Clay Shale Research Articles

Shear Strength and Durability Behaviors of Compacted Weathered Clay Shale Mixture Using Portland Cement

The use of weathered clay shale often has the potential to cause geotechnical problems as an embankment material, especially slope failures. In order for weathered clay shale to be used as embankment material, the weathered clay shale must be mixed with other materials. An example of a widely used mix is a mix with a Portland cement (PC). In general, this mixture will increase the shear strength of the embankment material. In addition to shear strength, it is very important to investigate whether the material mixture is susceptible to durability. Therefore, this study aims to evaluate the shear strength and durability behaviors of weathered clay shale mixture, using PC. The percentage of this cement was varied and did not exceed 20%, with the mixing material also compacted based on Proctor Standard procedure. This test included the determination of shear strength and durability index at the smaller and larger (dry and wet sides) than optimum moisture content (OMC). Shear strength and durability index were determined by Triaxial and slake durability index tests, respectively. The results showed that the weathered clay mixture with 10% PC and 8% larger OMC led to an increase in the normalized shear strength (∆σ/σ) and durability index at approximately 300% and 24%, respectively, compared to the original clay shale. This indicated that the optimum shear strength and durability of this shale mixture were highly observed at 10% PC and 8% larger OMC (wet side). This verified also although the durability index increased by 97% with the addition of 20% PC, whose utilization was found to be unrealistic

Geoelectric structure of the section on the Anmangynda ice (Magadan region) according to the data of the TEM method

In March of 2021, measurements were carried out by the TEM method on the Anmangynda aufeis in the Magadan region. The purpose of the research was to use the TEM method to determine the places of discharge of groundwater, which presumably feeds the aufeis, and to determine the geoelectric structure of the environment under the aufeis and its immediate surroundings to depths of 500 meters. Within the research area, profile soundings were performed by the TEM method, along 8 profiles of different lengths that intersect the aufeis itself and the bed of the Anmangynda River upstream and downstream. The measurements were carried out by a generator loop measuring 71 * 71 meters, a measuring loop measuring 20 * 20 meters with a distance of 100 meters from the center of the generator loop. When conducting research on the Anmangynda aufeis, 98 pickets were measured along 9 profiles. Within the research area, the electrical resistivity of rocks is determined primarily by their lithological features, degree of fracturing and material composition. This significantly complicates the geocryological interpretation of the sections and makes it almost impossible to isolate taliks in conditions of low-resistance clay shales. The places of groundwater outlets to the surface above the aufeis glade spatially coincide with the access to the surface of the steeply falling zones of the lowered UES, interpreted as faults along which the discharge of groundwater is likely to occur.

Parametric study of tunnel analysis in clay shale on short term and long-term conditions using finite element method

This study aims to determine the characteristics and classification of clay shale in West Java and their implications for tunnel stability under short and long-term conditions using the finite element method. Data were collected from projects in West Java containing clay shale spread over four rock formations, namely Cihoe (Tpc), Subang (Tms), Cantayan (Mtts/Mttc), and Jatiluhur (Tmj). Each formation has varying mechanical and engineering properties. The value of SPT from Cihoe formation can be categorized in three conditions, SPT < 40, SPT 40-60, and SPT > 60 for fully, highly to moderately, and slightly weathered, respectively. Meanwhile, the value of SPT of Subang formation can be categorized in two conditions, SPT < 60 and SPT >60 for fully and highly weathered, respectively. The data collected were analyzed to determine the Mohr-Coulomb (MC) and Hardening Soil (HS) parameters for Plaxis modeling. The model applied three variations of overburden thickness between 3 times, 6 times, and 9 times the tunnel diameter (3d, 6d, and 9d respectively). The greater the overburden thickness, the lower the surface deformation. The HS model gives better results than the MC model because it considers non-linearity. The minimum effective parameters needed to support tunnel during construction to meet the allowable deformation for 3d overburden conditions and the particular reinforcement system are c' 53kPa, ϕ' 28°, and E'50ref 30,000 kPa. Long-term conditions possess lower stability than short-term, while prolonged deformations increase after construction and provide a rise in tunnel lining stress that needs to be considered in the design stage. The application of 2D tunnel modeling needs to be carefully analyzed, thereby representing the behavior of a continuous or 3D tunnel structure.

