Kerogen Model Research Articles

Molecular simulation to understand the effect of methane concentration and moisture contents on hydrogen adsorption in kerogens

The adsorption behaviors of methane(CH4), hydrogen(H2), and their mixtures are crucial for estimating the underground hydrogen storage(UHS) capacity in depleted shale gas reservoirs. In this study, we developed four formulated kerogen models, each representing different maturation levels of type II kerogen molecules via utilizing molecular dynamics (MD) methodologies. We investigated the adsorption characteristics of pure H2 and mixed gases(H2 and CH4) on these kerogen models with varying gas components(0.5:0.5 to 0.9:0.1) and moisture content(0–3.0 wt percentage) through grand canonical Monte Carlo(GCMC) simulations. Our findings reveal that pure H2 lacks a pressure point for adsorption equilibrium below 100 MPa at ambient temperature(298K). The competitive adsorption dynamics between H2 and CH4 on kerogen models demonstrate that CH4 exhibits stronger selectivity at lower pressures. Yet, this selectivity diminishes compared to H2 as system pressure increases, indicating turning points in pressure buildup with kerogen maturity. Additionally, the presence of pre-loading H2O molecules reduces the adsorption capacity of mixed gases, particularly affecting CH4 more than H2. This study offers profound insights into the effect of kerogen maturity and moisture content on the interaction between H2/CH4 and kerogen at a microscopic scale.

Molecular simulation of the competitive adsorption of methane and carbon dioxide in the matrix and slit model of shale kerogen and the influence of water

Carbon dioxide enhanced shale gas recovery (CO2-EGR) technology is of great significance for shale gas extraction and carbon dioxide storage in subsurface, which involves the competitive adsorption in shale nanopores. Adsorption comparisons between the kerogen matrix and slit and between the pure gas and gas mixture are conducted in this study. Kerogen matrix and slit models are built with the type II-A kerogen macromolecules and adsorptions of CH4 and CO2 are modelled. It is seen: 1) The gas absolute adsorption increases with its molar fraction while decreases with temperature. The Langmuir pressure for CO2 decreases while that for CH4 increases with their molar fractions. The adsorption selectivity of CO2 over CH4 decreases with the increase in pressure and the CO2 fraction, while it is higher in the matrix than that in the slit. Water significantly reduces the gas adsorption especially for matrix. 2) CO2 has high affinity to the Sulfur and Nitrogen functional groups, while CH4 molecules mainly adsorb on the Sulfur, Nitrogen and Carbon functional groups, While water are strongly bound to the Oxygen functional groups with water cluster formed at high contents. 3) Lower interaction energies are shown in the matrix compared with the slit due to the adsorption superposition, which results in gases preferentially adsorbed in the matrix then on the slit surface in the kerogen slit model. The water interaction energy is lowest due to the hydrogen bond, while the interaction energy of CO2 is much smaller than that of CH4 indicating its adsorption advantage.

Insight into adsorption behaviors of shale oil in kerogen slit by molecular dynamics

Shale oil is predominantly stored in organic nanopores, with kerogen playing a pivotal role in its adsorption. This study utilizes molecular dynamics (MD) simulations to quantitatively investigate the adsorption behavior of multi-component shale oil within kerogen slits. Unlike previous studies that focused on single-component hydrocarbon fluids and simplified kerogen models to graphene, this research employs an authentic kerogen structure and a multi-component shale oil model, incorporating the effects of surface roughness. The research analyzes the interaction mechanisms and distribution patterns of light, medium, and heavy components within the pores. Additionally, it examines the effects of temperature, pore size, and wall surface physical and chemical properties on shale oil adsorption. Results indicate that strong adsorption between asphaltene and kerogen significantly reduces the width of the kerogen slits, inhibiting the adsorption of lighter components. The study introduces the concept of effective radius for kerogen slits and provides a quantitative analysis of the adsorption capacity of kerogen surfaces. Different kerogen surface roughness and chemical properties were compared to understand their influence on shale oil adsorption. Key findings show that with increasing temperature, the distribution of hydrocarbons becomes more uniform, with medium components showing the most significant response. Surface roughness of kerogen affects the adsorption capacity, although the component proportions remain consistent, while different chemical property surfaces influence selective adsorption and the overall adsorption amount of shale oil components. This research offers a deeper understanding of shale oil adsorption mechanisms and provides valuable insights for optimizing shale oil extraction and utilization.

