Gas Coexistence Research Articles

Gas Content and Geological Control of Deep Jurassic Coalbed Methane in Baijiahai Uplift, Junggar Basin

Deep coalbed methane (CBM) resources are abundant in China, and in the last few years, the country’s search for and extraction of CBM have intensified, progressively moving from shallow to deep strata and from high-rank coal to medium- and low-rank coal. On the other hand, little is known about the gas content features of deep coal reservoirs in the eastern Junggar Basin, especially with regard to the gas content and the factors that affect it. Based on data from CBM drilling, logging, and seismic surveys, this study focuses on the gas content of Baijiahai Uplift’s primary Jurassic coal seams through experiments on the microscopic components of coal, industrial analysis, isothermal adsorption, low-temperature CO2, low-temperature N2, and high-pressure mercury injection. A systematic investigation of the controlling factors, including the depth, thickness, and quality of the coal seam and pore structure; tectonics; and lithology and thickness of the roof, was conducted. The results indicate that the Xishanyao Formation in the Baijiahai Uplift usually has a larger gas content than that in the Badaowan Formation, with the Xishanyao Formation showing that free gas and adsorbed gas coexist, while the Badaowan Formation primarily consists of adsorbed gas. The coal seams in the Baijiahai Uplift are generally deep and thick, and the coal samples from the Xishanyao and Badawan formations have a high vitrinite content, which contributes to their strong gas generation capacity. Additionally, low moisture and ash contents enhance the adsorption capacity of the coal seams, facilitating the storage of CBM. The pore-specific surface area of the coal samples is primarily provided by micropores, which is beneficial for CBM adsorption. Furthermore, a fault connecting the Carboniferous and Permian systems (C-P) developed in the northeastern part of the Baijiahai Uplift allows gas to migrate into the Xishanyao and Badaowan formations, resulting in a higher gas content in the coal seams. The roof lithology is predominantly mudstone with significant thickness, effectively reducing the dissipation of coalbed methane and promoting its accumulation.

Accelerated corrosion by nitrate-reducing bacteria and gaseous hydrogen permeation of X80 steel

Effects of hydrogen gas and Nitrate-Reducing Bacteria (NRB) on the corrosion and hydrogen permeation dynamics of X80 steel in simulated pipeline condensate water was investigated. Weight-loss analysis and electrochemical tests show a direct correlation between NRB-induced corrosion of X80 steel and the presence of hydrogen gas, particularly evident with escalating hydrogen blending ratios. Notably, corrosion primarily manifested through the formation of localized pitting, exhibiting a positive correlation with hydrogen blending ratios. Furthermore, the hydrogen permeating results show there is a synergistic enhancement of hydrogen permeating facilitated by the coexistence of hydrogen gas and NRB. Quantitative assessments of hydrogen permeability parameters highlighted significant amplifications in hydrogen diffusion flux (J) and apparent hydrogen concentration (Capp) under the combined influence of hydrogen and NRB, underscoring the complex interplay between these factors.

Research and Experiment on Efficient Fracturing Technology of Tight Gas in Qaidam Basin

Abstract The lithology of the Huangguamao block reservoir in the Qaidam Basin is complex and dense. The difference between gas and water is not obvious. The gas reservoir has the characteristics of low porosity, low permeability, low saturation, and coexistence of gas and water. The problem of water output after fracturing and short stable production period has put forward higher requirements for improving production and efficiency in tight gas horizontal wells. Based on successful cases of tight gas reservoir fracturing both domestically and internationally, as well as the experience of unconventional reservoir volume transformation in the Qaidam Basin, this study conducts a historical analysis of reservoir characteristics and measures transformation in the Huangguamao block, evaluates the difficulties of measures transformation, optimizes the formation of two process ideas: high viscosity liquid controlled sand lifting fracturing and reverse composite fracture network fracturing, and prepares fracturing programme for MP2 and MP3 wells. After on-site fracturing, two processes were evaluated and analyzed, and the results showed that the reverse composite fracturing process, which takes into account both crack length and complexity, can effectively increase the transformation volume, provide more sufficient formation energy, increase production, and have stronger adaptability.

Wetting Preference of Silica Surfaces in the Context of Underground Hydrogen Storage: A Molecular Dynamics Perspective.

