Molecular simulation of displacement of shale gas by carbon dioxide at different geological depths

Review of Competitive Adsorption of CO2/CH4 in Shale: Implications for CO2 Sequestration and Enhancing Shale Gas Recovery

The injection of CO2 into shale gas reservoirs can not only enhance shale gas recovery (ESGR), but also realize CO2 geological storage (CGS). In this study, the competitive adsorption behaviors of CO2 and CH4 in shale were systematically reviewed, and the implication for shale gas recovery efficiency and CO2 storage potential were discussed. The adsorption advantage of shale for CO2 compared to CH4 provides a guarantee of the feasibility of supercritical CO2 (ScCO2) enhanced shale gas exploitation technology. The selective adsorption coefficient of CO2 and CH4 by shale (S CO2/CH4 ) is an important parameter in evaluating the competitive adsorption behavior of CO2/CH4 in shale gas reservoirs, which is closely related to the mineral composition, reservoir temperature, pressure conditions, water content, and mixed gas composition ratio. In addition, the injection type, injection mode, and injection rate of gases also exhibit different effects on CO2/CH4 competitive adsorption. Furthermore, the interaction between ScCO2 and the water-rock system will change the mineral composition and microstructure of shale, which will lead to changes in the adsorption behavior of shale on CO2 and CH4, so its influence on the competitive adsorption of CO2/CH4 cannot be ignored. Future research should integrate different research methods and combine with practical engineering to reveal the competitive adsorption mechanism of CO2/CH4 in shale reservoirs from both micro and macro aspects. This study can provide support for the integration technology of ScCO2 enhanced shale gas exploitation and its geological storage.

Molecular dynamics simulations of methane adsorption and displacement from graphenylene shale reservoir nanochannels

Methane is the main component of shale gas and is adsorbed in shale pores. Methane adsorption not only affects the estimation of shale gas reserves but also reduces extraction efficiency. Therefore, investigating the behavior of methane adsorption in shale reservoirs is important for evaluating shale gas resources, as well as understanding its desorption and displacement from the nanochannels of shale gas reservoirs. In this research, molecular dynamics simulations were used to investigate the adsorption behavior of methane gas in organic shale pores made of graphenylene, followed by its displacement by CO2 and N2 injection gases. The effects of pore size, pressure, and temperature on adsorption were examined. It was observed that increasing the pore size at a constant pressure led to a decrease in the density of adsorbed methane molecules near the pore surface, while a stable free phase with constant density formed in the central region of the nanopore. Moreover, adsorption increased with increasing pressure, and at pressures ranging from 0 to 3 MPa, 15 and 20 Å pores exhibited lower methane adsorption compared to other pores. The amount of adsorption decreased with increasing temperature, and the observed adsorption isotherm followed the Langmuir adsorption isotherm. The mechanism of methane displacement by the two injected gases differed. Carbon dioxide filled both vacant adsorption sites and directly replaced the adsorbed methane. On the other hand, nitrogen only adsorbed onto the vacant sites and, by reducing the partial pressure of methane, facilitated the displacement of methane.

Prediction of Adsorption and Diffusion of Shale Gas in Composite Pores Consisting of Kaolinite and Kerogen using Molecular Simulation

Natural gas production from shale formations is one of the most recent and fast growing developments in the oil and gas industry. The accurate prediction of the adsorption and transport of shale gas is essential for estimating shale gas production capacity and improving existing extractions. To realistically represent heterogeneous shale formations, a composite pore model was built from a kaolinite slit mesopore hosting a kerogen matrix. Moreover, empty slabs (2, 3, and 4 nm) were added between the kerogen matrix and siloxane surface of kaolinite. Using Grand-Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations, the adsorption and diffusion of pure methane, pure ethane, and a shale gas mixture were computed at various high pressures (100, 150, and 250 atm) and temperature of 298.15 K. The addition of an inner slit pore was found to significantly increase the excess adsorption of methane, as a pure component and in the shale gas mixture. The saturation of the composite pore with methane was observed to be at a higher pressure compared to ethane. The excess adsorption of carbon dioxide was not largely affected by pressure, and the local number density profile showed its strong affinity to kerogen micropores and the hydroxylated gibbsite surface under all conditions and pore widths. Lateral diffusion coefficients were found to increase with increasing the width of the empty slab inside the composite pore. Statistical errors of diffusion coefficients were found to be large for the case of shale gas components present at low composition. A larger composite pore configuration was created to investigate the diffusion of methane in different regions of the composite pore. The calculated diffusion coefficients and mean residence times were found to be indicative of the different adsorption mechanisms occurring inside the pore.