Construction of multidimensional structure model of the Mesoproterozoic Xiamaling shale kerogen, northern North China

In the southeastern Xuanlong sag of Northern China, the organic-rich black shale of the Mesoproterozoic Xiamaling Formation holds significant potential as a source rock. Through advanced analytical techniques including elemental analysis, 13C NMR, XPS, FTIR, and XRD, the elemental composition and chemical structure were thoroughly characterized, and the molecular structure of the Xiamaling shale kerogen was established. The 13C NMR analysis revealed the carbon composition of kerogen, indicating 75.19 % aliphatic carbon, 22.06 % aromatic carbon, and 2.75 % carbonyl carbon, with quantitative support from FTIR and XRD spectra. The dominant methylene carbons exhibited a low average methylene chain length (Cn) of 3.87, suggesting that they are primarily short-branched or alicyclic compounds rather than long straight chains. This conclusion was verified by the absence of a sharp peak at a wavenumber of 719.95 cm−1 in the FTIR spectrum. Oxygen-containing functional groups observed in 13C NMR spectra were also identified in FTIR spectra at a wavenumber range of 3500–1000 cm−1. XPS analysis indicated that nitrogen-containing groups were amino, quaternary, pyridine, and primary pyrrole groups, and sulfur existed as sulfoxide, aromatic sulfur, aliphatic sulfur, and sulfone. Finally, a relatively rational 2D molecular structure with the chemical formula of C278H346N6O26S2 was established by comparing the predicted and experimental 13C NMR spectra. Using MD simulation, five optimized 2D molecules were combined to construct a robust 3D kerogen model with a physical density of 1.067 g/cm3. Comparatively, the aliphatic structure of the Mesoproterozoic Xiamaling kerogen model consists of some monocyclic alkanes and polycyclic saturated structures, resembling Paleozoic kerogen models but differing from Mesozoic counterparts. The constructed kerogen model addresses the gap in the Precambrian kerogen modeling and sets the stage for in-depth exploration of pyrolysis mechanisms in the future.

Kinetic Characteristics of Kerogens in the Kuonamka Complex, Lower and Middle Cambrian of the Siberian Platform

Abstract —The kinetic characteristics of kerogen (activation energy and frequency factor distributions of the initial generative potential) determine the dynamics of naphthide generation during catagenesis. In the previous studies on quantitative modeling of naphthidogenesis in the Kuonamka source rocks, the kinetic characteristics were taken by analogy with the known kerogens from other complexes and provinces. It seems relevant to determine the effective kinetic characteristics of the kerogen from the Kuonamka source rocks from experimental data. To estimate the kinetic parameters, specialized multi-rate pyrolysis studies were performed. The variations in pyrolysis data was reported to be presumably associated with the conditions of kerogen and source rock formation during accumulation and diagenesis of organic matter. The effect of carbonate and organic carbon contents on the temperature range of the second pyrolysis peak is recorded. Reconstruction of the average (effective) kinetic characteristics of kerogens from the Kuonamka source rock, was conducted using data on kerogens from Serkinsky-5 and 10 and Tit-Ebya-6 wells, which are less degraded than kerogens from Ust’-Maiskaya-366 well and probably attained the early mesocatagenesis grades. The current average effective kinetic characteristics of kerogen were estimated and corrected for the possible partial degradation of reference samples. The obtained effective kinetic models of kerogen are compared with models of kerogens of various genetic types known from the literature. The estimated effective kinetic characteristics of kerogens or Menil-1 and Type B kerogens with similar characteristics are to be taken, as a first approximation, in regional historical-genetic (basin) modeling of naphthide generation in the Kuonamka source rock.

