Shale Oil Production Research Articles

A Grand Challenge: Update on Improved Recovery From Tight/Shale Reservoirs

_ This is the third of a series of six articles on SPE’s Grand Challenges in Energy, formulated as the output of a 2023 workshop held by the SPE Research and Development Technical Section in Austin, Texas. Described in a JPT article last year, each of the challenges will be discussed separately in this series: geothermal energy; net-zero operations; improving recovery from tight/shale resources; carbon capture, storage, and utilization; digital transformation; and education and advocacy. _ The exploration and extraction of hydrocarbons from tight or shale formations have revolutionized the global energy landscape, unlocking vast oil and gas reserves previously considered inaccessible. According to estimates, substantial oil production has been reported for various tight plays in the US in 2024 (Fig. 1). However, the production process in tight or shale formations is not without its own set of challenges ranging from technological hurdles to environmental concerns. Therefore, understanding the current outlook is essential for stakeholders to navigate the complex terrain. To address the significant issues outlined by SPE in July 2023 (Halsey et al. 2023), this article intends to explore the complexities of hydrocarbon extraction from tight and shale reservoirs. Description of Tight/Shale Formations First, we present a succinct description of tight or shale formations. Tight or shale oil and gas are hydrocarbons found in oil-bearing mudstone, and shale gas is produced from gas shale or associated with tight oil (Boak and Kleinberg 2020). Tight unconventional formations are characterized by the presence of fine-grained sedimentary rocks that are high in organic matter, such as shale, where hydrocarbons are firmly embedded within the rock matrix. Challenges in Tight or Shale Oil Production The Existence of Complex Geometry. Tight and shale formations are known for their complex geological structures, which can make it difficult to predict the distribution and behavior of hydrocarbons within the reservoir. Additionally, shale formations can exhibit significant variability in their thickness, composition, and the occurrence of natural fractures. Hence, it is crucial to understand and navigate the intricate geology of unconventional shale formations to achieve successful shale oil production (Jiang et al. 2016). Ultralow Reservoir Permeability. The low permeability of tight formations restricts the flow of hydrocarbons, necessitating techniques such as hydraulic fracturing to create artificial pathways for extraction. However, the effective stimulation of fractures requires the use of cutting-edge drilling and extraction methods, such as multistage hydraulic fracturing (fracking) and horizontal drilling, which in turn require precise engineering and a thorough understanding of reservoir characteristics (Pokalai et al. 2015).

Occurrence and mobility of shale oil in the Jiyang Depression, eastern China

The occurrence of shale oil is important for its mobility and holds great significance in determining the available shale oil resources. Here, shale samples from three representative wells with different shale oil production levels in the Jiyang Depression, eastern China, were collected to evaluate the mobility of shale oil from its occurrence. The samples were subjected to Rock-Eval pyrolysis, X-ray diffraction, soluble organic matter, and N2 adsorption measurements. The results indicate that the shales in the high production well have higher contents of clay or felsic minerals, more apolar oil components, and smaller pore sizes than those in the low production well. In particular, the shale oil occurrence exhibits two stages involving pore surface adsorption followed by pore void filling, which are separated by a total organic carbon (TOC) content of 0.6 wt% OC (equivalent to 7.7 mg/gRock, the lower threshold of shale oil content for mobility). The corresponding threshold of pore diameter is 10 nm. By integrating the shale oil occurrence with the oil properties at the producing intervals, the following indices are proposed to evaluate the shale oil mobility and high production level: shale oil content greater than 0.6 wt% OC or 7.7 mg/gRock, oil saturation index greater than 100 mg/gTOC, apolarity index greater than 1, and pore diameter greater than 10 nm. Our work provides an index regime to evaluate the mobility and high production probability of shale oil, which will benefit the exploration and development of shale oil resources.