Feasibility study on using CO2-rich industrial waste gas replacement for shale gas exploration based on adsorption characteristics

Clean utilization of CO2-rich industrial waste gas (CO2-rich IWG) is of great significance to realize carbon neutralization. Due to the strong adsorption capacity of clay and organic matter in shale to CO2, it has a good potential for CO2-rich IWG storage and replacement. So, the feasibility of using CO2-rich IWG replacement for shale gas exploration from the perspective of adsorption was studied. A shale composite model was constructed based on the adsorption experiments, and the molecular simulation results were fitted with experiments for the model reliability. Then, the slit models of different minerals were constructed to study the adsorption mechanisms. Finally, the competitive adsorption mechanisms of different components with CH4 were revealed. The results show that the overall adsorption capacity on shale minerals are ranked as HF > CO2 > SO2 > H2S > NO2 > CH4 > CO；Hydrogen bond and electrostatic force are the main adsorption modes of HF, and electrostatic force between negatively charged atoms of gas molecule and mineral surface is the main adsorption modes of CO2, SO2 and H2S. That of NO2, CH4 and CO is the weak van der Waals force. In the competition with CH4, the absolute adsorption capacity of gas decreases, but the relative adsorption capacity increases. The relative adsorption capacities of HF, CO2, H2S and SO2 increase by 35, 26, 15 and 12 times, respectively. During competitive adsorption, negatively charged atoms in HF, CO2, H2S and SO2 are more likely to bind with free cations on the surface of illite and smectite, and generate electrostatic repulsion for C atoms in CH4, leading to the weakening or even disappearance of CH4 adsorption capacity.

Lithofacies and Source Rock Quality of Organic-Rich Shales in the Cretaceous Qingshankou Formation, Songliao Basin, NE China

The organic-rich shale of the Qingshankou Formation (K2qn) is the most important target in the Songliao Basin. The relationship between lithofacies and source rock quality, however, is still controversial. Core observation, thin section identification, X-ray diffraction, organic geochemistry, and other analytical methods were adopted to investigate the petrology and its effects on hydrocarbon potential of the Qingshankou shale. Based on the differences in minerals, bioclastic, and fabric of laminae, four main lithofacies were defined as: (i) felsic shale (FS), (ii) clay shale (CS), (iii) bio-bearing shale (BS), and (iv) mixed shale (MS). The clay minerals content in the CS (average: 46.72 wt%) and MS (average: 41.11 wt%) was higher than that in FS (average: 39.97 wt%) and BS (average: 35.48 wt%). This classification allows the following comparative quantification of total organic carbon (TOC) content to be differentiated: CS > BS > MS > FS. Geochemical analysis indicated that the oil generation potential of the CS was the best, and the hydrocarbons generated from CS might migrate and accumulate in other lithofacies. All this knowledge could shed light on the lithofacies classification in shale systems with high clay mineral content, and may provide references for sweet spotting of the Qingshankou Formation in the Songliao Basin.

Quantitative Characterization for Pore Connectivity, Pore Wettability, and Shale Oil Mobility of Terrestrial Shale With Different Lithofacies—A Case Study of the Jurassic Lianggaoshan Formation in the Southeast Sichuan Basin of the Upper Yangtze Region in Southern China

