Kerogen Model Research Articles

Molecular simulation of kerogen-water interaction: Theoretical insights into maturity

The ubiquity of water and kerogen in source rocks at reservoir conditions is a clear indication that the study of kerogen-water system plays a key role and relevant to the current state of knowledge for fundamental and applied shale research. From the early experimental studies, it is acknowledged that kerogen insolubility in a typical organic solvent is due in part to their complex molecular structure. From a theoretical perspective, we aim to understand the molecular interaction of kerogen-water system at the atomic level. The petrophysical implication of this study is to understand the impact of maturity level of kerogen on the water recognition relative to other inorganic components of shale. For this purpose, we have calculated the adsorption free energies of water molecule on multi-configurations of kerogen model in a high mature state using B3LYP/Def2TZVP level of theory. Different models were built to reflect the diversity of kerogen composition (related to the heteroatoms). Furthermore, we have calculated the electrostatic potential and frontiers molecular orbitals to interpret and explain the trend observed in the adsorption free energies. The main finding of our study supported that there are no tight interactions between the water molecules and kerogen. The water-recognition by kerogen surface depends mainly on its constituents of the heteroatoms (N, S, and O). Indeed, nitrogen atoms displayed the highest affinity to ward moisture, followed by oxygen and sulfur. Given that kerogen in its high mature state does not consists much of nitrogen atoms, in that context, the matured kerogen would be less likely to adsorb water molecules in absence of gas.

Theoretical Study of Structural and Spatial Properties of Kerogen

In this study, the structural information and spatial properties of the Longkou kerogen model were obtained by using molecular simulation methods. The density evaluation of the molecular model was carried out to investigate the morphological properties of internal pore structures that are limited or constrained by kerogen density. The density of the lowest-energy kerogen model is 1.01 g/cm3, which is consistent with its experimental value and close to the reference value for Green River Shale kerogen. Radial distribution function analysis indicated that the carbon skeleton of kerogen consists of significant amounts of sp2- and sp3-hybridized carbons. To further investigate the distribution characteristics of internal pore structures and provide valuable insights into the microstructures of the kerogen model, the morphology of pore volumes was created using Connolly surfaces. Meanwhile, a simplified mechanism model of kerogen pyrolysis was proposed based on the Mayer bond order analysis, which provided a strategy to accurately, quickly predict the cleavage behavior in the intrinsic structure of kerogen. This present study reveals inherent chemical features of kerogen and will aid in understanding the microscopic morphology characteristics of the pore network inside kerogen at the molecular level.

Microemulsion Effects on Oil Recovery From Kerogen Using Molecular-Dynamics Simulation

Summary Source rocks contain significant volumes of hydrocarbon fluids trapped in kerogen, but effective recovery is challenged because of amplified fluid/wall interactions and the nanopore–confinement effect on the hydrocarbon–fluid composition. Enhanced oil production can be achieved by modifying the existing molecular forces in a kerogen pore network using custom–designed targeted–chemistry technologies. The objective of this paper is to show that the maturation of kerogen during catagenesis relates to the qualities of the kerogen pore network, such as pore size, shape, and connectivity, and plays an important role in the recovery of hydrocarbons. Furthermore, using molecular–dynamics (MD) simulations, we investigated how the transport of hydrocarbons in kerogen and hydrocarbon recovery can be altered with the delivery of microemulsion and surfactant micelles into the pore network. New 3D kerogen models are presented using atomistic modeling and molecular simulations. These models possess important chemical and physical characteristics of the organic matter of the source rock. A replica of Type II kerogen representative of the source rocks in the Permian Basin in the US is used for the subsequent recovery simulations. Oil–saturated kerogen is modeled as consisting of nine different types of molecules: dimethyl naphthalene, toluene, tetradecane, decane, octane, butane, propane, ethane, and methane. The delivered microemulsion is an aqueous dispersion of solvent–swollen surfactant micelles. The solvent and nonionic surfactant present in the microemulsion are modeled as d–limonene and dodecanol heptaethyl ether (C12E7), respectively. MD simulation experiments include two stages: injection of an aqueous–phase microemulsion treatment fluid into the oil–saturated kerogen pore network, and transient flowback of the fluids in the pore network. The used 3D kerogen models were developed using a representative oil–sample composition (hydrogen, carbon, oxygen, sulfur, and nitrogen) from the region. Simulation results show that microemulsions affect the reservoir by means of two different mechanisms. First, during the injection, microemulsion droplets possess elastic properties that allow them to squeeze through inorganic pores smaller than the droplet's own diameter and to adsorb at the kerogen surfaces. The solvent dissolves in the oil phase and alters the physical and transport properties of the phase. Second, the surfactant molecules modify the wettability of the solid kerogen surfaces. Consequently, the recovery effectiveness of heavier oil fractions is improved compared with the recovery effectiveness achieved with surfactant micelles without the solubilized solvent. The results indicate that solubilized solvent and surfactant can be effectively delivered into organic–rich nanoporous formations as part of a microemulsion droplet and aid in the mobilization of the kerogen oil.

