner and the temperatures of co-flow air and fuel were varied simultaneously from 293 K to 723 K. Planar distributions of soot volume fraction and soot temperature were measured using laser-induced incandescence and two-color pyrometry based on incandescence, respectively. The results show that the preheat temperature of gases has no significant effect on the shape of the CH4 and nitrogen-diluted C2H4 flames, but significantly influences that of the C2H4 flames with high soot loadings. With an increase in gas preheat temperature, both soot formation and oxidation are enhanced and soot loading increases for all flames. The magnitude of enhancement correlates with the soot loading, while the effect is more significant in the flames with lower soot loading. Soot temperature in the CH4 and nitrogen-diluted C2H4 flames also increases but is not as much as the change in the gas preheat temperature. However, for the C2H4 flames, an inverse tend is found in the central region of the flames, i.e., that lower soot temperatures are found in the flame with the higher gas preheat temperature.

Evidence for methane isotopic bond re-ordering in gas reservoirs sourcing cold seeps from the Sea of Marmara

The measurement of methane clumped isotopologues (Δ13CH3D and Δ12CH2D2) allows exploring isotope bond ordering within methane molecules, and may reveal equilibrium temperatures. Whether such temperature reflects the formation or re-equilibration temperature of the methane is not well understood, but would have critical implications for the use of methane clumped isotopologues as geo-thermometers. Here we investigate gas bubbles from vigorous emissions at cold seeps (n = 14) in the Sea of Marmara, Turkey. These cold seeps are sourced from deeper sedimentary reservoirs. Conventional geochemical tracers such as carbon and hydrogen bulk isotopic ratios (13C/12C and D/H) or n-alkane molecular ratios, suggest these gases reflect various degrees of mixing between thermogenic and microbial sources. Some samples would generally be considered purely microbial in origin (C1/C2+>1500; δ13C<−60‰). We report measurements of Δ13CH3D and Δ12CH2D2 showing that a fraction of those gases are in internal thermodynamic equilibrium, with the abundances of the two mass-18 isotopologues indicating concordant temperatures of ∼90°C and ∼130°C. These concordant temperatures are recorded by gases of putative microbial and thermogenic origin; the temperatures of equilibration are irrespective of the formation mechanism of the gases. We conclude that the two high-temperatures recorded by Δ13CH3D and Δ12CH2D2 are best explained by non-enzymatic re-equilibration at two local subsurface temperatures. First principles suggest that unequal rates of exchange are possible. Disequilibrium signatures where the two isotopologues yield discordant apparent temperatures are exhibited by other samples. In those cases the data define a trend of variable Δ13CH3D at nearly constant Δ12CH2D2. These signatures are enigmatic, and we investigate and reject multiple possible explanations including mixing, diffusion or Anaerobic Oxidation of Methane. Different rates of re-equilibration between the two rare isotopologues are implied, although lacks experimental foundation at present. In general, all of these data point towards re-equilibration of the mass-18 methane isotopologues as an important process.

Full-scale multi-physics numerical analysis of an isothermal chemical vapor infiltration process for manufacturing C/C composites

The internal architecture of a CVI reactor significantly influences the gas flow behavior, as well as the complex time-varying chemical reactions, but has been typically ignored in previous CVI models. Herein we developed, validated, and applied a fully three-dimensional (3D) physicochemical CVI model of an industry-scale reactor to simulate an isothermal CVI process for fabricating bulk carbon-carbon composites using methane as a precursor gas and a multi-layered preform consisting of a non-crimp fabric and felt. The flow inside the reactor was modeled using the Navier–Stokes equation, coupled with the convection-diffusion equation, to simulate the dispersive behaviors of the reactive gases inside the porous preform. The interactive molecular diffusion of methane (CH4), ethylene (C2H4), acetylene (C2H2), and benzene (C6H6) were modeled by considering the multi-step hydrocarbon reactions between the species. The hydrocarbon concentration changes, resulting from the carbon deposition on the preform surface, were computed to predict the evolution of the preform density and porosity. The current surface area of the preform was then determined based on the current porosity. The numerical results for the average preform density agreed well with the experimental data. In addition, the present model can provide detailed simulations of the temporal and spatial evolution of the preform density that cannot be experimentally observed. The effectiveness and utility of the developed model could benefit the design of CVI reactors and processes and minimize the need for test runs when processing conditions change.