Numerical analysis of microwave-enhanced oil shale pyrolysis by rotation turntable based on the Arbitrary Lagrangian-Eulerian method

Microwave heating technology has great potential for efficient pyrolysis of oil shale. However, the inhomogeneous heating characteristics of microwave heating can lead to secondary cracking of pyrolysis products, thus reducing the overall oil yield. In order to address this issue, a rotational movement of oil shale under microwave heating has been proposed to obtain a uniform temperature distribution of the oil shale. Then, a new coupled three-dimensional electromagnetic-thermal-chemical-hydraulic model incorporating the Arbitrary Lagrangian-Eulerian (ALE) method was developed to investigate the temperature distribution and pyrolysis characteristics of oil shale under the rotational motion during microwave pyrolysis. The effects of rotational speed, rotational direction, and microwave power on the pyrolysis process of oil shale were explored through this model. The results showed that the introduction of rotational movement effectively addressed the uneven temperature distribution within the oil shale. Compared with the static condition, the rotational condition reduced the secondary reactions of oil products during microwave pyrolysis and increased the oil yield of oil shale to 50.12 %. As the rotation speed increased, the temperature distribution within the oil shale became more uniform, but the total oil yield reached its peak at a moderate speed of 180 s/r. Research on microwave power indicated that rotation increased the maximum usable power and highlighted that reducing the residence time of pyrolysis products within the oil shale was key to increasing oil yield. This study provides a novel approach and numerical simulation method for enhancing the microwave pyrolysis efficiency of oil shale, and the findings of this study are applicable to both the ex-situ and in-situ exploitation of oil shale.

Submodels of 2-d model of the motion of fluid or gas in a porous medium with an external non-stationary source or absorption

The motion of fluid or gas in a porous medium is the main object of investigation of underground hydromechanics. The model describing this motion was called “Porous medium”.In our paper, we study a general 2-d model of the motion of fluid or gas in porous medium with an external non-stationary source or absorption. As the main research method, we use the method of group (symmetry) analysis, which is one of the most effective ways to obtain information about the properties of solutions to differential equations that define a physical model. A group classification of the differential equation defining studied physical model is carried out. As a result, all basic models with different symmetry properties are obtained. For the basic model, which admits the widest group a classification of all its invariant submodels is performed and all invariant submodels, defined by invariant solutions of rank 1 are studied. All solutions found have physical meaning.The set of explicitly found solutions forms a 47-parametric family of exact solutions. Each solution in this family depends on an arbitrary function defining an external non-stationary source or absorption. For each solution, for the specific values of the parameters on which it depends and for this specific function, we obtained a graph of the pressure distribution.Other solutions are used to describe the movement of a liquid or gas if either the pressure and its rate of change, or the pressure and one of its gradient components, are given at an initial time at a fixed point. The set of these solutions depends on 13 parameters and a function defining an external non-stationary source or absorption. For specific values of this function and specific values of the parameters on which each solution depends, the existence and uniqueness of a solution to the corresponding boundary value problem under some additional conditions has been established. Each such problem is solved numerically. The results of these decisions we reflected in the graphs.The significance of obtained solutions lies in the fact that we have obtained a database consisting of two multiparameter families of invariant solutions of rank 1 of the equation describing the motion of fluid or gas in a porous medium in the presence of an external non-stationary source or absorption. Every solution depends on an empirically determined function. These solutions describe specific physical processes. They can also be used as test solutions in numerical calculations related to the study of the motion of fluid or gas in a porous medium during fluids filtering, engineering surveys in the construction of building structures and in the extraction of shale oil or gas.

