Typical Shale Research Articles

An experimental investigation into the elastic and thermal properties of oil shales: Yanchang formation, Ordos basin

The relationship between oil shale's elastic behavior and geothermal parameters is vital in sweet spot identification and in-situ oil conversion techniques. To qualitatively study the elastic P- and S-wave velocities and thermal properties of oil shales, we simultaneously make ultrasonic measurement s and investigate geothermal properties in the Triassic Yanchang Formation. The results demonstrate that ultrasonic P-wave velocities are sensitive to the pressure of <5 MPa mainly due to the closure of the soft pores/cracks and kerogen content and distribution. Thermal experiments reveal that kerogen content has a significant influence on shale's thermal expansion coefficients (TEC) and heat capacity. Specifically, organic-rich shales show the highest thermal expansion coefficients in both directions in core plugs (0° and 90°), reaching a maximum of 5 × 10−5 °C−1 and the highest heat capacity compared to the other samples. On the contrary, mudstone has a relatively low thermal expansion coefficient of 1 × 10−5 °C−1. The variation trend of the TEC of typical shale samples with the change of TOC (total organic carbon) content indicates that existing four phases of TEC correspond to different dominant rock evolution. These observations imply that low ultrasonic velocities accompanied by high thermal properties (heat capacity, TEC) can be ‘sweet spot’ indicators for high TOC reservoir zones. The experiments prove that the quartz content has a significant influence on the ultrasonic P- and S-wave velocities and TEC of shale rocks. Both ultrasonic velocities and thermal parameters of shales exhibit a distinct change at ∼40% quartz volume, responding to a change in the rock skeleton, and a change in TOC content. These observations will thoroughly assist us in understanding the elastic signature and thermal properties of shales during in-situ conversion process of shale oil reservoirs in Ch7 Member of the Yanchang Formation, in the Ordos Basin, Northwest China.

Oil content and mobility in a shale reservoir in Songliao Basin, Northeast China: Insights from combined solvent extraction and NMR methods

The volatility of light hydrocarbons means that the conventional dichloromethane (DCM) extraction method may underestimate shale oil content, thereby limiting its usefulness in studies of oil content and mobility within shale pores. Here, a method involving a combination of DCM extraction and low-field (LF)–NMR. spectroscopy was developed for a study of a typical shale oil reservoir in Songliao Basin, Northeast China, measuring its total, adsorbed, mobile, and free oil contents and oil, gas, and water saturation. Factors affecting mobility were also considered. Total oil contents were in the range of 7.2–13.4 mg g−1 rock; adsorbed oil contents determined by DCM extraction accounted for 18%–67% of the total; and total oil saturation was in the range of 23%–40%. Mobile oil saturation ranged from 10% to 32% (17% in average), with free oil saturation averaging 9%. A total-oil correlation coefficient as a function of asphalt A extraction was first time developed. Increasing porosity made no contribution to favorable “sweet spots” for shale oil production. Mobile oil was stored in both meso and macro pores, rather than only in larger pores. Shale intervals with high contents of total organic carbon tended to store oil in an adsorbed state. These findings may improve resource estimation and shale oil sweet spot exploitation.

Palaeoenvironment characteristics of the Ediacaran Doushantuo Shale, Yichang area, South China: Implications for organic matter accumulation mechanism

The organic-rich Ediacaran Doushantuo Shale have been identified as a high-quality source rock and a main shale gas play in the Yichang area, South China, which was characterized by high TOC content, high thermal maturity, enormous cumulative thickness, and relatively high contents of brittle minerals. As the oldest shale gas stratum, the paleo-environmental characteristics and organic matter (OM) accumulation mechanism remain unclear in this special marine setting. In this study, core descriptions, TOC content, X-ray diffraction (XRD) mineralogical analysis, scanning electron microscopy (SEM) observation, and elemental analysis, including the major, trace, and rare earth elements, were combined to identify lithofacies, reconstruct paleo-environmental conditions, and document organic matter (OM) accumulation mechanisms for this potential Ediacaran Doushantuo Shale in the Yichang area, South China. Four major shale lithofacies were identified based on TOC content and mineral composition. The geochemical redox proxies, such as the enrichment factors of Uranium (U-EF) and ratio of organic carbon to total phosphorus (Corg/P), indicate that the Doushantuo Shale was probably deposited in a dysoxic-anoxic conditions, which is consist with the interpretation from size distribution of pyrite framboids. The terrigenous influxes suggest the Doushantuo Shale was deposited in arid climate in absence of chemical weathering. Meanwhile, the productivity proxies (biogenic SiO2, Ba, and P/Al) indicate that the organic-rich Doushantuo Shale was deposited under a relatively low primary productivity. In addition, hydrothermal input has limited influence on organic matter (OM) accumulation of the Doushantuo Shale, which was also reported in the Qinglinkou Section of Yichang area. It can be observed that the organic matter (OM) accumulation mechanism of the Doushantuo Shale, deposited after the Marinoan “Snow Earth” glaciation, in the Yichang area, South China, was mainly controlled by the redox conditions, which is much different from typical marine shales and transitional shales.