The impact of water on gas storage in organic-rich rocks

To achieve CO2 storage in organic-rich rocks such as coal and shale, the influence of H2O generated by organic matter and co-occurring CH4 and N2 cannot be neglected. Based on the molecular model of coal and kerogen established using Materials Studio, and verification of CH4 isothermal adsorption from 20 to 100 oC, the competitive adsorption between CH4、CO2、N2 and H2O is discussed. Both the coal and shale kerogen molecular models show similar relative adsorption capacities, namely H2O > CO2 > CH4 > N2. Competitive adsorption is more obvious in small pores with a greater apparent inhibiting effect of H2O on the gases. The adsorption of CO2, CH4 and N2 decreases to zero at 75 ℃ in the kerogen model. A positive correlation is shown between the charge transfer and the adsorption capacity, and the adsorption onto carboxyl group occurs indicate that H2O has the highest adsorption preference. The oxygen-containing functional group is the dominant adsorption site for H2O, with the carboxyl group being the best adsorption site. The hydroxyl group is the optimal site for all gases, while all lower than H2O. The adsorption heat released are of H2O>CO2 >CH4 >N2 sequences, which is consistent with the change in the amount adsorbed.

Dependence of Methane Transport on Pore Informatics in the Amorphous Nanoporous Kerogen Matrix.

Fluid transport in kerogen is mainly diffusion-driven, while its dependence on pore informatics is still poorly understood. It is challenging for experiments to identify the effect of pore informatics (such as pore connectivity and tortuosity) on fluid transport therein. Therefore, in this work, we use molecular dynamics simulations to study methane transport behaviors in amorphous kerogen matrices with broad pore properties. The pore properties including porosity, pore connectivity, pore size, and diffusive tortuosity are characterized. Next, self-diffusion coefficients in the connected pores (DeffS) and in the total pores without distinguishing its connectivity (DtotS) are calculated in all the kerogen matrices based on the free volume theory. We find that both DeffS and DtotS exponentially decreases with methane loading with two controlled parameters: fitting constant αeff and DeffS(0) (DeffS at infinitely small loading) for DeffS and fitting constant αtot and DtotS(0) (DtotS at infinitely small loading) for DtotS. However, in the kerogen models with relatively low pore connectivity, αeff and αtot as well as DeffS(0) and DtotS(0) can be quite different, inducing the different estimations of DeffS and DtotS. Since methane in the unconnected pores does not contribute to the actual transport, it is important to recognize connected pores when evaluating the fluid transport in kerogen. On the other hand, DeffS(0) strongly depends on the effective limiting pore size (rlim_eff) of the dominant flow path and effective diffusive tortuosity (τeff), in which DeffS(0) linearly increases with (rlim_eff/τeff)2. We also find that αeff is a multivariable function of ϕeff, τeff, and rlim_eff, but their generalized relation requires more data to obtain. This work provides important insights into fluid transport in kerogen based on the kerogen pore informatics, which are important to shale gas development.

Average molecular structure model of shale kerogen: Experimental characterization, structural reconstruction, and pyrolysis analysis

Shale can be both a source rock and a reservoir rock, which is a typical self-generating and self-storing type of oil accumulation. Its main organic component is kerogen. Using molecular dynamics (MD), a realistic kerogen molecular structure model is an alternative to experimental and analytical-only approaches for studying shale organic matter's microscopic properties. Therefore, it is useful to establish an average molecular structure model for shale kerogen. Moreover, the pyrolysis mechanism of shale kerogen is of great significance for developing and utilizing shale mineral resources. In this study, geochemical and spectroscopic measurements such as the elemental analysis, 13C nuclear magnetic resonance spectroscopy (13C NMR), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were used to obtain information about the carbon skeleton, aliphatic structures, and aromatic structures of the kerogen. Following information analysis and molecular fragment splicing, a 2D molecular structure model of kerogen from the Nanpu shale, with the chemical formula C199H240O20N6S2 and molecular weight of 3096 Daltons was constructed. After that, the accuracy of the 2D structure was verified by comparing the 13C NMR spectra simulated by the 2D structure to the experimental spectra. Using MD simulation, based on the kerogen density, 10 2D structures are then combined to create a realistic 3D molecular structure model of kerogen. Finally, pyrolysis analysis was performed based on the established 3D molecular structure model of kerogen to determine the effect of temperature on pyrolysis. Using the advantages of various technologies, this study not only provides a systematic method for establishing a realistic 3D molecular structure model of kerogen but also contributes to in-depth research on the reaction mechanism of kerogen pyrolysis, providing a reliable foundation for future research on the properties of shale at the molecular scale.