A Review of Molecular Models for Gas Adsorption in Shale Nanopores and Experimental Characterization of Shale Properties

Shale gas, as a promising alternative energy source, has received considerable attention because of its broad resource base and wide distribution. The establishment of shale models that can accurately describe the composition and structure of shale is essential to perform molecular simulations of gas adsorption in shale reservoirs. This Review provides an overview of shale models, which include organic matter models, inorganic mineral models, and composite shale models. Molecular simulations of gas adsorption performed on these models are also reviewed to provide a more comprehensive understanding of the behaviors and mechanisms of gas adsorption on shales. To accurately understand the gas adsorption behaviors in shale reservoirs, it is necessary to be aware of the pore structure characteristics of shale reservoirs. Thus, we also present experimental studies on shale microstructure analysis, including direct imaging methods and indirect measurements. The advantages, disadvantages, and applications of these methods are also well summarized. This Review is useful for understanding molecular models of gas adsorption in shales and provides guidance for selecting experimental characterization of shale structure and composition.

Selective adsorption of heavy metals from water by a hyper-branched magnetic composite material: Characterization, performance, and mechanism

The development of adsorbents to remove heavy metal ions from water with recyclable, high adsorption capacity, strong selectivity, safe, and economic performances has always been the focus and challenge of current research. A hyper-branched magnetic composite material (Fe₃O₄@SiO₂-S₄) was fabricated by a method combining "grafting,", "branching," and "modification,", and the structure was characterized by FTIR, XRD, SEM, TEM, SAED, VSM, TGA, and BET. In addition, the adsorption performance and mechanism for heavy metal ions in water were studied. The as-prepared composite material had excellent selective absorbability for Hg²⁺, Cd²⁺, and Ag⁺ in the presence of Fe³⁺, Fe²⁺, Cu²⁺, Mn²⁺, CO²⁺, Zn²⁺, and Ni²⁺, and when pH = 6, T = 30 °C, t = 4 h, it reached a saturated adsorption capacity of 2.42, 2.18, and 1.94 mmol/g to Hg²⁺, Cd²⁺, and Ag⁺, respectively. The adsorption isotherm was consistent with the Langmuir isotherm adsorption model, and the Dubinin Redushcke (D-R) model identified that the adsorption was chemical adsorption in nature. The adsorption kinetic followed the pseudo-second-order model and Boyd film diffusion models. The adsorption capacity of as-prepared material remained about 83% after five elutions. The adsorption mechanism and selective adsorption were revealed by FTIR, EDS, XPS, and DFT calculation. N atoms and O atoms of the active functional groups complexed with metal ions to form stable 2 heptachate chelates and 1 tridentate chelate to achieve the effect of adsorption; furthermore, the adsorption was mainly governed by N atoms of Schiff base groups. This work not only explored an innovative method for the construction of adsorbing materials but also provided a promising adsorbent to selectively remove heavy metal ions in water with potential application.

Release of methane from nanochannels through displacement using CO2

In this work, we investigate the release of methane in quartz nanochannels through the method of displacement using carbon dioxide. Molecular dynamics (MD) simulations and theoretical analysis are performed to obtain the release percentage of methane for nanochannels of various diameters. It is found that both the pressure of CO2 and the channel size affect the release percentage of methane, which increases with increasing pressure of CO2 and channel diameter. Without CO2, the majority of methane molecules are adsorbed by the channel surface. When CO2 is injected into the channel, CO2 molecules replace many methane molecules due to the relatively strong molecular interactions between CO2 and the channel, which leads to the desorption of methane, reduces the energy barrier for the transport of methane, and consequently increases the release rate. Theoretical predictions using the kinetic energy of methane and the energy barrier inside the channel are also conducted, which are in good agreement with the MD simulations.