Some major hydrocarbon-bearing basins are rich in shale with terrestrial facies in China, which may provide abundant terrestrial shale oil and gas resources. This work studied the Jurassic Lianggaoshan Formation in the Southeast Sichuan Basin of the upper Yangtze Region. Core samples were chosen for the total organic carbon content and mineral composition analyses to classify shale lithofacies. Afterward, pore connectivity, pore wettability, and shale oil mobility with different lithofacies were characterized by spontaneous imbibition, nuclear resonance, and centrifugation. Conclusions are as follows: the pore connectivity of organic-rich clay shale was mostly between moderate to good with oil-prone wettability and high mobile oil saturation. The organic-rich mixed shale has moderate to good pore connectivity, water-prone wettability, and the highest mobile oil saturation. Organic matter–bearing clay shale has bad to moderate pore connectivity. Meanwhile, its pore wettability covers oil wetting, mixed wetting, oil-prone wetting, and water-prone wetting. Its mobile oil saturation was moderate. Regarding organic matter–bearing mixed shale, the pore connectivity was bad to moderate with mixed-wetting pore wettability and moderate mobile oil saturation.

Study on the Wettability and Spontaneous Imbibition Characteristics of Lacustrine Shale

Wettability plays a significant role in the exploration and development of shale oil. The wettability affects the oil enrichment and restricts the selection of fracturing fluids. Shale is composed of complex minerals and organic matter. The pores composed of inorganic minerals have water wettability, while the pores composed of organic matter show the characteristics of oil wetting. The contact angle experiment and the spontaneous imbibition experiment are the most commonly used methods for characterizing wettability. The Qingshankou Formation in the Songliao Basin has thick source rocks, which is a favorable interval for shale oil exploration and development. Strengthening the wettability research in this area is of great significance for the exploration of shale oil. The wettability of different lithofacies shale in the northern Songliao Basin is seldom characterized, and there is a lack of comparative studies on contact angle and imbibition characteristics. In view of this situation, the shale of the Qingshankou Formation in the northern Songliao Basin has been classified. This article used the method of spontaneous imbibition combined with nuclear magnetic resonance to characterize the wettability of shale and analyze the influencing factors of the wettability of different shale lithofacies. Six samples with different lithological characteristics were used for this experiment. The study found that the imbibition results of samples with different lithofacies are different. The imbibition of sandy interlayer is less affected by the direction, while the imbibition of shale is more affected by the direction. The water imbibition volume of the sample is related to the content of clay minerals. The relationship of water imbibition volume in different lithofacies samples is as follows: low organic matter laminated siliceous shale > high organic matter massive clay shale > sandy interlayer > high organic matter laminated siliceous shale > high organic matter massive siliceous shale. Excessive content of clay minerals will cause shale to absorb water and expand and block pores, which is not conducive to further water imbibition by shale. The volume of oil imbibed is related to the organic carbon content. The relationship of oil imbibition volume in different lithofacies samples is as follows: high organic matter massive clay shale > high organic matter laminated siliceous shale > sandy interlayer > low organic matter laminated siliceous shale > high organic matter massive siliceous shale. The higher the total organic carbon content, the more developed the lipophilic pore network, and the more the volume of oil imbibed by the sample.

Using Waste from Oil Production in the Production of Lightweight Bricks

The possibility of using smeared soil from oil extraction and shale clay in the production of ceramic lightweight bricks without the use of natural traditional raw materials is shown. The suitability of the smeared soil containing an increased amount of organic matter and calorific value as a burnout additive for the production of lightweight bricks is investigated. The expediency of using interstitial clay (waste of combustible shale) as a binder for ceramic materials has been established. The resulting lightweight ceramic bricks have high physical and mechanical properties.

Investigation of subsurface connectivity and subsurface stormflow in low mountain ranges—Findings from the two research catchments Obere Brachtpe and Bohlmicke (Germany)