The growing interest in large-scale underground hydrogen (H2) storage (UHS) emphasizes the need for a comprehensive understanding of the fundamental characteristics of subsurface environments. The wetting preference of subsurface rock is a crucial parameter influencing the H2 flow behavior during storage and withdrawal processes. In this study, we utilized molecular dynamics simulation to evaluate the wetting preference of the silica surface in subsurface hydrogen systems, with the aim of addressing disparities observed in experimental results. We conducted an initial comprehensive assessment of potential models, comparing the wettability of five common silica surfaces with different surface morphologies and hydroxyl densities in CO2-H2/water/silica systems against experimental data. After introducing the INTERFACE force field as the most accurate potential model for the silica surface, we evaluated the wetting behavior of the α-quartz (101) surface with a hydroxyl density of 5.9 number/nm2 under the impact of actual geological storage conditions (333-413 K and 10-30 MPa), the coexistence of cushion gases (i.e., CO2, CH4, and N2) at various mole fractions, and pH levels ranging from 2 to 11 characterized through considering the negative charges of 0 to -0.12 C/m2 via deprotonation of silanol on the silica surface. Our results indicate that neither pressure nor temperature has a significant impact on the wetness of the silica in the case of pure H2 (single component UHS operations). However, when CO2 coexists with H2, especially at higher mole fractions, an increase in pressure and a decrease in temperature lead to higher contact angles. Moreover, when the mole fraction of cushion gas ranges from 0 to 1, the contact angle increases 20, 9.5, and 4.5° for CO2, CH4, and N2, respectively, on the neutral silica substrate. Interestingly, at higher pH conditions where the silica surface carries a negative charge, the contact angle considerably reduces where surface charges of -0.03 and -0.06 C/m2 result in an average reduction of 20 and 80% in the contact angle, respectively. More importantly, at a pH of ∼11 (-0.12 C/m2), a 0° contact angle is observed for the silica surface under all temperatures, pressures, types of cushion gases, and varying mole fractions.

Research on Production Behavior of A Fractured Well in Gas Hydrate Reservoirs Considering Multilayer

Abstract Natural gas hydrates are internationally recognized as a new clean energy source with high prospects for commercial development. However, the vast majority of hydrates are hosted in oceanic clayey silty sediments, which are inefficient to exploit due to the extremely low effective permeability. In this paper, a novel development mode, that is, multilayer commingled production with a hydraulically fractured well, is proposed for the multilayered reservoir where hydrate and free gas coexist, to enhance productivity. Especially, the co-production behavior was numerically investigated, with the multilayered reservoir containing a hydrate-bearing layer, a three-phase layer, and a free gas layer located in the Shenhu Area as the geological background. The results indicated that the fracture provided flow channels for gas production in the multilayer, as well as greatly increasing the pressure drop spreading zone and changing the hydrate decomposition mode from around the wellbore to along the fracture surface. Compared with improving the fracture conductivity, increasing the fracture length is more effective for improving hydrate decomposition efficiency and co-production capacity. Specifically, when the fracture half-length was 30 m and the fracture conductivity was 100 D·cm, the cumulative gas production reached 949×104 m3 at 3600 days, which was 3.28 times that of the unfractured scenario. Additionally, fracturing the free gas layer hardly affects productivity as the free gas can be well-recovered by the wellbore. Overall, the proposed mode is promising for multilayered gas hydrate reservoirs, provides references for the co-production plan design, and contributes to the scale mining of hydrates.

Metal-enhanced carbon-nitrogen material for selective detection of hazardous gases: Insights from interface electronic states

