Typical Shale Research Articles

Carbon dioxide adsorption on shale: A comparative study of isotherm models across diverse samples and temperatures

In order to address one of the most serious environmental concerns of our day, reducing carbon footprints, the globe has turned its attention to carbon dioxide (CO2) storage as a potential solution. Because of its unique features, shale is one of the most intriguing options in this area. Adsorption is the method through which CO2 is stored in shale, particularly in its supercritical condition. Adsorption isotherm models can be used to deduce the behaviour and mechanisms of this adsorption. Langmuir, Freundlich, Dubinin-Astakhov (D-A), and Brunauer-Emmett-Teller (BET) models are among the many available for CO2 modelling on shale. We attempted to fit these models to experimental data gathered from literature sources in this study, concentrating on four separate shale samples from various places in China. Among these samples are LMX1 and LMX2 from the Silurian Longmaxi Formation, WF1 from the Ordovician Wufeng Formation in the Sichuan Basin, and YC from the Ordos Basin's Yanchang Formation. The total organic carbon (TOC) content of these shales, three marine and one continental, ranged from 3.19 to 4.27. The experimental data used to fit the model was obtained at three different temperatures: 35, 45, and 55°C. The Langmuir and D-A models offered the best fit for the data across all samples and temperature. R2 values 0.93429 (for YC rock at 35°C) to 0.99287 (for WF1 at 35°C) for Langmuir and 0.88879 to 0.99201 LMX1 at 35°C. The theoretical underpinnings of these models, which account for the physical properties and adsorption dynamics of supercritical CO2 on shale, are responsible for their performance. Finally, this study adds to our understanding of CO2 adsorption on shale, giving useful insights for future research and potential practical uses in CO2 storage. More research is needed, however, to completely understand the mechanisms and influencing factors of CO2 adsorption in various types of shale, as well as to develop the models used to forecast this behaviour.

Detecting shale oil hidden in nanopores using a novel electron microscopy method

<p>Shale oil has revolutionized the global energy landscape, mostly existing in nanopores. However, there is no method for directly characterizing such a small oil cluster. This paper proposes a novel method for the direct characterization of shale oil at the nanoscale via controllable local charging imaging using electron microscopy. With this method, the potentially movable hydrocarbons of below one pmol (10<sup>?12</sup> mol) reserved in nanopores can be detected. The distribution of the shale oil clusters can be obtained through large-area two dimensional (2D) and three dimensional (3D) visualizations. Additionally, the amount of these resources can be calculated. Comparative analyses were conducted on reservoir samples from typical shale oil blocks in China and the US, and the results indicate the universality of the novel method, paving the way for accurate evaluation of shale oil resources and reasonable choice of exploitation technologies such as hydrofracturing and underground <i>in-situ</i> conversion processing.</p>

Microfacies and diagenetic alteration in a semi-deep to deep lacustrine shale: The Yanchang Formation in the Ordos Basin, China

The mineralogical development and diagenetic sequence of lacustrine shales in the Chang 7 Member of the Yanchang Formation in the Ordos Basin are detailed studied. A model of their depositional system and a diagenetic diagram are proposed in this study. Through detailed petrographic, mineralogical, and elemental analyses, four distinct shale types are identified: argillaceous shale, siliceous shale, calcareous shale, and carbonate, clay, and silt-bearing shale. The main diagenetic process in argillaceous shale is the transformation of illite to smectite, negatively impacting shale porosity. Siliceous shale undergoes carbonate cementation and quartz dissolution, contributing to increased porosity, particularly in mesopores. Calcareous shale experiences diagenesis characterised by carbonate formation and dissolution, with a prevalence of siderite. In carbonate, clay, and silt-bearing shale, the dissolution of K-feldspar contributes to illitization of kaolinite. Argillaceous shale, characterised by more clay minerals and lower mesopore volume, is identified as a potential hydrocarbon seal. Siliceous shale, with the highest pore volume and abundant inter-mineral pores, emerges as a promising shale oil reservoir. These findings contribute to a comprehensive understanding of shale properties, aiding in the prediction of shale oil exploration potential in the studied area.

