Methane Bubble Formation Research Articles

Multi-Mode Total Focusing Method (MTFM) to Detect High-Temperature Hydrogen Attack (HTHA) – A Review

High temperature hydrogen attack (HTHA) is a commonly observed harm component in carbon steels exposed to high temperature and pressure in a hydrogen-rich environment. Hydrogen together with carbon responds to produce methane. The formation of methane bubbles in steel can lead to loss of fracture toughness and lead to intergranular cracking. The main challenge of this problem lies in early warning systems that can detect these bubble clusters before they reach the advanced stage. Several advanced ultrasonic inspections have been developed over the years due to the challenges of inspecting materials for defects and discontinuities. These cover time-of-flight diffraction (ToFD), phased array ultrasonic testing (PAUT), total focusing method (TFM), multi-mode total focusing method (MTFM), and others. However, these ultrasonic techniques used are typically used to detect all possible material defects. This paper briefly discusses the advantages and disadvantages of these techniques. MTFM has been successfully applied to characterize isolated or clustered signs, whether tilted or not, using high-frequency probes. The defects grouped in this paper are believed to be methane bubbles or HTHA. ToFD defect screening before aims to save time and money.

The synergistic effect between imidazole reagents and kinetic hydrate inhibitors

In recent years, kinetic hydrate inhibitors (KHIs), as an important hydrate control method, have received extensive attention in the oil and gas industry. During the usage of KHIs, it is often necessary to add KHI synergists to maximize their hydrate inhibition performance. Therefore, the development of synergistic reagents with high-efficiency to improve inhibition performance of the KHIs is of great significance for the prevention and treatment of hydrates. In this work, we screened a series of imidazole reagents as KHIs synergists through experiments, and studied the synergistic mechanism between imidazole reagents and KHIs by molecular simulation. Our results demonstrate that the addition of low concentrations of imidazole reagents alone has no obvious inhibitory effect on methane hydrates, but show obvious synergistic hydrate inhibition effect with KHIs. Among the tested imidazole reagents, 2-propylimidazole shows the best synergistic effect with KHIs. The synergistic effect between 2-propylimidazole and KHIs is mainly reflected in two aspects. On the one hand, the adsorption of 2-propylimidazole molecules on the hydrate surface hinders the formation of hydrate. On the other hand, 2-propylimidazole molecules can promote the formation of methane bubbles and reduce the driving force for the formation of methane hydrates. Our research further expands the application of imidazole reagents in the prevention and control of hydrates, which is beneficial to enhance the commercial application of KHIs.

Effect of ethanol concentration on methane hydrate decomposition: MD simulation insights

The controllability of mining is a key factor affecting the commercial application of methane hydrates, and the addition of chemical additives can significantly accelerate the mining process. However, the effect of additive concentration on hydrate decomposition is not yet well understood. In this study, we systematically investigate the effect of ethanol concentration on the decomposition of methane hydrate under varying thermodynamic conditions using molecular dynamics (MD) simulations. To quantitatively characterize the decomposition process and mechanism of methane hydrates, the combination of angular order parameter (AOP), radial distribution function (RDF), mean square displacement (MSD), diffusion coefficients and system energy was for the first time used. The results showed that the addition of ethanol contributed to the formation of methane bubbles and accelerated the decomposition of hydrates. The mass transfer effect of ethanol molecules and the reconstruction of the hydrogen bond network of water molecules determined the stability of hydrates. From 0 to 40 mol% ethanol concentration, the hydrate decomposition increased with increasing the concentration of ethanol. Both increasing the temperature and decreasing the pressure are beneficial to the decomposition of the hydrate system. These results provide the selection of optimal ethanol concentration for the decomposition of methane hydrate and reveal its decomposition mechanism, and shed important light for the controllable production of gas hydrates.

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.