Study of geomechanical properties of the Pimienta formation with undrained multi-stage triaxial tests, Central-Western Tampico Misantla Basin, Mexico

The Pimienta Formation (J-Pi) of the Tampico Misantla Basin (TMB) in Mexico is known for its potential as unconventional onshore hydrocarbon reserves. The area of interest in this study is located in the western-central part of the TMB, where no significant production has been established from unconventional resources. Given the low permeability and low porosity characteristics of the carbonaceous mudstone in the Pimienta formation, it is necessary to implement techniques to increase the area of flow from the reservoir to the borehole, achieving profitable oil and gas extraction.The geomechanical properties of rock strata are essential inputs for the design and implementation of shale oil and gas production engineering, such as horizontal drilling and hydraulic fracturing. Understanding the capabilities of the rock to yield and contain hydraulic fractures is essential to designing safe and efficient well stimulations. Likewise, the wellbore stability is of critical importance to the successful development of secondary and tertiary recovery systems.In this study, core samples from the Pimienta Formation in the TMB are analyzed through undrained multi-stage triaxial tests, emphasizing the estimated in-situ geo stresses and the effect of the mineral composition of the rock. The geomechanical parameters of compressional strength, Poisson ratio (PR), and Young's modulus of the Pimienta formation are obtained from the analysis of the experimental data and finding correlations to the mineral composition of the samples. A detailed characterization is completed under the conventional rock mechanics methodologies of Mohr-Coulomb and Hoek-Brown failure criterion, as well as Brittleness Index (BI).

A review of the flow characteristics of shale oil and the microscopic mechanism of CO2 flooding by molecular dynamics simulation

Shale oil is stored in nanoscale shale reservoirs. To explore enhanced recovery, it is essential to characterize the flow of hydrocarbons in nanopores. Molecular dynamics simulation is required for high-precision and high-cost experiments related to nanoscale pores. This technology is crucial for studying the kinetic characteristics of substances at the micro- and nanoscale and has become an important research method in the field of micro-mechanism research of shale oil extraction. This paper presents the principles and methods of molecular dynamics simulation technology, summarizes common molecular models and applicable force fields for simulating shale oil flow and enhanced recovery studies, and analyzes relevant physical parameters characterizing the distribution and kinetic properties of shale oil in nanopores. The physical parameters analyzed include interaction energy, density distribution, radial distribution function, mean-square displacement, and diffusion coefficient. This text describes how molecular dynamics simulation explains the mechanism of oil driving in CO2 injection technology and the factors that influence it. It also summarizes the advantages and disadvantages of molecular dynamics simulation in CO2 injection for enhanced recovery of shale oil. Furthermore, it presents the development trend of molecular dynamics simulation in shale reservoirs. The aim is to provide theoretical support for the development of unconventional oil and gas.

Evaluation of movability of shale oil in organic matter in depletion production by REV-scale simulations

The oil transfer in shale oil formations includes the oil flow in inorganic and organic pores, and the diffusion in the organic matter (OM). A novel triple-continuum model (OM matrix, organic pores, inorganic matrix) is proposed coupling the oil flow (in organic and inorganic pores) and diffusion (in OM matrix). The proposed model is further validated by three-dimensional (3D) numerical simulations. The simulation results show that the proportion of oil production of the absorbed oil from OM matrix in the total shale oil production is small due to the relatively slow diffusion process. However, the diffusion of oil from OM matrix to the organic pores can provide extra energy for the oil expansion in inorganic pores, and thus can greatly increase the oil production in the depletion process. The high OM content can prolong the timespan of oil production. More absorbed oil in OM provides more elastic energy for oil depletion in organic and inorganic pores, and thus leads to higher flow rate in the late period of shale oil production. Smaller size of OM means shorter distance for oil transfer from OM to inorganic matrix, and thus accounts for higher movability of oil in OM.

Two-dimensional spectrum characteristics and oil movability study of the shale oil reservoir