Effects of adding loose materials on improvement of the physical properties of sediment and growth of green plants

Effects of different loose materials on the growth of green plants in the sediment and the improvement of the physical properties of the sediment were studied in pot experiments. Two types of loose materials, vermiculite and shale, were chosen. The addition ratio of loose materials and sediment were set to 1:1, 1:2, 1:5. Results showed that, the sediment after adding vermiculite was more conducive to the growth of ryegrass, and when the addition ratio was 1:1 and 1:2, the growth conditions of ryegrass were the best. The fresh weight of ryegrass in the treatment with the ratio of vermiculite and sediment at 1:1 increased by 43.94% compared to that in pure sediment; In the treatment with the ratio of 1:2, the plant height increased the most, which was 6 cm higher than that in pure sediment; When large amount of loose materials were added, the moisture content of the sediment increased compared to the pure sediment treatment; after the addition of loose materials, the bulk density of the sediment decreased, and the addition of too large amount would easily lead to a low bulk density, which was not conducive to water and fertilizer retention; The addition of vermiculite can reduce the bottom sediment. Bangladesh J. Bot. 52(2): 487-493, 2023 (June) Special

Effects of adding loose materials on improvement of the physical properties of sediment and growth of green plants

Effects of different loose materials on the growth of green plants in the sediment and the improvement of the physical properties of the sediment were studied in pot experiments. Two types of loose materials, vermiculite and shale, were chosen. The addition ratio of loose materials and sediment were set to 1:1, 1:2, 1:5. Results showed that, the sediment after adding vermiculite was more conducive to the growth of ryegrass, and when the addition ratio was 1:1 and 1:2, the growth conditions of ryegrass were the best. The fresh weight of ryegrass in the treatment with the ratio of vermiculite and sediment at 1:1 increased by 43.94% compared to that in pure sediment; In the treatment with the ratio of 1:2, the plant height increased the most, which was 6 cm higher than that in pure sediment; When large amount of loose materials were added, the moisture content of the sediment increased compared to the pure sediment treatment; after the addition of loose materials, the bulk density of the sediment decreased, and the addition of too large amount would easily lead to a low bulk density, which was not conducive to water and fertilizer retention; The addition of vermiculite can reduce the bottom sediment. Bangladesh J. Bot. 52(2): 487-493, 2023 (June) Special

Comparison of mechanical characteristics of different types of shales in the Ordos Basin

Shale gas is a very important unconventional energy. The mechanical properties of the three types of shale (laminated shale, sandwich shale and foliated shale) are different, and the difference in fracturing effectiveness is very significant. In this paper, the mineral composition, mechanical properties and conductivity of these three different types of shale were studied and compared by X-ray diffraction, triaxial mechanical experiments, and fracture conduction experiments. The study found that the foliated shale has the lowest content of rigid minerals (47.5%), lower elastic modulus and tensile strength (26.98 Gpa and 168.29 MPa, respectively), higher Poisson’s ratio (0.25), the smallest brittleness index (0.48), and larger fracture toughness (0.42). The laminated shale has a higher content of rigid minerals (68.50%), the lowest elastic modulus and tensile strength (25.77 Gpa and 122.46 MPa, respectively), the highest Poisson’s ratio (0.26), the highest brittleness index (0.56), and the lowest fracture toughness (0.18). The sandwich shale has the highest rigid mineral content (78.16%), the highest elastic modulus and tensile strength (35.31 Gpa and 197.37 MPa, respectively), the lowest Poisson’s ratio (0.24), a larger brittleness index (0.52), and larger fracture toughness (0.415). Furthermore, with the increase in the coring angle, the elastic modulus of all three shales increases. In addition, with the increase in closing pressure and the decrease in the sand laying concentration, the proppant embedding depth gradually increases and the conductivity decreases. This means that from the perspective of forming complex fracture networks, the fracturing effect of the foliated shale is unsatisfactory, while the fracturing effect of the laminated and sandwich shales is better. Moreover, it is recommended to prefer directional injection along vertical laminae or at high angles, which is conducive to the formation of complex fracture networks. For laminated shale with low strength, the sand laying concentration should be increased to ensure the conductivity of the fractured fracture. This study provides some technical guidance for the identification of different types of shale fracturing desserts and fracturing processes.