Quantifying leakage and dispersion behaviors for sub-sea natural gas pipelines

Accidental leakages in underwater natural gas pipelines pose significant risks for the environment. Therefore, it is essential to estimate the leakage rate and monitor the methane diffusion range outside the pipeline, which is challengeable due to the presence of water. The main contribution of this work is to establish the leakage rate calculation and gas dispersion models using the experimental and numerical methods. Firstly, the influences of the pressure, leakage orifice diameter and water depth on the leakage rate and gas dispersion process were revealed through experiments. Further, according to the established experimental facilities, the numerical models including the integral mathematical model and computational fluid dynamic model were established. According to the axial velocity, radius and spring height, the two model were compared and validated. In addition, a field simulation using computational fluid dynamic model was conducted to quantify the gas leakage rate and dispersion prediction, predicting the rising time and diffusion range. The results indicate that the quantifying estimation correlation can predict the leakage rate in the sea with errors less than 16.47%, and the computational fluid dynamic model could predict the dispersion process more accurately while the integral mathematical model could predict the dispersion process more quickly.

CH4 diffusion in intra-pore of montmorillonite: From self-diffusion to fick-diffusion

Given the low permeability of clay (the main component of shale), diffusion is the main method for transport of methane in shale. Diffusion is a slower process than flow and permeation. Therefore, diffusion is a rate-controlling step during shale gas extraction. Diffusion of CH4 in clay mineral with water is investigated in detail. Dependences of self-diffusivity, Maxwell–Stefan (M–S) diffusivity and Fick diffusivity on various geological conditions are discussed. The practically important Fick diffusivity of methane in montmorillonite is calculated through molecular dynamics for the first time. Results show that self-diffusivity conventionally increases with temperature and its dependence on methane concentration follows the similar behavior of dependence on temperature. However, self-diffusivity displays a maximum at the water concentration of 2.5 molecules/unit-cell. The displacement distribution of CH4 calculated from molecule trajectory follows a logarithmic normal distribution, whose mean reaches the maximum at the same water concentration, resulting in the above-mentioned unusual behavior of self-diffusivity. M–S and Fick diffusivities go up with CH4 concentration and temperature while peak at the water content of 3.125 molecules/unit-cell. On the basis of self-diffusivity and M–S diffusivity, diffusion correlation parameter, Dii, among methane molecules is computed. The diffusion correlation increases with methane concentration, water content, and temperature. Consequently, high methane concentration, temperature and moderately increasing water content result in fast methane diffusion, whereas excessively high water concentration hinders methane diffusion.

Soot aggregate morphology deduced from thermophoretic sampling in coflow laminar methane diffusion flames at pressures up to 30 bar

A high pressure combustion chamber, housing a co-flow burner and a novel thermophoretic soot sampling system, was used to collect soot samples from methane-air diffusion flames at pressures from 5 bar to 30 bar. To have tractable diffusion flames with the same residence times, constant methane flow rate of 0.55 mg/s was maintained throughout the experiments. Soot aggregates were collected on 3 mm diameter transmission electron microscope (TEM) grids and the images of the soot aggregates were captured by a TEM device. Sampling was done at three heights above the burner rim at 3, 6, and 8 mm. An automated image processing method was used to analyze the TEM images of soot aggregates to infer the primary soot particle diameters and the aggregate morphology. The mean primary soot particle diameters increased from 5 to 15 bar at all three sampling locations and seemed to reach a peak around 15–20 bar at 6 and 8 mm height and start decreasing with further increase in pressure. On the other hand, mean primary soot diameter at 3 mm height increased slightly beyond 15 bar. In view of the measured soot concentrations and temperatures at the sampling locations at 3, 6, and 8 mm above the burner, the soot number density is argued to be increasing with increasing pressure. Throughout the all experimental conditions, fractal dimension of the aggregates in these methane diffusion flames did not exceed 1.4 which is much smaller than the accepted universal value of about 1.85. Morphological characteristics inferred from TEM images indicate that the aggregation is less prominent in methane flames in comparison to ethylene diffusion flames under similar conditions.

Influence of methane diffusion on geochemical characteristics of natural gas: a case study of the Shiqiantan area in Junggar Basin, China

ABSTRACT The geochemical study of natural gas has made a great contribution to oil and gas exploration. However, the diffusion of natural gas components will affect the determination accuracy of natural gas origin. This study supplemented the organic carbon content, rock pyrolysis, gas component composition, and component carbon isotope analysis and testing of the source rocks and natural gas samples. The results indicated that the methane and ethane carbon isotope values of the natural gas in the Shiqiantan area are heavy. It should be originated from Carboniferous source rocks. When the maturity and genesis index of natural gas are highly matched with source rocks, the abnormal drying coefficient of natural gas may be related to the methane diffusion.