Shale oil is mainly stored in the nano–micro-pores of shale in the form of adsorption or in a free state. Among them, only free-state oil is the main contributor to shale oil production under natural elastic energy. Therefore, it is crucial to evaluate the movability of crude oil effectively. In this paper, focusing on the Jurassic shale oil reservoir core in the middle-eastern Sichuan region of China, low-field nuclear magnetic resonance (NMR) technology is used to analyze the basic characteristics of shale oil core samples. Experiments on the low-field NMR one-dimensional and two-dimensional spectrum characteristics of the original core, heat-treated core, and thermogravimetric-treated shale core are carried out. The effects of the TG-MS method, T2 method, and the volatilization of light oil components on the movability of shale oil reservoirs are analyzed, and the movability characteristics of shale oil reservoirs in the middle-eastern Sichuan region are preliminarily clarified. The results show that under thermal treatment, the distribution range of the two-dimensional spectrum of the core oil occurrence area is significantly reduced, and the T1 and T2 distribution ranges are reduced by 10.9% and 60.7%, respectively. According to the TG-MS method, the mobile oil, bound oil, and adsorbed oil account for 74.7%, 8.1%, and 17.2%, respectively. The quantitative calculation of movable oil saturation by NMR combined with heat treatment is 65.6%, which is lower than that calculated using the thermogravimetric method. In order to calculate the movability more accurately, the scale effect between different samples should be considered. This study could provide a theoretical basis for the subsequent shale oil development program.

Mechanisms of CO2-rich industrial waste gas enhanced shale oil recovery in kerogen slit based on adsorption behavior, gas flooding and surfactant synergy study

Enhanced oil recovery (EOR) by injection of CO2-rich industrial waste gas has been proposed as an energy-efficient and environmentally friendly method for shale oil reservoirs, in which the waste gas can be utilized and sequestered to reduce greenhouse gas emissions. In this work, the adsorption behaviors of shale oil and waste gas components in kerogen slits and the effects of gas flooding on EOR were investigated using molecular dynamics (MD) method, which can provide technical guide for shale oil development. In terms of the adsorption behaviors, an intensity decrease of oil adsorption is observed in kerogen slits after the injection of the waste gas components. Desorption of shale oil during injection of SO2 and CO2 is more pronounced in shallow reservoirs, whereas it is more evident for NO and N2 in deep reservoirs. The desorption efficiency can be 10.91 % for pure CO2 injection and 9.09 % for NO injection at a depth of 1000 m, and the relative stability ratios of CO2 and NO sequestration are 77.09 % and 76.06 %, respectively. For gas flooding, the existence of pressure difference provides favorable conditions for gas molecules to pass through the oil zone, and waste gas can be more effective than pure CO2 as driving gas. For the waste gas injection, the percentage of CO2 retained in the kerogen nanopores is much higher than that of pure CO2 injection, suggesting that more CO2 molecules can enter into kerogen matrix to take part in the displacement process. Furthermore, the effect of surfactants on oil desorption and displacement was also investigated and the interaction energy of the surfactant/oil/gas system was calculated and analyzed, finding that addition of the oil-soluble surfactants can be beneficial to shale oil displacement. This work can provide a comprehensive evaluation of the feasibility of CO2-rich industrial waste gas application for EOR, which is of great significance in the effective production of shale oil and the sequestration of waste gas in shale reservoirs.

The study of core and particle model of Fushun oil shale based on microwave field

As an unconventional oil and gas resource, oil shale can alleviate the shortage of oil and gas resources. As a new heating process for oil shale retorting and in-situ mining, microwave can effectively exploit and utilize oil shale. In this paper, the Fushun oil shale and particle model under microwave field are studied by means of experiment and numerical simulation. Based on the measurement of the basic physical properties of oil shale, the effects of microwave power and absorbent (semi-coke and graphite) on the pyrolysis of oil shale were investigated. The experimental results show that the heating rate is the fastest when the maximum microwave power is 700 W without adding absorbent or adding the same absorbent. At the same power, the effect of graphite is slightly better than that of semi-coke. In the aspect of numerical simulation, COMSOL simulation software is used to simulate the oil shale core and particles based on the three-dimensional electromagnetic thermal model. The experimental conclusion is verified. At the same time, it is found in the simulation results that microwave heating has better temperature distribution gradient and electric field distribution. Furthermore, the microwave port is extended to the underground for the simulation of microwave in-situ mining. It is found that the reservoir temperature reaches 450 °C after 1000 days of reservoir heating.