Pore system and methane adsorption capacity features of marine and marine-continental transitional shale in the Sichuan Basin, SW China

Recently, significant achievements have been made in the gas exploration of marine Longmaxi shale in China. As exploration efforts have advanced, the exploration targets have gradually expanded to other sedimentary systems (marine-continental transitional and lacustrine). Compared with marine shale, shale in other sedimentary systems shows stronger heterogeneity, rendering previous exploration experiences of marine shale ineffective in guiding exploration efforts. Therefore, there is a pressing need for comparative studies to support future exploration practices. In this paper, the marine Longmaxi Formation and the marine-continental transitional Longtan Formation shales in the Lintanchang area of the southeastern of the Sichuan Basin are selected as the research objects. The study aims to compare the mineralogical characteristics, pore systems, and methane adsorption capacities of these two sets of shales, thereby revealing the differences in controlling factors that affect their physical properties and methane adsorption capacities. Our results show that the Longtan shale exhibits a higher clay mineral content, while the Longmaxi shale demonstrates significantly higher siliceous mineral content. Compare with Longmaxi shale, the Longtan shale exhibits a wider distribution range and higher average value of TOC content. The pore system in the Longmaxi shale is primarily dominated by organic matter-related pores, whereas the Longtan shale is characterized by clay mineral-related pores as the primary pore type. Given the variance in sedimentary environments, the controlling factors of physical properties differ significantly between the two sets of shales. In the case of the Longmaxi shale, TOC content is the most influential factor governing physical properties, while clay mineral content exerts the most significant influence on physical properties in the Longtan shale. Furthermore, TOC content emerges as the primary factor affecting methane adsorption capacity in both the Longmaxi and Longtan shales, despite the presence of significant variations in their pore systems. Nevertheless, the specific mechanisms through which TOC content impacts methane adsorption capacity exhibit variations between the two distinct shale types under investigation. The difference in sedimentary environment leads to various effects of mineral composition on methane adsorption capacity. Therefore, in the future research, the influences of different factors on methane adsorption capacity should be studied in combination with the sedimentary background.

Tectonic evolution and accumulation characteristics of Carboniferous shale gas in Yadu-Ziyun-Luodian aulacogen, Guizhou Province, South China

The Yadu-Ziyun-Luodian aulacogen (YZLA) developed into being NW-trending in the Late Paleozoic, and was considered as an important passive continental margin aulacogen in Guizhou Province, South China. This tectonic zone is considered a large intracontinental thrust-slip tectonic unit, which has undergone a long period of development. It was ultimately determined in the Yanshanian, where the typical Upper Paleozoic marine shales were deposited. In 2021, Well QSD-1 was deployed in the Liupanshui area at the northwest margin of the aulacogen, and obtained a daily shale gas flow of 11011 m3 in the Carboniferous Dawuba Formation. It thus achieved a breakthrough in the invesgation of shale gas in the Lower Carboniferous in South China, revealing relatively good gas-bearing properties and broad exploration prospects of the aulacogen. Being different from the Lower Paleozoic strata in the Sichuan Basin and the Yichang area of the Middle Yangtze, the development of the Carboniferous Dawuba Formation in the aulacogen exhibits the following characteristics: (1) The Lower Carboniferous shale is thick and widely distributed, with interbedded shale and marlstone of virous thickness; (2) The total organic carbon (TOC) content of the shale in the Dawuba Formation ranges from 1% to 5%, with an average of 2%, and the thermal maturity of organic matter (Ro) varies from 1% to 4%, with an average of 2.5%, indicating good hydrocarbon generation capacity; (3) The main shale in the aulacogen was formed during the fault subsidence stage from the Middle Devonian to the Early Permian. Although the strong compression and deformation during the late Indosinian-Himalayan played a certain role in destroying the formed shale gas reservoirs, comparative analysis suggests that the area covered by the current Triassic strata has a low degree of destruction. It therefore provides good conditions for shale gas preservation, which can be regarded as a favorable area for the next exploration.©2023 China Geology Editorial Office.

