High-pressure CH4 Research Articles

Tertiary-Amine-Functional Poly(arylene ether)s for Acid-Gas Separations.

Competitive sorption enables the emergent phenomenon of enhanced CO2-based selectivities for gas separation membranes when using microporous polymers with primary amines. However, strong secondary forces in these polymers through hydrogen bonding results in low solvent solubility, precluding standard solution processing approaches to form these polymers into membrane films. Herein, we circumvent these manufacturing constraints while maintaining competitive-sorption enhancements by synthesizing eight representative microporous poly(arylene ether)s (PAEs) with tertiary amines. High-pressure H2S, CO2, and CH4 sorption isotherms were collected for these samples to demonstrate enhanced affinity for acid gases relative to the unfunctional control polymer. Although competitive sorption was observed for all samples, improvements were less pronounced than for primary-amine-functional analogs. For H2S-based separations, the benefits of competitive sorption offset decreases in selectivity due to plasticization. This detailed study helps to elucidate the role of tertiary amines for acid gas separations in solution-processable microporous PAEs.

Nonlinear Scattering of 248 nm Wavelength Light in High-Pressure SF6 and CH4 Gases for the Temporal Compression of a 20 ns KrF Laser Pulse

The nonlinear compression of narrowband (Δν ≈ 0.2 cm−1) 20 ns KrF laser pulses in SF6 at 10 atm and in CH4 at 50 atm pressure was studied. Both SBS and SRS optically phase-conjugated backward-reflected radiation was registered with an energy reflectivity of 10–14% in SF6 and CH4. In SF6, the SBS pulses gradually shortened from 10 ns to 2–3 ns with a decrease in pumping to the SBS threshold of ~10 mJ, while the SRS pulse had the shortest length of 30–60 ps for the maximal pumping of 120 mJ and broadened near the SRS threshold of ~30 mJ. For the SRS pulse energy, the ~2 mJ peak power 5 × 107 W was tenfold higher than the pump power. The theoretical model predicted a soliton-like SRS pulse compression to a temporal length of the order of the vibrational relaxation time. There was no pulse compression of backward SBS and SRS radiation in CH4, while, in the forward direction, SRS pulses shortened to 3–4 ns at reduced pumping.

Thermodynamic characteristics of high-pressure CH4 adsorption on longmaxi shale subjected to supercritical CO2-water saturation

The shale-CH4 interaction after supercritical CO2 (ScCO2) injection can be well interpreted by adsorption thermodynamics study. Using a gravimetric method, the methane adsorption experiments over wide range of pressure (up to 35 MPa) and temperature (30–90 °C) were conducted on Longmaxi shale before and after ScCO2-water saturation (60days/20Mpa/70°C). In addition, low-temperature CO2 and N2 adsorption experiments were performed to analyze the variations of pore structure of shale. Results indicates that the high-pressure CH4 adsorption curves of shale exhibit obvious excess adsorption characteristics, and the excess adsorption capacity of shale to CH4 decreased after ScCO2-water saturation, which is mainly caused by the decrease of the specific surface area of shale. Comparing to Brunauer-Emmett-Teller, Dubinin-Radushkevich and Ono-Kondo models, the excess adsorption data can be well-fitted by Langmuir model through combining Henry equation. Additionally, the thermodynamic parameters of shale-CH4 adsorption pair, including the isosteric heat of adsorption, enthalpy change, Gibbs free energy change and entropy change, displayed strong dependence on adsorption capacity and temperature, and altered in varying degrees after ScCO2-water saturation, demonstrating that the shale gas recovery can be enhanced by ScCO2-water-shale interaction from the perspective of thermodynamics. This study provides a reference for CO2 enhanced shale gas recovery.