The Feasibility Study of In Situ Conversion of Oil Shale Based on Calcium-Oxide-Based Composite Materia Hydration Exothermic Reaction

Oil shale, as a vast potential resource, is considered an important alternative energy source, and its effective development and economic utilization are of significant importance in alleviating the contradiction between energy supply and demand. Presently, the in situ conversion technology for oil shale has gained significant global attention, with numerous extraction methods undergoing active research and development. One of these methods is the in situ conversion of oil shale based on the hydration reaction of calcium-oxide-based composite material (CaO-CM). This approach harnesses the heat produced by the reaction between CaO-CM and water as a heat source for the pyrolysis of oil shale. This paper conducted experiments to assess the feasibility of temperature associated with this method. The feasibility study mainly includes two aspects: First, it is necessary to investigate whether the temperature generated by the hydration reaction of CaO-CM can meet the temperature requirements for the pyrolysis of oil shale. Through pyrolysis experiments of Xinjiang oil shale, the minimum temperature required for oil shale pyrolysis was determined to be 330 °C. High-temperature and high-pressure reaction vessels were employed to explore the temperature generated by the hydration reaction of CaO-CM. The results show that with the increase in environment pressure, environment temperature, and reaction mass, the maximum temperature generated by the hydration reaction of CaO-CM also increases (reach 455.5 °C), meeting the temperature requirements for the pyrolysis of oil shale. Second, the study evaluates whether the hydration reaction of CaO-CM can induce pyrolysis hydrocarbons of the oil shale. Through the pyrolysis experiments of oil shale based on the hydration reaction of CaO-CM, the changes in the content of pyrolysis hydrocarbons (S2) in oil shale before and after pyrolysis are measured. The results show that under 10 MPa pressure, the content of pyrolysis hydrocarbons in the oil shale decreased from 40.96 mg/g to 0.08 mg/g after pyrolysis. This confirms the feasibility of the temperature conditions for the in situ conversion of oil shale based on the hydration reaction of CaO-CM.

Energy-constrained production optimization for the in-situ conversion process of oil shale based on deep learning algorithms

The in-situ conversion process (ICP) involves complicated thermal-reactive-compositional flow processes where the production parameters are closely interdependent. Therefore, manual optimization of the production parameters of ICP through numerical simulation is arduous and time-consuming. This paper introduces a computational framework that integrates deep learning techniques and particle swarm optimization (PSO) to automatically optimize the values of production parameters of ICP, and thus locating the highest energy efficiency and the optimum energy usage. This approach utilizes a 3D CNN model to predict key metrics. The prediction process considers the detailed 3D heterogeneity of oil shale, resulting in remarkably high prediction accuracy, as evidenced by determination coefficients (R2) above 0.99. Subsequently, the trained CNN model is integrated to the PSO algorithm to automatically fine-tune the production parameters to optimize the energy efficiency of ICP.The optimization results yield three significant findings. First, as the energy consumption limit increases, the optimal number of heaters, well spacing, and heating temperature exhibit an upward trend, but stabilized beyond the threshold of 7 × 105 kWh. Secondly, the optimal input energy (7 × 105 kWh) is found for the given ICP model. Lastly, the analysis reveals that variations in initial reservoir pressure or bottomhole pressure have limited impact on cumulative oil production and energy usage.

Microwave heating of oil shale based on multiphysics field coupling: Positioning of the waveguide

Microwave heating has proven widespread applications in oil shale pyrolysis; however, the impact of waveguide port positioning on the microwave reactor's thermal performance remains underexplored. This work establishes a 3D coupled model that incorporates electromagnetic, thermochemical, and fluid dynamics effects employing multiphysics modeling methodology. The thermochemical characteristics of an oil shale sample under microwave irradiation are quantified for a broad range of waveguide port positions, specified via Latin hypercube sampling, which ensures the randomness of the port coordinates. The cavity field strength exhibits markedly high sensitivity to the waveguide positioning: the standard deviation of the cavity-averaged electromagnetic intensities for all considered positions is found to be 30.42% greater compare to the associated mean. The best port position yields a temperature rising rate an order of magnitude higher than the worst positioning, for which kerogen pyrolysis does not appear to be even activated in the same initial stage. The ranking of waveguide positions identified based on the temperature rising rate aligns well with the analysis of microwave utilization efficiency. The study offers insights for optimized microwave reactor design with enhanced oil shale thermal treatment.