Influence of shale bedding on development of microscale pores and fractures

The characteristics of bedding development have an important impact on the spatial distribution of shale pores and fractures. However, there is a lack of systematic study on the pore and fracture structure of bedding development characteristics. In this paper, the geological control mode of shale type on the development characteristics of pores and fractures is revealed; pore-throat models under different shale lithology conditions are constructed; the control mechanism of bedding development characteristics on the spatial distribution pattern of pores and fractures is clarified. Results show that: (a) The pore space size varies widely from nanoscale to micron-scale or even millimeter scale, and is mainly concentrated in the nanoscale range. (b) For shale samples No. 1-7-1 No. 21-7-1, although the proportion of organic matter and mineral area in different samples varies greatly, the difference between the proportion of organic pore area and the proportion of mineral pore area is not significant, and the proportion of organic pore area is much higher than that of mineral pore area. (c) The pore diameter of shale has strong heterogeneity; minerals have an important impact on throat diameter; maturity has an important impact on throat length; geological compaction and tectonism will have an impact on the tortuosity of shale reservoir pores; the controlling effect of pore connectivity on shale wettability is mainly reflected in the connectivity of hydrophilic and lipophilic pores.

Hydrocarbon generation differences of shales composed of green algal and cyanobacteria: A case study of Mesozoic and Cenozoic saline lacustrine shales in Junggar Basin, NW China

The Mesozoic and Cenozoic strata in the Junggar basin developed two sets of shallow to semi-deep lacustrine shale, namely, the Cretaceous Qingshuihe Formation (K1q) and the Paleogene Anjihaihe Formation (E2-3a). Through organic petrology and scanning electron microscope (SEM) observation, it is found that the primary hydrocarbon-generating organic matter (OM) in the two sets of strata is different. The biological precursor of the E2-3a OM is mainly green algae (Pediastrum), while the precursor of K1q kerogen is mainly cyanobacteria (Oscillatoria). Then, the E2-3a green algae-rich shale and K1q cyanobacteria-rich shale were subjected to hydrous pyrolysis and kinetic analysis, respectively. The results show that the evolution modes of hydrocarbon generation of the typical shales are very different. Green algae have the characteristics of a low oil generation threshold, heavy oil quality, and no prominent oil peak, while cyanobacteria have the characteristics of late oil generation, concentrated hydrocarbon generation, and relatively light oil quality. The characteristics of oil generation can also be well reflected in the composition evolution of the crude oil components. The carbon isotope of gas, kerogen, and extracts of the E2-3a green algae-rich shale are significantly heavier than the K1q cyanobacteria-rich shale, which may be related to the living habits of their biological precursors, carbon source usage, photosynthesis efficiency, and carbon fixation efficiency.

Sedimentary Facies Types and Their Control of Reservoirs in the Lower Jurassic Lacustrine Facies Shale of the Lianggaoshan Formation, Northeastern Sichuan Basin, China

