N2 Permeability Research Articles

Machine learning enables interpretable discovery of innovative polymers for gas separation membranes.

Polymer membranes perform innumerable separations with far-reaching environmental implications. Despite decades of research, design of new membrane materials remains a largely Edisonian process. To address this shortcoming, we demonstrate a generalizable, accurate machine learning (ML) implementation for the discovery of innovative polymers with ideal performance. Specifically, multitask ML models are trained on experimental data to link polymer chemistry to gas permeabilities of He, H2, O2, N2, CO2, and CH4. We interpret the ML models and extract valuable insights into the contributions of different chemical moieties to permeability and selectivity. We then screen over 9 million hypothetical polymers and identify thousands that lie well above current performance upper bounds, including hundreds of never-before-seen ultrapermeable polymer membranes with O2 and CO2 permeability greater than 104 and 105 Barrers, respectively. High-fidelity molecular dynamics simulations confirm the ML-predicted gas permeabilities of the promising candidates, which suggests that many can be translated to reality.

Hollow SAPO-34 incorporated mixed matrix membranes with the improved solubility selectivity and permeability for N2/CH4 separation

Hollow SAPO-34 with an average particle size of ca.370 nm was synthesized by selective etching its precursor under the controlled alkaline condition. Subsequently, freestanding mixed matrix membranes were synthesized by UV-induced in-situ polymerization of the mixture of hollow SAPO-34, the fluorinated monomer, and the trifunctional cross-linker. The structure and the physical characteristic of hollow SAPO-34 and membranes were characterized by scan electron microscope, wide-angle X-ray diffraction, etc. The effect of the hollow SAPO-34 loading amount on the gas permeation-separation performance of mixed matrix membranes was investigated. The results showed that hollowing out SAPO-34 improved the N2/CH4 sorption selectivity and converted the crystals from preferential adsorption of N2 to preferential adsorption of CH4, the incorporation of hollow SAPO-34 into membranes increased the N2/CH4 solubility selectivity and the permeability. The optimal membrane exhibited a 39-fold increase in N2 permeability from 6.9 × 10−13 to 2.7 × 10−11 cm3(STP)cm·cm−2·s−1·Pa−1 at 0.2 MPa and 35 °C without the loss of N2/CH4 permselectivity, which exceeded the 2008 Robeson upper bound.

Multi-lab study on the pure-gas permeation of commercial polysulfone (PSf) membranes: Measurement standards and best practices

Gas-separation membranes are a critical industrial component for a low-carbon and energy-efficient future. As a result, many researchers have been testing membrane materials over the past several decades. Unfortunately, almost all membrane-based testing systems are home-built, and there are no widely accepted material standards or testing protocols in the literature, making it challenging to accurately compare experimental results. In this multi-lab study, ten independent laboratories collected high-pressure pure-gas permeation data for H2, O2, CH4, and N2 in commercial polysulfone (PSf) films. Equipment information, testing procedures, and permeation data from all labs were collected to provide (1) accepted H2, O2, CH4, and N2 permeability values at 35 °C in PSf as a reference standard, (2) statistical analysis of lab-to-lab uncertainties in evaluating permeability, and (3) a list of best practices for sample preparation, equipment set-up, and permeation testing using constant-volume variable-pressure apparatuses. Results summarized in this work provide a reference standard and recommended testing protocols for pure-gas testing of membrane materials.

