Compact Sandstone Research Articles

ВИЗНАЧЕННЯ ЕФЕКТИВНОЇ ПОРИСТОСТІ ПОРІД-КОЛЕКТОРІВ ЗА ДАНИМИ СТРУКТУРИ ЇХ ПУСТОТНОГО ПРОСТОРУ У ВІЗЕЙСЬКИХ І ТУРНЕЙСЬКИХ ВІДКЛАДАХ БЕРЕЗІВСЬКОГО РОДОВИЩА

Background. Currently, it is quite important to track the patterns of the internal structure of reservoirs with their physical properties. The analysis of well-logging data in the well 203 of the Berezivske field of the Dnipro-Donetsk depression is necessary to assess the prospectivity of complexly structured reservoirs. Methods. The methodology for studying the void space structure in complexly structured reservoirs involved several stages: interpretation of well logging data; determination of parameters for the initial mathematical model of the reservoir; inversion of data from acoustic surveys into a curve of the distribution of different void formats; evaluation of reservoir rock types with determination of their effective porosity. Results. Based on the data of compressional and shear wave velocities, as well as density, three distinct rock groups were identified: compacted sandstone, calcareous sandstone and limestone. The initial approximation was obtained for each rock sample, including the set of void formats and their concentration within the reservoir layers. Direct analysis of the resulting data by the authors revealed that the most optimal void formats for the investigated intervals are: for intergranular voids – 0.07 to 0.9; for transitional voids and microcracks – 0.05 to 0.077; for microcracks – 0.007 to 0.0019; for caverns – 4 to 8. Based on the research results, it has been established that granular-cavernous porosity (39,2 %) and fracturedcavernous-granular porosity (29,5 %) are prevailing types of reservoirs. Conclusions. The calculated values of the void formats were used for these formations to quantitatively assess effective porosity based on the obtained data and specific electrical resistivity values at different depths of investigation. The resulting approximation of α = 0.6996 was selected, which reduced the error in calculating the effective porosity of reservoir rocks. The obtained results have demonstrated that the proposed methodologies are essential for understanding and quantitatively assessing the void space structure and filtration-capacitive properties of reservoir rocks, particularly when we are facing with a limited well logging data and restrictive core sampling.

Experimental analysis and elastoplastic damage modeling the mechanical properties of compact sandstone

Experimental analysis and elastoplastic damage modeling the mechanical properties of compact sandstone

Experimental Study of Compact Sandstone Deformation Under Axisymmetric Triaxial Loading Along Specific Paths in Stress Space

The deformation of rocks well below their ultimate strengths is frequently described through the constant stiffness moduli of the linear elasticity theory. In addition, the isotropic material approximation is usually used as the basic approach. Particularly in sedimentary rocks, local inhomogeneities, anisotropy, and irreversible processes, mainly related to the gradual breakage of grain joints, can affect their deformation behavior from the beginning of the loading. Consequently, the stiffness moduli measured under triaxial conditions are effective parameters, which may depend on the loading path in the stress space. Therefore, a complex experimental study of the deformation response of compact sedimentary rock (Brenna sandstone) along various paths in triaxial stress space was prepared to understand this dependence. In addition, a detailed analysis of the rock composition and structure of this compact sandstone was carried out. This paper presents an initial experimental study that is based on a loading method using alternative stress paths that correspond to different modes of the monotonic increase in differential stress from an initial isotropic compression state. In the experiment, the dependence of stiffness moduli on the loading path was found. Differences in rock deformation for conventional triaxial compression and extension can be attributed to a slight rock anisotropy originating from the stratification. The different behavior of deformation and related stiffness moduli along so-called reduced triaxial paths and conventional paths indicates that irreversible processes must be taken into account.

Analysis of the Hydromechanical Properties of Compact Sandstone and Engineering Application

The paper proposes a method to simulate the mechanical behavior of compact rock considering hydromechanics by combining physical experiments and numerical analysis. The effectiveness of the constructed method is validated by the comparison between the numerical and physical results of triaxial shear experiments on sandstone in seepage conditions. Based on the validated method, the stability of underground water-sealed oil and gas storage caverns in surrounding compact sandstone during excavation is analyzed. The main findings are as follows: The intrinsic permeability of compact sandstone has a power function relationship with the porosity; the combination of the porous media elastic model and the modified Drucker–Prager plasticity model can preciously represent the mechanical properties of compact sandstone; the proposed method can accurately replicate the hydromechanical response of compact sandstone in seepage conditions; the effects of hydromechanical effects have significant impacts on the stability of surround compact sandstone during the excavation of underground water sealed oil and gas storage caverns, which causes the obvious increase in stress, deformation and plastic deformation zones of the surrounding compact sandstone and remarkable decrease in the stability safety factor.

