Intraparticle Pores Research Articles

Pore-scale simulations of electrical and elastic properties of shale samples based on multicomponent and multiscale digital rocks

The special intrinsic characteristics of shale formations, such as multiscale pore systems and complex mineral components, make the simulation of shale properties more difficult than that of sandstone properties. As such, one prerequisite of numerically predicting the physical properties of shale samples is to construct accurate shale models that include those special features. To this end, the study presents a novel hybrid stochastic modeling algorithm, called DEM-QSGSA, which integrates the discrete element method (DEM) and the quartet structure generation set algorithm (QSGSA), to build the necessary multicomponent and multiscale shale models. To investigate the effects of the components on the shale properties, the DEM-QSGSA was used to generate a dozen digital shale models with different mineral components containing quartz, feldspar, calcite, clay minerals, pyrite, and organic matter. The accuracy of the models was verified by comparing the characteristics of pore systems and the percentages of mineral components of the generated models with the ones of the SEM images of real shale samples. The electrical resistivity, bulk modulus, and shear modulus of these shale models were obtained using FEM. The results of electrical and elastic properties of these digital rocks indicate that the increase in abundance of the organic matter (OM) pores, intraparticle (intraP) pores, clay minerals, or OM results in a decrease in the electrical resistivity and elastic moduli when the pore systems of shale models are saturated with water. However, when the volume fraction of pyrite becomes larger, the elastic moduli increases and electrical resistivity decreases. Moreover, comparison of the sensitivity indices of the variables shows that pyrite has the largest effect on the electrical and elastic properties of shale samples, whilst clay minerals exert a moderate impact on them.

Characteristics and controlling factors of dolomite karst reservoirs of the Sinian Dengying Formation, central Sichuan Basin, southwestern China

The Sichuan Basin is a typical superimposed oil- and gas-bearing basin with abundant hydrocarbon resources in deeply buried marine carbonate rocks. The petrological characteristics, origin, diagenetic evolution and controlling factors of the high-quality Sinian Dengying dolomite reservoirs were investigated based on outcrop and core observations in addition to data from various analyses, including thin section, casting thin section, scanning electron microscopy, cathodoluminescence, carbon and oxygen isotope, major and trace elements, and X-ray diffraction analyses. The main lithologies of the Dengying reservoirs are microbial dolomite, crystalline dolomite, granular dolomite and karst breccia dolomite. The reservoir spaces include microbial pores, interparticle dissolution pores, intraparticle dissolution pores, intercrystalline pores, karst caves and fractures. Three models to explain the origin of Dengying dolomites are proposed: (1) syngenetic microbial dolomitization, (2) penecontemporaneous reflux dolomitization and (3) burial dolomitization. Development of the high-quality Dengying dolomite reservoirs in the central Sichuan paleo-uplift was essentially controlled by two factors: sedimentation and supergene karstification. High-quality reservoirs were developed mainly in algal mound and grain-shoal sediments deposited on the platform margin and intraplatform margin. Affected by the multistage Tongwan movement, long-term weathered crust karstification resulted in the formation of a large number of dissolution-enlarged pores and caves in the Dengying reservoirs. Multistage fractures, especially the unfilled tectonic fractures that formed during the late Yanshanian–Himalayan period, connected the relatively isolated pores and karst caves and effectively improved the permeability of the reservoirs. The results of this study provide a deeper understanding of the origin and preservation mechanism of deeply buried paleocarbonate reservoirs and can also effectively guide future exploration in the Sinian Dengying Formation of the Sichuan Basin.

A taphofacies interpretation of shell concentrations and their relationship with petrophysics: A case study of Barremian-Aptian coquinas in the Itapema Formation, Santos Basin-Brazil