Shale oil development techniques and application based on ternary-element storage and flow concept in Jiyang Depression, Bohai Bay Basin, East China

The ternary-element storage and flow concept for shale oil reservoirs in Jiyang Depression of Bohai Bay Basin, East China, was proposed based on the data of more than 10 000 m cores and the production of more than 60 horizontal wells. The synergy of three elements (storage, fracture and pressure) contributes to the enrichment and high production of shale oil in Jiyang Depression. The storage element controls the enrichment of shale oil; specifically, the presence of inorganic pores and fractures, as well as laminae of lime-mud rocks, in the saline lake basin, is conducive to the storage of shale oil, and the high hydrocarbon generating capacity and free hydrocarbon content are the material basis for high production. The fracture element controls the shale oil flow; specifically, natural fractures act as flow channels for shale oil to migrate and accumulate, and induced fractures communicate natural fractures to form complex fracture network, which is fundamental to high production. The pressure element controls the high and stable production of shale oil; specifically, the high formation pressure provides the drive force for the migration and accumulation of hydrocarbons, and fracturing stimulation significantly increases the elastic energy of rock and fluid, improves the imbibition replacement of oil in the pores/fractures, and reduces the stress sensitivity, guaranteeing the stable production of shale oil for a long time. Based on the ternary-element storage and flow concept, a 3D development technology was formed, with the core techniques of 3D well pattern optimization, 3D balanced fracturing, and full-cycle optimization of adjustment and control. This technology effectively guides the production and provides a support to the large-scale beneficial development of shale oil in Jiyang Depression.

Formation damage mechanism and control strategy of the compound function of drilling fluid and fracturing fluid in shale reservoirs

For the analysis of the formation damage caused by the compound function of drilling fluid and fracturing fluid, the prediction method for dynamic invasion depth of drilling fluid is developed considering the fracture extension due to shale minerals erosion by oil-based drilling fluid. With the evaluation for the damage of natural and hydraulic fractures caused by mechanical properties weakening of shale fracture surface, fracture closure and rock powder blocking, the formation damage pattern is proposed with consideration of the compound effect of drilling fluid and fracturing fluid. The formation damage mechanism during drilling and completion process in shale reservoir is revealed, and the protection measures are raised. The drilling fluid can deeply invade into the shale formation through natural and induced fractures, erode shale minerals and weaken the mechanical properties of shale during the drilling process. In the process of hydraulic fracturing, the compound effect of drilling fluid and fracturing fluid further weakens the mechanical properties of shale, results in fracture closure and rock powder shedding, and thus induces stress-sensitive damage and solid blocking damage of natural/hydraulic fractures. The damage can yield significant conductivity decrease of fractures, and restrict the high and stable production of shale oil and gas wells. The measures of anti-collapse and anti-blocking to accelerate the drilling of reservoir section, forming chemical membrane to prevent the weakening of the mechanical properties of shale fracture surface, strengthening the plugging of shale fracture and reducing the invasion range of drilling fluid, optimizing fracturing fluid system to protect fracture conductivity are put forward for reservoir protection.