In recent years, new breakthroughs have been made in the field of shale oil and gas exploration in the Lower Jurassic Lianggaoshan Formation in Sichuan Basin. At present, there is a lack of systematic studies on reservoir properties and sedimentary facies of the Lianggaoshan Formation shale. Therefore, in this study, taking the Lianggaoshan Formation in Sichuan Basin as an example, the sedimentary facies types of shale reservoirs and their control over shale oil and gas are systematically studied, based on a large number of outcrops, experimental testing, logging, and seismic interpretation methods. The results show that five sedimentary microfacies are developed in the Lianggaoshan Formation in the study area, namely, semi-deep lake mud, shallow lake mud, wave-influenced shallow lake mud, delta-influenced shallow lake mud, and underwater interbranch bay microfacies. The stratum thickness of the Lianggaoshan Formation is in the range of 26–315 m, and mainly distributed in the eastern region, but rapidly thinned in the northwestern region. The sedimentary sequence framework of the Lianggaoshan Formation has been constructed. Moreover, the lithology of the Lianggaoshan Formation shale has been divided into three types, including shale, massive mudstone and silty mudstone. The brittleness index and total organic carbon (TOC) value of three types of shale show a negative correlation. Silty mudstone has the highest brittleness, while that of black shale is the lowest. For porosity and permeability, massive mudstone is better than silty mudstone, and silty mudstone is better than black shale. There are many kinds of matrix pores in the Lianggaoshan Formation shale, and the development degree of inorganic pores is higher than that of organic pores. Finally, based on the analysis of oil-bearing, pore types, physical properties and productivity, it is considered that black shale facies is the most favorable lithofacies type. The deep–semi-deep lacustrine facies belt obviously controls the shale oil enrichment of the Lianggaoshan Formation.

Clay mineral transformation mechanism modelling of shale reservoir in Da’anzhai Member, Sichuan Basin, Southern China

Shale reservoirs often undergo intense clay mineral transformation, which plays a crucial role in the formation and evolution of pores. The reservoir lithofacies types of Da’anzhai Member in the Sichuan Basin are complex, the heterogeneity is strong, and the transformation mechanism of clay minerals is unclear, limiting the understanding of reservoir diagenesis and reservoir formation mechanism. In this study, we selected the typical shale reservoir in the Da’anzhai Member of the eastern Sichuan Basin and innovatively introduced the multiphase fluid-chemical-thermal multi-field coupled numerical simulation technique to focus on the dissolution, precipitation and transformation laws of diagenetic minerals in the shale reservoir. We calculated the transformation of diagenetic minerals and their physical response under different temperatures, pressure and fluid conditions and identified the main controlling factors of mineral transformation in shale reservoirs in the study area. The results show that the transformation of smectite to illite in the Da’anzhai Member is a complex physicochemical process influenced by various factors such as temperature, pressure, fluid, and lithology. The increase in temperature can promote illitization until the critical temperature of 110°C–115°C, below which the conversion rate of smectite to illite increases as the temperature increases. However, when it is higher than the critical temperature, the degree of illitization decreases. In specific K-rich fluids, organic acids significantly affect the conversion of clay minerals in the Da’anzhai Member of the formation. The acidic fluid promotes the dissolution of minerals such as K-feldspar and releases K+, thus provides the material basis for illitization. The research results provide theoretical support for the diagenetic and formation mechanism of the shale reservoir in the Da’anzhai Member of the Sichuan Basin and even for the efficient exploration and development of shale gas.

Lithofacies influence characteristics on typical shale pore structure

Clarifying the three-dimensional pore throat structure characteristics of shale is the foundation for efficient development of shale oil and gas. In this paper, firstly, by using FIB-SEM high-tech characterization technology, three-dimensional quantitative characterization of shale pore-throat structures under different lithological conditions was carried out. Secondly, reconstruct through binary images to establish a three-dimensional pore-throat structure model. Thirdly, the coupling mechanism of “pore distribution, form factor, coordination number, throat diameter” is quantitatively clarified. Results show that: (a). For six shale samples, the volume fraction is mostly concentrated in the range of pore diameter greater than 20 nm and less than 300 nm. When the pore diameter is greater than 300 nm, the proportion of volume fraction is lower. (b). For six shale samples, the pore quantity distribution is mostly concentrated in the range of pore shape factor greater than 0.04 and less than 0.07. When the pore diameter is less than 0.04 or greater than 0.07, the pore quantity distribution is lower. (c). For six shale samples, the pore volume distribution is mostly concentrated in the range of coordination number greater than 1 and less than 4. When the pore diameter is less than 1 or greater than 4, the pore volume distribution is lower. (d). For six shale samples, the throat volume distribution is mostly concentrated in the range of throat diameter greater than 8 nm and less than 120 nm. When the throat diameter is less than 8 nm or greater than 120 nm, the throat volume distribution is lower. The pore structure of shale is very complex, mainly manifested by a variety of pore types.