Modeling competitive adsorption and diffusion of CH4/CO2 mixtures confined in mature type-II kerogen: Insights from molecular dynamics simulations

In this study, the adsorption and competitive diffusion of methane (CH4) and carbon dioxide (CO2) mixtures through a molecular model of mature type-2 kerogen was investigated using molecular dynamics simulations. The Dual Control Volume-Grand Canonical Molecular Dynamics method was used to analyze the system and the results were focused on the selectivity of kerogen towards mixtures under a wide range of thermodynamic conditions. The results showed that the adsorption selectivity of CO2/CH4 was around 2–4 and decreased with temperature in the range 350–450 K. The Extended Langmuir (EL) model was found to underestimate the selectivity, and a correction was proposed in the form of the Corrected Extended Langmuir (CEL) model, which improved the estimations of the adsorption of CH4/CO2 mixtures in kerogen with only one adjustable parameter independent of thermodynamic conditions. Interestingly, we noticed that, out of equilibrium, the adsorption selectivity was not affected by the flow conditions when a pressure gradient was imposed, and the CEL model can thus be used to describe the contribution of adsorption to the selectivity under flow conditions. The results showed that the ratio of the collective diffusion coefficients of CH4 and CO2 was around 0.3−0.4, which was about two times lower than other simulations found in the literature. To describe the global selectivity, a simple approach combining the CEL model and the average ratio of diffusion coefficients was proposed, resulting in a selectivity of around 1.

Molecular Simulation of Argon Adsorption and Diffusion in a Microporous Carbon with Poroelastic Couplings.

Neglected for a long time in molecular simulations of fluid adsorption and transport in microporous carbons, adsorption-induced deformations of the matrix have recently been shown to have important effects on both sorption isotherms and diffusion coefficients. Here we investigate in detail the behavior of a recently proposed 3D-connected mature kerogen model, as a generic model of aromatic microporous carbon with atomic H/C ∼ 0.5, in both chemical and mechanical equilibrium with argon at 243 K over an extended pressure range. We show that under these conditions the material exhibits some viscoelasticity, and simulations of hundreds of nanoseconds are required to accurately determine the equilibrium volumes and sorption loadings. We also show that neglecting matrix internal deformations and swelling can lead to underestimations of the loading by up to 19% (swelling only) and 28% (swelling and internal deformations). The volume of the matrix is shown to increase up to about 8% at the largest pressure considered (210 MPa), which induces an increase of about 33% of both pore volume and specific surface area via the creation of additional pores, yet does not significantly change the normalized pore size distribution. Volume swelling is also rationalized by using a well-known linearized microporomechanical model. Finally, we show that self-diffusivity decreases with applied pressure, following an almost perfectly linear evolution with the free volume. Quantitatively, neglecting swelling and internal deformations tends to reduce the computed self-diffusivities.

Experimental and molecular investigation of water adsorption controls in marine and lacustrine shale reservoirs

The restriction of hydrocarbon migration within shale reservoirs by pre-adsorbed water necessitates an investigation into water adsorption control mechanisms within shales. In this study, water vapor adsorption isotherms obtained from marine and lacustrine shale samples along with isotherms from their organic matter and experimentally verified kerogen molecular models were used to evaluate water adsorption mechanisms, with simulated wet kerogen-quartz nanocomposites. Furthermore, these were also used to investigate organic matter-inorganic interaction controls on water adsorption. Pore size distribution (PSD) controls on water distribution in organic matter were obtained via water vapor adsorption and low-pressure nitrogen adsorption analysis, along with small angle neutron scattering (SANS) analysis. The results reveal water adsorption is linked to changes in mesopore volume within organic matter and porosity in kerogen models. PSD obtained from the shales only reveal mesopore controls at high RH, with water adsorption mainly controlled by oxygen functional groups in organic matter pores and swelling clays. Inorganic controls on water adsorption are also observed in adsorbed water not tightly bound to the shale surface due to a high montmorillonite content in clay interlayers, and organic matter interaction with quartz with a negative relationship between the adsorbed water and the quartz content. Collectively, these findings indicate that CO2 migration in shale reservoirs could be inhibited by water that is distributed in potential CO2 adsorption sites within organic matter pores and montmorillonite.

Multistage Hydrocarbon Generation of Saline Lacustrine Source Rocks in Hydrous Pyrolysis: Insights from Clay Mineral-Organic Matter Interactions.