PH-switchable W/O drag reducer emulsions based on dynamic covalent stabilizers for efficient stabilization and rapid dissolution of drag reducers

As the global emphasis on shale oil extraction grows, there is a rising demand for slickwater fracturing fluids for reservoir stimulation. Polyacrylamide drag reducers plays a crucial role in slickwater and are available in two primary forms: powder and emulsion. Powder-based drag reducers suitable for long-term transportation and storage, but increasing their dissolution rate remains a challenge. While emulsion-based drag reducers are valued for the efficient dissolution facilitated by hydrophilic phase-inversion surfactants, the incorporation of the surfactants compromise the stability of W/O emulsions. In this work, an imine-functionalized pH-switchable stabilizer was synthesized, and the W/O emulsion constructed with the stabilizer underwent demulsification at a pH of 4.9. The pH-switchable emulsion system was optimized, and inverse emulsion polymerization using the stable monomer emulsion was performed for preparing pH-switchable drag reducer emulsions. The drag reducer emulsion features remarkable resistance to instability even after 30 days of storage owing to the absence of phase-inversion surfactants. The imine bond on the stabilizer was cleaved in acid solution, allowing the switchable drag reducer emulsion to be rapidly released and dissolved in water at pH 2.75, boasting an impressive dissolution rate of only 150 s. Furthermore, the pH-switchable drag reducer emulsion demonstrated effective drag-reduction performance, achieving a drag-reduction rate as 73 % at a concentration of 0.06 wt%. The pH-switchable drag reducer emulsion effectively resolves the conflict between rapid dissolution and efficient stabilization of drag reducer, thus offering potential improvements in the formulation process of slickwater and the efficiency of shale oil development.

A study on the structures and primary reaction products of kerogens in Longkou oil shale with different densities through supercritical ethanolysis

The difference in the density of oil shale significantly affects the structural characteristics of the kerogen, which brings the change in the pyrolysis behavior, thus leading to different retorting oil yields. The primary reaction (the cleavage of weak covalent bonds into free radicals) in the pyrolysis of oil shale greatly affects the retorting oil yield and quality. In this work, the structural characteristics and primary reaction products of the kerogens in Longkou oil shale (LKOS) with three different densities (<1.3, 1.4 − 1.5, 1.8 − 1.9 g/cm3) were investigated by supercritical ethanolysis, 13C nuclear magnetic resonance (13C NMR) and Fourier transform infrared spectroscopy (FTIR). The weak covalent bonds in the kerogens were broken by ethanolysis at 350 °C to obtain small molecular substances (SMSs) with the carbon conversions of 28 %, 47 % and 54 %, respectively. The SMSs can be divided into aliphatic acid esters, aromatics, alkanes and N-containing organic compounds (NCCs). Among the SMSs, the relative contents of the aliphatic acid esters, whose aliphatic chain lengths distribute in C3-C23, C1-C24 and C1-C28, are the highest. The aromatics are mainly monocyclic, and the NCCs mainly exist as amines. The possible pathways for these ethanolysis products were proposed. The 13C NMR and FTIR analyses of the kerogens and their ethanolysis residues show that the aliphaticity (fal) and average methylene chain length (Cn) increase, while the aromaticity (fa) and average aromatic cluster size (Xb) decrease with the increasing oil shale density. After ethanolysis, the value of fa gradually increases, while that of fal and Cn decrease. Furthermore, the contents of C=O and O-C=O decrease, while that of O-C-O and C-OH increase. The results suggest that the SMSs are connected to the kerogen matrix by weak covalent bonds. The cleavage of C=O, O-C=O and C-O bonds dominates the early stage of the oil shale pyrolysis. During the pyrolysis of oil shale, these weak covalent bonds are first broken and the above SMSs are the most primary reaction products and then undergo the second reactions to yield oil and gas.

Real-time imaging of oil shale pyrolysis dynamics at nanoscale via environmental scanning electron microscopy

Pyrolysis is a promising technology to increase pore and fracture connectivity in oil shale and thereby accomplish commercial recovery of unconventional hydrocarbons. Herein, real-time oil shale pyrolysis dynamics are imaged at nanoscale via environmental scanning electron microscopy (imaging resolution of 58 nm/pixel, imaging speed of 1 frame per second, and heating temperature up to 750 °C). It is counterintuitively observed that 1) inorganic nano-fractures started to appear below 100 °C; 2) inorganic nano-fracture width had a non-monotonous relationship with temperature; and 3) organic kerogen areas decreased monotonously with increasing temperature. These findings will establish a standard benchmark for unconventional resource recovery, promote fundamental understanding of oil shale pyrolysis dynamics at nanoscale and provide key guidance on oil shale extraction at reservoir scale.