AbstractThe two small research catchments Obere Brachtpe (2.6 km2; 50.989986, 7.752013) and Bohlmicke (1 km2, 51.079319, 7.892988) are located in the Rhenish Massif, a low mountain range in Germany. Land use in both catchments is dominated by pasture land, spruce stands and mixed forests. Mean annual temperature is 9.1°C, and mean annual total precipitation is 1250 mm, with 15%–20% of the annual precipitation falling as snow. The geology is characterized by sandy silty clay shale from the Lower and Middle Devonian. Loamy Cambisols derived from periglacial slope deposits, complemented by Leptosols and Stagnosols, are the most prominent soils in the catchments. Long‐term hydrological datasets of precipitation, throughfall, discharge, groundwater levels and soil moisture (at different soil depths) in a high temporal and spatial resolution are available for further scientific analysis. Both catchments were monitored within the time period 1999 and 2009, in order to understand how the antecedent soil moisture, stratified soils (periglacial cover beds) and topography (slope form) impacted the subsurface connectivity, and the subsurface stormflow generation ‐ a dominant runoff generation process in humid mountainous catchments. Detailed physically based investigations on runoff processes were carried out, and the obtained results helped to better understand subsurface stormflow generation and subsurface connectivity dynamics. The process knowledge gained, which was presented at several conferences, as well as publications, was the basis for the discussion of open questions within the scientific network ‘Subsurface Stormflow ‐ A well‐recognized, but still challenging process in Catchment Hydrology’ (2016–2021), and the research unit ‘Fast and invisible: conquering subsurface stormflow through an interdisciplinary multisite approach’ (2022–2025), both financed by the German Research Foundation (DFG).

Variation in the brittle-ductile transition of Longmaxi shale in the Sichuan Basin, China: The significance for shale gas exploration

The Longmaxi Formation has abundant shale gas resources. High-yield wells have been obtained in medium-shallow and deep areas in the Sichuan Basin. Quantitative research on the brittle-ductile change in rocks is a basic geological component for caprock evaluation, and it can also be applied to shale gas exploration. This paper fully considers the difference between the lower shale (N1) and upper shale (N2) of the Longmaxi gas-bearing shale. Through total organic carbon (TOC), mineral components, field emission scanning electron microscopy (FE-SEM), and triaxial mechanics experiments, the two groups of shale samples, N1 and N2, were compared and analyzed, and the brittle-ductile transition was quantitatively studied. Based on previous work, a method for the fast calculation of the key over consolidation ratio (OCR) was proposed. The results show that N1 is mainly organic-rich siliceous shale, N2 is primarily organic-poor clay shale and mixed shale, and N1 is generally more brittle than N2. The key OCRs of N1 and N2 are 2.16 and 2.60, respectively. The key confining pressures at which N1 and N2 become fully ductile are 87.87 MPa and 70.28 MPa, respectively. Therefore, the key depths of the brittle zones of N1 and N2 (H b1 and H b2 ) and the key depths of the ductile zones of N1 and N2 (H d1 and H d2 ) are obtained. On this basis, a comprehensive model for the Longmaxi shale gas' brittle-ductile transition exploration zone in the Sichuan Basin and H b1 and H b2 maps of the Longmaxi gas-bearing shale have been established. Through comprehensive vertical and horizontal analysis, Longmaxi shale gas exploration risks in different blocks of the Sichuan Basin have been clarified. Therefore, this study of the variation in the brittle-ductile transition has an important guiding role in refining the exploration of Longmaxi shale gas in the Sichuan Basin. • There are obvious differences between the lower shale and the upper shale. • Difference of brittle-ductile transition makes shale gas exploration more complicated. • The new method can quickly calculate the threshold value of over consolidation ratio. • The new model provides more comprehensive guidance for shale gas exploration.

Evolution of excavation damaged zones in Opalinus Clay shale inferred from seismic investigations