The Itapema Formation in the Santos Basin is described as a carbonate coquina interval, a type of sedimentary rock composed mainly by shells and their fragments. These rocks have different textural characteristics related to depositional and diagenetic processes, which influence the pore system and consequently the permeability. Understanding how the different facies are distributed and their relationship with petrophysics is essential to preview the quality of a reservoir. In this study, we classify the shell beds of the Itapema Formation through taphonomic characteristics, applying the concept of taphofacies, where taphonomic patterns such as orientation of shells, degree of packing, fragmentation of shells, abrasion, rounding, and sorting (type of grains) are used to differentiate shell concentrations. Six taphofacies (Tf-1 to Tf-6) were described in a core interval of 12 m, which corresponds to the Itapema Formation. Taphofacies Tf-1 and Tf-2 are well-sorted and unsorted grainstones/rudstones with parallel-oriented and densely packed shells with a braded valves. Taphofacies Tf-3 and Tf-4 are well-sorted and unsorted grainstones/rudstones with oblique oriented shells, generally well-preserved but often with high shell dissolution. Taphofacies Tf-5 and Tf-6 are well-sorted and unsorted grainstones/rudstones with randomly oriented shells and densely packed deposits. In general, all the well-sorted taphofacies are composed mainly of shells and their fragments, do not show mud matrix or even shell fragments smaller than 0.2 mm, the size of valves varies from 0.5 to 5 mm and shell fragments are the smallest components. The unsorted taphofacies contain grains smaller than 0.2 mm, most of them peloids, and very small shell fragments. Through the analysis of thin sections, x-ray tomography (CT), high resolution tomography (H-CT) and Nuclear Magnetic Resonance (NMR) it was possible to quantify the porosity in the described taphofacies and classify them according to Choquette and Pray (1970) and Ahr (2008). In well-sorted taphofacies primary pores are preserved; the total porosity varies from 9% to 21% in thin sections, and mainly consists of interparticle, intraparticle, and moldic pores and vugs. The control of porosity varies from depositional to hybrid 1, according to Ahr (2008), and pores are generally connected. In unsorted taphofacies, preserved primary porosity is uncommon and most pores are moldic and vugs. The total porosity in this facies varies from 2 to 15%, although almost all values are between 10% and 15%. The control of porosity varies from hybrid 1 to diagenetic, based on Ahr (2008), and the connectivity varies. Despite these textural differences between well-sorted and unsorted taphofacies, in general, the porosity is good and varies between 10% and 25%. The permeability in well-sorted taphofacies varies from 0.3 to 8 D, the high values being associated with a connected porous system, whereas in unsorted taphofacies the permeability varies from 0.01 to 0.4 D due to the occurrence of isolated vugs and moldic pores. The well-sorted coquinas were interpreted as deposits influenced by storm-induced currents and waves where the energy was sufficient to winnow the finer grains and preserve the primary pores. The taphonomic characteristics indicate tempestites to shoreface deposits, whereas in unsorted taphofacies the energy was not sufficient to remove finer grains generally associated with distal tempestites and backshore deposits. • Shell concentration (coquinas) pore types and petrophysics. • Taphofacies interpretation and their relationship with porosity and permeability. • Porosity control in carbonate coquina deposits. • Permeability distribution in carbonate coquinas reservoir.

SEM, XRD and FTIR analyses of both ultrasonic and heat generated activated carbon black microstructures

The microstructures of the activated carbon black microparticles (ACBMPs) generated through both treatments of 20 min ultrasonic and 400 °C thermal energy equivalent have been analyzed properly using scanning electron microscope (SEM), X-ray diffraction (XRD) and Fourier-transformed infrared (FTIR) spectroscopy methods. The research was aiming to generate binding or active sites points on the outer surface of the ACBMPs body of which commonly plays an important role in both adsorption and catalytic processes. It was observed that around 150 nm up to 400 nm in average diameter super macro voids with many various turns of nano-scale wells, and around 1.84 angstrom (Å) up to 15.98 Å intraparticle pores were generated. In addition, the parallel planes spacing of the carbonaceous framework sheets, namely dhkl in Miller indexes terminology, of about 4.44 Å up to 2.98 Å constructed the inner particles of the ACBMPs body. A new nomenclature method for the binding or active site shapes identification and classifying them into four categories based on the quadrants terminology, i.e. quadrant one (Q1), two (Q2), three (Q3) and four (Q4) is proposed. Each the quadrants contains four categories of turns types, i.e. sharp, semi sharp, obtuse and non-significant turns depending on the angle of the associated turn in radian angle, θ. Finally, it can be concluded that the combination of ultrasonic and thermal energy treatments in fabricating ACBMPs could generate binding or active site points with unique shapes as a transit terminal for any guest molecules, in this context is methyl red (MR) molecules to enter into the suitable intra-particles pores of the ACBMPs body.