Transformation and degradation processes of dissolved organic matter in shale oil fracturing flowback water: Implications for environmental management

Shale oil extraction yields shale oil fracturing flowback water (SOFFW) rich in recalcitrant soluble organic compounds, the mismanagement of which can cause ecological harm. The current lack of research on dissolved organic matter (DOM) in SOFFW hampers the informed selection and optimization of oxidative methodologies. This study employed diverse characterizations to investigate the degradation and transformation mechanisms of DOM in SOFFW during Fenton oxidation. It identified three primary components in SOFFW, predominantly humic-like substances, the intensity of which was considerably reduced through oxidation. 2D-COS-FTIR analysis revealed that during oxidation, phenolic organic compounds showed preferential reactivity, followed by compounds containing double bonds and, finally, aliphatic compounds. Fenton oxidation effectively degraded highly unsaturated phenolics and polycyclic aromatic hydrocarbons, simultaneously generating an abundance of aliphatic and oxygen-poor unsaturated phenolic compounds. Through carboxylation, demethylation, and hydrogenation reactions, the proportion of large organic molecules (400–700 Da) decreased, leading to the formation of smaller molecules in the 200–400 Da range. Our findings demonstrate that hydroxyl radicals in the Fenton oxidation process react rapidly with electron-rich compounds, such as highly oxygenated unsaturated phenolics, while showing lower reactivity toward aliphatic substances. These findings substantially contribute to the efficient treatment of SOFFW and offer valuable insights for practical applications.

Study on the Influence of Supercritical CO2 with High Temperature and Pressure on Pore-Throat Structure and Minerals of Shale.

Injection of carbon dioxide offers substantial social and economic advantages for reduction of carbon emission reduction. Utilizing CO2 in shale formations can significantly enhance the extraction of shale oil or gas. Conducting fundamental research on how CO2 affects shale rock's physical properties is crucial for enhancing its porosity and permeability. Particularly for deep shale layers, the effects of supercritical CO2 on shale physical properties should be investigated at a high temperature and pressure, differing from the standard conditions applied in shallower layers. A study examined the impact of supercritical CO2 under such conditions on the pore-throat structure and mineral composition of the shale. The experimental parameters included immersing shale rock in supercritical CO2 at pressures ranging from 10 to 70 MPa and temperatures between 55 and 95 °C. This study evaluated changes in mineral composition, pore-throat structure (using scanning electron microscopy and nitrogen adsorption tests), and the porosity and permeability of the shale rocks. Findings indicated that the dissolution of CO2 altered the relative content of certain minerals. The average quartz content rose and, potassium feldspar and the average contents of plagioclase declined conversely. When increasing the pressure, an increase in the relative content of I/S mixed layer and a decrease in illite content were observed and kaolinite content experienced minor changes. When increasing the temperature, kaolinite, I/S mixed layer, and chlorite all exhibited a decreasing trend with increasing temperature, while the relative contents of illite increased. Some of the pores become rounded in a high-magnification view under the impact of CO2 dissolution. Additionally, the Brunauer-Emmett-Teller specific surface area, pore volume, porosity, and permeability generally improved with increasing pressure and temperature. With the temperature and pressure of CO2 increased, the curves of nitrogen absorption had moved first upward and then downward. However, under specific CO2 conditions, the permeability enhancement effect could diminish or even negatively impact the shale's permeability. These findings underscore the need to optimize supercritical CO2 injection parameters under high-temperature and high-pressure conditions. This research aims to provide theoretical guidance for the efficient use of CO2 in deep shale applications to increase the shale gas output.