The role of sediment structure in gas bubble storage and release

AbstractEbullition is an important pathway for methane emission from inland waters. However, the mechanisms controlling methane bubble formation and release in aquatic sediments remain unclear. A laboratory incubation experiment was conducted to investigate the dynamics of methane bubble formation, storage, and release in response to hydrostatic head drops in three different types of natural sediment. Homogenized clayey, silty, and sandy sediments (initially quasi‐uniform through the depth of the columns) were incubated in chambers for 3 weeks. We observed three distinct stages of methane bubble formation and release: stage I—microbubble formation‐displacing mobile water from sediment pores with negligible ebullition; stage II—formation of large bubbles, displacing the surrounding sediment with concurrent increase in ebullition; and stage III—formation of conduits with relatively steady ebullition. The maximum depth‐averaged volumetric gas content at steady state varied from 18.8% in clayey to 12.0% in silty and 13.2% in sandy sediment. Gas storage in the sediment columns showed strong vertical stratification: most of the free gas was stored in an upper layer, whose thickness varied with sediment grain size. The magnitude of individual ebullition episodes was linearly correlated to hydrostatic head drop and decreased from clayey to sandy to silty sediment and was in excess of that estimated from gas expansion alone, indicating the release of pore water methane. These findings combined with a hydrodynamic model capable of determining dominant sediment type and depositional zones could help resolve spatial heterogeneities in methane ebullition at medium to larger scales in inland waters.

Methane bubble formation and dynamics in a rectangular bubble column: A CFD study

Methane is a suitable material in biomass production industries due to its non-toxicity and volatility. The study of bubble formation of methane provides detailed information about the methane bubble hydrodynamics (i.e., bubble size, shape, velocity and interaction), which assist in improvement of biomass productions. This paper employs the volume of fluid (VOF) method to simulate the methane bubble dynamics in a bubble column filled with quiescent water. Excellent agreement between simulation results of air bubble dynamics and the experimental data approve the viability of the method in prediction of bubble formation and rising. Since various bubble densities have almost identical formation characteristics, particularly similar necking process, this study compares the methane bubble formation with that of existing experiments of air bubble dynamics to validate methane bubble dynamics. We simulate the effect of orifice gas velocity and orifice diameter size on the methane bubble dynamics. Furthermore, we examine the effect of orifice arrangement on the bubble detachment and coalescence. The results show that the specific distance between orifices results in producing small and uniform bubbles, which improves the gas and liquid interfacial area and bubble column efficiency.

Prediction of the phase equilibria of methane hydrates using the direct phase coexistence methodology.

The direct phase coexistence method is used for the determination of the three-phase coexistence line of sI methane hydrates. Molecular dynamics (MD) simulations are carried out in the isothermal-isobaric ensemble in order to determine the coexistence temperature (T3) at four different pressures, namely, 40, 100, 400, and 600 bar. Methane bubble formation that results in supersaturation of water with methane is generally avoided. The observed stochasticity of the hydrate growth and dissociation processes, which can be misleading in the determination of T3, is treated with long simulations in the range of 1000-4000 ns and a relatively large number of independent runs. Statistical averaging of 25 runs per pressure results in T3 predictions that are found to deviate systematically by approximately 3.5 K from the experimental values. This is in good agreement with the deviation of 3.15 K between the prediction of TIP4P/Ice water force field used and the experimental melting temperature of ice Ih. The current results offer the most consistent and accurate predictions from MD simulation for the determination of T3 of methane hydrates. Methane solubility values are also calculated at the predicted equilibrium conditions and are found in good agreement with continuum-scale models.

Dissociation of methane hydrate in aqueous NaCl solutions.

Molecular dynamics simulations of the dissociation of methane hydrate in aqueous NaCl solutions are performed. It is shown that the dissociation of the hydrate is accelerated by the formation of methane bubbles both in NaCl solutions and in pure water. We find two significant effects on the kinetics of the hydrate dissociation by NaCl. One is slowing down in an early stage before bubble formation, and another is swift bubble formation that enhances the dissociation. These effects arise from the low solubility of methane in NaCl solution, which gives rise to a nonuniform spatial distribution of solvated methane in the aqueous phase. We also demonstrate that bubbles form near the hydrate interface in dense NaCl solutions and that the hydrate dissociation proceeds inhomogeneously due to the bubbles.

