CH4 Recovery Research Articles

Characterization of shales from Damodar valley coalfields for CH4 recovery and CO2 sequestration

Anthropogenic emission of greenhouse gases particularly, CO2 is believed to be responsible for the global climate change. Carbon sequestration has been identified as one of the technological alternatives of reducing CO2 emission from the atmosphere. Disposal of CO2 in unconventional reserves such as shale formations may help in achieving the objectives of sequestration of the CO2 coupled with enhanced recovery of methane. The methane/CO2 storage potential of shale mainly depends on its porous nature which is very complex due to the presence of micro-, meso-, and macro-pores. Understanding the pore size distribution of shales is important because the storage and transport mechanisms of methane and CO2 vary as per the size of the pore. Further, the clay minerals present may also contribute significantly to the adsorption capacity of shale and may thus control the gas storage. In the present study, an attempt is made to investigate the pore structure of some shale samples from Damodar valley coalfields (Jharia and Raniganj coalfields), India in detail and correlate the pore parameters with adsorption of methane. Secondly, the clay mineral composition of studied shales is analyzed and its effect on porosity and adsorption are examined. Low pressure nitrogen and carbon dioxide adsorption tests, mercury intrusion and helium pycnometry tests are carried out to ascertain the pore sizes, surface area, and pore volume. It is observed that the porosity for most of the studied shales are dominated by mesopores. A moderate correlation is observed between nitrogen BET surface area and porosity suggesting lesser amount of micropores and dominance of mesopores. Porosity of the studied shales varied with clay mineral content exhibiting a U-shaped relationship.

Selective adsorption of CO2/CH4 mixture on clay-rich shale using molecular simulations

Shale rocks comprise of a lager proportion of clay minerals; shale gas potentially adsorbs on the clay minerals due to its large specific surface area. In this work, selective adsorption behaviors of CO2/CH4 are investigated in clay-mineral nanopores using the grand canonical Monte Carlo (GCMC) simulation method. In addition, the subsequent implications for CH4 recovery and CO2 sequestration are summarized. Results show that CH4 exhibits the strongest adsorption on kaolinite, followed by that on illite and montmorillonite, while adsorption capacity of CO2 is calculated in the order of montmorillonite > illite ≈ kaolinite, suggesting that CO2 injection has the best performance for CH4 recovery when implemented in montmorillonite-rich shale reservoirs. Comparatively, smaller pores are dominated by the adsorbed gas and are also favorable for CO2 sequestration. The adsorption selectivity of CO2 over CH4 on clay minerals is predicted in the order of kaolinite < illite < montmorillonite; moreover, the adsorption selectivity is reduced as pressure decreases for illite and montmorillonite, implying that the injection pressure of CO2 should be low enough to achieve an efficient CH4 recovery and CO2 sequestration. However, the ideal injection pressure of CO2 should be around the pressure where it has the maximum adsorption selectivity for kaolinite-rich shale. This study may achieve a deeper understanding of the adsorption behavior of CO2/CH4 in clay-rich gas-bearing reservoirs; furthermore, it may provide the guidelines for future optimization design of CO2 injection for CH4 recovery and CO2 sequestration in the field applications.

Review of CO2 injection techniques for enhanced shale gas recovery: Prospect and challenges

CO2 injection is a promising technique that not only enhances shale gas recovery but also achieves geological storage of CO2. This paper reviewed the performance of CO2 injection techniques based on simulations and field test studies. We observed that CO2 injection can be practical and successful in a hydraulically fractured shale. The techniques can lead up to 26% more methane production after primary recovery, and sequester more than 60% of the injected CO2 for continuous CO2 injection, while for huff-n-puff higher amount of CO2 is reproduced. Reservoir pressure gradient, competitive adsorption, flow dynamics, and shale properties were found as essential factors controlling CH4 recovery and CO2 storage. Despite the flow dynamics of gases being important for predicting gas production and storage, most simulations described it based on models suitable for well connected fractures and homogenous shale. Moreover, these models are incapable of analyzing fluid flow in stimulated fractures. Future studies on CO2 injection should address the issue of higher CO2 reproduction during the huff-n-puff, the effects of moisture content, induced effects on shale matrix properties by CO2 injection, the kinetics of CO2–CH4 competitive adsorption, flow dynamics of multicomponent gas, and consider the complex pore system of a heterogeneous shale.