We present the results of new and repeat gallery- and borehole-scale seismic experiments carried out in two excavation damaged zones (EDZs) of 10 (gallery 08) and 20 (gallery 98) years of age at the Mont Terri Underground Rock Laboratory in St. Ursanne, Switzerland. The initial geometry and rock mass properties of the EDZs were studied during and shortly after excavation—allowing for a direct comparison of measurements collected in 2018 to those completed in the past.In gallery 08 parts of the EDZ remain unchanged over ten years, whilst other zones show improvement (i.e., self-sealing) or considerable degradation. In addition, velocity tomograms from 2018 measure extensive areas of p-wave velocity reduction in comparison to the surrounding rock mass at depths shallower than 3.5 m. Isolated extensile fractures are still found in boreholes up to a maximum radial extent of 4.2 m. Data from borehole interval velocity measurements and cross-hole seismic surveys from this gallery suggest the damaged zone still extends up to 1.9 m and 3.1 m radial depth.Refraction seismic results in gallery 98, in contrast, suggest a less pervasive, shallower (<1.0 m) EDZ with a lower frequency of extensile fractures. The smaller circumference of this gallery can in part explain this; however, when compared to various measurements collected during and after excavation, some reduction in EDZ extent is inferable. Both borehole interval velocity measurements and seismic refraction data record p-wave velocity values higher on average than those in gallery 08 and without obvious areas of velocity reduction. Most interval velocity measurements likewise do not measure zones of decreased velocity (e.g., corresponding to rock mass damage) at this location. This suggests more self-sealing has occurred over the twenty-year post-excavation period in gallery 98 in comparison to the ten-year period in gallery 08. As repository safety depends upon the integrity of the rock mass surrounding an excavation, these results are relevant for mid-term assessments of safety (i.e., especially prior to repository closure).

Получение сейсмостойкого кирпича на основе вторичных энергетических ресурсов без применения природного традиционного сырья

Smeared soil from oil production with an increased content of calorific value is advisable to use not only as a thinning agent, but also as burnout additives for the production of ceramic materials. Obtained on the basis of large-tonnage waste: combustible shale – shale clay, oil and gas processing complex – smeared soil and nanotechnogenic waste of petrochemical production — catalyst IM - 2201 without the use of natural traditional materials earthquake-resistant brick with high physical and mechanical properties.

Middle Devonian Skały Formation in the Holy Cross Mountains (Poland) – formal description and subdivision based on new field data

The well-known fossiliferous and lithologically variable clay-carbonate series in the Łysogóry Region (northern part of the Holy Cross Mts, central Poland), enclosed between the Middle Devonian Amphipora dolomites and limestones (Kowala Formation) and siliciclastics (Świętomarz Beds), is defined formally as the Shaly-Calcareous Skały Formation. This Upper Eifelian to Middle Givetian, ca. 250–280 m thick unit, consists of marly and clay shales, interbedded many times with various limestone types (including encrinite and biohermal varieties), as well as with marls and siltstones. Its diagnostic feature is the presence of variable skeletal accumulations, formed by exceptionally numerous, well-preserved and diverse macrofauna (including brachiopods, corals, crinoids, bryozoans), described since the 19th century. The stratotype is located in the eastern slope of the Dobruchna stream near the Skały village and belongs to the Silurian to Upper Devonian Grzegorzowice-Skały section. Compared to the previously used term, Skały Beds sensu Pajchlowa (1957), the lower boundary is redefined, owing to a new exposure in the active Skała Quarry, and placed higher, at the base of the famous brachiopod shales (set XIV of Pajchlowa), instead of the formerly accepted lower boundary at the base of set XIII. Set XIV is formally distinguished as the Dobruchna Brachiopod Shale Member. The higher part of the Skały Fm (sets XV–XXVA) is not subdivided further, as the poorly exposed succession, including in particular the type area, precludes a more accurate recognition of lithological variability. The upper boundary of the Skały Fm is placed at the top of set XXV sensu Pajchlowa (1957), corresponding to the boundary between subsets XXVA and XXVB sensu Malec and Turnau (1997). A hypostratotype of the upper boundary is selected in the outcrop M0 at Miłoszów, 2.5 km westwards from the type section, allowing recognition of the diachroneity of lithological change defining the transition from the Skały Fm to Świętomarz Beds. A borehole situated in a key location would be an obvious next step in the further elucidation of the stratigraphic sequence of the Łysogóry Region.