Corrosion Mechanism of Typical Shale Gas Gathering and Transportation Pipelines in Southwest China

The shale gas surface gathering and transportation pipeline in southwest China has experienced serious internal corrosion, which affects the production of shale gas wells and has great potential safety hazards. To find out the corrosion type and corrosion mechanism, the corrosion analysis and mechanism research of gathering and transportation pipelines are carried out, including corrosion morphology observation, corrosion product analysis, fluid mechanics calculation, etc. The results show that the corrosion products of the gathering and transportation pipeline are mainly FeCO3 with high S content, and the corrosion type is sulfate-reducing bacteria (SRB) and CO2 corrosion. At the same time, the corrosion pits are mainly distributed on the approach stream side, which is closely related to the scouring effect of the medium flow in the vertical branch pipe on the wall surface of the tee. The corrosion weight loss test shows that the corrosion rate of sterile solution is significantly lower than that of bacterium solution, indicating that the presence of SRB significantly expedites the corrosion of materials.

Failure Analysis of Casing in Shale Oil Wells under Multistage Fracturing Conditions

During the multistage fracturing in shale oil and gas wells with tieback and liner, one of the major challenges is the wellbore temperature variation due to the high-rate fracturing. In such a case, the axial shrinkage trend of the casing string could be caused due to the sudden drop in temperature, but the actual axial length of the casing string would not change due to the cement constraints. Therefore, this could lead to cementation damage between the casing and cement due to excessive load from the casing string. A wellbore seal that is out of control often leads to irreversible consequences, even well abandonment. In order to study the mechanism of casing deformation in shale oil and gas wells with tieback and liner quantitatively, in this paper, take LS1 well (a typical shale oil and gas well with tieback and liner, and casing deformation is caused) for example, the transient changes of temperature and pressure in the whole wellbore during multistage fracturing are studied. Moreover, the cementing strength test of the interface between casing and cement is also tested. Then, the testing results are carried out and extended to model the finite element (FE) model with the whole vertical section casing string with tieback and liner. The model is used to simulate the internal force changes under fracturing conditions with different stages of fracturing. Meanwhile, the casing deformation mechanism in LS1 well is analyzed and studied in detail. Our simulation results indicated the failure process and mechanism of cementation between casing and cement in shale oil and gas wells with tieback and liner. Our work can provide a detailed theoretical reference and a basis for field application.

Geochemical and gas bearing characteristics of the marine-continental transitional Longtan shales in southern Sichuan, China

Marine-continental transitional shale, as an important shale type, gains less attention than marine and continental shale, which restricts the exploration and development process of marine-continental transitional shale gas. In this study, the Upper Permian Longtan Formation shale in the southern Sichuan Basin was taken as the research object, and the organic matter development characteristics, hydrocarbon generation ability, mineral composition, physical properties, and gas bearing characteristics of the Longtan Formation shale were systematically analyzed. In addition, the effects of organic matter abundance, maturity, and mineral components on shale gas adsorption capacity have been discussed in detail. The results show that the abundance of organic matter in the marine continental transitional shale of the Longtan Formation in southern Sichuan varies greatly, with the TOC value of the vast majority of shale being greater than 2.0%, with the carbon shale (TOC&amp;gt;12%) accounting for about 5%. The main type of organic matter is Type III, with part of Type II2. The maturity of organic matter is in the stage of high maturity to over maturity, which is conducive to the generation of dry gas. There is a good positive linear correlation between the reflectance of vitrinite (Ro) and the maximum thermal decomposition peak temperature (Tmax) of rock. The higher the abundance of organic matter, the greater the hydrocarbon generation potential of shale, and the carbonaceous shale shows good shale gas generation potential. The shale of the Longtan Formation is rich in clay minerals, with the highest content of the illite/smectite mixed layer. The abundance and maturity of organic matter jointlypromote the enrichment of Longtan shale gas. The enrichment of clay minerals is beneficial to shale gas adsorption, but poses a challenge to production fracturing.