An insight into CO2 sequestration and EGR in Longmaxi and Niutitang shale formations via experimental analysis

In the global effort toward climate change mitigation, geologic storage of CO2 in shale plays is becoming critical while it can improve production from these reservoirs. In this study, the gas storage capacity and CO2 sequestration suitability of the Longmaxi and Niutitang shale gas reservoirs is investigated and compared via XRD analysis, programmed pyrolysis, low pressure N2 and CO2 adsorption, high pressure CH4 adsorption, and MICP analysis. Results showed that clay and TOC controls on micropore heterogeneity and connectivity happens only within the Longmaxi Formation. Furthermore, both components play a role in the adsorbed quantities of gas in the formation which revealing a higher storage capacity in mesopore-macropores. This was verified by a positive correlation between adsorbed volume within the mesopore-macropore space. The Longmaxi Formation was also found to own better pore connectivity and PSD homogeneity of mesopore-macropores, indicating easier pathways for CO2 movement channels within the pore network. Furthermore, the samples from the Longmaxi exhibited a positive relationship between adsorbed volume with the CO2 breakthrough pressure and median pore diameter, alike between the mesopore-macropore volume and CO2 breakthrough pressure. This means, the injected CO2 will displace adsorbed methane within the mesopore-macropore and will be adsorbed and stored in such pores. Overall, superiority of the Longmaxi shale over the Niutitang for CO2 storage and EGR would be significant for future operations in similar shale gas reservoirs in China and around the globe.

Hybrid Pore-Scale Adsorption Model for CO2 and CH4 Storage in Shale

Making reliable estimates of gas adsorption in shale remains a challenge because the variability in their mineralogy and thermal maturity results in a broad distribution of pore-scale properties, including size, morphology and surface chemistry. Here, we demonstrate the development and application of a hybrid pore-scale model that uses surrogate surfaces to describe supercritical gas adsorption in shale. The model is based on the lattice density functional theory (DFT) and considers both slits and cylindrical pores to mimic the texture of shale. Inorganic and organic surfaces associated with these pores are accounted for by using two distinct adsorbate–adsorbent interaction energies. The model is parametrized upon calibration against experimental adsorption data acquired on adsorbents featuring either pure clay or pure carbon surfaces. Therefore, in its application to shale, the hybrid lattice DFT model only requires knowledge of the shale-specific organic and clay content. We verify the reliability of the model predictions by comparison against high-pressure CO2 and CH4 adsorption isotherms measured at 40 °C in the pressure range 0.01–30 MPa on four samples from three distinct plays, namely, the Bowland (UK), Longmaxi (China), and Marcellus shale (USA). Because it uses only the relevant pore-scale properties, the proposed model can be applied to the analysis of other shales, minimizing the heavy experimental burden associated with high-pressure experiments. Moreover, the proposed development has general applicability meaning that the hybrid lattice DFT can be used for the characterization of any adsorbent featuring morphologically and chemically heterogeneous surfaces.

Relationship between the Geological Origins of Pore-Fracture and Methane Adsorption Behaviors in High-Rank Coal.

Coal is characterized by a complex pore-fracture network and functional groups, which are derived from various geological origins and which further affect methane adsorption. To explore the relationship between the geological origins of pore-fractures and methane adsorption behaviors, we conducted pore structure tests and adsorption isotherms on six Qinshui high-rank coals. The pores and fractures were observed using an optical microscope (OM), a field emission scanning electron microscope (FESEM), and a high-resolution transmission electron microscope (HRTEM), and the pore structure parameters were determined using mercury intrusion and low-pressure N2 and CO2 adsorption. High-pressure CH4 adsorption isotherms were obtained at 30 °C using the manometric method. Results show that the Qinshui high-rank coals develop five stages of pore size distribution, consisting of the smaller micropore stage (0.3–1 nm), the larger micropore and smaller mesopore stage (1–10 nm), the mesopore and smaller macropore stage (10–110 nm), the microfracture stage (0.11–40 μm), and the larger macropore stage (>40 μm). The micropores dominate the total pore volume (PV) and specific surface area (SSA). Pores and fractures of various morphologies and sizes have different geological origins, which are related to coalification and stress field evolution. Methane adsorption on coals mainly occurs in the micropores as a form of volume filling. The maximum pore size for complete gas filling (MPSCGF) ranges from 0.60 to 0.88 nm in Qinshui high-rank coals. The coal-forming geological processes, such as coalification and stress field evolution, contribute to various pores and fractures, which show different pore sizes and functional groups. The geological origins of pores and fractures control the methane adsorption behaviors in coals by way of the pore size and functional groups. Surface coverage-related methane adsorption behavior occurs in fractures, primary pores, and large-scale secondary pores, while micropore filling is the methane adsorption behavior in macromolecular pores and small-scale secondary pores. The aim of this study is to provide a new insight into the methane adsorption on coals from the geological process of the formation and modification of pores and fractures.