Structural model of Longkou oil shale kerogen and the evolution process under steam pyrolysis based on ReaxFF molecular dynamics simulation

ABSTRACT The product distribution and reaction mechanism of steam pyrolysis of Longkou oil shale kerogen was researched by molecular dynamics simulation. Molecule structural model used in the simulation was constructed according to the analysis results of a series of detection about kerogen extracted from Longkou oil shale. Reactive force field molecular dynamics (ReaxFF MD) was used to simulate both steam pyrolysis and direct pyrolysis process of the kerogen at the temperature of 1600, 2000, 2400 and 2800 K. The results show that temperature is a critical factor affecting product distribution in steam pyrolysis, and 2000 K is a proper set temperature for studying steam pyrolysis via molecular simulation method. Besides that, adding the H2O molecules during steam pyrolysis can form complexes with heterogeneous atoms, thus destroying the intermolecular interactions in kerogen. Moreover, as the hydrogen radicals come from H2O molecules can inhibit cross-linking reactions between small fractions, it can reduce the average molecular weight of organic molecules product. These conclusions could be helpful for rational use of oil shale.

The Constructions and Pyrolysis of 3D Kerogen Macromolecular Models: Experiments and Simulations.

Kerogens are extracted from deep shales to study pyrolysis of deep shale samples. The 2D molecular models of kerogens are obtained by a series of physical and chemical experiments by which the macromolecular models of kerogens are constructed. Then, the reasonable 3D macromolecular models are established by molecular mechanics and global energy minimization. The effects of temperature and heating rate on the chemical kinetics of kerogen pyrolysis are studied using reactive force field (ReaxFF). The hybrid molecular dynamics/force‐biased Monte Carlo (MD/fbMC) approach is used to simulate the pyrolytic process at the experimental temperature, which is lower than the conventional one. The gaseous products and residues obtained by the simulations agree with the experimental results, which means a reliable simulation method for pyrolysis at experimental temperature is provided. This study constructs the rational macromolecular models of kerogen by experiments, and proposes the mechanisms of typical reactions of kerogen pyrolysis, which may help in understanding the formation of shale oil and gas.

Evaluation of the diffusive tortuosity by analyzing the molecular thermal motion displacement

Molecular thermal motion is a very meaningful process. Plenty of useful information can be extracted from molecular travel displacement. In this paper, a kerogen model with a random and complex pore network is constructed. Based on the molecular thermal motion process, the diffusive tortuosity caused by the confined pore network is investigated by the molecular dynamics simulations. The influence of thermodynamic parameters on the diffusive tortuosity is carefully studied. The results showed that the diffusive tortuosity ranges from 1.57 to 2.70 depending on the pressure. However, with the variation of temperature and porosity, the diffusive tortuosity has a little change, mainly distributed from 1.79 to 1.95. The thermal diffusive tortuosity of the complex pore network is successfully calculated by analyzing molecular thermal motion property.