Dynamic displacement of adsorbed methane by fracturing fluid during soaking in a shale gas reservoir based on low-field nuclear magnetic resonance

At present, the stimulation technique involving soaking after fracturing of shale gas reservoirs has received increasing attention. In this study, gas-water displacement experiments were carried out to simulate the contact state between the fracturing fluid and the shale during the soaking stage after hydraulic fracturing of the shale gas reservoir. By using low-field nuclear magnetic resonance to dynamically monitor the occurrence state of the methane in shale cores, the changes in the masses of the methane in different occurrence states were calculated. Furthermore, the displacement of the adsorbed methane in the shale by the fracturing fluid was studied. In addition, the influences of the fracturing fluid's salinity and the shale clay content and type on the displacement process were also analyzed. The results of this study show that the fracturing fluid has a stronger adsorption on clay minerals than that of methane, so it can replace the adsorbed methane in the shale. Compared with pure fracturing fluid solution, the high salinity fracturing fluid solution containing cations has a stronger adsorption on the clay minerals in the shale, and thus, it has a stronger displacement ability. The mineral composition of the shale reservoir also has an important influence on the displacement efficiency. The higher the content of clay minerals with high adsorption capacities, the more adsorbed methane is replaced. The results of this study are important for gaining a better understanding of the stimulation technique involving soaking after fracturing of shale gas reservoirs and for optimizing the design of soaking.

Swelling and swelling pressure of a clayey soil: Experimental data, model simulations and effects on slope stability

The behaviour of soils and rocks containing clay minerals is the effect of complex chemo-mechanical processes of interparticle interaction. This paper presents experimental results and model simulations of the influence of mineral composition and pore fluid composition on swelling and swelling pressure of clayey soils frequently affected by instability problems. The soil from a landslide in tectonized clay shales, characterized by wide ranges of plasticity and activity, was analysed. Soil fractions at different liquid limit wL were reconstituted with distilled water or salt solutions at different concentrations and they were compressed in oedometric conditions. The tendency to swell was induced by unloading or by exposure to a fluid different from the pore fluid. Swelling was permitted in a set of specimens and it was prevented in another set by applying increasing pressures. The temporal evolution of swelling and swelling pressure was monitored, and ion concentration in the pore fluid was evaluated by monitoring the cell fluid composition. The chemically induced swelling was very large in the most plastic soil and negligible in the least plastic. Swelling pressure too attained different values, however, it was also very large in the least plastic soil.The observed coupled chemo-mechanical processes were successfully modelled with the FEM method by using a user-defined UEL subroutine that was implemented in the commercial code ABAQUS. The subroutine takes account of the chemo-mechanical interactions in the solid skeleton, through a suitable constitutive model, and of the coupled fluxes in the porous space, in addition to water desorption/adsorption at clay platelets. In the proposed constitutive model, chemical interactions are assumed to affect both the plastic and the elastic response, as it was observed in experiments, thus an extended elasticity law is proposed. A simplified analysis, calibrated on the model results, shows that the role of swelling/swelling pressure on slope stability is significant even in the case of the least active soil. A decrease in pore solution concentration thus provokes two effects that are both negative for slope stability: a decrease of strength parameters and an increase of horizontal stresses.

Shale types and sedimentary environments of the Upper Ordovician Wufeng Formation-Member 1 of the Lower Silurian Longmaxi Formation in western Hubei Province, China