Contribution of clay on storage and adsorption capacities of the Middle Permian marls, South China

A significant gas show appeared in JY66-1 well when it was drilled into the Middle Permian limestone-marl alternation interval, high-resolution scanning electron microscopic (SEM) observation shows that clay-related pores were developed extensively. There have been a lot of comprehensive studies on the origin of organic-matter pores and their pore size distribution, but few studies on the scales of clay-related pores and their contributions to the total pore volume and surface area. In this paper, we conducted thin section and SEM observation, mercury intrusion, low pressure N2 adsorption and high-pressure CH4 adsorption experiments, to characterize the pore volume and surface area of Mg-clay-bearing samples. The result suggests that the pore volume of clay-related pores mainly derives from the Intra-aggregate pores of 5–100 nm pore size, while the absolute and relative contributions of Interlayer pores (smaller than 2 nm) and Intra-tactoid pores (2–5 nm) to the total pore volume are small. Based on TOC contents and inorganic mineral compositions, a multiple regression analysis was carried out on the contributions of organic-matter and clay-related pores to pore volume in the pore-diameter ranges of smaller than 2 nm, 2–5 nm, 5–100 nm and larger than 100 nm. Although the contribution of clay-related pores (including inter-aggregate pores, Intra-aggregate pores, Intra-tactoid pores, Interlayer pores) to the total pore volume is comparable to that of organic-matter pores, the latter still contributes most of the pore surface area and CH4 adsorption site. It can be interpreted by the fact that the specific surface area of the dominant Intra-aggregate pores is significantly smaller than that of organic-matter pores. The conclusion of this study can be applied for the analyses of storage and adsorption capacity in all gas shales containing both organic and clay minerals.

Quantitative characterization of pore network and influencing factors of methane adsorption capacity of transitional shale from the southern North China Basin

Quantitative characterization of pore structure and analysis of influencing factors of methane adsorption are important segments in shale gas reservoir and resources evaluation and have not been systematically carried out in marine–continental shale series. A series of integrated methods, including total organic carbon (TOC) contents, Rock-Eval pyrolysis, mineral composition analysis, pore structure measurement, high-pressure CH4 adsorption analysis and FE-SEM observation, were conducted on 12 transitional shale samples of well WBC-1 in the southern North China Basin (SNCB). The results indicate that TOC contents of the transitional shales range from 1.03 to 8.06% with an average of 2.39%. The transitional shale consists chiefly of quartz, white mica and clay minerals. Interparticle pore, intraparticle pore, dissolution pore and microfracture were observed in the FE-SEM images. The specific surface area (SSA) of BET for the samples ranges from 3.3612 to 12.1217 m2/g (average: 6.9320 m2/g), whereas the DR SSA for the samples ranges from 12.9844 to 35.4267 m2/g (average: 19.67 m2/g). The Langmuir volume (VL) ranges from 2.05 to 4.75 cm3/g (average = 2.43 cm3/g). There is unobvious correction between BET and DR SSA with TOC contents, which means inorganic pores are the main component of pore space in the transitional shale from the SNCB. The relationship of SSA and pore volume shows that micropore has a greater impact on the CH4 adsorption capacity than mesopore–macropore in the transitional shale. Different from shales in other petroliferous basin, clay minerals are the primary factor affecting adsorption capacity of CH4 for transitional shale in this study. The pore structure of the transitional shale for this study is characterized by higher fractal dimension and more heterogeneous pore structure compared to shale in other petroliferous basin. This study provides an example and new revelation for the influencing factors of pore structure and methane adsorption capacity of marine–continental transitional shale.

Experimental study on influence of adsorption equilibrium time on methane adsorption isotherm and Langmuir parameter

The aim of this work was to examine the impact of equilibrium judgment criteria on adsorption test of samples with different adsorption kinetics and determine how long it takes for different particle size samples to attain equilibrium adsorption data. The adsorption data of coal samples with three different particle sizes was obtained by manometric method at the different time intervals. High pressure CH4 adsorption equilibrium measurements and low pressure N2 and CO2 adsorption measurements were carried out for coal samples of eight various particle sizes. The results demonstrated that both the equilibrium and disequilibrium adsorption data satisfied the Langmuir equation. The Langmuir volume (VL) increased on the equilibrium time increase and it became more remarkable with increasing particle size. The Langmuir pressure (PL) showed an opposite and more obvious trend. When the particle size of the coal sample decreased from 2 to 4 to <0.074 mm, the adsorption equilibrium time (AET) required for the adsorption isotherm testing declined rapidly from 389 to 5 h. This research is of some enlightening significance for comparing the adsorption characteristics of samples with different adsorption kinetics (adsorbent, temperature, particle size, etc.).