Porous mullite-bonded SiC filters prepared by foaming-sol-gel-tape casting for high-efficiency hot flue gas filtration

Porous mullite-bonded SiC filters was prepared by combining foaming-sol–gel-tape casting and in situ mullite reaction bonding. Silica sol served two main functions: (i) it was used to create a sol–gel reaction at room temperature to consolidate foam, and (ii) it was used as a silicon source for in-situ mulletization reactions with alumina at high temperatures to bind silicon carbide to produce high mechanical strength. In this study, gelation process, reaction bonding behavior and hot flue gas filtration were thoroughly investigated. The produced filters comprise a high number of interconnected big pores with sizes ranging from 50 to 150 µm. The size of the pores connecting channels with filtration functionality was less than 10 µm. High porosity of 64.8 to 80.5 % and high N2 permeation of 4.82 × 107 to 7.44 × 107 Lm−2 h−1 bar−1 were attained by adjusting the amount of foaming agent. In hot flue gas filtration, the unique structure of the filter allows for high filtration efficiency (>89.6% for PM2.5), ultra-low pressure drop (30 Pa), and exceptional cyclic stability.

Modeling CO2 and N2 Permeation through Mixed Matrix Membranes Containing Oxygenated Carbon Nanotubes Dispersed in Polysulfone

Mixed matrix membranes (MMMs) consist of a polymeric phase and a dispersed solid filler such as zeolites or carbon nanotubes. We prepared symmetric and asymmetric polysulfone-based MMMs containing functionalized multi-walled carbon nanotubes (MWNT-O) for CO2/N2 separation. Maxwell’s model was used to predict the permeabilities of MMMs. The permeability achieved using the model exhibited a good fit of the model in symmetric MMMs, especially for CO2. This can be potentially attributed to the high affinity of the system toward the polymer matrix. The permeability recorded using Maxwell’s model could not reflect the properties of the asymmetric membranes, and this could be attributed to the generation of voids around the fillers or defects present on the surface of the skin. Finally, a mathematical fit was proposed to improve prediction accuracy for both the MMM systems.

Interfacially-confined polyetherimide tubular membranes for H2, CO2 and N2 separations

Capturing CO2 and H2 purification from gas mixtures such as CO2/N2 and H2/N2 are vital in addressing greenhouse gas emission abatement and global energy strategy. Here, we present a systematic investigation of tubular polyetherimide (PEI) membranes prepared by combining dip-coating and multilayer assembly techniques to target CO2 and H2 transports and CO2/N2 and H2/N2 selectivities. The effects of deposition cycle during dip-coating on H2, CO2 and N2 single gas permeations and binary gas separations were evaluated. The membranes prepared by three deposited layers produced gas permeances (10−12 mol m−2 s−1 Pa−1) of 310, 150 and 1 for H2, CO2 and N2 respectively, corresponding to CO2/N2 and H2/N2 permselectivities of 149 and 308. For binary gas mixture separation, an exclusive 100% CO2 and H2 permeate purity was observed irrespective of feed gas concentration. The PEI membrane exhibited a suppression of glass transition temperature and crystallinity compared to the bulk polymer indicating interfacial confinement phenomenon, which was further confirmed by X-ray diffraction and Raman spectroscopic analyses. Due to a subtle change of imide group conformation of the obtained membrane layer, it was inferred that an increase in the polymer amorphicity and interfacial confinement facilitated the exclusive H2 and CO2 transports without much affecting the N2 permeance.

A novel poly (4-methyl-1-pentene)/polypropylene (PMP/PP) thin film composite (TFC) artificial lung membrane for enhanced gas transport and excellent hemo-compatibility.

Extracorporeal membrane oxygenation is a technique that provides short-term supports to the heart and lungs. It removes CO2 from the blood and provides enough oxygen, which is a huge help in the fight against COVID-19. As the key component, the artificial lung membranes have evolved in three generations including silicon, polypropylene and poly (4-methyl-1-pentene). Herein, we for the first time design and fabricate a novel poly (4-methyl-1-pentene)/polypropylene (PMP/PP) thin film composite (TFC) membrane with the anticoagulant coating composed of poly (sodium 4-styrenesulfonate) and cross-linked poly (vinyl alcohol). Poly (sodium 4-styrenesulfonate) provides sulfonic acid groups to inhibit the coagulant factors (FVIII and FXII), and cross-linked poly (vinyl alcohol) increase the stability of the anticoagulant coating and further improve the hydrophilicity via abundant hydroxyl groups to depress the protein adsorption. Long-term anticoagulant property was demonstrated by whole human blood for 28 days. Blood compatibility was evaluated by hemolysis rate, anticoagulation activity (APTT, TT and PT), complement activation, platelet activation and contact activation. Pure CO2, O2 and N2 permeation rates were determined to evaluate the mass transfer properties of PMP/PP TFC membranes. Gas permeation results revealed that gas permeation flux increased in the TFC membranes because of the decrease of crystallinity. Overall, the so prepared PMP/PP membrane shows good CO2/O2 selectivity and blood compatibility as novel TFC artificial lung membrane.