Abstract By performing scanning electron microscopy, microscopic observations, whole-rock X-ray diffraction analysis, organic geochemistry analysis, and elemental analysis on drill core specimens and thin sections, in this study, we classified the shale types of the Wufeng Formation-Member 1 of the Longmaxi Formation in western Hubei, southern China, and explored the development characteristics and formation environments of the different shale types. The results show that (1) the shales of the Wufeng Formation-Member 1 of the Longmaxi Formation are composed of three types of shale: siliceous shale, mixed clay-siliceous shale, and clay shale. The siliceous shale is a type of shale unique to deep-water environments; clay shale is the main type of shale formed in shallow-water environments; and mixed clay-siliceous shale falls between the two. (2) The changes in shale type are characterized by multiple depositional cycles in the vertical direction with strong heterogeneity and an obvious tripartite character, and the siliceous shales gradually thicken as they laterally extend northwestward, with their last depositional cycle gradually ending at a later time. (3) The Late Ordovician-Early Silurian paleoenvironment can be divided into six evolutionary stages (A, B, C, D, E, and F) from early to late. In particular, the sea level was relatively lower in stages A and F when the bottom water was mainly oxygen rich with higher terrigenous inputs and a lower paleoproductivity, which led to the formation of clay shales poor in organic matter but rich in terrigenous quartz clasts. The sea level was higher in stages B, C, and D when the bottom water was anoxic with lower terrigenous inputs and a higher paleoproductivity, which led to the formation of siliceous shales rich in organic matter and biogenic silica. The total organic carbon (TOC) contents of siliceous shales decrease in the order of stage C &gt; stage D &gt; stage B, which is mainly attributed to the different degrees of water restriction in the three stages and the consequently different paleoproductivities. Stage E corresponds to the mixed clay-siliceous shales, the depositional environment of which is between those of the siliceous shales and the clay shales, thereby resulting in the mineral composition and TOC content of the mixed clay-siliceous shales being between those of the other two shale types.

Quantitative Characterization for the Micronanopore Structures of Terrestrial Shales with Different Lithofacies Types: A Case Study of the Jurassic Lianggaoshan Formation in the Southeastern Sichuan Basin of the Yangtze Region

Due to the specificity of the geological background, terrestrial strata are widely distributed in the major hydrocarbon-bearing basins in China. In addition, terrestrial shales are generally featured with high thickness, multiple layers, high TOC content, ideal organic matter types, and moderate thermal evolution, laying a solid material foundation for hydrocarbon generation. However, the quantitative characterization study on their pore structure remains inadequate. In this study, core samples were selected from the Middle Jurassic Lianggaoshan Formation in the southeastern Sichuan Basin of the Upper Yangtze Region for analyses on its TOC content and mineral composition. Besides, experiments including oil washing, the adsorption/desorption of CO2 and nitrogen, and high-pressure mercury pressure experiments were carried out. The pore structure of different petrographic types of terrestrial shales can be accurately and quantitatively characterized with these works. The following conclusions were drawn: for organic-rich mixed shales and organic-rich clay shales, the TOC content is the highest; the pore volume, which is primarily provided by macropores and specific surface area, which is provided by mesopores, was the largest, thus providing more space for shale oil and gas reservation. The pores take on a shape either close to a parallel plate slit or close to or of an ink bottle. For organic-matter-bearing shales, both the pore volume and specific surface area are the second-largest and are provided by the same sized pores with organic-rich mixed shales. Its pores take on a shape approximating either a parallel plate slit or an ink bottle. Organic-matter-bearing mixed shales have the lowest pore volume and specific surface area; its pore volume is primarily provided by macropores, and the specific surface area by mesopores and the shape of the pores are close to an ink bottle.

Favorable lithofacies types and genesis of marine-continental transitional black shale: A case study of Permian Shanxi Formation in the eastern margin of Ordos Basin, NW China