Improved Kerogen Models for Determining Thermal Maturity and Hydrocarbon Potential of Shale

Kerogen is the insoluble component of organic-rich shales that controls the type and amount of hydrocarbons generated in conventional and unconventional reservoirs. Significant progress has recently been made in developing structural models of kerogen. However, there is still a large gap in understanding the evolution of the molecular components of kerogen with thermal maturation and their hydrocarbon (HC) generative potential. Here, we determine the variations in different molecular fragments of kerogen from a Marcellus Shale maturity series (with VRo ranging from 0.8 to 3) using quantitative 13C MultiCP/MAS NMR and MultiCP NMR/DD (dipolar dephasing). These molecular variations provide insight into the (1) evolution of the molecular structure of kerogen with increasing thermal maturity and, (2) the primary molecular contributors to HC generation. Our results also indicate that old model equations based on structural parameters of kerogen underestimate the thermal maturity and overestimate the HC generation potential of Marcellus Shale samples. This could primarily be due to the fact that the kerogen samples used to reconstruct old models were mostly derived from immature shales (VRo <1) acquired from different basins with varying depositional environments. We utilized the kerogen molecular parameters determined from the Marcellus maturity series samples to develop improved models for determining thermal maturity and HC potential of Marcellus Shale. The models generated in this study could also potentially be applied to other shales of similar maturity range and paleo-depositional environments.

A Simple Molecular Kerogen Pore-Network Model for Transport Simulation in Condensed Phase Digital Source-Rock Physics

Transport of small hydrocarbon molecules inside organic nanopores is of great interest in the oil and gas production from source rocks such as shale. Because of the small pore sizes, it is difficult to measure transport directly in the laboratory but require alternative approaches such as molecular simulation. Existing molecular models use nanoscale capillaries and do not describe the exact geometric structure of the organic nanopores in real source rocks, especially for mesopores (2–50 nm in diameter, defined by IUPAC), which widely exist in organic solids and are crucial for fluid transport in the source rocks. Here we present a new method to simplify the existing molecular kerogen models. Pore structures can be built to reflect the exact porous structure of hydrocarbon-bearing kerogen pores for fluid transport without losing accuracy. The kerogen pore-network skeleton can be populated based on 3D digital segments obtained from high-resolution TEM tomographs, combining with other material properties, including the elemental composition and density of the organic solid. The basic idea of the simplified molecular model is to ignore the detailed chemical information of the kerogen molecules and to use a uniform “representative” atom to replace all various atom types, while maintaining the geometric structure intact. To test the transport simulation accuracy using molecular dynamics for this simplified model, sample pore-network structures are populated with both the existing exact molecular model and the simplified model, and the permeability of reservoir fluid flowing through the media is simulated and compared. Specifically, simple reservoir fluids are charged into the space of the 3D porous networks with grand canonical Monte Carlo simulation at given subsurface condition; then, the fluids are driven by an external force field to flow through the network using non-equilibrium molecular dynamics simulation. The simplified kerogen model can be easily applied to larger organic porous material samples to reduce spatial uncertainty, provided enough computational resources.

Chemical structure evolution of kerogen during oil generation

Abstract Kerogen was isolated from the source rock of Well L69, Zhanhua depression, Bohai Bay Basin and an artificial pyrolysis experiment was carried out in a closed gold tube system with a heating rate of 2 °C/h. Products were collected at eleven temperature points every ten degrees from 350 °C to 450 °C. The soluble organic matter and residual kerogens from the experiment were quantified by weighing. Furthermore, the residual kerogens were analysed by element analysis, X-ray photoelectron spectroscopy (XPS) and solid 13C nuclear magnetic resonance (13C NMR) spectroscopy to determine the chemical structural of kerogen during oil generation. Combining the data from the analyses, information on the elemental compositions as well as on how functional groups were connected in residues was obtained. Seven molecular models of initial and residual kerogens were established to trace the structure changes in kerogen with maturity. The results suggest that as the temperature increases, aliphatic chains and small clusters of aromatic groups break down from kerogens, generating soluble hydrocarbons at temperatures of less than 390 °C (Easy% Ro1.15). At higher thermal maturity, a few short aliphatic carbons remain in the structure of residues. The number of aromatic groups increases and their size grows with increasing maturity, with condensation occurring at aromatic groups at high temperature, while these residues have little potential for oil generation.