Fabrication of metal-organic framework-mixed matrix membranes with abundant open metal sites through dual-induction mechanism

Metal-organic framework (MOF)-mixed matrix membranes (MMMs) have the potential to overcome the trade-off limitations between gas separation performance and large-scale fabrication. However, most conventional MOF MMMs are used for CH4/N2 separation systems, the preparation of MOF-MMMs with high permeance and good selectivity for N2/CH4 separation has rarely been reported. Here in, we propose supported thin MOF-MMMs with abundant open metal sites to preferentially adsorb N2 and improve the N2/CH4 separation performance. The formation of the selectivity layer with abundant open metal sites was realized through dual-induction mechanism of in-situ Co2+ ion doping and polydimethylsiloxane (PDMS) solution post-modification. The introduction of Co2+ ions restricted the coordination of Zn-N and increased the number of open metal sites in the ZIF layer during its formation. The post-treated PDMS solution with weak acid could further induce the formation of open metal sites. Meanwhile, the electron paramagnetic resonance and synchrotron radiation-based XAFS results also confirmed that the number of open metal sites in the separation layer increased gradually with the in-situ Co2+ ion decoration and PDMS solution post-modification. In addition, the PDMS also could repair the intercrystalline defects of the MOF membranes and reduce their non-selectivity area. Therefore, the synthesized ZIF-8 MMMs showed a high N2 permeance of 1367 GPU and a moderate N2/CH4 selectivity of up to 6.

Permeability characteristics of CH4, CO2, and N2 during the whole process of adsorption in coal with accumulated pressure under triaxial stresses

This study aimed to explore the seepage characteristics of N2, CH4, and CO2 in coal during the whole process of adsorption with cumulative gas pressure. A self-developed device was utilized to conduct the adsorption isotherm experiment, and the theoretical method based on the gas state equation was utilized to calculate the real-time permeability of gases during adsorption. The gas flow and velocity at the inlet and outlet were calculated on the basis of the pressure and time change law. Results showed that, during adsorption with accumulated pressure, the average pore pressure of N2 increased, whereas that of CO2 decreased. As cumulative pressure increased, the gas flow at the inlet gradually decreased, whereas that at the outlet increased. The flow and velocity of CO2 at the inlet were the largest, followed by CH4. On the contrary, the flow and velocity of N2 at the outlet were the smallest, followed by CH4. On the basis of the gas flow and velocity at the inlet and outlet, the adsorption amount and permeability of gases in coal were calculated. Gas adsorption isotherm conformed to Langmuir characteristics. Under certain gas pressure, the real-time permeability showed a quadratic form with adsorption amount, and the real-time gas permeability decreased with the increase of cumulative pressure. The real-time permeability of N2 was greater than that of CH4 and CO2, followed by CH4. The research can provide theoretical basis for deep coalbed methane development and carbon sequestration in coal seam, even gas flow in porous media.

Gas Permeability, Fractional Free Volume and Molecular Kinetic Diameters: The Effect of Thermal Rearrangement on ortho-hydroxy Polyamide Membranes Loaded with a Porous Polymer Network.