Insights into Pore Structure and Fractal Characteristics of the Lower Cambrian Niutitang Formation Shale on the Yangtze Platform, South China

Shales from the Lower Cambrian Niutitang Formation of Yangtze Platform have been widely investigated due to its shale gas potential. To better illustrate the pore structure and fractal characteristics of shale, a series of experiments were conducted on outcrop samples from the Lower Cambrian Niutitang Formation on Yangtze Platform, including X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and low-temperature nitrogen adsorption. Frenkel-Halsey-Hill (FHH) model was adopted to calculate the fractal dimensions. Furthermore, the relationships between fractal dimensions and pore structure parameters and mineral composition are discussed. FE-SEM observation results show that interparticle pores are most developed in shale, followed by intraparticle pores. This study identified the fractal dimensions D1 (ranging from 2.558 0 to 2.710 2) and D2 (ranging from 2.541 5 to 2.765 2). The pore structure of the Niutitang Formation shale is primarily controlled by quartz and clay content. Fractal dimensions are able to characterize the pore structure complexity of Niutitang Formation shale because D1 and D2 correlate well with average pore diameter and quartz content.

Petrographic characterization and petroleum potential of Mesozoic carbonate rock in Block 106, Northern Red river basin

Petroleum exploration and exploitation in Red River basin has been carried since the early 1960s of the 20th century, however until now its effectiveness has been still limited. Recently, the oil price is constantly changing so the efficiency of petroleum exploration and exploitation is particularly considered. Therefore, the assessment of petroleum potential and the direction of exploration are not only scientific research but also economic problem for developing countries in which there is Vietnam. The article considers that characteristic of carbonate petrography is along with intergration of interpreted seismic –stratigraphy and well logs, geochemistry analytic results of source rocks and related literatures as well. The purpose is to predict the petroleum potential of carbonate rock in block 106 and serve effectively in Petroleum exploration and exploitation in Red River basin Based on the analytic results, carbonate rock in the study area was impacted by tectonic activities such as mechanic compaction; dissolution forming fractures, stylolites; and post-depositional processes as recrystallization of minerals, creating vuggy, mouldic and intraparticle pores and dolomitization as well. Carbonate rock contains fossils as foraminifera, coral, algae, echinoderm with subordinate brachiopod, bryozoa. Most of them are mudstone, wackestone with mud-supported and packstone is made up of abundant fossils. Locally, carbonate rock was fractured and filled up by calcite and silic. Oil and gas traces have been discovered in Mesozoic carbonate rock, block 106, northern Red River basin. Fractured carbonate rock and weathered carbonate rock in the structures as A, C and E are oil fields. Oil migrates into traps that were early formed in fractured carbonate basement rock masses that were buried in pre-Kainozoi.

Prediction and experimental evaluation of soil-water retention behavior of skeletal calcareous soils

Skeletal calcareous soils refer to soils consisting of skeletal remains of marine organisms. Although calcareous deposits are prevalent along coastal plains throughout the world, there is no study which investigates their behavior in unsaturated conditions. In these conditions, soil-water retention capacity is a fundamental parameter in relation to many geotechnical, environmental, and agricultural aspects. Water retention capacity of a soil is expected to be influenced by intra-particle pores unique to calcareous soils. Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) used in the current study clearly revealed this special microstructural characteristic. To study water retention and hydraulic hysteresis behavior of skeletal calcareous soil (obtained from the Hormuz Island of Iran) compared with a reference silicate soil (obtained from the Netherlands), a series of tests were conducted using pressure plate and controlled-suction oedometer apparatuses. Test results showed that in similar grain-size distribution of these soils, water retention curves of calcareous and silica soils are almost the same in low suctions, but calcareous soil retains more water at higher suctions due to its intra-particle voids. In addition, owing to these microstructural pores, ratio of Sr reduction on wetting path of a hysteresis loop was found to be more for calcareous soil than for silicate soil. Besides, to find and/or modify a prediction method for water retention curve of calcareous soil with acceptable accuracy, several existing pedotransfer functions were evaluated. Results showed that models proposed by Zapata et al. (Geotech Spec Publ 99:84–124, 2000) and Saxton et al. (Soil Science Society of America Journal 50(4): 1031–1036, 1986) after some simple modifications provide accurate estimations for water retention curve of calcareous soils.