Characteristics and potential cytotoxicity of halogenated organic compounds in shale gas wastewater-impacted surface waters in Chongqing area， China

Recent screening surveys have shown the presence of unknown source halogenated organic compounds (HOCs) in shale gas wastewater. However, their occurrence, profile, transport in surrounding surface water and environmental risk potentials remain unclear. Here, a method for the extraction and quantitative determination of 13 HOCs in water by solid phase extraction combined with gas chromatography–mass spectrometry (GC–MS) was established. All of the targeted HOCs were detected and peaked at the outfall, while these contaminants were generally not detected in samples upstream of the outfall, suggesting that these contaminants originated from the discharge of shale gas wastewater; this was further supported by the fact that these pollutants were generally detected in downstream samples, with a tendency for pollutant concentrations to decrease progressively with increasing distance from the outfall. However，different HOCs had different transport potential in water. In addition, the toxicological effects of typical HOCs were evaluated using HepG2 as a model cell. The results indicated that diiodoalkanes suppressed HepG2 cell proliferation and induced ROS generation in a concentration-dependent manner. Mechanistic studies showed that diiodoalkanes induced apoptosis in HepG2 cells via the ROS-mediated mitochondrial pathway, decreasing mitochondrial membrane potential and increasing intercellular ATP and Ca2+ levels. On the other hand, RT–qPCR and Western blot assays revealed that the SLC7A11/GPX4 signaling pathway and HO-1 regulation of ferritin autophagy-dependent degradation (HO-1/FTL) pathway were involved in the ferroptosis pathway induced by diiodoalkane in HepG2 cells. Our study not only elucidates the contamination profiles and transport of HOCs in surface water of typical shale gas extraction areas in China, but also reveals the toxicity mechanism of typical diiodoalkane.

Geochemical Characteristics and Geological Significance of Mud Shale of the Second Member, Permian Lucaogou Formation, Jimsar Sag, Junggar Basin, NW China

Abstract ––The Lucaogou Formation in Jimsar sag is an important shale oil-producing layer in the Junggar basin. In this paper, core data, organic carbon, rock pyrolysis, biomarker compounds, and organic matter group component separation were used to study the sedimentary environment, parent material type, organic maturity and oil-bearing property of mud shale of the Second member of the Lucaogou Formation. The results showed that the sedimentary environment is a semi-oxidation and semi-reduction environment. The parent material is mainly aquatic with minor input of higher plants. The average TOC is 7.43%, the average value of hydrocarbon generation potential (S1+S2) is 50.54 mg/g, and the average value of Tmax is 446 °C, showing high maturity of the mud shale. The chloroform bitumen “A” features high saturated hydrocarbon content (34.10% on average), low aromatic hydrocarbon content (15.39% on average), and high saturated/aromatic ratio (2.54). The carbon number of saturated hydrocarbons is distributed between n-C13 and n-C33. Their peak are mainly n-C17 and n-C23, which are biased toward medium-low molecular weight alkanes. The average of the ΣC21–/ΣC22+ is 1.19, indicating that the alkane has advantage of short chain. The mud shale in the Second member of the Lucaogou Formation has a large sedimentary thickness, high organic matter content, and high oil generation potential. It is an important replacement target for shale oil in Jimsar sag and has important exploration significance in the future.