Mixed-matrix membranes (MMMs) consisting of an ortho-hydroxy polyamide (HPA) matrix, and variable loads of a porous polymer network (PPN) were thermally treated to induce the transformation of HPA to polybenzoxazole (β-TR-PBO). Two different HPAs were synthesized to be used as a matrix, 6FCl-APAF and tBTpCl-APAF, while the PPN used as a filler was prepared by reacting triptycene and trifluoroacetophenone. The permeability of He, H2, N2, O2, CH4 and CO2 gases through these MMMs are analyzed as a function of the fraction of free volume (FFV) of the membrane and the kinetic diameter of the gas, allowing for the evaluation of the free volume. Thermal rearrangement entails an increase in the FFV. Both before and after thermal rearrangement, the free volume increases with the PPN content very similarly for both polymeric matrices. It is shown that there is a portion of free volume that is inaccessible to permeation (occluded volume), probably due to it being trapped within the filler. In fact, permeability and selectivity change below what could be expected according to densities, when the fraction of occluded volume increases. A higher filler load increases the percentage of inaccessible or trapped free volume, probably due to the increasing agglomeration of the filler. On the other hand, the phenomenon is slightly affected by thermal rearrangement. The fraction of trapped free volume seems to be lower for membranes in which the tBTpCl-APAF is used as a matrix than for those with a 6FCl-APAF matrix, possibly because tBTpCl-APAF could approach the PPN better. The application of an effective medium theory for permeability allowed us to extrapolate for a 100% filler, giving the same value for both thermally rearranged and non-rearranged MMMs. The pure filler could also be extrapolated by assuming the same tendency as in the Robeson’s plots for MMMs with low filler content.

Separation of CH4/N2 gas mixture using MFI zeolite nanosheet: Insights from molecular dynamics simulation

The performance of the MFI zeolite membrane for the separation of the CH4/N2 gas mixtures was investigated in this research. The studied systems comprised MFI zeolite membrane and different ratios of CH4/N2 gas mixture. Molecular dynamics simulations, under the applied hydrostatic pressure, were done to study the separation of these gas molecules from each other. In addition, the effect of increasing system temperature on the separation process was explored. Also, the effect of feed composition on the separation of the CH4/N2 mixture was investigated. Various analyses, including the potential of the mean force of gas molecules, density map, density profile, permeance, and the van der Waals interactions energy, were done in detail. Results show that the N2 molecules can permeate across the channels of MFI zeolite and the methane molecules remain in the feed side in pure form. The maximum N2 permeance at 298 K through MFI zeolite was observed 3.43 × 105 GPU, which indicates the remarkable performance of MFI zeolite in the separation of CH4/N2 gas mixture. Therefore, this zeolite can be an appropriate membrane for the separation of the gas mixture with acceptable performance.

High performance of PES-GNs MMMs for gas separation and selectivity

In this study, graphene nanosheets (GNs) were incorporated into polyethersulfone (PES) by phase inversion approach for preparing PES-GNs mixed matrix membranes (MMMs). To investigate the impact of filler content on membrane surface morphology, thermal stability, chemical composition, porosity and mechanical properties, MMMs were constructed with various GNs loadings (0.01, 0.02, 0.03, and 0.04 wt%). ​The performance of prepared MMMs was tested for separation and selectivity of CO2, N2, H2 and CH4 gases at various pressures from 1 to 6 bar and temperature varying from 20 to 60 °C. It was observed that, compared to the pristine PES membrane, the prepared MMMs significantly improved the gas separation and selectivity performance with adequate mechanical stability. The permeability of CO2, N2, H2 and CH4 for the PES + 0.04 wt% GNs increases from 9 to 2246, 11 to 2235, 9 to 7151, and 3 to 4176 Barrer respectively, as compared with pure PES membrane at 1 bar and 20 °C due to improving the membrane absorption and porosity. In addition, by increasing the pressure, the permeability and selectivity of CO2, N2, H2 and CH4 are increased due to the increased driving force for the transport of gas via membranes. Furthermore, the permeability of CO2, N2, H2 and CH4 increased by increasing the temperature from 20 to 60 °C due to the plasticization in the membranes and the improvement in polymer chain movement. This result proved that the prepared membranes can be used for gas separation applications.