Spatial distribution of reservoir fluids in mature kerogen using molecular simulations

This work analyses the distribution of volatile oil and other reservoir fluids in kerogen belonging to the gas condensate/volatile oil window. The kerogen models representing kerogen Type II-C contain pores in the range of micropores (diameter <2 nm) and mesopores (2 nm < diameter < 50 nm). A few of the micropores are isolated pores whereas the mesopores are connected in all spatial directions. Methane is observed to fill the pores while longer n-alkane chains transition from filling the pore to adsorbing onto the pore surface. The heavier components in the reservoir fluids adsorb onto the surface of the pore and can partially or completely clog the pore throats. As a general trend, the longer the alkane chain, the more preferentially the molecule adsorbs onto the pore surface. Additionally, the molecular simulations correctly capture the hydrophobic environment in which water molecules cluster together to form a droplet that resides in or close to a pore throat. Some molecules of carbon dioxide seem to preferentially adsorb onto either the kerogen internal surface or on the molecules of adsorbed heavier components, while the rest of the molecules diffuse into the kerogen body. This observation has not been previously reported to the best of our knowledge. Also the adsorption of the heavier species onto the pore walls significantly decreases the surface area available for adsorption of other species. This work underscores the significance of knowing fluid distributions to model primary recovery as well as secondary or tertiary recovery.

Enhanced gas recovery by CO2 sequestration in marine shale: a molecular view based on realistic kerogen model

Injection of CO2 into shale reservoir is regarded as one potential scenario for CO2 sequestration and enhanced gas recovery (CS-EGR). In this work, a realistic molecular model of kerogen in Chinese Silurian marine black shale was generated using molecular dynamics (MD) simulations. The competitive adsorption of CH4 and CO2 was simulated by the grand canonical Monte Carlo (GCMC) method under different reservoir pressures, temperatures, geological depths, CO2 mole ratios, and moisture contents of kerogen model. Results show that CO2/CH4 adsorption selectivity decreases with increasing reservoir pressure, indicating that CS-EGR can be more efficient if CO2 injection is conducted at the late development stage. The temperature has a negative effect on the selectivity, which indicates that thermal stimulation has an adverse effect on the efficiency of CS-EGR. Also, the selectivity decreases with increasing geological depth, suggesting that shallow shale formations are more suitable for CS-EGR. At low pressures, the selectivity increases with increasing CO2 mole ratio, while at high pressures, the selectivity decreases with the increase of CO2 mole ratio. This result suggests that CO2 mole ratio should be dynamically adjusted with the production so as to adapt to the changing reservoir pressure. At higher pressure condition, both the amounts of CO2 sequestration and CH4 desorption increase with the increase of CO2 mole fraction. However, the adsorption stability of CO2 weakens with increasing injection amounts of CO2. Moreover, the adsorption selectivity decreases initially, and then increases with the moisture content of kerogen. Thus, the performance of CS-EGR may be improved by increasing the kerogen moisture content for Silurian shale gas reservoirs. This study gains enhanced insights on the effect of reservoir pressure, temperature, geological depth, CO2 mole ratio, and kerogen moisture content on CO2/CH4 competitive adsorption, and the results can provide applicable guidances for CS-EGR in shale gas reservoirs.

Detecting Marine Kerogen from Western Canada Basin Using Terahertz Spectroscopy.

In this paper, terahertz time-domain spectroscopy was employed to study the properties of marine kerogen from Western Canada Basin at various temperatures. On the basis of terahertz absorption coefficients of samples, the evolution model of oil and gas generation in kerogen was established, explained, and verified by IR and principal component analysis. In addition, the molecular models of kerogen were simulated by means of the quantum chemistry. Then the vibration characteristics of functional groups in terahertz band were analyzed, and the reasons for different absorptions of kerogen at different temperatures were explained. This study will provide a reference for thermal evolution kinetics of kerogen, as well as an effective complement to the potential evaluation of oil and gas resources.