CO2 injectability and CH4 recovery of the engineering test in qinshui Basin, China based on numerical simulation

An important CO2-ECBM (CO2 enhanced coalbed methane) engineering test will be carried out in #3 coal seam of Qinshui basin, China this year. This study is a preliminary work of CO2-ECBM engineering test with an aim at optimizing the engineering proposal and evaluating the effectiveness of CO2-ECBM. In this study, considering the competitive adsorption and diffusion of CO2 and CH4, seepage of gas and water, coal deformation, the dynamic evolution of coal seam porosity and permeability, and heat conduction and convection between coal and fluid, a fully coupled THM (thermo-hydro-mechanical) mathematical model for CO2-ECBM was established. The modelling was followed by a detailed discussion of the engineering well group’s CO2 injection and CH4 output characteristics under different CO2 injection pressures. The results confirm that CO2 injection has a positive effect on improving CH4 production. Within the simulated time, the CH4 recovery was enhanced by 4.26%–12.80%. Although the CO2 injectability of the engineering well group reduced significantly due to the drop in permeability near the CO2 injection well, the engineering well group has considerable CO2 storage potential. Under simulation conditions, the CO2 storage capacity for single CBM well is up to 0.73×105 - 2.54×105 m3 (standard temperature and pressure, STP). Increasing CO2 injection pressure can expand the coverage of CO2, thereby significantly improving the CH4 production rate and CO2 storage capacity. In addition, the permeability decrease caused by CO2 injection mainly affects the CO2 injection rate and storage capacity with weak influence on CH4 production and recovery.

The Effect of CO2 Partial Pressure on CH4 Recovery in CH4-CO2 Swap with Simulated IGCC Syngas

To investigate the influence of CO2 partial pressure on efficiency of CH4-CO2 swap from natural gas hydrates (NGHs), the replacement of CH4 from natural gas hydrate (NGH) is carried out with simulated Integrated Gasification Combined Cycle (IGCC) syngas under different pressures, and the gas chromatography (GC), in-situ Raman, and powder X-ray diffraction (PXRD) are employed to analyze the hydrate compositions and hydrate structures. The results show that with the P-T (pressure and temperature) condition shifting from that above the hydrate equilibrium curve of IGCC syngas to that below the hydrate equilibrium curve of IGCC syngas, the rate of CH4 recovery drastically rises from 32% to 71%. The presence of water can be clearly observed when P-T condition is above the hydrate equilibrium curve of IGCC syngas; however the presence of water only occurs at the interface between gas phase and hydrate phase. No H2 is found to present in the final hydrate phase at the end of process of CH4-CO2 swap with IGCC syngas.

A study on the carbon dioxide injection into coal seam aiming at enhancing coal bed methane (ECBM) recovery

Coal seams, particularly deep unmineable coal reservoirs, are the most important geological desirable formations to store CO2 for mitigating the emissions of greenhouse gas. An advantage of this process is that a huge quantity of CO2 can be sequestrated and stored at relatively low pressure, which will reduce the amount of storage cost required for creating additional platform to store it. The study on CO2 storage in coal seam to enhance coal bed methane (ECBM) recovery has drawn a lot of attention for its worldwide suitability and acceptability and has been conducted since two decades in many coalmines. This article focuses on the coal seam properties related to CO2 adsorption/desorption, coal swelling/shrinkage, diffusion, porosity and permeability changes, thermodynamic/thermochemical process, flue gas injection, etc. Here, the performance analysis of both CO2 storage and ECBM recovery process in coal matrixes is investigated based on the numerical simulation. In this study, a one-dimensional mathematical model of defining mass balances is used to interpret the gas flow and the gas sorption and describe a geomechanical relationship for determining the porosity and the permeability alteration at the time of gas injection. Vital insights are inspected by considering the relevant gas flow dynamics during the displacement and the influences of coal swelling and shrinkage on the ECBM operation. In particular, pure CO2 causes more displacement that is more efficient in terms of total CH4 recovery, whereas the addition of N2 to the mixture assists to make quicker way of the initial methane recovery. However, this study will support future research aspirants working on the same topic by providing a clear conception and limitation about this study.