Pore‐Scale 3D Dynamic Modeling and Characterization of Shale Samples: Considering the Effects of Thermal Maturation

AbstractThe development of shale gas necessitates accurate modeling and characterization of these complicated formations, at both small and large scales. At the small scale, the studies based on experimental techniques have uncovered that the components and pore types in shales vary dramatically. Several pore types are identified and varied when the thermal maturation of shales changes. Therefore, accurate modeling and characterization of shale samples require taking such dynamic variations into consideration. This study presents a novel, dynamic, and three‐dimensional (3D) modeling technique considering pore‐system variation when the thermal maturation changes. The technique can construct dynamic shale models based on quartet structure generation set algorithm and morphological operation. This method can include various elements available in the shale samples in a very accurate way when the dynamic processes are reproduced. To evaluate the performance of the presented technique, 12 dynamic 3D shale models of three cases are constructed. These models are then characterized by analyzing the fractions of components and pores, pore and throat size distributions, coordination number distribution, fractal dimension, and tortuosity. Besides, gas transport in these dynamic 3D shale models is also simulated using pore network modeling to demonstrate the permeability variation of these models. Moreover, the fractions of interparticle and intraparticle pores and cementation degree are changed to further illustrate the capability of the developed algorithm. This study reveals such a dynamic modeling technique is a robust tool to construct various porous media with complicated elements and pores, which is not limited to shale samples.

Safe Li-ion batteries enabled by completely inorganic electrode-coated silicalite separators

The high wettability and intra-particle pores of the silicalite separator homogenize the Li-ion availability for the viscous LiFSI/TMP electrolyte. This results in a completely safe, scalable, thermally stable, and high-rate capable battery.

Porosity characteristics of different lithofacies in marine shale: A case study of Neoproterozoic Sinian Doushantuo formation in Yichang area, China

This study evaluates the pore network systems in the Neoproterozoic Sinian Doushantuo (Z1d) shale from the Yichang area in the Middle Yangtze region. All samples were classified based on their lithologies and examined by geochemical and petrographic analysis, assisted with carbon dioxide/nitrogen (CO2/N2) adsorption, mercury injection capillary pressure (MICP) and focused ion beam-scanning electron microscopy (FIB-SEM) to fully characterize the nanoscale porosity of the different shale lithofacies. The calcareous shale lithofacies group (I-C), mixed shale lithofacies group (II-M) and siliceous shale lithofacies group (III-S) were identified via ternary mineralogy diagrams. Clearly, different lithofacies had different porosities, pore volume, surface area and pore-size distribution (ranging from 0.3 nm to 36 μm). The mixed shale lithofacies group (II-M) had the highest total pore volumes, followed by the calcareous shale lithofacies group (I-C) and the siliceous shale lithofacies group (III-S). Peak values (2–20 nm, 20–50 nm, 50–60 nm and 90–200 nm) obtained from N2 adsorption were identified as organic matter (OM) pores, dissolution intraparticle (intraP) pores and intercrystalline pores (using SEM images analysis). Porosity shows a non-monotonic trend with TOC content and a maximum at 4.5–5.5 wt% TOC, similar relationships between TOC and mesopore/macropore volumes, due to pore collapse and compaction. The FIB-SEM images revealed solid bitumen and two types of kerogen in the examined shale samples; these different organic matter types had clearly distinct pore characteristics. While the smaller OM particles mobilized and converted into solid bitumen in which OM pores were larger and more abundant, the large-scale OM particles were most likely non-porous inert maceral and sponge-like porous amorphous kerogen. Overall TOC content and organic types are the key controlling factors to the nanoscale porosity development in the Doushantuo Shale, and the Sinian Doushantuo shale (in the Yichang area) has great potential for shale gas exploration and exploitation in South China.

Process-based and dynamic 2D modeling of shale samples: Considering the geology and pore-system evolution

Accurate characterization of shale samples is of great importance for predicting and evaluating their intrinsic properties as they undergo various geological processes. As such, constructing shale models not only entails accounting for the numerous components and pores, but also requires considering the effects of thermal maturation on the evolutions of the pore systems and components. In this study, a new dynamic and process-based modeling for considering the geological evolutions occurred in pore systems is developed, which makes a significant improvement in reproducing the complex geological features in shale samples. Three types of pores, including organic matter (OM), interparticle (interP), and intraparticle (intraP) pores, and several other components including kerogen, pyrite, clay minerals, globigerinids, calcite, feldspar, quartz, and petroleum products, are all considered in our process-based modeling. In order to demonstrate the performance of our process-based method, two-dimensional (2D) dynamic shale models of multiple cases with different geological processes are constructed. Then, to characterize the constructed samples, the occurrences and distributions of all components are studied using a two-point correlation function. Aside from statistical comparisons, the gas flow in these shale models is also simulated to exhibit the permeability variations produced in the process-based models. Moreover, the degree of cementation, the fractions of interP, intraP, and OM pores are varied to mimic new geological scenarios, which aims to further evaluate the performance of the proposed technique. The results indicate that our process-based model presents an excellent performance. Furthermore, the presented modeling technique can be extended to investigating the effects of various components and pores on the intrinsic properties of shales in the future.