In this study, utilizing density functional theory, the C3N1 monolayer modified by Ir, Pd, Pt, and Rh atoms (Ir/Pd/Pt/Rh-C2N1) was chosen for selective adsorption of C2H2 amidst multiple gases (H2O, C2H2, and C4H10O2). According to the results of cohesive energy and ab initio molecular dynamics simulations, it is indicated that precious metal atoms can be stably anchored on the monolayer while enhancing the conductivity of the material The analysis of the electrostatic potential and work function determined the highly active sites and electron release capacity. In addition, the adsorption energy and distance disclosed the gas-solid interface structure of multiple gases on the Ir/Pd/Pt/Rh-C2N1 monolayer. Importantly, C2H2 exhibits strong responses to p-type semiconductor Pt-C2N1 and n-type semiconductor Ir-C2N1, respectively. Crystal Orbital Hamilton Population reveals the difference in adsorption energy due to modifications involving four precious metals. Interestingly, for the first time, the density of states calculation reveals that under the coexistence of multiple gases, the Pt/Ir-C2N1 monolayer effectively eliminates the interference of other gases and has a unique response only to C2H2. In real situations, with the basis of Gibbs free energy and Einstein's law of diffusion, it was determined that Pt-C2N1 and Ir-C2N1 showed excellent hydrophobicity, a wider temperature range, and a low diffusion activation energy barrier. In summary, Pt-C2N1 and Ir-C2N1 detect C2H2 without interference, maintaining fundamental principles, responsiveness, stability, and versatility unaffected by external factors.

Water-bearing properties of high rank coal reservoir and the effect on multiphase methane

Coal reservoirs commonly exhibit the coexistence of gas and water, wherein the distribution of water plays a crucial role in influencing methane storage and transportation. The presence of water affects methane adsorption, and models predicting adsorbed methane that do not account for water conditions can introduce errors, posing challenges in meeting practical production needs. In this study, through a series of centrifugation experiments and nuclear magnetic resonance (NMR) adsorption experiments, the distribution characteristics of bound water and movable water in reservoirs and their influence on multiphase methane were assessed. The results indicate that under water-bearing conditions, there exists specific methane in the adsorption space, exhibiting properties similar to free methane. In terms of the relaxation characteristics, the presence of water affects both the peak area and morphology of the T2 spectra of multiphase methane. Based on the NMR response characteristics of adsorbed methane at different water saturations, we proposed and established a theoretical model for predicting the content and density of adsorbed methane under varying water saturations. Additionally, utilizing two-dimensional NMR technology, a simple identification spectrum for multiphase methane was established. The study suggests that the presence of water delays the time of adsorption equilibrium for methane. At low water saturations, water primarily inhibits the adsorption rate of methane. However, with increasing pressure and water saturation, water predominantly suppresses the adsorption capacity of methane. At low water saturations, the bound water has a significant effect on adsorbed methane. Conversely, as water saturations increase, movable water gradually occupies adsorption sites with weaker adsorption potential. It is worth noting that with an increase in water saturation, the content of adsorbed methane decreases, but the adsorption phase density may not necessarily decrease. These research findings provide new insights into the interaction mechanisms between water and methane in water-bearing coal reservoirs.

Coexistence of Methane Hydrate and Gas Associated with the Difference in Thermodynamic Stabilities of Hydrate in the Pores of Sediments from Shenhu, South China Sea

Coexistence of Methane Hydrate and Gas Associated with the Difference in Thermodynamic Stabilities of Hydrate in the Pores of Sediments from Shenhu, South China Sea

Study on the Influence of Different Gas Concentrations on Coal Spontaneous Combustion

ABSTRACT The coexistence of gas and spontaneous coal combustion poses a serious threat to mine production safety. Therefore, studying the compound disaster of gas and spontaneous coal combustion and revealing the impact of gas concentration on the characteristic parameters of coal spontaneous combustion are of great significance for understanding the mechanism of compound disasters involving coal and gas and for comprehensive management. Based on theoretical analysis, this paper selects four typical types of cannel coal、non-caking coal、meager coal、anthracite coal as research subjects, and conducts programmed heating experiments on coal samples under different gas concentration conditions, aiming to provide a theoretical basis for the prevention and control of spontaneous coal combustion in a gas environment. Experimental results show that with the increase in temperature of the coal samples, the oxygen consumption rate and heating emission intensity exhibit a trend of slow increase followed by rapid increase. With the increase in gas concentration in the environment, the production of CO and CO2, oxygen consumption rate, and heat emission intensity of coal spontaneous combustion all show a decreasing trend. This is mainly because the high gas concentration occupies the adsorption sites on coal molecules, thereby hindering the reaction between coal and oxygen. Additionally, by calculating the thermodynamic parameters of the coal samples, it was found that the apparent activation energy increases with the increase in gas concentration, further confirming that the increase in gas concentration in the environment significantly inhibits the spontaneous combustion process of coal.