Pore Fluid Movability in Fractured Shale Oil Reservoir Based on Nuclear Magnetic Resonance

Gulong shale oil is found in a typical continental shale oil reservoir, which is different from marine shale oil reservoirs. The Gulong shale oil reservoir is a pure shale-type oil reservoir with abundantly developed nanoscale pores, making it extremely difficult to unlock fluids. Pressure drive does not easily achieve fluid unlock conditions; thus, it is necessary to utilize imbibition to unlock nanoscale pore fluids. In this study, experiments were conducted on oil displacement by high-speed centrifugal pressure and imbibition under different conditions, respectively, and simulations were used to evaluate the effects of pressure differential drive and imbibition efficiency on the utilization of crude oil following fracturing. Combined with the mixed wettability of the reservoir, the imbibition efficiency was analyzed, and the imbibition efficiency at different soaking stages was evaluated. When the fracturing pressure was higher than the matrix pore pressure, the imbibition efficiency was the most obvious, which was 27.9%. Spontaneous imbibition depending solely on capillary force had poor efficiency, at 16.8%. When the fracturing pressure was lower than the matrix pore pressure, the imbibition efficiency was the lowest, at only 1.3%. It is proposed that strengthening fracture pressure and promoting pressurized imbibition are the keys to improving shale oil development.

Structural characteristics of continental carbonate-rich shale and shale oil movability: A case study of the Paleogene Shahejie Formation shale in Jiyang Depression, Bohai Bay Basin, China

Based on rock mineral and geochemical analysis, microscopic observation, physical property measurement, and thin laminae separation test, etc., the characteristics of typical laminae of the Paleogene Shahejie Formation carbonate-rich shale in the Jiyang Depression were analyzed, and the organic matter abundance, reservoir properties, and oil-bearing properties of different laminae were compared. Typical shale storage-seepage structures were classified, and the mobility of oil in different types of shale storage-seepage structure was compared. The results show that the repeated superposition of mud laminae and calcite laminae are the main layer structure of carbonate-rich shales. The calcite laminae are divided into micritic calcite laminae, sparry calcite laminae and fibrous calcite vein. The mud-rich laminae are the main contributor to the organic matter abundance and porosity of shale, with the best hydrocarbon generation potential, reservoir capacity, and oil-bearing property. The micritic calcite laminae also have relatively good hydrocarbon generation potential, reservoir capacity and oil-bearing property. The sparry calcite laminae and fibrous calcite vein have good permeability and conductivity. Four types of shale storage-seepage structure are developed in the carbonate-rich shale, and the mobility of oil in each type of storage-seepage structure is in descending order: sparry calcite laminae enriched shale storage-seepage structure, mixed calcite laminae enriched shale storage-seepage structure, fibrous calcite vein enriched shale storage-seepage structure, and micritic calcite laminae enriched shale storage-seepage structure. The exploration targets of carbonate-rich shale in the Jiyang Depression Shahejie Formation are different in terms of storage-seepage structure at different thermal evolution stages.

Study on the influence of different rock/coal mixture ratio on thermal storage and low temperature oxidation of coal

Coal mines in the goaf are formed by the collapse and accumulation of coal rock, and the process of oxidizing and heat storage of singular coal does not reflect the real environmental conditions within the airspace. In this study, coal and rock mixing on coal spontaneous combustion (CSC) is experimentally investigated, and the bituminous coals and typical siltstones, mudstones and shales in the overlying rock layers of the goaf are selected. The different rock/coal blending ratios and real low oxygen conditions in the goaf are considered. The results show that in addition to coal, shale also contains some substances such as graphite and fixed carbon, which would participate in oxidization and heat storage processes. The thermal conductivity and thermal diffusivity of the rock/coal mixtures increased with the increase of the rock blending ratio. The heat storage capacity of the three rocks is shale > mudstone > siltstone. After blending rocks in bituminous coal, the contents of functional groups –OH, –CH3, –CH2, CC, etc. is decreased, and the degree of decrease is greater with the increase of blending ratio, and the degree of decrease of shale/coal mixture is relatively small compared with the others. As the proportion of rock in the rock/coal mixture increases, all oxidized combustion stages are delayed to varying degrees. Under the same oxygen concentration, the activation energy of rock/coal mixtures is higher than bituminous coal, and the activation energy of siltstone/coal, mudstone/coal, and shale/coal increased by 25.28 %, 20.12 %, and 14.84 %, respectively. The degree of CSC inhibition by the three rock/coal mixtures is siltstone/coal > mudstone/coal > shale/coal. The research results are of great significance to the prevention and control of CSC in the real environment of goaf.