Molecular Simulation of CO2/CH4 Competitive Adsorption on Shale Kerogen for CO2 Sequestration and Enhanced Gas Recovery

The adsorption behavior and underlying mechanism of CO2 and CH4 binary mixture in shale kerogen significantly affect the CO2 sequestration with enhanced gas recovery project (CS-EGR). In this study, we investigated the competitive adsorption behaviors of CO2 and CH4 in shale kerogen nanopores using grand canonical Monte Carlo (GCMC) method. Kerogen model takes into effect of matrix and slit nanopores and moisture content based on Ungerer’s molecular model and scanning electron microscope (SEM) analysis, and is successfully validated against experimental data. The effects of temperature, CO2 and CH4 distribution, moisture content, adsorption selectivity, and optimal formation for injection were discussed. The results show that adsorption amount of CH4 on the kerogen increases with increasing pressure and decreases with increasing temperature. The adsorption selectivity of CO2 over CH4 is 2.53–7.25, which indicates that CO2 is preferentially adsorbed over CH4 under different temperatures. H2O prefers to ads...

Impact of Nanoporosity on Hydrocarbon Transport in Shales' Organic Matter.

In a context of growing attention for shale gas, the precise impact of organic matter (kerogen) on hydrocarbon recovery from unconventional reservoirs still has to be assessed. Kerogen's microstructure is characterized by a very disordered pore network that greatly affects hydrocarbon transport. The specific structure and texture of this organic matter at the nanoscale is highly dependent on its origin. In this study, by the use of statistical physics and molecular dynamics, we shed some new lights on hydrocarbon transport through realistic molecular models of kerogen at different level of maturity [ Bousige et al. Nat. Mater. 2016 , 15 , 576 ]. Despite the apparent complexity, severe confinement effects controlled by the porosity of the various kerogens allow linear alkanes (from methane to dodecane) transport to be studied only via the self-diffusion coefficients of the species. The decrease of the transport coefficients with the amount of adsorbed fluid can be described by a free volume theory. Ultimately, the transport coefficients of hydrocarbons can be expressed simply as a function of the porosity (volume fraction of void) of the microstructure, thus paving the way for shale gas recovery predictions.

Molecular simulation of methane adsorption on type II kerogen with the impact of water content

Based on realistic kerogen model, the effects of water content on methane (CH4) adsorption capacity were studied qualitatively and quantitatively using the molecular dynamics (MD) and Monte Carlo (MC) simulation methods. The methane single component adsorption process under the dry condition, methane and water two components competitive adsorption on dry kerogen, and methane single component adsorption on moist conditions (0.6, 1.2, 1.8, 2.4, and 3.0 wt%) were simulated. Adsorption processes under different temperatures (298, 323, and 348 K) were modeled, and the pressure was up to 20 MPa. Simulation results show that the absolute adsorption capacity of CH4 on the dry kerogen increases with the increasing pressure but decreases with the increasing temperature. The excess adsorption capacity increases up to a specific pressure and then declines with the increasing pressure. The competitive adsorption simulation results indicate that kerogen prefers to adsorb water (H2O) than CH4. At the pressure of 20 MPa and temperature of 298 K, the competitive adsorption capacity of CH4 (0.53 mmol/g) is only a seventh of the single component adsorption capacity of CH4 (3.92 mmol/g) on dry kerogen. The CH4 adsorption capacity on moist kerogen also decreases with the increasing moist content. Compared with the absolute adsorption capacity on the dry kerogen (3.92 mmol/g), the adsorption capacity on the moist kerogen reduces about 16%, 30%, 40%, 47%, and 55% at 20 MPa, respectively. The reduction of the adsorption capacity is attributed to the strong attraction between kerogen and water. At last, the effect of the water content on the shale gas adsorption is discussed under the micro-scale, and the adsorption capacity reduction curves were calculated under the reservoir conditions. The results show that the adsorption capacity of shale gas is significantly affected by water at the economically recoverable depth. The dry condition is considered as the situation of the maximal adsorption capacity of the shale gas reservoir. This study can serve as a reference for better understanding of the transportation and storage mechanism of shale gas under the reservoir conditions.