Broad ion beam-scanning electron microscopy pore microstructure and multifractal characterization of shale oil reservoir: A case sample from Dongying Sag, Bohai Bay Basin, China

Pore structure and its heterogeneity are critical factors controlling the storage capacity and transportation properties of hydrocarbons. Broad ion-beam-milling scanning-electron microscopy allows for the study of a larger planar at high resolution than other methods and can provide insight into shale microstructures. In this study, we investigate the microscopic pore structure of a shale oil reservoir sample from Paleogene Shahejie Formation in Dongying Sag, Bohai Bay Basin, based on the broad ion-beam cross-section, and discuss the heterogeneity of the major pores using multifractal theory. The representative elementary area of the sample was first inferred to be ∼100 × 100 µm2 (25 single images) for the broad ion-beam cross-section with an area of 1.054 × 0.915 mm2. Five pore types (interparticle, intraparticle clay, dissolution, inter-crystalline, and organic) were subsequently identified and analyzed in the selected typical representative elementary area. The results showed that interparticle, intraparticle clay, and dissolution pores were the major pore types and made a significant contribution to the total visible surface porosity (98.34%), whereas inter-crystalline and organic pores were not of great importance. Interparticle pores exhibited the most complex pore morphologies, the largest average pore diameter, and the simplest pore structure. Moreover, interparticle pores that were sub-parallel to the bedding plane showed the best connectivity. Intraparticle clay pores, on the other hand, had the smallest average pore diameter, the most complex pore structure, and their distribution in a two-dimensional plane was the most homogeneous. Dissolution pores were characterized by the least complex pore morphologies but more heterogeneous pore distribution. Both intraparticle clay and dissolution pores were abundant but possessed poor connectivity. We conclude that for shale oil storage and transportation in the Dongying Sag, interparticle pores play an important role in shale oil seepage, whereas intraparticle clay and dissolution pores provide the main space for the occurrence of shale oil.

Pore structure characteristics and its effect on adsorption capacity of Niutitang marine shale in Sangzhi block, southern China

Characteristics of shale pore structures may play an important role in natural gas accumulation and consequently estimating the original gas in place. To determine the pore structure characteristics of Niutitang marine shale in the Sangzhi block, we carried out [Formula: see text] adsorption-desorption (LP-[Formula: see text]GA), [Formula: see text] adsorption (LP-[Formula: see text]GA), and methane isothermal adsorption on shale samples to reveal the pore size distribution (PSD) and its impact on the adsorption capacity. Results indicate that the Niutitang Shale is in stages of maturity and overmaturity with good organic matter, and they also indicate well-developed interparticle, intraparticle, and organic pores. Quartz and clay are found to be the main minerals, and the high illite content means that the Niutitang Shale is experiencing the later stage of clay mineral transformation. Various-sized shale pores are well-developed, and most of them are narrow and slit-like. For pores with diameters of 2–300 nm measured with LP-[Formula: see text]GA, mesopores (2–50 nm) contribute most of the total specific surface area (SSA) and total pore volume (TPV) in comparison to macropores (50–300 nm). For micropores ([Formula: see text]) tested by LP-[Formula: see text]GA, the PSD appears to be multimodal; shale pores of 0.50–0.90 nm diameter contribute most of the SSA and TPV. [Formula: see text]-SSA and [Formula: see text]-SSA indicate positive correlations with their corresponding TPV. The total organic matter (TOC) has good correlation with the SSA and TPV of micropores. The Langmuir volume positively correlates with the total SSA. Additionally, the TOC content has a good correlation with the Langmuir volume, which is consistent with the observation of well-developed fossils of diatoms and organic pores. As an important source of organic matter, more diatoms mean more organic matter, larger TOC values and quartz content, larger SSA and TPV of micropores, and, of course, stronger shale adsorption capacity. The results provide important guidance for the exploration and development of shale gas existing in the Sangzhi block.