Based on core description, thin section identification, X-ray diffraction analysis, scanning electron microscopy, low-temperature gas adsorption and high-pressure mercury intrusion porosimetry, the shale lithofacies of Shan23 sub-member of Permian Shanxi Formation in the east margin of Ordos Basin was systematically analyzed in this study. The Shan23 sub-member has six lithofacies, namely, low TOC clay shale (C-L), low TOC siliceous shale (S-L), medium TOC siliceous shale (S-M), medium TOC hybrid shale (M-M), high TOC siliceous shale (S-H), and high TOC clay shale (C-H). Among them, S-H is the best lithofacies, S-M and M-M are the second best. The C-L and C-H lithofacies, mainly found in the upper part of Shan23 sub-member, generally developed in tide-dominated delta facies; the S-L, S-M, S-H and M-M shales occurring in the lower part of Shan23 sub-member developed in tide-dominated estuarine bay facies. The S-H, S-M and M-M shales have good pore structure and largely organic matter pores and mineral interparticle pores, including interlayer pore in clay minerals, pyrite intercrystalline pore, and mineral dissolution pore. C-L and S-L shales have mainly mineral interparticle pores and clay mineral interlayer pores, and a small amount of organic matter pores, showing poorer pore structure. The C-H shale has organic micro-pores and a small number of interlayer fissures of clay minerals, showing good micro-pore structure, and poor meso-pore and macro-pore structure. The formation of favorable lithofacies is jointly controlled by depositional environment and diagenesis. Shallow bay-lagoon depositional environment is conducive to the formation of type II2 kerogen which can produce a large number of organic cellular pores. Besides, the rich biogenic silica is conducive to the preservation of primary pores and enhances the fracability of the shale reservoir.

Influence of different shut-in periods after fracturing on productivity of MFHW in Duvernay shale gas formation with high montmorillonite content

Drilling multi-stage fracturing horizontal wells (MFHWs) is the most effective way to unlock shale gas resources economically. This paper investigates the influence of different shut-in periods after fracturing, and their impact on the productivity of MFHWs in the Duvernay shale gas formation. This western Canadian formation has a high montmorillonite content which impacts the findings. A Duvernay shale core with high montmorillonite content was used to study gas–water relative permeability. Unsteady state tests were conducted under a variety of soaking times. The experiments demonstrated that the Permeability Jail for relative permeability curves exist in a water saturation range of 60% – 80%. This phenomenon partly explains the rapid gas production decline and a long period of little water production during the formal production in a typical shale gas well. Clay in shale mainly contains illite and montmorillonite. They have surface hydration, which occurs fast enough to cause induced fractures that improves the seepage capacity initially when the core is soaked. As the soaking time prolongs, there is an osmotic hydration expansion of the montmorillonite blocking pore throats and a corresponding reduction of the seepage capacity. The relative permeability increases during the soaking days one to four. After four soaking days, relative permeability begins to slowly decrease over time. Study calculations found that 61.4% – 75.6% of the total fracturing fluid filtrates into the formation. During days one to six of the soaking, the filtration volume increases significantly due to a high initial pressure in the fractures after pump shutdown. After seven days of soaking, the filtration velocity decreases due to the pressure drop and the difficulty of compression caused by more and more fluid in the formation. Based on reservoir simulations of 600 days, the relative permeability value in the near fracture zone and the cumulative gas production are the greatest on day four of the shut-in. The longer the shut-in period, the greater the initial water production rate and cumulative water production reaching a maximum at four shut-in days. In this Duvernay area with high montmorillonite content, the optimal shut-in period after fracturing was determined to be four days.

Classification and Disintegration Characteristics of the Carboniferous Rocks in Guangxi, China

To investigate the disintegration characteristics of the carbonaceous rocks in Guangxi Province, the typical carbonaceous rocks in the section exposed by the Hechi-Baise Expressway were investigated in this study. First, based on their mineral compositions and contents, the carbonaceous rocks were divided into four types: carbonate chert, carbonaceous argillaceous limestone, carbonaceous illite clay mudstone, and carbonaceous illite clay shale. Then, through indoor wet-dry cyclic disintegration test, the disintegration characteristics of the four types of rocks were studied. The test results showed the following: (1) the disintegration residues of the carbonate chert and the carbonaceous argillaceous limestone decrease linearly as the number of wetting-drying cycles increases. (2) The disintegration index of the carbonaceous illite clay mudstone and the carbonaceous illite clay shale decreases exponentially as the number of wetting-drying cycles increases. (3) As the number of wetting-drying cycles increases, the disintegration index curves of the carbonaceous illite clay mudstone and the carbonaceous illite clay shale samples gradually become stable until the disintegration of the samples is completed. (4) The disintegration of carbonaceous rocks is mainly affected by the clay content, followed by the structural form, but it cannot be ignored.