Adsorption Mechanisms of High-Pressure Methane and Carbon Dioxide on Coals

Carbon dioxide (CO2) geological sequestration and coal-bed methane (CBM) recovery in deep coal seams are usually operated with a pressure higher than 10 MPa. The adsorption mechanisms of methane (CH4) and CO2 on coals in such a situation, however, are not yet revealed. With the help of a high-pressure gas adsorption system, CH4 and CO2 adsorption isotherms were first conducted on two coal samples. Simplified local density (SLD) theory was then tailored and applied to describe the adsorption characteristics of specific CH4 and CO2 on coals. Next, the adsorption mechanisms of high-pressure CH4 and CO2 on coal samples were revealed on the basis of adsorbed and bulk density distributions within the matrix pores. The results show that the high-pressure gas adsorption on coals is different from that at low pressure. The excess adsorption capacity first increases and then decreases with pressure, and the maximum value occurs at a specific pressure referring to reverse pressure. The maximum excess adsorption capacity of CH4 is less than that of CO2, while its reverse pressure is greater than that of CO2. The reversal of excess adsorption with pressure depends on the relative increase in adsorption and bulk phases. However, the mechanisms of adsorption reversal vary with gas types. The reversal of CH4 excess adsorption is due to the saturation near the pore wall, while the CO2 excess adsorption reversal is due to the dramatic bulk density changes near the critical point. Advances in the mechanism of high-pressure gas adsorption on coals suggests that coal seams deeper than 1000 m have more recovery potential, and CO2-enhanced coal-bed methane recovery success depends on the optimal management of its injection pressure and the associated coal swelling.

Laboratory investigation on pore characteristics of coals with consideration of various tectonic deformations

Coal structure is highly related to paleo-stress variation and the pore structure of coals can be modified through tectonic activities. This study characterizes the pore structure variations for various coal samples with different tectonic deformation intensities. The low temperature N2 and pressure CO2 adsorption analyses were conducted to quantify the pore structures, and high pressure CH4 adsorption measurements were carried out for the sorption analysis. The pores were classified into ultramicropores (<2 nm), micropores (2–10 nm), mesopores (10–100 nm) and macropores (>100 nm). The results show that with the increase of coal deformation intensity, the proportion of pore size of 50–300 nm decreased, showing that more macropores and mesopores were deformed to the smaller pores (<50 nm). Micropores (2–10 nm) in granulated and mylonitic coals obviously increased. The predominant ultramicropore in four kinds of coal structures were distributed at the range of 0.45–0.6 nm and 0.80–1.0 nm. Tectonic coals formed more ultramicropores lower than 0.65 nm. The adsorption of CH4, N2 and CO2 increased as the coal deformation degree increased, following the order: mylonitic coal > granulated coal > cataclastic coal > intact coal. Fractal dimensions show that tectonic coals characterized the rougher pore surface (higher D1) and more homogeneous pore structures (lower D2), which led to the higher adsorption capacity. The development of micropores was positive with Langmuir VL, but the ultramicropore SSAs were significantly larger than the contribution to SSAs of micropores and mesopores. Thus, ultramicropore provided the main adsorption sites for CH4. The variations of adsorption potential increased with the coal deformation intensity increased, and the smaller the adsorption space volume was, the larger adsorption potential would be, illustrating that adsorption in micropores was higher than mesopores and macropores. Tectonic coals have the higher reduced rate of cumulative surface free energy, which shows that tectonic damages promote the adsorption potential and surface free energy in coals.