CO2 abatement and CH4 recovery at vehicle exhausts: Comparison and characterization of Ru powder and pellet catalysts

The catalytic conversion of CO2 to CH4 (Sabatier reaction) has been studied to develop an after-treatment process at vehicles exhausts. Three different formulations of Ru commercial catalysts, two in powder and one in pellets shape, were tested and characterized by means of X-ray powder diffraction, scanning electron microscopy (SEM-EDX), N2 adsorption at −196 °C and temperature-programmed reduction (TPR).Experimental results show a high CO2 conversion (XCO2 = 0.96 @ T = 280 °C) for one powder catalyst formulation whereas the other one has maximum CO2 conversion = 0.69. In both cases a high CH4 selectivity is measured. High CO2 conversion (XCO2 = 0.92 @ T = 300 °C) is obtained also with pellet catalysts but only at lower GHSV values. The different behavior of the catalysts was ascribed to the different physicochemical properties and the key parameters for the application development of the process were identified. In particular, the possibility to use pellets or monolithic reactors, thus minimizing the pressure drops in the reactor, makes possible a commercial application in the treatment of vehicles exhausts.

Time-dependent observation of a cage-specific guest exchange in sI hydrates for CH4 recovery and CO2 sequestration

CH4–CO2 replacement in naturally occurring gas hydrates has been considered a promising method for both energy recovery and CO2 sequestration. In this study, the time-dependent guest exchange behaviors and guest distributions during CH4–CO2 replacement were closely examined at two different CO2 injecting pressures (2.2 and 3.5 MPa) using nuclear magnetic resonance (NMR), in-situ Raman spectroscopy, powder X-ray diffraction (PXRD), and gas chromatography. The 13C NMR spectra confirmed that the cage occupancy ratio of the CH4 molecules in the large 51262 and small 512 cages (θL/θS,CH4) after the replacement was significantly smaller than that before the replacement because of the preferential occupation of CO2 in the large 51262 cages. The time-dependent Raman spectra revealed that the rate of CO2 inclusion and the resultant CH4 depletion in the hydrate phase during the replacement was faster at a higher CO2 injecting pressure. The Rietveld refinement of the PXRD patterns offered a quantitative cage occupancy of CH4 and CO2 molecules before and after the replacement. The time-dependent cage occupancy values of CH4 and CO2 during the replacement obtained from a multi-methodological approach, which is a combination of PXRD analysis and in-situ Raman measurement, demonstrated that a significant guest exchange in the large 51262 cages had a greater effect on the extent of replacement and that the kinetics of the CH4–CO2 replacement was accelerated at a higher CO2 injecting pressure. The results provide a better understanding of the kinetics and mechanism of the cage-specific CH4–CO2 replacement occurring in the sI hydrates for CH4 recovery and CO2 sequestration.

Experimental investigation of the constant and time-dependent dynamic diffusion coefficient: Implication for CO2 injection method

Gas diffusion in coal is an important transport mechanism that plays a crucial role in carbon dioxide (CO2) storage in and methane (CH4) extraction from coal seams. Studies on coal diffusion are largely based on experimental work on different coal particles and the establishment of uni-pore or bidisphere theoretical models. However, problems remain in both experimental and theoretical analyses. Therefore, the initial desorption time of adsorbed gas, something that is not done using conventional experimental devices, was revealed in this study. The initial desorption time was obtained by calculating the relationship between the amount of gas collected and the theoretical amount of free gas. An improved diffusion model considering lost gas was established based on the initial desorption time of adsorbed gas. The mpdel provides diffusion coefficients for different equilibrium pressures and particle sizes on the basis of a unipore diffusion model. By correlating diffusion coefficient and time, an empirical time-dependent diffusion model was established to solve the dynamic diffusion coefficient under different conditions. Finally, in order to substantiate that the variation in diffusion length is the primary cause of the variation of diffusion coefficients with time, the variation of diffusion length with desorption time was obtained. As a result, a stepwise pressure-rasing method was proposed. This new injection method can effectively improve CO2 storage and CH4 recovery from coal seams. The study has a certain guiding significance in engineering practice.