Lacustrine Shale Diagenesis—A Case Study of the Second Member of the Funing Formation in the Subei Basin

Shale diagenesis differs from that of sandstone and carbonate rocks with regard to the type, evolution stage, and evolution mode. The quality of shale reservoirs is closely linked to the extent of diagenetic evolution. This study identifies the types and characteristics of shale diagenesis using thin sections and scanning electron microscopy (SEM) observations. The stages of shale diagenesis are determined by analyzing organic matter evolution and clay mineral transformation and establishing a diagenetic evolution sequence. This paper describes the comprehensive diagenetic evolution of organic matter, clay minerals, clastic particles, and carbonate minerals to determine the diagenesis types, diagenetic sequences, and pore evolution occurring during diagenetic evolution. The results show that the diagenesis types of shale in the second member of the Funing Formation include compaction, dissolution, cementation, metasomatism, dolomitization, syneresis, and transformation of clay minerals, as well as thermal evolution of organic matter. The middle diagenetic A stage is prevalent, with some areas in the early and middle diagenetic B stages. The shale underwent a diagenetic evolution sequence, including the collapse and shrinkage of montmorillonite interlayers in the early stage; the rapid formation and transformation of illite and smectite mixed layers, massive hydrocarbon generation of organic matters, and dissolution of unstable components in the middle stage; and the occurrence of fractures filled with gypsum, quartz, ferrocalcite, or other authigenic minerals in the later stage. Dissolution pores and fractures are the dominant shale reservoirs of the second member of the Funing Formation in the Subei Basin. The results provide new insights into understanding the formation and evolution of reservoir spaces during shale diagenesis and information for the exploration and development of lacustrine shale oil and gas.

Petrography and mineralogy control the nm-μm-scale pore structure of saline lacustrine carbonate-rich shales from the Jianghan Basin, China

Carbonate-rich shale in salt lakes has become a potential target for petroleum exploration. A full understanding of storage space development characteristics is the basis for the efficient development of this kind of shale. In this study, the complex mineral composition and multiple pore structure of a typical carbonate-rich shale from the Paleogene Xin'gouzui Formation in the Jianghan Basin were systematically analyzed. The mixed shale and dolomitic shale were the main lithofacies types in the study area. Both of them were dominated by interparticle (interP) pores. However, their pore sizes are noticeably different. Pores with a size of <100 nm are the main compositions of the mixed shale, with an average volume proportion of 78.24%. Conversely, the dolomitic shale develops mainly micro-sized pores and micro-fractures, with a ratio of 70.46%. The difference in pore structure for different types of shales is closely associated with mineral composition. When particles of different sizes are mixed, most of the interP pores are prone to be filled by fine minerals (e.g., clay), resulting in relatively small pore sizes for the mixed shale. Homogeneous carbonate minerals are favorable for the residual of larger size interP pores. However, recrystallization and secondary enlargement of carbonate minerals can also lead to a significant decrease in pore diameter of interP pores. With these results, a storage space development model for different lithofacies shales was established. This work is crucial to further reveal the reservoir mechanisms and hydrocarbon scales of an argillaceous dolomite reservoir.

Hybrid geothermal energy and hydrocarbon resources production by repurposing horizontal wells in shale gas reservoirs in Horn River Basin, British Columbia, Canada

The production of geothermal energy from the depleted oil and gas reservoirs could represent one of the practical pathways of the clean energy transition for the hydrocarbon energy industry. In this study, a new strategy for repurposing oil and gas wells into geothermal energy production system is proposed to co-produce hydrocarbon and geothermal resources during the transition. At a certain stage in a hydrocarbon production system of fractured horizontal wellbores, one vertical injection well is added to connect to the end of the horizontal wellbores to form a U-loop system with open fractures in the lateral part. Cold water is injected through the injection well, transported along the U-loop system, and geothermal energy is produced along with natural gas for utilization. Natural gas production continues as long as the pressure of the horizontal well remains lower than the reservoir pressure. An example application case from a typical shale gas production system in the Horn River Basin of British Columbia, Canada has been examined in this study. The results show that reduction in cumulative gas production is limited if the injection well is added at a middle or later stage. The outlet temperature from this system behaves like those in a closed-loop geothermal system and could be stable for several decades of operation. Although the time of conversion is sensitive to gas production, it does not affect the bottom hole temperature much. The water flow rate does not affect the cumulative gas production but considerably affects the formation temperature. The horizontal length of the fractured wellbore will significantly affect the gas production performance and geothermal energy output. Fracture length and permeability have critical impact on gas production but less influence on geothermal energy recovery.