Motility-induced phase separation of soft active Brownian particles

Motility-induced phase separation (MIPS) is the hallmark of non-equilibrium phase transition in active matter. Here, by means of Brownian dynamics simulations, we determine the phase behavior and the critical point for phase separation induced by motility of a two-dimensional system of soft active Brownian particles, whose interaction is modeled by the generalized purely repulsive Weeks–Chandler–Andersen potential. We refer to this model as soft active Brownian particles. We determine and analyze the influence of particle softness on the MIPS and show that the liquid–gas coexistence region is wider, the softer the interparticle interactions becomes. Moreover, the critical value of the self-propulsion velocity at which diluted and dense phases start to coexist also increases; as a consequence, the softer the particle interaction is, the bigger self-propulsion velocities are needed in order to observe a MIPS.

CO2 Compression and Liquefaction Processes Using a Distillation Column for the Flexible Operation of Transportation

Impurities in the CO2 stream should be removed to prevent eventual phase changes in CO2 transportation because a two-phase flow caused by the phase change in the pipeline necessitates additional overpressure and can induce equipment damage. In this study, CO2 compression and liquefaction (CCL) processes with a distillation column were used to remove non-condensable impurities and were compared with those with a flash. Three different feeds with a flow rate of 50.1 t/h (400,500 t/y) were supplied to the CCL processes and compressed to 65 bar to gauge pressure (barg) and 20 °C. Although the CO2 mixtures obtained through dehydration and flashing met the purity requirements for transportation and storage recommended in literature, the flash-separated CO2 product at 65 barg demonstrated the coexistence of gas and liquid phases, which restricted the temperature window for liquid CO2 transportation. When the distillation column was used instead of the flash, the operating temperature window at 65 barg widened by 3–6 °C owing to the high purity of CO2. However, the levelized cost of CO2 liquefaction (LCCL) increased by 2–4 $/t-CO2 varying with the feed purity because the distillation column consumed more cooling and heating duties than the flash. This study highlighted that a two-phase flow existed under certain operating conditions despite a high purity of CO2 (over 97 mol%), and the distillation column enhanced the operability of liquid CO2 transportation.

Deformation-induced phase separation of active vesicles.

Many active materials, such as bacteria and cells, are deformable. Deformability significantly affects their collective behaviors and movements in complex environments. Here, we introduce a two-dimensional deformable active vesicle (DAV) model to emulate cell-like deformable active matter, wherein the deformability can be continuously adjusted. We find that changes in deformability can induce phase separation of DAVs. The system can transition between a homogeneous gas state, a coexistence of gas and liquid, and a coexistence of gas and solid. The occurrence of deformation-induced phase separation is accompanied by nonmonotonic changes in effective concentration, particle size and shape. Moreover, the degree of deformability also impacts the motility and stress within the dense phase following phase separation. Our results offer new insights into the role of deformability in the collective behavior of active matter.

Structure and Dynamic Inhomogeneity of Liquids on the Liquid–Gas Coexistence Curve Near the Triple Point

Structure and Dynamic Inhomogeneity of Liquids on the Liquid–Gas Coexistence Curve Near the Triple Point

Study on the Oxygen Content of Oxygen-Reducing Air Flooding Based on Anticorrosion Considerations

Abstract Oxygen-reduced air flooding (ORAF) is an important efficient development means for tight and low permeability reservoirs because of its low cost, low risk, and ability to penetrate into voids and substrates. However, the critical value of oxygen-reduced, which can take into account the cost and corrosion protection requirements, is uncertain. Therefore, the corrosion behavior of N80, J55, and 3Cr steels under simulated high pressure (30 MPa) gas injection with different oxygen content (1%–12% vol.) was studied by weight loss method and Ultra-Depth Three-Dimensional Microscope. Under this oxygen content, the corrosion behavior of N80, J55, and 3Cr steels under different total pressure (20–35 MPa), temperature (25℃–70℃), and coexistence of gas and water was further studied. The results show that under the condition of high temperature and high-pressure drying, when the oxygen content in the oxygen-reduced air is reduced to 5%, all three kinds of steels belong to slight corrosion (<0.025 mm/a), and the oxygen-reduced cost can still be kept at a low level. In addition, under the condition of 5% oxygen content, when the corrosion medium of the wellbore is only deoxidized gas, the corrosion is slight, so N80 steel at a lower cost can be used. However, when the wellbore contains liquid, the corrosion is serious, and anticorrosion measures need to be taken. This study can provide certain data support for the selection of oxygen content and the corrosion protection of gas-injection wells shaft in the process of high-pressure gas injection by ORAF.