Construction and Operation of a Respiration Chamber of the Head-Box Type for Methane Measurement from Cattle.

Simple SummaryThe aim of the present work is to describe the construction and operation of a respiration chamber of the head-box type for measuring methane emissions from bovines. Methane is a greenhouse gas 28 times more potent than CO2 in its capacity of producing the greenhouse effect and global warming. This gas is produced in considerable amounts by cattle as part of its normal digestion process; approximately 37% of the global anthropogenic methane emissions originate from the livestock industry. Measuring emissions of methane by cattle is necessary for inventory calculation and the evaluation of mitigation policies of this gas. The gold standard technique for measuring methane emissions from cattle is the respiration chamber; however, respiration chambers are expensive pieces of equipment that are not easily available for developing countries. Since a large proportion of the world’s cattle population is in the developing countries, a cheaper option is necessary. A respiration chamber of the head-box type is an option because of its low cost and high accuracy in estimating emissions. This chamber can be used to determine in vivo methane emission factors for those countries that do not have full respiration chambers. It can also be used to conduct experiments to evaluate the anti-methanogenic effects of different compounds. This paper aims to describe the construction and operation of a respiration chamber of the head-box type for methane (CH4) measurements in bovines. The system consists of (1) a head box with a stainless steel frame and acrylic walls, floor, and ceiling; (2) a stainless steel feeder; (3) an automatic drinking water bowl; (4) a hood made from reinforced canvas; (5) an infrared (IR) CH4 gas analyzer, a mass flow generator, a data-acquisition system; and (6) a steel metabolic box. Six assays were conducted to determine the pure CH4 recovery rate of the whole system in order to validate it and comply with standards of chamber operation. The gravimetrical method was used for the recovery test and the recovery rate obtained was 1.04 ± 0.05. Once the system was calibrated, measurements of CH4 were conducted using eight animals consisting of four Holstein cows with a live weight of 593.8 ± 51 kg and an average milk yield of 23.3 ± 1.8 kg d−1 and four heifers with a live weight of 339 ± 28 kg. The CH4 production values were 687 ± 123 and 248 ± 40 L CH4 d−1 for cows and heifers, respectively. The CH4 yield was 19.7 ± 3.4 g and 17.1 ± 3.4 g CH4 kg−1 of dry matter consumed for cows and heifers, respectively. These results are consistent with those reported in the literature.

Mechanistic insight into the optimal recovery efficiency of CBM in sub-bituminous coal through molecular simulation

In this study, a newly created Fortran program was used to assist in the constructing of a sub-bituminous coal model (SCM). Followed by grand canonical Monte Carlo simulations, the adsorption behaviors of CBM adsorption on dry-/moisture-SCM systems have been systematically studied at different pressure-temperature conditions. A geological model with typical geothermal and pressure gradients was established to investigate the effect of coupled pressure and temperature on the absorption and recovery. As a result, pure-N2 injection has been confirmed to paly a negative role on CH4 recovery, whereas CO2 could effectively improve the CO2 sequestration and CH4 displacement. Meanwhile, the adsorption percentage (δ) and the recovery efficiency (η) were first put forward in terms of theoretical calculations. The results showed that, the δ was determined by pore size distribution of the SCM, adsorption temperature/pressure, moisture-containing in coal and the types of adsorbates. The value of ηCH4 is 53.07% in dry coal seam, whereas 47.20% is evaluated in moisture coal seam, the corresponding optimum mining depths are 600 m and 800 m, respectively. Remarkably, water-containing in coal seam has an obviously inhibitory effect on simultaneous extraction of coal and gas, and it should be valued in the follow-up studies. Based on this principle, the optimum mining depth for CO2-ECBM was predicted to be ∼800 m in actual deep coal seams through our geological model. Our results provide a molecular-scale insight into CBM recovery efficiency in coal at different burial depths, and offer useful guidance for realistic exploitation.