The organic matter (OM) in shale is closely associated with clay minerals, and its maturation is usually accompanied by the diagenesis of these minerals, especially smectite illitization. However, the effect of mineral transformation and its accompanying change of mineral-OM interactions in shale on hydrocarbon generation is still unclear. To investigate this question, smectite-rich immature shale was selected to carry out hydrous pyrolysis. Organic geochemistry and mineralogy of pyrolysates at different temperatures show that the maturation of OM is accompanied by the transformation of bulk and clay minerals. Based on the change in hydrocarbon yield, Rock-Eval parameters, and mineral composition, hydrocarbon generation in this study is divided into three stages: 25-300, 300-400, and 400-500 °C, which are the result of the synergistic evolution of clay minerals and OM. Multistage hydrocarbon generation can be attributed to the mineral transformation-induced desorption of mineral-bound soluble OM (SOM), decarboxylation and hydrocracking of kerogen promoted by solid acids, and cross-linking and cracking reactions of free SOM and residual kerogen under high temperatures. Although different from the classical hydrocarbon generation model of kerogen, this multistage hydrocarbon generation is consistent with the characteristics of the saline lacustrine source rocks in nature. The mineral transformation-induced desorption of SOM is a new pathway for petroleum formation, which can well explain the formation of low-mature oils in nature. In addition, the release of mineral-bound and kerogen-bound biomarkers results in two reversals of isomerization ratios. Considering mineral transformation and mineral-OM interactions can help us better understand and refine the hydrocarbon generation theory of OM.

Molecular interactions of CO2 and CH4 and their adsorption behaviour in kerogens with different grades of maturity

ABSTRACT CO2 sequestration (CS) into the shale formations can reduce not only carbon emissions but also enhance gas recovery (EGR). The adsorption and diffusion behaviour of CO2 and CH4 on kerogens with different maturity play a crucial role in CS-EGR as they determine the efficiency of CO2 storage and energy recovery. In this work, GCMC and MD simulations were performed to investigate the adsorption and diffusion behaviour of CO2 and CH4 on kerogen models at different grades of maturity. It indicated that, in the same maturity, the CO2 adsorption capacity was more significant than that of CH4. With increasing maturity, the adsorption capacity and diffusion rate increased. With the increase of water contents, the swelling ratio of kerogens increased, and the adsorption capacity and the diffusion coefficients of CH4 and CO2 decreased. However, the adsorption selectivity of CO2 over CH4 significantly increased. H2O and CO2 molecules both preferred to adsorb on functional groups of oxygen, nitrogen and sulfur. This study consolidated our hypothesis that an injected CO2 to shale gas and oil formations contributed positively to enhanced energy recovery owing to the difference in adsorption and diffusion behaviour of CH4 and CO2 in kerogens with different maturity in gas and oil reservoirs.

Hydrocarbon Generation Mechanism of Mixed Siliciclastic–Carbonate Shale: Implications from Semi–Closed Hydrous Pyrolysis

Affected by the complex mechanism of organic–inorganic interactions, the generation–retention–expulsion model of mixed siliciclastic–carbonate sediments is more complicated than that of common siliciclastic and carbonate shale deposited in lacustrine and marine environments. In this study, mixed siliciclastic–carbonate shale from Lucaogou Formation in Junggar Basin was selected for semi–closed hydrous pyrolysis experiments, and seven experiments were conducted from room temperature to 300, 325, 350, 375, 400, 450, and 500 °C, respectively. The quantities and chemical composition of oil, gases, and bitumen were comprehensively analyzed. The results show that the hydrocarbon generation stage of shale in Lucaogou Formation can be divided into kerogen cracking stage (300–350 °C), peak oil generation stage (350–400 °C), wet gas generation stage (400–450 °C), and gas secondary cracking stage (450–500 °C). The liquid hydrocarbon yield (oil + bitumen) reached the peak of 720.42 mg/g TOC at 400 °C. The saturate, aromatic, resin, and asphaltine percentages of bitumen were similar to those of crude oil collected from Lucaogou Formation, indicating that semi–closed pyrolysis could stimulate the natural hydrocarbon generation process. Lucaogou shale does not strictly follow the “sequential” reaction model of kerogen, which is described as kerogen firstly generating the intermediate products of heavy hydrocarbon compounds (NSOs) and NSOs then cracking to generate oil and gas. Indeed, the results of this study show that the generation of oil and gas was synchronous with that of NSOs and followed the “alternate pathway” mechanism during the initial pyrolysis stage. The hydrocarbon expulsion efficiency sharply increased from an average of 27% to 97% at 450 °C, meaning that the shale retained considerable amounts of oil below 450 °C. The producible oil reached the peak yield of 515.45 mg/g TOC at 400 °C and was synchronous with liquid hydrocarbons. Therefore, 400 °C is considered the most suitable temperature for fracturing technology.