Diffusion characteristics of methane in various rank coals and the control mechanism

The methane diffusion behavior is one of the key factors that influence the production potential of coalbed methane (CBM) reservoirs. In this study, low temperature nitrogen, carbon dioxide adsorption and methane isotherm adsorption experiments for various rank coal samples were carried out to investigate the characteristics of pore size distribution and gas adsorption-diffusion behavior. A mathematical model similar to the Langmuir equation was established to describe the methane diffusion in coals. The variation law of the Langmuir effective diffusion coefficient (DL) with coal rank and its influencing factors, control mechanism were analyzed. With the increase of coal rank, the development degree of micropore rises, and both the specific pore volume and specific surface area first decrease and then increase. The diffusion coefficient of methane in coals generally rises with the rise of pressure. The Langmuir volume rises with the rise of Ro,m (maximum vitrinite reflectance with oil). The influence of moisture and coal molecular structure on gas diffusion varies with coal rank. The moisture content has a more significant influence on the gas diffusion in the medium-rank coals than in the low- and high-rank coals. The outcomes may deepen our understanding of gas transport in coals and benefit the exploration of CBM.

Effects of benzene, cyclo-hexane and n-hexane addition to methane on soot yields in high-pressure laminar diffusion flames

Effects of doping high pressure methane diffusion flames with benzene, cyclo-hexane and n-hexane were investigated to assess the sooting propensity of three hydrocarbons with six carbons at elevated pressures. Amount of liquid hydrocarbons added to methane constituted 7.5% of the total carbon content of the fuel stream. The pressure range investigated extended up to 10 bar and the experiments were carried out in a high pressure combustion chamber capable of establishing stable laminar diffusion flames with various fuels at elevated pressures and was used in similar experiments previously. Temperatures and soot volume fractions were measured using the spectral soot emission technique capturing spectrally-resolved line-of-sight intensities which were subsequently inverted using an Abel type algorithm to obtain radial distributions assuming that the flames are axisymmetric. The total mass carbon flow of the fuel stream was kept constant at 0.524 mg/s in neat methane, benzene-doped methane, cyclo-hexane-doped methane, and n-hexane-doped methane flames to have tractable measurements at all pressures. Measured maximum soot volume fractions and evaluated maximum soot yields showed that benzene-doped methane flame had the higher values than cyclo-hexane doped methane flames which in turn had higher values than n-hexane doped methane flames at all pressures. Sooting propensity dependence of the three hydrocarbons on pressure can be ranked as, in descending order, n-hexane, cyclo-hexane, and benzene; however, the difference between pressure dependencies of n-hexane and cyclo-hexane was within the measurement error margins. Ratio of soot yields of benzene to n-hexane doped flames changed from about 2 at 2 bar to 1.2 at 10 bar; the ratio of benzene to cyclo-hexane doped flames showed similar trends.

Evidence on the formation of dimers of polycyclic aromatic hydrocarbons in a laminar diffusion flame

The role of polycyclic aromatic hydrocarbons (PAHs) in the formation of nascent soot particles in flames is well established and yet the detailed mechanisms are still not fully understood. Here we provide experimental evidence of the occurrence of dimerization of PAHs in the gas phase before soot formation in a laminar diffusion methane flame, supporting the hypothesis of stabilization of dimers through the formation of covalent bonds. The main findings of this work derive from the comparative chemical analysis of samples extracted from the gas to soot transition region of a laminar diffusion methane flame, and highlight two different groups of hydrocarbons that coexist in the same mass range, but show distinctly different behavior when processed with statistical analysis. In particular, the identified hydrocarbons are small-to-moderate size PAHs (first group) and their homo- and heterodimers stabilized by the formation of covalent bonds (second group).

The impact of diffusive transport of methane on pore-water and sediment geochemistry constrained by authigenic enrichments of carbon, sulfur, and trace elements: A case study from the Shenhu area of the South China Sea