Influence of distinct testing methods on the mode-I fracture toughness of Longmaxi shale

Abundant shale gas in shale reservoirs can be successfully extracted by the fracking technique. The formation of hydraulic fractures is related to the mode-I fracture toughness that represents the resistance to cracking propagation. Two typical Longmaxi shale samples with horizontal and vertical bedding planes are selected in this work. To adequately measure the mode-I fracture resistance of shale, this research adopts four mode-I test specimens with different geometries (cuboid, cylinder, disk, and semi-disk). The experimental results demonstrate that there is an acceptable linear correlation between mode-I fracture resistance and critical T-stress. Further, the discrepancies in mode-I fracture resistance are due to distinct T-stresses that exist in the mode-I test specimens. Consequently, the mode-I fracture toughness KIc for the shale with horizontal bedding is 1.57 MPa·m0.5, while that for the shale with vertical bedding is 1.36 MPa·m0.5. Considering the influences of singular and non-singular stress terms, the GMTSN (generalized maximum tangential strain) and GMTSEDF (generalized maximum tangential strain energy density factor) fracture criteria can provide theoretical explanations for the variation of mode-I fracture resistance measurements. Based on the AE (acoustic emission) technique, the classical RA-AF method can identify the fracture mechanisms of distinct mode-I test specimens. However, the differences in the high-density zones of the RA-AF distribution are conspicuous for distinct mode-I testing methods.

Micropore Structure Characteristics and Recoverability Evaluation of Typical Shale Oil Reservoirs

Abstract In view of the weak research on the availability of typical shale oil reservoirs from the perspective of development, this study introduced a two-dimensional nuclear magnetic resonance (NMR) evaluation method on the basis of the previous one-dimensional NMR combined with centrifugal physical simulation experiments. Not only the production characteristics of typical shale oil reservoirs were studied but also the microscopic production laws of different occurrence states were studied. The results show that the pore distribution of Jilin shale is more concentrated than that of Qinghai shale. The oil of the two blocks mainly occurs in 0.01–10 ms pores, and the occurrence ratio of Jilin shale in the pores is higher, which is more than 90%. The oil production of the two blocks is mainly dominated by 0.01–10 ms pores, and the utilization efficiency contribution of these pores in Jilin shale is higher, accounting for about 80%. The utilization efficiency (UE) increases logarithmically with centrifugal force, and the growth rate of Jilin shale is greater than that of Qinghai shale. The proportion of free oil in Jilin block is less than that in Qinghai block. The shale oil in the two blocks is both at 15% final UE, and the UE of free oil in Jilin shale is about 9% and that of Qinghai shale is about 12%. The recoverability of Jilin shale is lower than that of Qinghai shale.