Molecular Investigation on the Displacement Characteristics of CH4 by CO2, N2 and Their Mixture in a Composite Shale Model

The rapid growth in energy consumption and environmental pollution have greatly stimulated the exploration and utilization of shale gas. The injection of gases such as CO2, N2, and their mixture is currently regarded as one of the most effective ways to enhance gas recovery from shale reservoirs. In this study, molecular simulations were conducted on a kaolinite–kerogen IID composite shale matrix to explore the displacement characteristics of CH4 using different injection gases, including CO2, N2, and their mixture. The results show that when the injection pressure was lower than 10 MPa, increasing the injection pressure improved the displacement capacity of CH4 by CO2. Correspondingly, an increase of formation temperature also increased the displacement efficiency of CH4, but an increase of pore size slightly increased this displacement efficiency. Moreover, it was found that when the proportion of CO2 and N2 was 1:1, the displacement efficiency of CH4 was the highest, which proved that the simultaneous injection of CO2 and N2 had a synergistic effect on shale gas production. The results of this paper will provide guidance and reference for the displacement exploitation of shale gas by injection gases.

Interfacial CO2-mediated nanoscale oil transport: from impediment to enhancement

CO2-based enhanced oil recovery is widely practiced. The current understanding of its mechanisms largely focuses on bulk phenomena such as achieving miscibility or reducing oil density and viscosity. Using molecular dynamics simulations, we show that CO2 adsorption on calcite surfaces impedes decane transport at moderate adsorption density but enhances decane transport when CO2 adsorption approaches surface saturation. These effects change the decane permeability through 8 nm-wide pores by up to 30% and become negligible only in pores wider than several tens of nanometers. The strongly nonlinear, non-monotonic dependence of decane permeability on CO2 adsorption is traced to CO2's modulation of interfacial structure of long-chain hydrocarbons, and thus the slippage between interfacial hydrocarbon layers and between interfacial CO2 and hydrocarbon layers. These results highlight a new and critical role of CO2-induced interfacial effects in influencing oil recovery from unconventional reservoirs, whose porosity is dominated by nanopores.

Mathematical Model of Carbon Dioxide Injection into a Porous Reservoir Saturated with Methane and Its Gas Hydrate

In this paper, the process of methane replacement in gas hydrate with carbon dioxide during CO2 injection into a porous medium is studied. A model that takes into account both the heat and mass transfer in a porous medium and the diffusion kinetics of the replacement process is constructed. The influences of the diffusion coefficient, the permeability and extent of a reservoir on the time of full gas replacement in the hydrate are analyzed. It was established that at high values of the diffusion coefficient in hydrate, low values of the reservoir permeability, and with the growth of the reservoir length, the process of the CH4-CO2 replacement in CH4 hydrate will take place in the frontal regime and be limited, generally, by the filtration mass transfer. Otherwise, the replacement will limited by the diffusion of gas in the hydrate.

CO2/CH4 Competitive Adsorption in Shale: Implications for Enhancement in Gas Production and Reduction in Carbon Emissions

CO₂/CH₄ interaction determines the prospects for complementary enhanced gas recovery (EGR) associated with CO₂ sequestration in shale. We characterize the competitive adsorption of CO₂ and CH₄ in shale using low-field NMR. Competitive sorption of CO₂ relative to CH₄ is defined as the CO₂/CH₄ competitive adsorption ratio (CO₂/CH₄ CAR for short) when CO₂ and CH₄ have the same original partial pressure in shale. Results indicate the CO₂/CH₄ CAR decreases with the logarithm of increasing pressure. Observed CO₂/CH₄ CARs are on the order of 4.28-5.81 (YDN-1) to 3.43-5.57 (YDN-2), describing the remarkable competitive advantage of CO₂ sorption relative to CH₄ for shale. Results also indicate that increasing the CO₂/CH₄ pressure ratio (1) increases the adsorption capacity of shales to CO₂ and decreases that to CH₄ logarithmically with pressure, and (2) boosts CO₂-CH₄ displacement and generates greater EGR efficiency in shale, where the EGR efficiency can be inferred by the CO₂/CH₄ pressure ratio using a Langmuir-like function. Furthermore, the maximum sequestration capacity of adsorbed CO₂ during CO₂-CH₄ competition is on the order of ∼3.87 cm³/g (YDN-1) to ∼5.13 cm³/g (YDN-2). These promising results for EGR and CO₂ storage reveal the considerable potential for carbon capture and geological sequestration in shale.