Organic geochemical and petrophysical characteristics of saline lacustrine shale in the Dongpu Depression, Bohai Bay Basin, China: Implications for Es3 hydrocarbon exploration

Saline lacustrine shales are widely developed in the Es3 strata of the Dongpu Depression. Recently, a high-yield oil flow from the upper sub-member of Es3 (Es3U) of the PS18-1 well first confirmed the presence of shale reservoirs. However, the detailed difference of geochemistry, hydrocarbon generation potential, and reservoir quality between the upper (Es3U), middle (Es3M), and lower (Es3L) sub-members of Es3 saline shales remained unclarified. Both geochemical and petrophysical analyses were performed to answer above questions in this study. The results indicate that compared with Es3U and Es3M shales, Es3L shale has a higher TOC content (mean 1.94%). Oil-prone type I-II kerogen is the main kerogen type of Es3U and Es3L shales, while Es3M shale is dominated by type II-III kerogen. Es3L shale shows a relatively higher original hydrocarbon generation potential (mean 10.26 mg HC/g Rock) with an average original total organic carbon (TOCo) value of 1.62%, and a higher generated hydrocarbon (QHC) than other two sets of shales. Compared with the dual organic matter source of marine and terrestrial organisms of Es3M shales, the main organic matter of Es3U and Es3L shales derived from aquatic organisms, and developed in saline water and brackish–saline water, respectively, with strong reducing environment. Organic-matter pores are undeveloped in Es3 shales, whereas interparticle and intraparticle pores are easily to be found. Higher brittle minerals are found in Es3U and Es3L shales, which are beneficial to form fracturing fractures during the development process, and widely developed bedding plane fractures can provide good conditions for hydrocarbon migration. Based on discussion, Es3L shales with present-day TOC over 1.5%, higher brittle minerals and gypsum salt around can be considered to be the most promising area for further shale oil exploration in the northern Dongpu Depression.

Full-Scale Pore Structure and Fractal Dimension of the Longmaxi Shale from the Southern Sichuan Basin: Investigations Using FE-SEM, Gas Adsorption and Mercury Intrusion Porosimetry

Pore structure determines the gas occurrence and storage properties of gas shale and is a vital element for reservoir evaluation and shale gas resources assessment. Field emission scanning electron microscopy (FE-SEM), high-pressure mercury intrusion porosimetry (HMIP), and low-pressure N2/CO2 adsorption were used to qualitatively and quantitatively characterize full-scale pore structure of Longmaxi (LM) shale from the southern Sichuan Basin. Fractal dimension and its controlling factors were also discussed in our study. Longmaxi shale mainly developed organic matter (OM) pores, interparticle pores, intraparticle pores, and microfracture, of which the OM pores dominated the pore system. The pore diameters are mainly distributed in the ranges of 0.4–0.7 nm, 2–20 nm and 40–200 μm. Micro-, meso- and macropores contribute 24%, 57% and 19% of the total pore volume (PV), respectively, and 64.5%, 34.6%, and 0.9% of the total specific surface area (SSA). Organic matter and clay minerals have a positive contribution to pore development. While high brittle mineral content can inhibit shale pore development. The fractal dimensions D1 and D2 which represents the roughness of the shale surface and irregularity of the space structure, respectively, are calculated based on N2 desorption data. The value of D1 is in the range of 2.6480–2.7334 (average of 2.6857), D2 is in the range of 2.8924–2.9439 (average of 2.9229), which indicates that Longmaxi shales have a rather irregular pore morphology as well as complex pore structure. Both PV and SSA positively correlated with fractal dimensions D1 and D2. The fractal dimension D1 decreases with increasing average pore diameter, while D2 is on the contrary. These results suggest that the small pores have a higher roughness surface, while the larger pores have a more complex spatial structure. The fractal dimensions of shale are jointly controlled by OM, clays and brittle minerals. The TOC content is the key factor which has a positive correlation with the fractal dimension. Clay minerals have a negative influence on fractal dimension D1, and positive influence D2, while brittle minerals show an opposite effect compared with clay minerals.