Extractable and kerogen-bound hopanoids from typical Eocene oil shales in China

This study conducted a comparative analysis of extractable hopanoid hydrocarbons and those released via stepwise pyrolysis of typical Eocene immature oil shales in China, namely the Huadian, Maoming, and Fushun shales. Both the Huadian and Maoming shales exhibit immature indicators in extractable and kerogen-bound hopanoids (notably, high abundance of C29 to C32 17β,21β-hopanes and unsaturated hopenes). In contrast, the Fushun oil shale's hopanoids from extracts and pyrolyzates suggest a higher maturity level. The absence of neohopenes in the pyrolyzates of the shales underpins that the kerogen-bound hopanoid skeletons resist rearrangement. However, the Huadian oil shale's asymmetric distribution of C29 and C30 hopenes and neohopenes hints at the presence of an additional source. Novel unsaturated hopenes, such as hop-20 (21)-enes, identified in pyrolyzates of the three kerogens at various pyrolysis temperatures, reveal the occurrence of double bonds in kerogen-bound hopanoid skeletons without methyl rearrangements. The absence of hop-20-(21)-ene in extracts suggests that it might act as an intermediate of these novel hopenes during the epimerization of hopanoid skeletons within kerogen. The extractable and pyrolytic hopanoids' stereochemical alignment indicates that epimerization may occur in both ring systems and alkyl side chains of kerogen-bound hopanoid skeletons. Sequential stepwise pyrolysis proves to be a quick screening method for geological hopanoids without causing any significant alteration to the original skeletons even when cracking multiple covalent bonds is necessary.

Effect of environmental factors on accumulation of trace metals in a typical shale gas exploitation area: A comprehensive investigation by machine learning and geodetector models

Whether a certain relationship is exist between shale gas exploitation and accumulation of trace metals in soil is a controversial issue in recent years. To date, few study clearly reveal the intrinsic contributions of natural and anthropogenic factors to accumulation of trace metals in soil. In this study, machine learning and geodetector models were integrated to investigate to contribution of environmental factors to variations of trace metals concentration. Before modeling, there are 10.33%–25.87% of soil considered as metal pollution, and the value of Pn further suggest that the Ba contribute the most to the comprehensive pollution index of trace metals in soil. The initial prediction of trace metals concentration by machine learning models is less effectively indicating the need for alternative approaches. To address this problem, post-constraints approach was used, and the post-constraint MSLR model demonstrates superior performance (R2 = 0.81) Additionally, through the utilization of geodetector model, the explanatory power (q) of CEC and SOM were identified as dominant natural factors with value of 0.055 and 0.089. respectively. Moreover, distance from working sites and working status were identified as the dominant anthropogenic factors associating to the spatial heterogeneity of trace metals in soil. The interaction between natural and anthropogenic factors showed a siginifacnt nonlinear enhancement effect on accumulation of Cr, Ba and Sr, and the highest value of q was 0.38 for SOM and distance. This study indicated that the potential metal contamination was related to shale gas exploitation and provide reference for controlling soil pollution in shale gas exploitation area and making management strategy.

Influences of shale microstructure on mechanical properties and bedding fractures distribution

The difference in microstructure leads to the diversity of shale mechanical properties and bedding fractures distribution patterns. In this paper, the microstructure and mechanical properties of Longmaxi marine shale and Qingshankou continental shale were studied by X-ray diffractometer (XRD), field emission scanning electron microscope (FE-SEM) with mineral analysis system, and nanoindentation. Additionally, the typical bedding layers area was properly stratified using Focused Ion Beam (FIB), and the effects of microstructure and mechanical properties on the distribution patterns of bedding fractures were analyzed. The results show that the Longmaxi marine shale sample contains more clay mineral grains, while the Qingshankou continental shale sample contains more hard brittle mineral grains such as feldspar. For Longmaxi marine shale sample, hard brittle minerals with grain sizes larger than 20 μm is 18.24% and those with grain sizes smaller than 20 μm is 16.22%. For Qingshankou continental shale sample, hard brittle minerals with grain sizes larger than 20 μm is 40.7% and those with grain sizes smaller than 20 μm is 11.82%. In comparison to the Qingshankou continental shale sample, the Longmaxi marine shale sample has a lower modulus, hardness, and heterogeneity. Laminated shales are formed by alternating coarse-grained and fine-grained layers during deposition. The average single-layer thickness of Longmaxi marine shale sample is greater than Qingshankou continental shale sample. The two types of shale have similar bedding fractures distribution patterns and fractures tend to occur in the transition zone from coarse-grained to fine-grained deposition. The orientation of the fracture is usually parallel to the bedding plane and detour occurs in the presence of hard brittle grains. The fracture distribution density of the Longmaxi marine shale sample is lower than that of the Qingshankou continental shale sample due to the strong heterogeneity of the Qingshankou continental shale. The current research provides guidelines for the effective development of shale reservoirs in various sedimentary environments.