Parametric Analysis of Two-dimensional Equation of State used to Predict High Pressure CO2 and CH4 on NaY zeolite and babassu activated carbon

In this work, the potential of two-dimensional equations of state (2D) was explored in terms of high pressure adsorption prediction and parametric analysis. To attain this objective, firstly, low pressure experimental data for CO2 and CH4 adsorption on NaY zeolite and babassu activated carbon were correlated with two-dimensional equations of state of van der Waals (2D vdW), Peng-Robinson (2D PR), Zhou et al. (2D ZGR) and 2D ZGRm (m as a fitted paramter). Then, high pressure isotherms were predicted for the same systems and compared to experimental data. Thus, a parametric analysis was made varying 2D ZGRm parameters: adsorbate-adsorbate interaction (a2); co-area (b2); Henry's Law constant (K); heterogeneity parameter (m); adsorbent specific area (A). Results indicate that these parameters present a critical value at a given pressure that separates homogeneous and heterogeneous adsorbed phases. Depending on the set of parameters applied, different types of isotherms can be described by 2D ZGRm. Therefore, it was verified that 2D EOS have the potential to be applied for both low pressure characterization of adsorbent materials and high-pressure adsorption processes.

Hyperthermophilic methanogenic archaea act as high-pressure CH4 cell factories

Bioprocesses converting carbon dioxide with molecular hydrogen to methane (CH4) are currently being developed to enable a transition to a renewable energy production system. In this study, we present a comprehensive physiological and biotechnological examination of 80 methanogenic archaea (methanogens) quantifying growth and CH4 production kinetics at hyperbaric pressures up to 50 bar with regard to media, macro-, and micro-nutrient supply, specific genomic features, and cell envelope architecture. Our analysis aimed to systematically prioritize high-pressure and high-performance methanogens. We found that the hyperthermophilic methanococci Methanotorris igneus and Methanocaldococcoccus jannaschii are high-pressure CH4 cell factories. Furthermore, our analysis revealed that high-performance methanogens are covered with an S-layer, and that they harbour the amino acid motif Tyrα444 Glyα445 Tyrα446 in the alpha subunit of the methyl-coenzyme M reductase. Thus, high-pressure biological CH4 production in pure culture could provide a purposeful route for the transition to a carbon-neutral bioenergy sector.

Carbon dioxide and methane sorption characteristics of Damodar Valley and Upper Assam shale, India

Shale gas has become one of the most significant unconventional hydrocarbon resources. The gas is adsorbed in shale micropores, more precisely in the pores of clay minerals and organic matter. In this work, studies were done on the shale samples of Damodar valley and Upper Assam Basin. High pressure adsorption isotherms of methane (CH4) and carbon dioxide (CO2) were analysed to assess the gas storage capacity. The organic geochemistry, thermal maturity and mineralogy are analysed by Rock-Eval pyrolysis, petrographic study and XRD study respectively. The purpose of this study was to understand the factors controlling the variations in the nature of high pressure CH4 and CO2 sorption capacities. The factors include organic-inorganic constituents, and pore characteristics. Results show that the shale samples have a higher CO2 sorption capacity compared to that of methane. However, the CO2 sorption capacity shows a sharp decrease after 70 bar. Under similar temperature and pressure condition CO2: CH4 ratio increases with increasing pressure up to a pressure of 70 bar and then shows a sharp decrease. Petrographic study shows that vitrinite is the most dominant maceral group. Unlike clay minerals, organic matter shows a robust control over gas sorption in the studied shale samples. Sorption potential of carbon dioxide and methane increases positively with vitrinite content but, decreases with rising mineral matter and thermal maturity. Vitrinite content also controls the CO2: CH4 ratio, and the ratio bears no relation with thermal maturity. Micropores, rather than mesopores, mostly influence the gas sorption.