Study on the advantageous effect of nano-clay and polyurethane on structure and CO2 separation performance of polyethersulfone based ternary mixed matrix membranes

In recent years, many efforts have been made to develop new membrane materials for natural gas sweetening (CO2/CH4) and post-combustion CO2 capture from flue gas (CO2/N2), especially in coal-fired and natural gas-fired power plants. This study focused on the effect of polyurethane and nano-clay on structure and CO2 separating properties of polyethersulfone (PES) based three-component mixed matrix membranes (MMMs). The different weight fractions polyurethane (PU) (5−30 wt.%) together with clay nano-sheets (Cloisite® 15A) (0.5−10 wt.%) were blended to fabricate a novel PES-based ternary MMMs. The principal purpose was to investigate the effect of loading PU polymer and layered clay particles with 2D channels on the gas separation performance of the PES membranes. Pure gas permeability for N2, CH4, and CO2, and ideal gas separation for CO2/N2 and CO2/CH4 mixtures were examined through neat PES and modified PES membranes. By incorporating 20 wt.% of PU into PES, the permeability of CO2 and selectivities of CO2/N2 and CO2/CH4 were improved by 4, 1.4, and 1.7 times higher than the neat PES membrane, while by loading 20 wt.% of PU and 2 wt.% of nano-clay, the CO2 permeability, and CO2/N2, and CO2/CH4 selectivities were improved by 7.8, 1.8, and 2.2 times higher than the neat membrane, respectively. In general, PU and clay nano-sheets together as a filler into the PES matrix exhibited a meaningful improvement in the gas permeation and separation properties of PES and can be admitted as a worthy membrane for CO2 separation from the flue gas and natural gas.

Gas permeation properties of nickel nanoparticle-embedded PPO/PVP polymers based carbon membrane

Mixed-matrix membranes (MMM) of thermally stable poly (2,6-dimethyl-1,4-phenyleneoxide) (PPO) polymer and thermally labile Polyvinylpyrrolidone (PVP) polymer, PPO/PVP, and its nickel nanoparticle (NP)-incorporated, PPO/PVP/Ni, were prepared to study their gas separation properties. Field emission scanning electron microscopy (FESEM) images were used to study their surface morphology, surface porosity, average pore size, and X-ray diffraction (XRD) patterns for crystallite size of Ni NPs and their phase purity. In particular, surface study of MMMs revealed that due to the inclusion of nickel NPs, the pore structure of the PPO/PVP/Ni membrane has been decreased. The gas permeation of H2, O2 and N2 was measured by single gas permeation method. The PPO/PVP/Ni exhibits much higher permeation results as compared to that of PPO/PVP. Additionally, the former provides higher selectivity for the H2/N2 gas pair as compared to O2 /N2 gas pair. Furthermore, the permeation results show that the incorporating of nickel NPs (1 wt%) to the PPO/PVP/Ni membrane caused the permeances to increase by nearly 45 times when compared to the PPO/PVP membrane.

Hydrogen Separation Property from Gas with Nitrogen of PI and PEEK Films

For the decarbonized society ammonia (NH3) has attracted attention as a new hydrogen (H2) carrier and we should develop a high-quality membrane separating H2 from nitrogen (N2) after decomposing NH3 and would focus on amorphous polyimide (PI) and crystalline polyetheretherketone (PEEK) films as the membrane. Since H2 permeation experiments had been conducted in our research group, we carried out N2 gas permeation experiment of each PI and PEEK with different film thicknesses under several temperatures and measured N2 gas permeability coefficients. As the results, the N2 gas permeability of PI depends on the film thickness. This suggests that that of PI 7.5 and more PI 12.5 relate the dissolution and the diffusion processes, respectively. The N2 permeability of PEEK was found to increase with increasing free volume in the polymer above the glass transition temperature. Next, H2/N2 separation factor was determined from this and the previous study. The results showed that the H2/N2 separation performance of the PI and PEEK films was 0.032 times lower than that of the Pd alloy and silica films, making them impractical.