Enrichment characteristics and exploration potential of the Permian marine shale gas in the Wujiaping Formation, Sichuan Basin, SW China

Shale gas exploration has achieved global success in marine shales. This study analysed geochemical, mineralogical, porosity, permeability and gas‐bearing parameters of organic‐rich shales in the Permian Wujiaping Formation in the Sichuan Basin. A comprehensive comparison was conducted with the Longtan Formation transitional shales and typical commercially developed marine shales. The results showed that: (1) siliceous shale within the Wujiaping Formation exhibits total organic carbon content ranging from 0.60% to 9.53% (average 4.45%) and vitrinite reflectance (Ro) ranging from 1.83% to 2.14% (average 1.98%), indicating an overmature stage of hydrocarbon generation. Porosity varies from 0.48% to 11.8% (average 4.94%), with an average content of 51.4% brittle minerals (quartz+calcite+dolomite) and calculating gas content exceeding 3 m3/t based on logging data. Effective sealing is achieved by the overlying carbonate of the Changxing Formation and the underlying carbonate of the Maokou Formation. (2) The primary storage space of the siliceous shale in the Wujiaping Formation encompasses diverse organic matter pores, clay mineral pores and microfractures. The porosity and permeability of the Wujiaping Formation marine shale ranges from 0.48% to 11.8% and 0.0053 to 0.8450 mD. Total gas content was estimated between 3 to 12 m3/t averaging 6 m3/t. (3) By comparing Permian Wujiaping Formation shales with Longtan Formation transitional shales, Wufeng Formation‐Longmaxi Formation shales and the typical North American marine gas shales (such as Barnett, Ohio, Antrim, New Albany, etc.), we conclude that marine siliceous shales within the Wujiaping Formation exhibit enrichment conditions comparable to established marine shales and superior to Permian transitional shales. The favourable upper and lower seals of the Wujiaping Formation resemble those of the Barnett shale gas reservoirs. In conclusion, the marine shale in Wujiaping Formation holds potential for substantial shale gas reservoirs and is poised for strategic exploration and development breakthroughs in the northeast and east Sichuan Basin.

Influence of Mineralogy, Organic Matter and Thermal Maturity on the Diagenetic Evolution of Pore Systems in the Shales of Cambay and Raniganj Basin, India

Abstract The mineral composition, organic matter, thermal maturity, and diagenesis control the shale reservoir properties, including the pore distribution, variable fluid distribution, microstructure, heterogeneity, etc. The shale layers of the Cambay Formation and the Raniganj Formation are considered prospective shale gas resources in India. This paper focuses on the detailed analysis of shale from the Cambay Formation (Eocene age) of the Cambay Basin and the Raniganj Formation (Permian age) of the Raniganj Basin, India. The whole rock X-ray diffraction data indicates that quartz is the dominant mineral in both the shales (Raniganj and Cambay shale) of the Raniganj and the Cambay Basin. The main clay minerals are clinochlore, smectite-kaolinite, illite, and kaolinite in the Raniganj shale, while kaolinite, montmorillonite, and illitesmectite are the dominant clay minerals in the Cambay shale. The total organic content (TOC) ranges from 4.97 to 17.88 wt.% and 1.16-7.6% in the Raniganj and Cambay shales respectively. For the studied shale section, the TOC shows a gradual decrease with respect to depth in both shale types. The Raniganj shale has 3.8 to 8.3 percent total porosity, and the Cambay shale contains 1.16 to 6.5% total porosity. The increasing thermal maturity with the higher TOC corresponds to the more developed organic pores in the Raniganj shale. In contrast, the secondary pores are developing in the Cambay shale due to dissolution activities and enhancing the storage capacity. The study infers that the mineral composition and diagenesis are playing a crucial role in the pore system evolution of the shale in both the basins.