Effect of organic type and moisture on CO2/CH4 competitive adsorption in kerogen with implications for CO2 sequestration and enhanced CH4 recovery

Although research attentions for CO2 injection in gas-bearing reservoirs have been drawn to CO2 sequestration with enhanced gas recovery (CS-EGR), the microscopic competitive adsorption mechanism of methane (CH4) and carbon dioxide (CO2) considering the effect of organic type and moisture remains to be determined. In this work, we focus on the competitive adsorption behaviors of CH4 and CO2 on dry and moist realistic kerogen models of different organic types by performing combined molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations. The effects of organic type and moisture content on kerogen pore structures, moisture distribution and interaction between CH4/CO2 and kerogen surfaces are discussed in details. Simulation results show that CO2/CH4 adsorption capacity and adsorption selectivity are in the order of kerogen IA < IIA < IIIA, consistent with the sequence of enterable pore volume fraction (IA, 9.38%; IIA, 13.59%; IIIA, 28.88%). H2O molecules are preferentially adsorbed on the sulfur- and oxygen-containing groups at low moisture, and then migrate and aggregate into clusters in the middle of enterable pores at high moisture. The CO2/CH4 adsorption capacity decreases with increasing moisture content, while the CO2/CH4 adsorption selectivity, specific adsorption energy and CO2 isosteric heat decrease at the beginning, and then increase with the moisture content. Moisture has a bigger effect on the adsorption of CO2 than that of CH4. This study indicates that kerogen IIIA is the optimized organic type for CS-EGS due to its large and stable CO2 storage capacity. Despite its negative effect on gas adsorption capacity, moisture can potentially boost the displacement of CH4 by CO2 at certain moisture conditions. Results of this study lay the foundation for future optimization design of CS-EGR projects with application to coal and shale systems.

Molecular simulation of adsorption behaviors of methane, carbon dioxide and their mixtures on kerogen: Effect of kerogen maturity and moisture content

Abstract The adsorption behaviors of methane (CH 4 ), carbon dioxide (CO 2 ) and their mixtures are vital to understand the process of CO 2 sequestration and shale gas exploitation. In this work, four realistic kerogen models with different maturities (immature (IIA), beginning of oil window (IIB), middle of oil window (IIC), postmature (IID)) were built by the molecular dynamics (MD) method. The adsorption characteristics of CH 4 , CO 2 and their mixtures on these kerogen models with various moisture contents (0, 0.7, 1.4, 2.1, 2.8 wt%) were investigated by the grand canonical Monte Carlo (GCMC) simulations. The influences of kerogen maturity and moisture content on the adsorption capacity, isosteric heat of adsorption and adsorption selectivity of gas molecules were discussed. Simulation results show that the maximum adsorption capacity of gas molecules increases with increasing kerogen maturity, but decreases with increasing moisture content, and the reduction decreases as the maturity increases at high moisture contents. The average isosteric heat of CO 2 adsorption is relevant to the sulfur/oxygen content of kerogen models. The pre-adsorbed water (H 2 O) has a small effect on the gas isosteric adsorption heat when located in the middle of pores, but can reduce the CO 2 isosteric adsorption heat by occupying the hydrophilic groups. Moreover, H 2 O molecules are observed to migrate and aggregate into growing clusters at higher moisture contents for kerogen IIC and IID models, increasing the gas isosteric adsorption heat. The CO 2 /CH 4 adsorption selectivity gradually decreases to the equilibrium value with the rise of bulk pressure. Also, the selectivity decreases with increasing CO 2 mole fraction for lower mature kerogen models (IIA and IIB), but increases with the CO 2 mole fraction at low pressure for kerogen models of higher maturity (IIC and IID). Meanwhile, the selectivity increases for IIA, IIC and IID models, while decreases for IIB model as the moisture content increases. This study gains deep insights into the effect of kerogen maturity and moisture content on the interaction between CH 4 /CO 2 and kerogen at microscopic scale.

Estimation of adsorbed-phase density of methane in realistic overmature kerogen models using molecular simulations for accurate gas in place calculations

Methane adsorption in the organic matrix (kerogen) of gas shales is an important factor in determining the storage of natural gas and volumetric calculations of gas in place. The pore system in shales is very heterogeneous, ranging from micropores, mesopores, and macropores, and the pore size affecting the adsorption behavior. For example, in micropores the proximity between opposite walls causes the enhancement of the interactions of methane with the surfaces, which affects adsorbate packing and density. Grand canonical Monte Carlo simulations were carried out to study methane adsorption, its density under confinement, and estimate the density of the adsorbed phase in realistic overmature kerogen models that account for the chemical functionality of the surfaces and pore size distribution of kerogens of such maturity. This will provide better estimates of fluids in place. It is demonstrated that errors in the adsorbed phase have a considerable impact on gas in place calculations.