N2/CH4 separation behavior at elevated temperature on P84 hollow fiber carbon membrane

Recently, natural gas consumption has grown over the years and this large demand requires efficient technology to purify it. Nitrogen (N2) is one of the main impurities in natural gas and could reduce its heat value. Therefore, it is necessary to separate nitrogen from natural gas. Since the diameter kinetic between CH4 and N2 is similar, it is rather hard to separate this gas mixture. This research aims to study the N2/CH4 separation at elevated temperature through a hollow fiber carbon membrane (HFCM) derived from P84 co-polyimide. A comprehensive investigation of P84 HFCM fundamental transport from 273 K to 373 K provided a better understanding of gas permeability as a function of permeation temperature. Moreover, the influence of carbonization heating rate towards the microstructure of HFCM and the N2/CH4 separation behavior was carefully investigated. The XRD analysis confirmed that the carbon membrane formation referred to the amorphous structures at 2θ of 22° and aromatic graphite at 42°, which refers to the (002) and (100), respectively. In addition, the SEM demonstrated the HFCM's dense structure with finger-like pores. The highest selectivity of N2/CH4 occurred at the 3 °C/min heating rate with 373 K permeation temperature (9.09) by N2 permeability of 186.92 Barrer because of the contribution of activation energy. Higher energy activation for N2 (13.91 kJ mol−1) makes a higher permeability improvement than CH4 (8.37 kJ mol−1). Thermodynamic studies confirm the selective adsorption effect of gas on HFCM. The influence of heating rate on N2/CH4 gas separation performance was studied at 1 to 3 °C/min. Adsorption and activated surface diffusion contributed to gas diffusion at 298 K and 373 K permeation temperatures) and 5 °C/min exhibited good separation performance above the Robeson upper bound. Overall, the study gives another alternative in separating the N2/CH4 mixture through the optimization of the operating process.

Switchable gas permeability of a polypropylene‐liquid crystalline composite film

AbstractThe development of functionalized polyolefins for use as stimuli‐responsive commodity polymers has recently received much attention. In this work, a microporous polypropylene (PP) scaffold is used to align and fortify a smectic liquid crystalline network (LCN) which can switch its gas permeability upon pH changes. The LCN is a photopolymerized liquid crystalline mixture of a dimerized benzoic acid derivative monoacrylate and a diacrylate crosslinker. In the hydrogen‐bonded state, the composite membrane shows a high‐molecular order and a low permeability for He, N2, and CO2 gases. By pH switching from the hydrogen‐bonded state to the salt form, the molecular order is reduced, and the gas permeability is increased by one order of magnitude. This increase is mainly attributed to a loss in order of the system, increasing the free volume, resulting in an increased diffusibility. By exposing the composite film to basic or acidic environments, reversible switching between low and high gas permeability states is observed, respectively. The physical constraints imposed by the PP scaffold strengthens the membrane while the reversible switching inside the liquid crystalline polymer is maintained. This switching of gas permeation properties is not possible with the fragile freestanding LCN films alone.

Investigation on effect of ionic liquid on CO2 separation performance and properties of novel co-casted dual-layer PEBAX-ionic liquid/PES composite membrane