Geophysical Characterization and Attenuation Correction Applied for Hydrate Bearing Sediments in Japan Sea

Estimation of rock properties from seismic data is important for exploration and production activities in the petroleum industry. Considering the compressional velocity—the speed of propagating body waves in formations—and the quality factor (Q)—a measure of the frequency-selective energy losses of waves propagating through formations—both properties are usually estimated from multichannel seismic data. Velocity is estimated during multichannel processing of seismic reflection data in either the time or depth domain. In marine seismic acquisition, Q can be estimated from the following sources: Vertical Seismic Profile (VSP) surveys, where sources are located near the sea surface and geophones are distributed at depth along a borehole; and multichannel reflection data, where sources are also located near the sea surface and receivers are distributed either at the sea surface (conventional seismic survey with streamers) or on the sea floor (use of nodes or Ocean Bottom Cables (OBC)). The aforementioned acquisition devices, VSP, conventional streamers, nodes, and OBCs are much more expensive than single-channel acquisition with one receiver per shot due to the cost of operation. There are numerous old and new datasets from academia and the oil industry that have been acquired with single-channel acquisition devices. However, there is a paucity of work addressing the estimation of velocity and Q from this type of equipment. We investigate the estimation of Q and velocity from single-channel seismic data. Using the windowed discrete Fourier transform for a single seismic trace, we calculate the peak and dominant frequency that changes with time. In the geologic environment, higher frequencies are attenuated at shallow depths (time), while lower frequencies remain at deeper positions. From the rate at which higher frequencies are attenuated with time, we estimate the effective quality factor (Qeff). However, when using Kirchhoff migration to process single-channel seismic data, events far from the vertical projection of the receiver contribute to the trace at a given time. Then, an underestimation of the effective quality factor occurs. To compensate for the effects of more distant events with lower-frequency content contaminating the shorter events, we propose a linear equation to correct the effective quality factor estimated from migrated seismic data. Effective Q and its correction are estimated in five single-channel seismic lines surveyed along the Joetsu Knoll, a SW-NE anticline structure on the eastern margin of the Sea of Japan. These results are linked to geomorphological and geological features and the velocity field. Joetsu Knoll is a known site of massive gas hydrates (GH), which occur in the first hundred metres of Neogene sediments and, together with gas chimneys, play an important role in seismic wave absorption. Qeff estimated from migrated seismic data maintains the spatial relationship between high and low Q regions. The region of low Q, which is below 124 and has an average value of 57, occurs near the anticlinal hinge and tends to coincide with the region in which the Bottom Simulating Reflector (BSR) resides. The coexistence of GH and free gas coincides with the very low P velocity gradient of 0.225 s−1. BSR occurrence, Qeff and the geometry of the Joetsu anticline testify to progressive gas hydrate depletion northward along the dome.

Gas-triggered resistive switching and chemiresistive gas sensor with intrinsic memristive memory

Combining gas sensing and resistive switching within a single device allows for its utilization as a gas sensor with built-in memory, capable of remembering a temporal change of the target gas concentration. In this work, coexistence of the hydrogen gas sensing and resistive switching in a capacitor-like Pt/TiO2/Pt structure is examined in detail. In particular, influence of the forming process on the sensor’s response to hydrogen and influence of the hydrogen concentration on the resistive switching properties are studied. An equivalent device model based on the obtained data is proposed. The built-in memory operation, i.e. triggering the resistive switch by a change of the hydrogen gas concentration, is demonstrated for various devices at various temperatures and various humidity levels. It is shown that the response magnitude in the high resistance state (HRS) and variance of the voltage which triggers the set process are critical parameters for a reliable device operation.