Research on the Processes of Injecting CO2 into Coal Seams with CH4 Recovery Using Horizontal Wells

The paper presents a research study on modeling and computer simulation of injecting CO2 into the coal seams of the Upper Silesian Coal Basin, Poland connected with enhanced coal bed methane (ECBM) recovery. In the initial stage of the research activities, a structural parameter model was developed specifically with reference to the coal-bearing formations of the Upper Carboniferous for which basic parameters of coal quality and the distribution of methane content were estimated. In addition, a lithological model of the overall reservoir structure was developed and the reservoir parameters of the storage site were analyzed. In the next stage of the research, the static model was supplemented with detailed reservoir parameters as well as the thermodynamic properties of fluids and complex gases. The paper discusses a series of simulations of an enhanced coalbed methane recovery process with a simultaneous injection of carbon dioxide. The analyses were performed using the ECLIPSE software designed for simulating coal seam processes. The results of the simulations demonstrated that the total volume of CO2 injected to a designated seam in a coal mine during the period of one year equaled 1,954,213 sm3. The total amount of water obtained from the production wells during the whole period of the simulations (6.5 years) was 9867 sm3. At the same time, 15,558,906 sm3 of gas was recovered, out of which 14,445,424 sm3 was methane. The remaining 7% of the extracted gas was carbon dioxide as a result of reverse production of the previously injected CO2. However, taking into consideration the phenomena of coal matrix shrinking and swelling, the total amount of injected CO2 decreased to approximately 625,000 sm3.

Correlating Physicochemical Properties of Commercial Membranes with CO2 Absorption Performance in Gas-Liquid Membrane Contactor

The gas-liquid membrane contactor (GLMC) is a promising alternative gas absorption/desorption configuration for effective carbon dioxide (CO2 ) capture. The physicochemical properties of membranes may synergistically affect GLMC performances, especially during the long-term operations. In this work, commercial polypropylene (PP) and polyvinylidene fluoride (PVDF) hollow fiber (HF) membranes were applied to explore the effects of their physicochemical properties on long-term CO2 absorption performances in a bench-scale GLMC rig. PP membranes with pore size of 19 nm, thickness of 0.046 mm, and porosity of 58% achieved high CO2 flux when feeding pure CO2 (5.4 and 24.4×10-3 mol/m2 .s using absorbents of water and 1M monoethanolamine (MEA), respectively) whereas PVDF membranes with pore size of 24 nm, thickness of 0.343 mm, and porosity of 84% presented a good CO2 separation performance from the simulated biogas using 1M MEA (6.8×10-3 mol/m2 .s and 99.9% CH4 recovery). When using water as absorbent, the coupled phenomena of membrane wetting and fouling restricted CO2 transport and resulted in continuous flux loss during the long-term operations. When using MEA as absorbent, both PP and PVDF membranes suffered dramatic flux decline. A series of membrane characterization tests revealed that the morphology, pore size, hydrophobicity, and stability of selected commercial membranes were greatly affected by MEA attack during long-term operations. Therefore, the selection criterion of microporous membranes for high-efficiency and long-term stable CO2 absorption in GLMC processes was proposed. It is envisioned that this study can shed light on improving existing membrane fabrication procedures and the application of novel membrane surface modification techniques to facilitate practical applications of the GLMC technology.

Adsorption of CO2 by synthetic zeolites

The paper reports on a possible way to recycle fluid catalytic cracking catalysts (FCCCs), widely used in oil refining operations. This research proposes a novel approach that leads to a near zero-waste process. The spent FCCC was leached by 1.5 mol/L of HNO3, HCl and H2SO4 solutions at 80°C, for 3 h with a solid to liquid ratio of 20 %wt/vol. The leaching yields for cerium and lanthanum were in the range 69-82 %. The solid residues from the leaching stage were used as base material for the synthesis of the zeolites by means of a combined thermal-hydrothermal treatment. The characterization of the zeolites demonstrated that the Na-A phase was predominant over the Na-X phase. The zeolites were tested as sorbent material for CO2 separation from CH4, in order to simulate the upgrading of biogas to biomethane. The maximum adsorption rate of CO2 was 0.778 mol CO2/kg of zeolite at 3 bar, with a resulting CH4 recovery of 62 % and purity of 97 %vol. The zeolites synthesized from spent FCCC represent a feasible solution to recover such industrial waste.