Dual-layer composite membranes composed of Pebax 1657, different 1-Ethyl-3-methylimidazolium tetrafluoroborate ([Emim][BF4]) contents and Polyethersulfone (PES) were fabricated by co-casting method, to investigate CO2/light gases separation, due to the high affinity of CO2 to Pebax and the ionic liquid (IL). Gas permeation test, which was performed at 25 °C and pressure range of 4–8 bar, indicated remarkable increment in CO2, CH4 and N2 permeance through membranes, especially CO2, as well as ideal CO2/CH4 and CO2/N2 selectivities, with IL increment. At 6 bar, CO2 permeance increased 152.97% meanwhile, at 8 bar, CO2/CH4 and CO2/N2 selectivities increased 59.22% and 28.64%, respectively, in the membrane containing 20 wt. % IL, which is the optimum IL content. In addition, CO2 permeance and selectivity increased with pressure. FTIR, XRD, DSC, SEM, dynamic frequency sweep, contact angle and tensile test results revealed the formation of defect-free dense Pebax/IL top-layer on porous PES sub-layer. IL addition led to formation of new bonds between the IL and Pebax which cause changes in the chain packing and crystallinity of Pebax and fabrication of more amorphous top-layer and more porous sub-layer. These results are confirmed by decrement in the mechanical properties of Pebax/IL casting solutions and composite membranes, by IL increment.

Blocking chemical warfare agent simulants by graphene oxide/polymer multilayer membrane based on hydrogen bonding and size sieving effect

Chemical warfare agents (CWAs) are toxic materials that cause death by contact with the skin or by respiration. Although studies on detoxification of CWAs have been intensively conducted, studies that block CWAs permeation are rare. In this study, for blocking CWAs, a multilayer thin film composed of linear polyethylenimine (LPEI) and graphene oxide (GO) is simply prepared through a spray-assisted Layer-by-Layer (LbL) assembly process. LPEI could change its morphology dependent on pH, which is known as a representative hydrogen donor and acceptor. By controlling the shape of the polymer chain, a heterogenous film could have a loose or dense inner structure. CWAs mainly move through diffusion and have hydrogen bonding sites. Therefore, the heterogeneous film can limit CWAs movement based on controlling pathways and hydrogen bonds within the film. The protective effect of this membrane is investigated using dimethyl methylphosphonate (DMMP), a nerve gas simulant. DMMP vapor transmittance rate (DVTR) and N2 permeance of LPEI/GO are 67.91 g/m2 day and 34,293.04 GPU. It means that the protection efficiency is 72.65%. Although this membrane has a thin thickness (100 nm), it shows a high protective effect with good breathability. And water/DMMP selectivity of the membrane is 66.63. Since this multilayer membrane shows efficient protection performance with a simple preparation method, it has a high potential for applications such as protective suits and masks.

Experimental Studies on the Enhancement of Permeability of Anthracite by Acidizing: A Case Study in the Daning Block, Southern Qinshui Basin.

As the most active and top producing area of coalbed methane (CBM) in China, the southern Qinshui Basin (SQB) is dominated by anthracite. Due to the low permeability of coals, plenty of non-gas-producing and low production CBM wells exist in the SQB. The permeability enhancement through some technological means is the key to increasing the CBM production of this area. In this paper, some typical anthracites were selected from the Daning block of the SQB to assess the effect of acidification treatments on permeability enhancement. The maceral composition determination shows that approximately 15% of minerals exist in the collected coal samples, and the X-ray diffractometer (XRD) results reveal that the minerals consist primarily of clay minerals, along with a little amount of quartz, calcite, and dolomite. Two types of acidizing fluids were used to conduct acidification treatments on the anthracites for different lengths of time. The N2 permeability of the anthracites before and after acidification was measured and compared. The results show that the original samples exhibit low permeability. As the acidification time increases, the permeability of all of the samples shows an increasing trend, and the acid sensitivity index Ia increases rapidly first and then levels off, and finally approaches 1. After 48 h of acidification, the samples show an increase ranging from 8.75 to 22.67 times (avg. 14.3 times) the original permeability. The permeability enhancement of the SQB anthracites is mainly attributed to the dissolution of acid-soluble minerals in the cleat system of coal. The minerals in the cleats are completely or partially dissolved by the acids, generating some soluble and insoluble substances; when the fluid flows through, the cleat space is reallocated. Overall, the cleat demineralization by acids frees up a lot of cleat spaces, leading to an increase in cleat connectivity. As a result, the fluid movement becomes smooth and the permeability of coal improves.