Introducing realistic distributions of gas bubble size and inclusion aspect ratio to model the coexistence of gas hydrate and free gas

The coexisting free gas within the hydrate stability zone is recognized by laboratory and field studies. The conventional resistivity method fails to obtain hydrate saturation in the sediment with gas hydrate and free gas because both of them are electrical insulators. Hydrate saturation can be obtained from a velocity log based on a certain rock physics model. However, few studies of rock physics have been conducted on gas and hydrate coexistence. In this study, a more realistic distribution of gas bubble size and inclusion aspect ratio was introduced into an effective medium model to elucidate how such coexistence affects velocity and attenuation at a broad-band frequency. The modeling results obtained indicate that such coexistence decreases P-wave velocity at low frequencies, while increases P- and S-wave velocities at high frequencies. The velocity decrease of P-wave at low frequencies is attributed to the overwhelming effect of gas bubble vibration, while it seems not to affect P-wave velocity at high frequencies significantly. Moreover, coexisting gas hydrate and free gas may strengthen P-wave attenuation at the broad-band frequency range. It is found that the damping of oscillating gas bubbles dominates P-wave attenuation at low frequencies, while squirt flow in the porous hydrate is more likely to contribute to P-wave attenuation at high frequencies. Hydrate morphology is also a significant factor affecting P-wave velocity and attenuation at high frequencies. This research reveals that rock physics modeling can provide insight into how to characterize the coexisting gas hydrate and free gas as referring to the acoustic velocity and attenuation at seismic, sonic, and ultrasonic frequencies.

Effect of gravity on phase transition for liquid–gas simulations

Direct simulations of phase-change and phase-ordering phenomena are becoming more common. Recently, qualitative simulations of boiling phenomena have been undertaken by a large number of research groups. One seldom discussed limitation is that large values of gravitational forcing are required to simulate the detachment and rise of bubbles formed at the bottom surface. The forces are typically so large that neglecting the effects of varying pressure in the system becomes questionable. In this paper, we examine the effect of large pressure variations induced by gravity using pseudopotential lattice Boltzmann simulations. These pressure variations lead to height dependent conditions for phase coexistence and nucleation of either gas or liquid domains. Because these effects have not previously been studied in the context of these simulation methods, we focus here on the phase stability in a one-dimensional system, rather than the additional complexity of bubble or droplet dynamics. Even in this simple case, we find that the different forms of gravitational forces employed in the literature lead to qualitatively different phenomena, leading to the conclusion that the effects of gravity induced pressure variations on phase-change phenomena should be very carefully considered when trying to advance boiling and cavitation as well as liquefaction simulations to become quantitative tools.

Dust Explosion Risk Assessment for Dry Dust Collector Based on AHP-Fuzzy Comprehensive Evaluation

Dry dust collectors are a typical dust and gas coexistence space. Dust explosion risk assessments should be performed for effective prevention and control of dust explosion accidents. In this paper, a dust explosion risk assessment index system for dust removal systems was constructed following the dust characteristics and the actual operation of a dry dust collector. The proposed system consisted of three first-level indexes (dust explosion characteristic parameters, environmental parameters in the dust collector box, use state of explosion prevention and control device) and seven second-level indexes (dust explosion sensitivity, dust explosion severity, temperature in the dust collector box, pressure difference between inlet and outlet, operating state of spark detection, operating the explosion venting disc, and operating state of the lock gas ash discharge valve). The analytic hierarchy process was adapted to calculate the weight of each index. Additionally, a dust explosion risk assessment model for the dust removal system was constructed using the fuzzy comprehensive evaluation method to form a set of dust explosion risk assessment methods suitable for dry dust collectors. The risk of explosion was assessed at level II through the use of paper powder with a particle size of 75 μm, which means this method is reliable.

Unique extension of the maximum entropy principle to phase coexistence in heat conduction

The maximum entropy principle determines the values of thermodynamic variables in thermally isolated equilibrium systems. This paper extends the principle to a variational principle that applies to liquid--gas coexistence in heat conduction. We show the uniqueness of the extension under the assumption that the variational principle and the fundamental thermodynamic relation are simultaneously extended in the linear response regime with the total energy fixed. Using the extended variational principle, we calculate the thermodynamic quantities in this steady state and find that the temperature of the liquid--gas interface deviates from the equilibrium transition temperature, which should be verified in experiments.