Wireline Sand-Detection Tool Locates, Quantifies Downhole Sand Production

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 198566, “Locating and Quantifying Downhole Sand Production With Wireline Sand-Detection Tool and Examples of Application in Wells Offshore Caspian Sea,” by Andrey Timonin, Schlumberger, and Eldar Mollaniyazov, Dragon Oil, prepared for the 2019 SPE Gas and Oil Technology Showcase and Conference, Dubai, 21-23 October. The paper has not been peer reviewed. Most of the wells in the Dzheitune (Lam) field in the Caspian Sea were completed with dual tubing. Depletion in reservoir pressure caused formation failure in the A sand reservoir, and wells producing from this formation began to produce sand. Field examples presented in the complete paper describe principles of data acquisition with a sand-detection tool when run in combination with a production logging string and results of logging in slightly deviated wells completed with sand screens. Comparison of multiphase-inflow profiles with sources of sand production showed that most sand was produced through eroded intervals in the sand screens. Introduction Most Lam wells were completed with 9.625-in. casing and 7-in. liner across the reservoir section. The A sand reservoir, located at depths between approximately 1500 and 1800 m, and the CH sand reservoir, located below 2000 m, are produced separately by using dual-string tubing to avoid crossflow from the high-pressure CH reservoir into the low-pressure A reservoir. When wells in the A reservoir began to produce sand, the operator had to change the completion strategy for newly drilled wells and had to recomplete some old wells by pulling out of hole dual-string completions and running in hole single tubing with sand screens across existing perforations. The top of sand accumulation in the annulus was identified with a slim sonic tool that measures cement-bond-log and variable-density-log data. This change in completion strategy helped prevent sand production to the surface, but, eventually, several wells completed with sand screens across the A reservoir began to produce sand on the surface, damaging production facilities and plugging separators, which had to be cleaned periodically. To identify the source of sand production downhole and to verify the integrity of sand screens, a sand-detection tool was run on wireline in a few such wells.

A reference high-pressure CH4 adsorption isotherm for zeolite Y: results of an interlaboratory study

This paper reports the results of an international interlaboratory study led by the National Institute of Standards and Technology (NIST) on the measurement of high-pressure surface excess methane adsorption isotherms on NIST Reference Material RM 8850 (Zeolite Y), at 25 °C up to 7.5 MPa. Twenty laboratories participated in the study and contributed over one-hundred adsorption isotherms of methane on Zeolite Y. From these data, an empirical reference equation was determined, along with a 95% uncertainty interval (Uk=2). By requiring participants to replicate a high-pressure reference isotherm for carbon dioxide adsorption on NIST Reference Material RM 8852 (ZSM-5), this interlaboratory study also demonstrated the usefulness of reference isotherms in evaluating the performance of high-pressure adsorption experiments.

Influences of moisture on adsorption and desorption of methane on gas shales

ABSTRACT The moisture (H2O) within gas shale reservoirs significantly affects methane (CH4) adsorption and desorption on shales. To further address these influences, H2O adsorption on four shales selected from the Lower Silurian Longmaxi Formation located in southern Sichuan Basin of China was investigated. Moreover, CH4 adsorption and desorption behaviors on moist shales were also studied and elucidated. Results verify that the modified Brunauer-Emmet-Teller (BET) model with the assumption of dual adsorption sites can well describe H2O adsorption on shales, the multiple correlation coefficients of which are within the range of 0.9926–0.9979. The monolayer adsorption capacity of H2O on shales derived from the modified Brunauer-Emmet-Teller (BET) model varies between 0.0052 and 0.0087 g/g. For relative pressure (P/P s) less than 0.65–0.80, the oxygenic functional groups accommodate the primary adsorption sites for H2O on shales. Then, H2O clusters formed on the secondary adsorption sites, i.e. the previously adsorbed H2O molecules and the clay minerals, play a dominant role for H2O adsorption on shales at P/P s greater than 0.65–0.80. The elevated moisture content of shales leads to a sharp decrease in CH4 adsorption capacity. Particularly, the decreasing magnitude in maximum CH4 adsorption capacity generated based on the Langmuir model varies between 11.80% and 48.30% for all the moist shale samples. The desorption study indicates that CH4 adsorption/desorption hysteresis is more pronounced for the moist shales than the dry shales. Two possible mechanisms that the pore throat of shales is blocked by the desorbed H2O molecules, and CH4 molecules are readsorbed on the adsorption sites previously occupied by H2O clusters, could explain the influences of moisture on CH4 adsorption/desorption hysteresis on shales. The moist shales at reservoir saturation condition are strongly suggested to perform CH4 adsorption test to accurately estimate the total shale gas resources. Furthermore, aiming to deeply explore the mechanism regarding H2O dependence of CH4 adsorption and desorption within shales, the terahertz waves with superior accuracy of monitoring trace H2O under high-pressure CH4 condition, are recommended into the volumetric or gravimetric method to generate highly reliable data regarding CH4 adsorption and desorption on moist shales.