Gas hydrate exploitation and carbon dioxide sequestration under maintaining the stiffness of hydrate-bearing sediments

To improve the stratum mechanical properties and prevent the geological disaster, we present a new approach by executing CH4 hydrate exploitation and CO2 sequestration below the freezing point of water to maintain the stratum stiffness during and after hydrate exploitation. An experimental method to clearly show the evolution of the stratum mechanical properties was built and a series of experiments were conducted to simulate the CH4 exploitation and CO2 sequestration process below and above the freezing point of water. The stratum stiffness was found to be maintained during the whole hydrate exploitation process when below the freezing point. Moreover, the presence of ice improves the stratum stiffness due to cementing the sandy grain. The CO2 injection into the hydrate-bearing sediments would further enhance the mechanical properties and realize a CO2 geological sequestration. Compared with hydrate exploitation above the freezing point, the CH4 recovery efficiency is a little lower when under below the freezing point and it decreased with the increased hydrate saturation due to the CH4 hydrate being surrounded by the generation of ice from hydrate dissociation. This phenomenon could be modified by injecting CO2, from which the CH4 recovery efficiency obviously improved for hydrate exploitation below the freezing point.

Highly rapid mechanochemical synthesis of a pillar-layer metal-organic framework for efficient CH4/N2 separation

Efficient capture of methane (CH4) from nitrogen (N2) by adsorption separation remains a challenge. In this work, we demonstrate the first solvent-free mechanochemical synthesis of a zinc-based pillar-layer MOF Zn2(5-aip)2(bpy) (1) within minutes, making 1 accessible via a rapid and green route. By rational control of grinding time, the resulting product of 1–4 (which stands for grinding 4 min) has the optimal porosity confirmed by using CO2 as the probe gas. For the recovery of CH4 from N2 at 5.0 bar and 298 K, the adsorption uptakes of CH4 and N2 for 1–4 are 1.10 and 0.27 mmol/g, exhibiting preferential adsorption CH4 over N2 with high CH4/N2 selectivity of 7.0. The high selectivity of CH4/N2 in 1–4 is mainly ascribed to the difference in affinity interactions between the pore walls and gas molecules, confirmed by experimental adsorption heat and molecular simulation. Two CH4 molecules can enter in the same pores, but most of the pores can only adsorb one N2 molecule. Furthermore, the dynamic separation performance for CH4/N2 mixture in 1–4 is verified by simulated breakthrough curves. The unprecedentedly rapid solvent-free mechanochemical synthesis within minutes makes 1 easily accessible at the gram scale via an environmentally friendly method for accelerating potential application of CH4/N2 separation.

Numerical study on enhancing coalbed methane recovery by injecting N2/CO2 mixtures and its geological significance

AbstractAs an effective carbon utilization technology, the injection of N2/CO2 mixtures into coal seams has significant potential for improving coalbed methane recovery. Considering the technical barrier that injection of pure CO2 decreases the well injectivity index and pure N2 injection leads to the rapid methane recovery, a method of injecting N2‐enriched gas mixtures with a constant component is proposed. In this study, a thermo‐hydro‐mechanical (THM) coupling numerical model for enhanced coalbed methane (CBM) recovery by injecting N2/CO2 mixtures is established. This model includes complex interactions of coal deformation, competitive adsorption, ternary gas seepage, and heat transfer. The THM coupling model is first validated, and then applied to investigate the evolution of mixed gas concentrations, reservoir permeability, reservoir temperature, CH4 production, and N2/CO2 storage during N2/CO2 enhanced CBM recovery. The results show that the displacement radius and concentration of the mixed gas in the coal seam increased with gas injection pressure increase. The concentration of CH4 gradually decreased with time, and the early decline is faster than the later stage. The sweep of the N2 flow accelerates CH4 desorption and migration, promoting a reduction in reservoir temperature near the production well. Reservoir permeability evolution results from the combined effects of ternary gases (CH4, CO2, N2) competitive adsorption, gas pressure, and geostress on the coal seam within the THM fields. At the methane natural depletion stage (within 250 days), the permeability of coal reservoir first decreases and then increases. With the arrival of the N2/CO2 mixture, the permeability decreases dramatically. From the perspective of cumulative CH4 production, the optimal composition is dominated by the synergistic effect of maximizing breakthrough time and minimizing coal matrix swelling. For 30% CO2‐70% N2, the CH4 recovery ratio reached 71.76%, representing an increase of 16.67% compared to natural depletion.