Development of Atomistic Kerogen Models and Their Applications for Gas Adsorption and Diffusion: A Mini-Review

With the emergence of shale gas, numerous atomic-scale models of kerogen have been proposed in the literature. These models, which attempt to capture the structure, chemistry, and porosity of kerogens of various types and maturities, are nowadays commonly─if not routinely─used to gain nanoscale insights into the thermodynamics and dynamics of complex and important processes such as hydrocarbon recovery and carbon sequestration. However, modeling such a complex, disordered, and heterogeneous material is a particularly challenging task. It implies that important underlying assumptions and simplifications, which can significantly affect the predicted properties, have to be made when constructing the kerogen models. In this mini review, we discuss the existing atomistic models of kerogen by categorizing them according to the different approaches and assumptions used during their construction. For each type of model, we also describe how the construction strategy can impact the prediction of certain properties. Important work on kerogen interactions with gas and oil, from both the point of view of equilibrium adsorption (including adsorption-induced deformation) and transport, are described. Possible improvements and upscaling strategies─to better account for kerogen in its geological environment─are also discussed.

Sorption of Deep Shale Gas on Minerals and Organic Matter from Molecular Simulation

Deep shale gas is one of the key targets of China’s natural gas exploitation in the future. Abnormally high reservoir pressures cause an unclear occurrence state of shale gas. In this paper, molecular modeling techniques are first used to establish slit-pore models of illite, quartz, and kerogen with different widths. Monte Carlo and molecular dynamics methods are second applied to simulate the sorption of methane under the geological conditions of deep reservoirs. Then, the gas distributions in the pores and changes of energies in sorption are analyzed. The applicability of the Langmuir model is also verified. The results show that (1) shale gas is approximately adsorbed as strong monolayers in a pore; the density of the adsorbed gas in mesopores is not sensitive to the pore size; (2) the Langmuir equation is applicable for the sorption of shale gas over a wide range of pressures; the surface excesses and maximum adsorbed amounts for the mesopores follow the order of illite > quartz > kerogen, which results from the different actual surface areas depending on the roughness and holes on the surfaces; (3) in a mesopore, total gas energy reduces more in adsorption under higher pressure; van der Waals force dominates gas adsorption, while electrostatic force affects weakly; under high pressure, the total gas energy for kerogen matrix decreases less when pressure increases, which is related to the extremely dense absorbed gas and nonmonotonic variation of van der Waals force with the distance between molecules; the interactions between shale gas and different types of pores follow the order of kerogen matrix > kerogen mesopore > illite > quartz. This study clarifies the sorbing behaviors of deep shale gas as well as the applicability of the Langmuir model, which can be a reference for developing the deep shale gas.

Adsorption and Diffusion Behaviors of CO2 and CH4 Mixtures in Different Types of Kerogens and Their Roles in Enhanced Energy Recovery

CO2 geological sequestration in subsurface shale formations is a promising strategy to store CO2 and to increase shale gas production. The understanding of gas adsorption and diffusion mechanisms in microporous media is critical for CO2 storage-enhanced gas recovery (CS-EGR). The type of kerogens is one of the important factors that influence the adsorption and diffusion behaviors of gases. In this work, the Grand Canonical Monte Carlo and Molecular Dynamics simulations were utilized to develop kerogen models and further investigate gas and water adsorption and diffusion behavior on the type IA, IIA, and IIIA kerogen models. The results indicated that the adsorption and diffusion capacities of CO2 are larger than those of CH4. The adsorption and diffusion capacity decreased with increasing water content. However, the CO2/CH4 adsorption selectivity increased with the increase in water content. Type IIIA demonstrated the best potential for adsorption and diffusion. This study provides insights into the role of the adsorption and diffusion behavior of CO2 and CH4 mixtures on kerogens of different types under different water contents at a microscopic scale, and can facilitate further understanding of the processes involved in CO2 storage coupled with enhanced energy recovery.