Effects of Moisture and Salinity on Methane Adsorption in Kerogen: A Molecular Simulation Study

The adsorption characteristics of methane in shales play a critical role in the assessment of shale gas resources. The microscopic adsorption mechanism of methane considering the effect of moisture and especially salinity remains to be explored. In this work, combined molecular dynamics and grand canonical Monte Carlo simulations are conducted to investigate the adsorption behaviors of methane in the realistic kerogen matrixes containing different moisture contents (0-6 wt %) and various salinities (0-6 mol/L NaCl). Adsorption processes are simulated under realistic reservoir conditions at four temperatures in the range from 298.15 to 358.15 K and pressures up to 40 MPa. Effects of the moisture content on methane adsorption capacities are analyzed in detail. Simulation results show that the methane adsorption capacity declines as the moisture content increases. In comparison to the dry kerogen matrix, the reduction in the maximum CH4 adsorption capacity is as high as 42.5% in moist kerogen, with a moisture content of 6.0 wt % at 338.15 K. The overlap observed in the density distributions of water molecules and decrease in adsorbed methane indicates that the water molecules occupy the adsorption sites and, thus, lead to the reduction in methane adsorption capacity. Besides, the effects of salinity on CH4 adsorption isotherms are discussed. The salinity is found to have a negative influence on the methane adsorption capacity. The maximum CH4 adsorption capacity reduces around 6.0% under the salinity of 6 mol/L at 338.15 K. Adsorption of methane in kerogens of constant salinity but different moisture contents are further discussed. Results from the present study show that the moisture content has a greater impact on the adsorption of methane compared to that of salinity. The findings of this study have important implications for more accurate estimation of shale gas in place.

Competitive Sorption of CO2 with Gas Mixtures in Nanoporous Shale for Enhanced Gas Recovery from Density Functional Theory

CO₂ competitive sorption with shale gas under various conditions from simple to complex pore characteristics is studied using a molecular density functional theory (DFT) that reduces to perturbed chain-statistical associating fluid theory in the bulk fluid region. The DFT model is first verified by grand canonical Monte Carlo simulation in graphite slit pores for pure and binary component systems at different temperatures, pressures, pore sizes, and bulk gas compositions for methane/ethane with CO₂. Then, the model is utilized in multicomponent systems that include CH₄, C₂H₆, and C3+ components of different compositions. It is shown that the selectivity of CO₂ decreases with increases in temperature, pressure, nanopore size, and average molecular weight of shale gas. Extending the model to more realistic situations, we consider the impact of water present in the pore and consider the effect of permeation of fluid molecules into the kerogen that forms the pore walls. The water-graphite interaction is calibrated with contact angle from molecular simulation data from the literature. The kerogen pore model prediction of gas absolute sorption is compared with experimental and molecular simulation values in the literature. It is shown that the presence of water reduces the CO₂ adsorption but improves the CO₂ selectivity. The dissolution of gases into the kerogen matrix also leads to the increase in CO₂ selectivity. The effect of kerogen type and maturity on the gas sorption amount and CO₂ selectivity is also studied. The associated mechanisms are discussed to provide fundamental understanding for gas recovery by CO₂.

Competitive adsorption phenomenon in shale gas displacement processes

Displacement of methane (CH₄) by injection gas is regarded as an effective way to exploit shale gas and sequestrate carbon dioxide (CO₂) simultaneously. To remarkably enhance the rupture and extension of fractures, an original and comprehensive simplification for the real shale composition model is established to study the shale gas displacement by gas injection. In the present model, besides the consideration in the existence of organic matter in shale, the choice of silica as inorganic minerals is firstly taken into account considering its brittleness characteristic to meet the demand of fracture stretch. Based on the model, the displacement methane process and competitive adsorption behaviors were studied by using the grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) respectively. As the results, the strong interaction between carbon dioxide and shale results in the higher efficiency of displacing methane. We also find that the optimum operating conditions for CO₂ and N₂ displacing methane are at the pore width of 30 Å, the result being slightly different from the previous studies indicating that the displacement efficiency of small pores is higher. Moreover, the displacement efficiency by using different gases can all reach higher than 50% when the injection pressure is greater than 30 MPa. It is expected that this work can reveal the mechanisms of competitive adsorption between shale gas and gases, and provide a guidance for displacement exploitation of shale gas by gas injection and sequestration of carbon dioxide.

Displacement of methane (CH₄) by injection gas is regarded as an effective way to exploit shale gas and sequestrate carbon dioxide (CO₂) simultaneously.