Effects of sequence stratigraphy on coal characteristics and CH4 adsorption capacity of the low-rank coal in Santanghu Basin, China

This paper focusses on sequence stratigraphy of coal-bearing strata, coal petrography, coal chemistry, and the CH4 adsorption capacity of the Badaowan Formation in Santanghu Basin, China. The comprehensive coal characteristics of 12 coal samples were determined, coupled with maceral composition, proximate analysis, elemental analysis, reflectance (Ro), and high-pressure CH4 adsorption. The results indicated that maceral composition and chemical properties of coal were controlled by the sequence stratigraphic framework. During the TST, the content of vitrinite, the ratio of vitrinite to inertinite (V/I) and the content of carbon in coal increased gradually from bottom to top. While during HST, an opposite trend was observed. Furthermore, due to the intrusion of the lake water, the ash yield in the TST is higher than that in HST. The CH4 adsorption capacity is positively correlated with the content of vitrinite and V/I. There is a moderate positive correlation between fixed carbon contents and VLad. Ash content was strongly correlated to Langmuir volume under the air-dry basis, with increasing Ad associated with the reduction of the VLad. It was observed that the CH4 adsorption capacity generally gradually increased during the TST, and gradually decreased in the HST. Gas content also shows a similar change rule. CH4 adsorption capacity is functionally dependent on coal characteristics, which again are dependent on sequence stratigraphy and thus the coal-forming environment. The base level and cycles of the sequence establish a theoretical and practical framework for CBM extraction from low-rank coal reservoirs.

Nanoscale pore structure of Carboniferous coals from the Ukrainian Donets Basin: A combined HRTEM and gas sorption study

Various compositional, depositional and maturity related influencing factors affect the complex pore structure of coal. To study the pore structural evolution at nanoscale, a well characterized sample set of vitrinite-rich Carboniferous coals from the Ukrainian Donets Basin, covering a maturity interval from 0.69 to 1.47%Rr, was selected. Conventional bright field transmission electron microscopy (BF TEM) and high-resolution TEM (HRTEM) imaging was used to directly determine pore size distributions, pore morphology, geometry factors and other structural features, while gas invasion techniques such as low-pressure gas adsorption (CO2 and N2) were used for the investigation of micro- and mesopore structural parameters. High-pressure CH4 sorption experiments revealed changes in the methane storage capacity within the investigated maturity range, while associated structural changes of vitrinite were monitored by Raman spectroscopy. The results indicate pore occlusion in vitrinite mainly at peak oil window maturity, the sensibility of micro- and mesopore structure to thermal maturity and the importance of organic sulphur as a catalyst for kinetics of structural modification. Observed structural changes at 1.10%Rr were related to the onset of wet-gas generation.A structural control on micromechanical properties of vitrinite is indicated by the correlation between reduced elastic moduli from a previous study and average nanopore diameters obtained by HRTEM. The applied comprehensive approach improved the understanding of depositional and maturity-related processes that may affect pore evolution and resulting gas storage capacity of coals.

New insights into the CH4 adsorption capacity of coal based on microscopic pore properties

The objective of this study is to quantitatively describe the CH4 adsorption capacity of coal through microscopic pore properties. In this work, the micropore volume distributions (MPVDs) of granular coal samples from six collieries were obtained by low-pressure CO2 adsorption (LPGA-CO2), and the external specific surface areas (SSAs) were calculated using low-pressure N2 adsorption (LPGA-N2). Based on micropore filling and monolayer coverage theories, the micropores were assumed to be cylindrical to obtain the distribution of CH4 in pore structures with different scales. For the coal samples used in this study, micropores were common, whose SSAs accounted for most of the total SSAs (90.39–99.58%), and the micropore volumes accounted for 75.61–96.55% of total pore volumes. The quantity of CH4 adsorbed in the form of micropore filling account for 74–99% of the total amount adsorbed, while the amount of CH4 adsorbed by micropores in the size range of 0.38–0.76 nm was 38–55% of the total amount adsorbed and accounted for the largest proportion in the nine adsorption areas partitioned according to the number of CH4 molecules occupying micropores with different scales. Based on a combination of the high-pressure CH4 adsorption (HPGA-CH4) results, the estimated Langmuir volumes are in close agreement with the measured Langmuir volumes, which demonstrates that the CH4 adsorption capacity of the coal is determined by both the accessible MPVD and external SSAs. The results of this study may be significant for understanding the pore networks and the CH4 adsorption mechanism in coal under in situ conditions.