Characterization and identification of bioturbation-associated high permeability zones in carbonate reservoirs of Upper Cretaceous Khasib Formation, AD oilfield, central Mesopotamian Basin, Iraq

Abstract High-permeability zones (HPZ) are common occurrences in carbonate reservoirs of the Middle East and act to improve productivity in primary depletion but lead to water breakthrough in secondary depletion. Bioturbation aggravated the reservoir heterogeneity of the Khasib Formation carbonates in AD oilfield, Iraq, resulting in the development of HPZ. The observations of cores and thin sections, measurements of porosity, permeability, capillary pressure and probe permeability were conducted to investigate the characteristics and genesis of the HPZ. Well log responses of the HPZ in cored wells were analysed to identify the HPZ in uncored wells and predict its distribution. Results demonstrate that the HPZ is demarcated by Glossifungites ichnofacies and Thalassinoides ichnofabric. The burrow systems filled passively with overlying sediments that were apparently different from the unbioturbated matrix, in terms of the lithologic associations, pores types, and pore connectivity. The burrow systems, which contribute to the high permeability of the HPZ, were merely composed of grainstone with well-connected interparticle pores and vugs. The matrix comprises packstone and hardground-associated strongly cemented grainstone, which developed some fractures and exhibited poorly connected pores, including intraparticle, moldic, and residual interparticle pores. The arithmetic mean permeability of the HPZ is 241.0 mD, which is much higher than that of the non-HPZ (17.3 mD). The entry pressure and pore throat radius of the burrows in HPZ is 0.02–0.05 MPa and 5–20 μm, respectively, which reveal the high-quality pore structure of the burrow systems. There is strong heterogeneity within the HPZ. The permeability of burrow systems could reach up to 10133.5 mD with an arithmetic mean permeability of 712.2 mD, while the permeability of the matrix is only 0.05 to 69.29 mD, with an arithmetic mean permeability of 8.7 mD. Interconnected burrow networks in the HPZ were formed by bioturbation. The efficiency of connectivity of the HPZ was further enhanced by the early dissolution occurred within burrow architecture and matrix in the meteoric freshwater environment. These resulted in the formation of the high-permeable network composing of interparticle pores, vugs, and the dissolution-enlarged fractures. The high-permeable burrow systems and low-permeable matrix jointly contributed to the distinct well log responses, which are markedly different from the conventional shoal HPZ. The bioturbated HPZ can be identified by medium natural gamma (GR), high bulk density value (RHOB), low acoustic transit-time value (DT), low compensated neutron (NPHI), high deep induction value (RILD), and high microspherically focused resistivity value (MSFL). A well log-model based three cutoffs, namely RHOBnor/DTnor, RILD*MSFL and GR, was built and shows good prediction of the HPZ.

Study of subcritical and supercritical gas adsorption behavior in different nanopore systems in shale using lattice Boltzmann method

Adsorption has been studied for decades in either petroleum or chemical industry. However, the detailed adsorption behavior is yet not well understood in nanoporous media with complex pore structures and surface properties, especially for subcritical gases when capillary condensation might occur. Shale (mudrock) has complex pore systems as a result of heterogeneous mineralogy and diagenesis, and interparticle (interP) and intraparticle (intraP) pores are two main components. Understanding gas adsorption and phase transition behavior in different pore systems is essential for petrophysical characterization, reserve estimation and production forecast. In this study, we use lattice Boltzmann method (LBM) to study nitrogen (subcritical) and methane (supercritical) adsorption behavior in reconstructed shale nanopore systems. The model accounts for the van der Walls (vdW) forces between fluid molecules by incorporating a modified pseudopotential model based on Peng-Robinson equation of state (PR-EOS). Rock-fluid interaction is also considered by introducing phenomenological surface forces, which take two different forms for subcritical and supercritical gases respectively. The correlation between parameters defined in LBM and physical properties (i.e. density, pressure, isosteric heat of adsorption) is established systematically. Using the developed model, we observe quantitative agreement between LBM and lattice density functional theory (LDFT)/experimental data in terms of adsorption isotherms and density distributions. We then compare adsorption behavior in two ‘model’ interP and intraP pore systems and a synthesized polyethylene porous medium with more complex pore shapes. We observe three adsorption stages for subcritical gas adsorption: a) early capillary condensation near grain contacts or sharp corners, forming isolated pendular rings or fluid clusters; b) growing, merging, and spreading of the isolated condensed phase, forming circular gas/liquid meniscus; c) the condensed phase becomes fully-connected across the geometry, leaving isolated gas bubbles. We observe a signature behavior for the adsorption isotherm of intraP pore system, where a stepped feature occurs around relative pressure of 0.8, and the adsorption uptake for the polyethylene porous medium is bounded by the upper and lower limits given by the interP and intraP pore systems. On the other hand, we find that supercritical methane adsorption in mesoporous media is less sensitive to pore shape, but rather controlled by the specific surface area. The existence of micropores also affects the adsorption uptake.

Fabrication of Maghemite Nanoparticles with High Surface Area.