Methane and CO2 Adsorption Capacities of Kerogen in the Eagle Ford Shale from Molecular Simulation.

Over the past decade, the United States has become a world leader in natural gas production, thanks in part to a large-fold increase in recovery from unconventional resources, i.e., shale rock and tight oil reservoirs. In an attempt to help mitigate climate change, these depleted formations are being considered for their long-term CO2 storage potential. Because of the variability in mineral and structural composition from one formation to the next (even within the same region), it is imperative to understand the adsorption behavior of CH4 and CO2 in the context of specific conditions and pore surface chemistry, i.e., relative total organic content (TOC), clay, and surface functionality. This study examines two Eagle Ford shale samples, both recovered from shale that was extracted at depths of approximately 3800 m and having low clay content (i.e., less than 5%) and similar mineral compositions but distinct TOCs (i.e., 2% and 5%, respectively). Experimentally validated models of kerogen were used to the estimate CH4 and CO2 adsorption capacities. The pore size distributions modeled were derived from low-pressure adsorption isotherm data using CO2 and N2 as probe gases for micropores and mesopores, respectively. Given the presence of water in these natural systems, the role of surface chemistry on modeled kerogen pore surfaces was investigated. Several functional groups associated with surface-dissociated water were considered. Pressure conditions from 10 to 50 bar were investigated using grand canonical Monte Carlo simulations along with typical outgassing temperatures used in many shale characterization and adsorption studies (i.e., 60 and 250 °C). Both CO2 and N2 were used as probe gases to determine the total pore volume available for gas adsorption spanning pore diameters ranging from 0.3 to 30 nm. The impacts of surface chemistry, outgassing temperature, and the inclusion of nanopores with diameters of less than 1.5 nm were determined for applications of CH4 and CO2 storage from samples of the gas-producing region of the Eagle Ford Shale. At 50 bar and temperatures of 60 and 250 °C, CH4 adsorption increased across all surface chemistries considered by 60% and 2-fold, respectively. In the case of CO2, the surface chemistry played a role at both 10 and 50 bar. For instance, at temperatures of 60 and 250 °C, CO2 adsorption increased across all surface chemistries by 6-fold and just over 2-fold, respectively. It was also found that at both 10 and 50 bar, if too low an outgassing temperature is used, this may lead to a 2-fold underestimation of gas in place. Finally, neglecting to include pores with diameters of less than 1.5 nm has the potential to underestimate pore volume by up to 28%. Taking into consideration these aspects of kerogen and shale characterization in general will lead to improvements in estimating the CH4 and CO2 storage potential of gas shales.

Model representations of kerogen structures: An insight from density functional theory calculations and spectroscopic measurements

Molecular structures of kerogen control hydrocarbon production in unconventional reservoirs. Significant progress has been made in developing model representations of various kerogen structures. These models have been widely used for the prediction of gas adsorption and migration in shale matrix. However, using density functional perturbation theory (DFPT) calculations and vibrational spectroscopic measurements, we here show that a large gap may still remain between the existing model representations and actual kerogen structures, therefore calling for new model development. Using DFPT, we calculated Fourier transform infrared (FTIR) spectra for six most widely used kerogen structure models. The computed spectra were then systematically compared to the FTIR absorption spectra collected for kerogen samples isolated from Mancos, Woodford and Marcellus formations representing a wide range of kerogen origin and maturation conditions. Limited agreement between the model predictions and the measurements highlights that the existing kerogen models may still miss some key features in structural representation. A combination of DFPT calculations with spectroscopic measurements may provide a useful diagnostic tool for assessing the adequacy of a proposed structural model as well as for future model development. This approach may eventually help develop comprehensive infrared (IR)-fingerprints for tracing kerogen evolution.