Characterizing rock transverse anisotropic spatial variations using digital drilling

Transverse anisotropy plays an important role in the engineering behavior of rock masses. This study examines the transverse anisotropic spatial variations and evaluates them using the coefficient of variation (CV) and anisotropy index (AI) through digital drilling tests. Four types of rocks, shale (SH), mudstone (MU), sandstone (SA), and schist (SC), are studied in four directions, revealing significant transverse anisotropy at the cutting point. The results demonstrate that the effect of the depth per rotation on the difference is weaker compared to the scaled weight and torque. The ranking of transverse anisotropy in the tested rocks is: MU > SA > SH > SC. Transverse anisotropy changes along the drilling depth vary across rock types. With the increase of drilling depth, SH experiences a decrease, rebound, and another decrease. MU shows an initial increase followed by a decrease. For SA, the degree of transverse anisotropy decreases and is sustained at a lower extent. SC exhibits slight fluctuations with the exception of the abrupt change. The time when the drilling bit is in perfect contact with the rock and the cutting efficiency in different drilling directions are the main factors causing the transverse anisotropy. The intricate evolution of transverse anisotropy should be worth investigating on a wider scale.

Deformation Characteristics and Permeability Properties of Cap Rocks in Gas Storage of Depleted Reservoirs under Alternating Load

Gas reservoirs have significant engineering characteristics of injection and extraction. The reservoir cap rock is subjected to cyclic alternating loading and has the potential risk of seal failure. Therefore, it is necessary to study the stress−percolation−damage mechanism of the reservoir cap rock in depleted gas reservoirs. The rock Mechanics Test System (MTS) was used to study the permeability characteristics of a typical mud shale cap layer under different loading and unloading rates, analyze the deformation characteristics and permeability performance evolution law of the rock under the confining pressure alternation, and study the effects of loading and unloading rate, confining pressure and number of cycles on the permeability of the cap rock. The test results show that with the increase in the number of cycles, the hysteresis loop moves in the direction of axial strain increase offset. The overall morphology is presented as an elongated type, and the damage of the specimen is small in the cyclic confining pressure; At the beginning of the cycling period, the permeability decreases with the increase in the confining pressure in the form of a negative exponential. At the later stage of the cycling period, the permeability basically stays unchanged, and is maintained at a low level; At low confining pressure, permeability also decreases in a negative exponential form with the increase in the number of confining pressure cycles; The greater the loading and unloading rate at the beginning of the cycle, the more the permeability decreases; There is a tendency of sealing improvement under cyclic loading in the gas storage of depleted reservoirs with mud shale as the cap layer. The results of the study can provide technical parameters for the evaluation of the cap sealing of gas storage of depleted reservoirs.

Extracting oil from oil shale using internal distillation (in situ retorting)

Abstract The method of internal distillation (in situ retorting) and internal treatment for extracting shale oil is one of the modern methods developed with good economic quality but at a higher cost. Therefore, the net of fossil oil can be considered alternative energy. Hydrocarbon-rich hydrogen turns into oily shale, which distills by heat and turns into gases and petroleum liquids suitable for use as fuel. The fumes from burning oil shale are considered cancer-causing, so advanced treatment is required before extraction. As the temperature increases, the speed of oil extraction increases dramatically, but at 510°C, the production rate begins to decline. Oil extraction produces more than gas; the highest production peak is around 550 h. The permeability is closely proportional to the oil producers, and the permeability is inversely proportional to the highest temperature in the case of high heat through the combustion of coal with oxygen internally. The type of oil shale is affected by the surrounding environment and the temperature of the earth’s crust.