Favorable lithofacies and pore characteristics of the Permian Longtan Formation shale in the southern Sichuan Basin

The marine–continental transitional shale (MCTS) reservoirs of the Longtan Formation (LTF) are widely distributed in the Sichuan Basin. However, the LTF shale exhibits considerable variations in mineral composition and pore characteristics, which makes identifying the 'sweet spot'a challenging task. To address this issue, 10 samples from four typical shale gas wells in the LTF in the southern Sichuan Basin were selected and analyzed for total organic carbon (TOC) content, whole-rock composition using X-ray diffraction (XRD), low-pressure gas adsorption, and high-pressure mercury intrusion. The lithofacies distribution and pore structure of the MCTS were studied to determine the pore structural characteristics and the primary factors influencing pore formation in different types of shale lithofacies in the LTF. The lithofacies of the LTF shale in the study area can be classified into three categories: siliceous clay shale, clay shale and mixed shale. Mineral content has a significant impact on the pore characteristics, while TOC content has a minor effect on the pore volume and specific surface area of micropores and mesopores. It can be inferred that the mesopores in the MCTS are mainly related to clay mineral pores, and mineral dissolution and TOC content are not the primary factors contributing to pore formation.

Differences of Organic Matter Types of High Maturity Marine Organic-Rich Shale from Wufeng-Longmaxi Formation: Implication for Shale Gas “Sweet Spots” Prediction

Based on the identification of visual kerogen assessment, a subdivision method of organic matter types was proposed. Combined with GR curve, the distribution was further studied, the relationship between shale reservoir parameters and organic matter types was analyzed, and the significance was discussed. The results show that the kerogen types of organic rich shale in the lower part of Wufeng-Longmaxi Formation in the south of the Sichuan Basin are mainly of type I, II1a and II1b, while type II2 and type III are not developed. In Wufeng Formation, the shale organic matter is mainly of type II1a and II1b. From Long111 to Long114, the shale displays a trend of “Type I-II1b-II1a-II1b”. Type I was characterized by higher TOC, porosity and gas bearing capacity of shale reservoirs than the other organic matter. The reservoir parameter of type II1a was better than type II1b, indicating that the organic matter type control on the quality of shale reservoirs. Type difference is closely related to the bio-precursors component, among which type I and type II1a are related to the content of planktonic algae, while type II1b is related to the content of benthic algae. This study provides a good reference for the subdivision of marine shale organic matter types and the evaluation and prediction of gas bearing properties.

Experimental investigation of shale/hydrogen geochemical interactions

Underground Hydrogen Storage (UHS) is an effective technique that provides safe storage and recovery of the stored hydrogen for long-term energy storage. Shale caprocks are regarded as effective seals of the stored gas, however, one of the key challenges is the in-situ geochemical shale/hydrogen reactions that can impact the seal integrity during injection/production of hydrogen. To address this issue, this work investigates the geochemical reactions between hydrogen and shales, and evaluates the possible alterations in shales’ petro-physical properties. Calcite-rich shale samples are collected from EagleFord (EFS) and Wolfcamp (WCS) shale formations and exposed to pure hydrogen for 108 days under 9.65 MPa at 348 K. Analytical methods including X-ray diffraction, X-ray fluorescence and Scanning Electron Microscope analysis are performed. The results indicate calcite precipitation after hydrogen treatment in both samples due to the presence of organic contents. Sulfate and pyrite reduction reactions with hydrogen are not observed as hydrogen did not all react with sulfate minerals. Consequently, minor changes in the surface pore structure and pore spaces are observed after the treatment, confirming the low reactivity of calcite-rich shale rocks with hydrogen. Gas chromatography analysis shows neither considerable hydrogen loss nor the generation of other gases such as CH4 or CO2 occurred during the experiments. The obtained results suggest that the integrity of shale-caprocks are expected to remain strong during the injection and production of hydrogen, which is favorable for UHS. This work can be a suitable experimental framework to assist in evaluating the hydrogen reactivity to different types of shales with various mineralogy, and assessing the suitability of shale rock for UHS.