CFD and experimental study on methane hydrate dissociation. Part II. General cases

AbstractRate equations proposed in Part I were extended to dissociation processes induced by different methods such as depressurization, raised temperature, or simultaneous changes in temperature and pressure. For all cases, the dissociation rate of hydrate into the two‐phase coexisting region (V‐LW) can be expressed by a general form of the product of the rate constant and the logarithm of the solubility ratio of methane in the aqueous solution, xR to xS: xR is the solubility of methane in the aqueous solution that is hypothetically in equilibrium with the hydrate phase under the dissociation pressure and temperature, and xS is the equilibrium solubility of methane in water under the same dissociation condition. The virtual solubility of methane, xR, can be estimated by extrapolating the solubility curve of methane under the conditions in equilibrium with the stable hydrate to the region at which the dissociation occurs by a simple thermodynamic treatment. The dissociation equation was applied to experimental results of dissociation processes induced by depressurization with methane bubble formation. The experimentally observed dissociation rates were well correlated by the rate equation with the dissociation constants determined in Part I. The results suggest that the dissociation equation can be applied generally to dissociation processes that occur by different methods, and the rate constant is a universal parameter within the experimental range examined in this work. © 2007 American Institute of Chemical Engineers AIChE J, 2007.

Analysis of dissolved noble gases in the porewater of lacustrine sediments

Here we present a new method for the sampling and quantitative extraction of dissolved He, Ne, Ar, Kr, and Xe from lake sediment samples leading to determinations of porewater noble gas concentration profiles and the isotopic ratios 3He/4He, 20Ne/22Ne, and 40Ar/36Ar. Bulk sediment is transferred from a sediment core into standard Cu sample tubes without exposure to the atmosphere or other gas reservoirs. The noble gases are then extracted from the porewater by degassing the sediment in an evacuated extraction vessel and analyzed following standard mass spectrometric procedures. In tests of the new method using 0.8 to 1.4 m long sediment cores from two Swiss lakes, analytical uncertainties were only slightly greater than those of standard water samples. The majority of porewater noble gas concentrations and isotopic ratios were found to correspond closely to those measured in the overlying lake water. Because these values reflect water temperature and salinity during atmospheric equilibration at the lake surface, historical conditions are expected to be archived further downcore in the sediment porewater. This method therefore has great potential for paleolimnological reconstructions. The formation of methane bubbles in anoxic sediment layers is one process that may alter gas distributions. However because the lighter noble gases are most sensitive to degassing effects, noble gas data can be used to detect this process. In addition, noble gas data can yield information on the transport processes occurring in the sediment pore space and on the input of water or gas to the sediment from external sources.

A study of fatigue crack propagation in prior hydrogen attacked pressure vessel steels

A study has been made of the effects of prior hydrogen attack damage on fatigue crack propagation behavior in commercial pressure vessel steels. Quenched and tempered Mn-Mo-Ni steel (ASTM A533B Class 2) and normalized and tempered 2.25Cr-1Mo steel (ASTM A387 Class 2 Grade 22) were exposed to gaseous hydrogen atmospheres for up to 1480 hours at hydrogen pressures of 12.4 to 17.2 MPa and temperatures of 550° to 600°C and tested in fatigue. Mild degrees of hydrogen damage, characterized by limited methane bubble formation with no appreciable decarburization, were found to increase growth rates slightly at near-threshold stress intensities. Severe degrees of hydrogen damage, characterized by extensive intergranular bubble formation and decarburization with associated large reductions in strength and toughness, were found to have no further influence on near-threshold growth rates. The minor influence of prior hydrogen damage on fatigue crack extension, even for cases of severe attack, is attributed to result from two mutually competitive mechanisms, namely, the creation of methane-filled voids on prior austenite grain boundaries, whichincreases growth rates, and the enhancement in crack closure from decarburization-induced softening and rough cavitated intergranular fracture surfaces, whichdecreases growth rates.

The kinetics of hydrogen attack of steels

The hydrogen attack of steel involves the formation of methane bubbles along the grain boundaries and their subsequent link-up to form fissures. This paper presents a detailed model for the kinetics of growth of such methane bubbles. The model considers two parallel processes which can control the growth—one involving the bubble growth by direct power-law creep and the other involving combinations of surface diffusion, grain boundary diffusion and matrix accommodation processes. The proposed model is more general and complete than the earlier ones and considers for the first time the possibility of bubble growth being controlled by surface diffusion and accommodation processes. The predictions of the model are shown to compare well with the experimental results obtained in our lab and with the literature data. The model also indicates the relative importance of lower carbon activity and increased creep stength of steel to its hydrogen attack resistance.