Combining molecular simulation and experiment to understand the effect of moisture on methane adsorption in kerogens

There is a limited understanding of the critical impact moisture has on shale gas resource estimation by affecting gas adsorption and pore structure. Laboratory experiments on dry and 95% relative humidity (R.H.) isolated kerogens are combined with Grand Canonical Monte Carlo (GCMC) and Molecular Dynamic (MD) simulations for kerogen models, including matrix and slits (0.5, 1.0, 1.5, and 2.0 nm) with a range of moisture contents (0–42 wt% on a total organic carbon content (TOC) basis) to better understand how moisture impacts methane adsorption. Higher methane adsorption capacities (Qm) and micropore volumes (Vmicro) are observed for simulated kerogens since all pores in GCMC are accessible. Moisture has a negative effect on Qm, displaying ‘rapid’, ‘gentle’, and ‘slow’ stages with increasing moisture in simulation. Reductions in Qm (61–75%) and Vmicro (88–93%) are obtained for isolated kerogens containing moisture of 38–70 wt% TOC with up to 56% of the moisture in micropores. The same Qm and Vmicro reductions can be reached for the simulated kerogens with moisture contents of 4–24 wt% TOC for matrix and slits. The relative coordination number (Cr) from MD simulation indicates water has a stronger affinity than methane for all functional groups with preferred sorption sites like carboxyl (COOH) under reservoir conditions. The microporosity controls condensed water cluster size. Water adsorbed in ultra-micropores (<0.7 nm) leads to ‘rapid’ reduction, the ‘gentle’ Qm reduction stage arises from water condensing, and filling of remaining pores at the highest moisture is related to the ‘slow’ Qm reduction stage. Therefore, water reduces the methane adsorption capacity of kerogen mainly by occupying and blocking the pore volume rather than competing directly with methane for sorption sites.

Direct imaging of micropores in shale kerogen

Although micropores (<2 nm) play an important role in shale gas storage, these pores have not been imaged previously. Aberration-corrected scanning transmission electron microscope (AC-STEM) with a secondary electron detector (SED) has been used here to study such pores in kerogen isolated from Longmaxi shale. In addition, a three-dimensional (3D) kerogen molecular model was constructed based on 13C Nuclear Magnetic Resonance (13C NMR), Fourier transform infrared spectroscopy (FT-IR) and CO2 adsorption experiments, to further study the relationship between micropores and kerogen molecule. The results show that AC-TEM with SED can directly image pores smaller than 2 nm in the kerogen particle by operating in the scanning transmission electron microscopy (STEM) mode. These micropores can occur everywhere on the kerogen surface, which is consistent with the large micropore volume and surface area determined by gas (CO2 and N2) adsorption experiments. The surface of the 3D kerogen model is similar to that observed in the SED-STEM images in terms of surface morphology and scale. This suggests that the 3D model of the kerogen molecule is a good representation of the kerogen sample. This study links the direct images of micropores in kerogen sample and the kerogen molecular models, which is important for further understanding of shale gas storage, as these micropores and molecules provide the sites and surface energy for methane storage.

Free Volume Model for Transport in Flexible Kerogen of Source Rock's Organic Matter.

We build a model of transport of adsorbed fluid within the microporous network of kerogen's porosity, especially accounting for the adsorption-induced swelling exhibited by flexible kerogen structures. This model, based on Fujita-Kishimoto free volume theory that was historically developed for swellable polymers, is built over extensive results for the self-diffusion coefficients obtained by molecular dynamics calculations for a representative molecular model of kerogen designed to study the importance of flexibility effects on the properties of kerogen. To do so, we first highlight that transport within flexible kerogen incorporating the coupling between the dynamics of the fluid molecules and the kerogen matrix atoms does not introduce any significant collective effects in the usual long time limit. Then, we show that despite the slightly anisotropic diffusion properties, averaging over all the dimensions can still be performed in order to model the behavior of the transport properties with the amount of adsorbed fluid. Lastly, we link the increase of the self-diffusion coefficients and that of the accessible free volume with the fluid loading via the Fujita-Kishimoto model. We conclude by commenting on the evolution and significance of the model parameters over a broad range of thermophysical conditions.