Sulfate-driven anaerobic oxidation of methane (SD-AOM) controls methane release from marine sediments to the ocean. A variety of authigenic precipitates within sediments has been widely used to identify past occurrences of SD-AOM. However, the lack of a systematic evaluation of the formation conditions of these precipitates sometimes impedes the recognition of SD-AOM in past environments characterized by diffusive transport of methane. Carbon, sulfur, and trace element geochemistry of pore-water and sediments was investigated at a site affected by upward methane diffusion in the Shenhu area of the South China Sea. Here, the sulfate-methane transition zone (SMTZ) is located ~7.6 m below the seafloor based on sulfate and methane concentrations. The slope of δ34S vs. δ18O values of sulfate is consistent with diffusive transport of methane. Concentration and isotope profiles of pore-water species point to diffusive rather than advective transport of solutes. Enhanced sulfate reduction inferred from δ34S values of sulfate agrees with the local occurrence of relatively abundant, 34S-enriched iron sulfide minerals. The time required to produce the observed authigenic iron sulfides around the SMTZ is estimated to be ~1.1 ka based on the amount of iron sulfide minerals and the present diffusive flux of sulfate. No enrichment of authigenic carbonate, barium (Ba), and molybdenum (Mo) was detected in the studied sediments. This observation is consistent with the calculation that the current fluxes of pore-water calcium, magnesium, Ba, and Mo are too low to allow for authigenic enrichments. The low fluxes are largely controlled by diffusion, which facilitates the formation of 34S-enriched iron sulfide minerals. The observed enrichment patterns – unlike those of sediments affected by advective seepage – are expected to be prevalent in modern and ancient continental-margin sediments, and may contribute to the identification of past methane-rich zones and overlying SD-AOM zones as sinks for methane in the geological record.

A 6M digital twin for modeling and simulation in subsurface reservoirs

A 6M digital twin for modeling and simulation in subsurface reservoirs

Coalbed methane production of a heterogeneous reservoir in the Ordos Basin, China

Macrolithotypes control the pore-fracture distribution heterogeneity in coal impacting gas adsorption, diffusion coefficient, and the gas flow. However, the behaviors that are impacted by coal macrolithotype are traditionally ignored and the drainage radius that linked to the macrolithotype contributions are lack of systematic research all the way. Thus, to evaluate the gas production relationship for macrolithotype, four blocks (bright, semi-bright, semi-dull, and dull) were obtained from the same seam. The methane absorption, diffusion, and permeability data were obtained and used in mathematical modeling to identify well production, drainage radius, and well-interference locations. Production data for 104 wells and 11 test wells were also obtained and with COMET3 simulation software to validate the mathematical model. From the bright to dull coal: there was a lower micropore contribution, a lower gas capacity, and lower gas diffusion coefficient. The cleat frequency and aperture differences impacted the reservoir permeability with bright coal having 195% higher than the dull coal. As these macrolithotype differences impact the coalbed methane (CBM) development and cause heterogeneous gas flow systems within the resource, a coupled mathematical model for comprehensive consideration of the adsorbent-diffusion-seepage was established capturing the macrolithotype contribution to the pressure propagation, gas flow, and gas drainage radius estimation. The bright coal had the higher Langmuir volume, CH4 diffusion coefficient, and permeability thus, producing a higher gas production volume, and a larger gas drainage area than the dull macrolithotype. Thus, the macrolithotype diversity and its stratification cause partitioning of the gas flow with multi-stage pressure drop effecting exploration and development. In addition, the well interference was also observed and show that the wells of bright and semi-bright coal have a continuous pressure drop, which achieving the purpose of overall pressure reduction.

Effect of fuel and oxygen concentration toward catalyst encapsulation in water-assisted flame synthesis of carbon nanotubes

Early catalyst deactivation through the encapsulation of catalyst nanoparticles by amorphous carbon layer due to the oversupply of carbon source is one of the key elements that inhibit efficient carbon nanotubes (CNT) synthesis in flame environment. The present study utilizes methane diffusion flame with water vapor additive to analyze the effectiveness of the novel water-assisted synthesis of CNT in reducing amorphous carbon formation. Flame shape, temperature, and CNT growth region within the water-assisted flame were analyzed at varying fuel and oxygen concentration. The amorphous carbon layer thickness (ACLT) is analyzed through cross-sectional analysis of the CNT growth region using a developed bend wire method. A 50% increase in fuel concentration results in the increase in axial extent of the growth region by 37.5% followed by an increase in the ACLT by 20%. Meanwhile, an 8% increase in oxygen concentration results in the decrease of growth region axial extent by 82% with a 35% reduction in ACLT. On average, water vapor additive produces an additional 17.3% reduction in ACLT in any flame composition. Both the reduction in ACLT as oxygen and water concentration is independently increased happen due to the carbon supply regulation. However, since the change in the growth temperature that happens at varying oxygen concentration is not observed at varying water concentration, the carbon supply regulation in both situations is remarkably different as explained by the water-gas shift reaction.