Maghemite nanoparticles with high surface area were obtained from the dehydroxylation of lepidocrocite prismatic nanoparticles. The synthesis pathway from the precursor to the porous maghemite nanoparticles is inexpensive, simple and gives high surface area values for both lepidocrocite and maghemite. The obtained maghemite nanoparticles contained intraparticle and interparticle pores with a surface area ca. 30 × 103 m2/mol, with pore volumes in the order of 70 cm3/mol. Both the surface area and pore volume depended on the heating rate and annealing temperature, with the highest value near the transformation temperature (180–250 °C). Following the transformation, in situ X-ray diffraction (XRD) allowed us to observe the temporal decoupling of the decomposition of lepidocrocite and the growth of maghemite. The combination of high-angle annular dark-field imaging using scanning transmission electron microscopy (HAADF-STEM) and surface adsorption isotherms is a powerful approach for the characterization of nanomaterials with high surface area and porosity.

Fluid distribution and gas adsorption behaviors in over-mature shales in southern China

To better understand the mechanism of hydrocarbon accumulation and fluid migration in Chinese shales, several samples from two leading marine shales (Longmaxi and Niutitang) were collected for geochemical and mineral analysis, the characterization of pore structure, preliminary studies on fluid flow, and the distribution and investigation of different methane adsorption behaviors. Firstly, the Longmaxi and Niutitang marine shales were found to be in an over-matured stage and dry gas window. Pore structures and connectivity were characterized by field emission - scanning electron microscopy (FE-SEM). The SEM images showed that the micro-fracture, interparticle pores, and organic pores had better connectivity than intraparticle pores. Results from contact angles indicated that the Longmaxi and Niutitang shales tended to have higher affinities for oil rather than for water. The connectivity of pores, distribution of fluids (water and gas) and fluid flow behaviors in the over-matured shale were analyzed using spontaneous imbibition (SI) and tracer diffusion tests. It was found the tracer fluids flow in the way of network flow inside of shale matrix. The connected pore network may be the main path for fluid migration in shale. Additionally, a gravimetric isothermal adsorption experiment was applied to assess methane adsorption capacity in shale. Shale samples with higher total organic carbon (TOC) contents had higher methane adsorption capacities. Organic matter (including kerogen and bitumen), had much greater adsorption capabilities than inorganic minerals (quartz, clay, and others). Finally, the fluid distribution patterns in over-matured marine shale were studied. Shale gas was originally generated in organic matter and then migrated out through connected pores, under differential pressure, between internal and external organic matter hydrocarbon generation areas. These hydrocarbons replaced the water in the pores. Therefore, shale gas may adsorb in organic pores and is freely stored in connected inorganic pores, with bound water, whereas isolated pores are saturated with water. Two different adsorption patterns on mineral surfaces (organic matters and inorganic minerals) were established to explain the different shale gas adsorption behaviors in organic pores and connected pores, with bound water.

A model for porosity evolution in shale reservoirs: An example from the Upper Devonian Duvernay Formation, Western Canada Sedimentary Basin

The influence of thermal maturity on porosity in shale samples from the Upper Devonian Duvernay Formation is examined. The samples span a maturity range from immature to the wet gas window. Porosity decreases from immature to the oil window, primarily because of compaction. Relatively high porosity of wet gas window samples is ascribed to formation of secondary organic pores, feldspar dissolution pores, and primary pore preservation by the quartz framework. The final decline in the porosity of the dry gas window samples is explained by greater compaction, the disappearance of secondary organic pores, and feldspar dissolution pores. Porosity correlates positively to quartz content and negatively to carbonate content; no relationship was evident between porosity and clay or total organic carbon content. No obvious correlations exist between rock composition and permeability except that SiO2 content shows a weakly positive correlation to permeability. Permeability is highest in immature samples, which have the greatest pore and pore-throat sizes. Nitrogen adsorption and mercury injection analysis show that pore and pore-throat sizes decrease with increasing maturity. Visible pores, imaged by scanning electron microscopy and helium ion microscopy, exist as organic pores, including bubblelike pores developed within organic matter (OM) and fissure-type pores, intraparticle pores mainly developed within carbonate grains, and interparticle pores either within a clay-rich matrix or between rigid mineral grains. In immature samples, the primary pores are interparticle pores between clay minerals and other mineral grains. The OM fissures are ubiquitous in oil window samples, and secondary bubblelike OM–hosted pores are well developed within gas window samples.