Analysis of membrane unit performance in presence of wet CO2-containing mixtures

Membrane gas separation is more and more gaining room of application in biogas upgrading to biomethane to be directly fed to pipelines. The water vapour contained in the biogas stream affects the separation performance of membranes; therefore, it is important to consider this aspect for the design of the separation stage.In this work, we developed performance maps taking into account the water vapour effect on the performance of a membrane gas separation by considering different CO2/CH4 selectivities (from 25 to 100), pressure ratio and flow rate/membrane area ratio. The presence of water vapour induced significant changes in membrane performance, with consequent repercussions in CH4 recovery, which in some cases, reduced from 85.3% to 80.9% from dry to wet conditions.

Variation in permeability during CO2–CH4 displacement in coal seams: Part 1 – Experimental insights

The injection of CO2 into deep coal seams can improve the recovery of CH4 and contribute to the geological sequestration of CO2. To reveal the controlling factors associated with CO2 displacing CH4, experiments were conducted to simulate the process under different confining pressures (8, 12, 16, and 20 MPa) and CO2 injection pressures (1–6 MPa). The results show that coal can adsorb more CO2 than CH4. However, the permeability of CH4 is greater than that of CO2 under inlet gas pressures of 1–5 MPa. For CH4 saturated coals, the CH4 desorption volume is higher under CO2 displacing conditions compared to pure CH4 desorption processes under the same pressures. The coal adsorbed CO2 decreases under high pore and confining pressures, indicating the displacement results may be better in shallow buried coals. The axial, radial, and bulk strains of coal show steady, sharp, and then slow declining trends with decreasing pore pressures, and the inflection points generally occur at 5.25 and 3.25 MPa. Meanwhile, the strain in three directions decreases exponentially with the increase of effective stress, and the radial strain is higher than the axial strain under each confining pressure. With the combined influences of effective stress, matrix swelling/shrinkage, and gas slippage, the permeability decreases first and then increases with the decrease of pore pressure. With the decrease of pore pressure and the increase of confining pressure, the combined influence of the three factors on coal permeability gradually weakens. The results would be beneficial for controlling the injection process and permeability variation predicting during CO2 sequestration and the replacement of CH4 in coals at different burial depths.

Microsecond simulation study on the replacement of methane in methane hydrate by carbon dioxide, nitrogen, and carbon dioxide–nitrogen mixtures

Replacement of methane (CH4) in CH4 hydrate by carbon dioxide (CO2) can enable recovery of CH4, which is a potential future energy resource, while sequestering CO2 to mitigate the effects of global warming. However, little work has been done to address the effects of impurities on CO2 replacement, and the detailed mechanisms. Here, microsecond molecular dynamics simulations were performed to understand the influence of nitrogen (N2) gas on the process of replacing CH4 in CH4 hydrate with CO2 at 280 K and 6 MPa. The results show that CO2 molecules can penetrate more deeply into CH4 hydrate phase when it is mixed with N2. This is mainly because N2 can favor the decomposition of CH4 hydrate and expand the replacement area of CH4 by guest molecules. We confirm that the replacement of CH4 by CO2 and N2 preferably occurs in large and small cages, respectively. In most cases, a mixture hydrate reforms at the outmost layer of the hydrate surface. The CO2/N2 mixture shows an overall higher replacement efficiency than pure CO2 case. Our work demonstrates that CH4 recovery by CO2 injection in CH4 hydrate can be facilitated by N2. The penetration depth of replacement is sensitive to the ratio of N2 to CO2. The knowledge obtained in this study will be helpful for the effective utilization of CO2/N2 mixtures to maximize the recovery percentage of CH4 from hydrate.