Літогеохімія чорних аргілітів фанерозою Заходу України ‒ нетрадиційних колекторів вуглеводнів

The purpose of this work is to compare the lithological, geochemical and mineralogical features of carbonaceous clayey and siliceous-clay rocks of Cretaceous-Paleogene flysch of the Ukrainian Carpathians and Lower Silurian of the Volyn-Podillya edge of the East European Platform and to determine the factors that contributed to the formation of zones of “unconventional reservoirs” in these sedimentary strata of cracked and mixed types. Data from the lithology, geochemistry and mineralogy of bituminous siliceous-clay rocks and siliceous rocks of Lower Cretaceous and Oligocene of the Carpathians and black argillites of Lower Silurian of the Volyn-Podillya edge of the East European Platform are presented. Sedimentogenesis of Lower Cretaceous and Oligocene bituminous deposits of the Carpathians and Lower Silurian deposits of Volyn-Podillya took place in anoxic conditions (phases of oceanic anoxic events: OAE-1 (Barrem–Albian), OAE-4 (Oligocene) and at the border of Ordovician and Silurian. Paleoceanographic conditions of their sedimentation differed significantly. The first are deep-sea formations at the foot of the continental slope of the Carpathian segment of the Tethys Ocean, the second were accumulated in the warm shallow sea on the eastern shelf of the West European Sea Basin. The layered texture of carbonaceous deposits, as well as the catagenetic transformation of rock-forming clay and siliceous minerals and their hydrophobization, played a significant role in the formation of the filtration capacity properties of “unconventional reservoirs”. In Cretaceous-Paleogene flysch deposits of the Carpathians, “unconventional reservoirs” are usually terrigenous-clay or siliceous-clay rocks with shale and layered texture or compacted sandstones localized in conventional oil, gas or condensate deposits. Lower Silurian clay deposits of Volyn-Podillya are promising for the search for “shale gas”.

Petrography and diagenetic evolution of the Barail sandstones of Naga Schuppen belt, North East India: implication towards reservoir quality

Rock-thin section, scanning electron microscopy and X-ray diffraction analyses have been employed to describe in detail the mineralogical constituents, diagenetic alterations and their impact on reservoir quality of Oligocene Barail sandstones of Naga Schuppen belt, North East India. The Barail Group comprises of alternate beds of hard and compact sandstones with siltstone, shale, carbonaceous shale, and a few thin intermittent coal seams in the upper part of the rock sequence. Petrographic analysis indicates that quartz (42.02–55.02%) is the most dominant mineral constituent followed by rock fragments (6.85–15.67%) and feldspars (0.00–1.97%) with different types of cement in the studied sandstones. Quartz overgrowth, formation of pseudo matrix, authigenic growth of secondary mine­rals and precipitation of clay within the pore spaces tend to reduce the primary and secondary porosities of the rocks. However, in certain samples, the grain coating restricts or hinders cementation and preserves porosity during deep burial, but decreases permeability at pore throats. Partial dissolution and intragranular fracturing of the framework minerals provide sites for pore growth. Pyrite framboids and iron oxides inhibit quartz cementation, but infill pore spaces. The present study shows that original pore morphologies, as well as secondary porosities within the sandstones tend to be destroyed to a large extent by the diagenetic processes

Long lasting habitable periods in Gale crater constrained by glauconitic clays.

In situ investigations by the Mars Science Laboratory, Curiosity rover, have confirmed the presence of an ancient lake in Gale crater for up to 10 million years. The lake was filled with sediments that eventually converted to a compacted sandstone. However, it remains unclear whether the infilling of the lake was the result of background sedimentation processes or represents punctual flooding events in a largely isolated lake. Here we used the XRD data obtained with the Chemistry and Mineralogy instrument (CheMin), on board the Curiosity rover, to characterize the degree of disorder of clay minerals in the Murray formation (MF) at Gale crater. Our analysis shows that they are structurally and compositionally related to glauconitic clays, which are a sensitive proxy of quiescent conditions in liquid bodies for extended periods of time. Such results provide evidence of long periods of extremely low sedimentation in an ancient brackish lake on Mars, signature of an aqueous regime with slow evaporation at low temperatures. More in general, the identification of lacustrine glauconitic clays on Mars provides a key parameter in the characterization of aqueous Martian paleoenvironments that may once have harbored life.

Petrophysical and petrographic characteristics of Barail Sandstone of the Surma Basin, Bangladesh

The Barail sandstone in the Surma Basin is a medium- to coarse-grained pinkish-colored rock exposed near the northeastern margin of Bangladesh. In this study, we evaluated the reservoir quality of the Barail sandstone based on its petrophysical and petrographic characteristics. Petrophysical analyses of outcropped samples showed that sandstones are made up of 16.48% porosity and 132.48 mD permeability. Sandstone density ranges from 1.94 g/cm3 to 2.37 g/cm3, with a mean value of 2.12 g/cm3, shown as moderately compacted sandstone. Integrated data such as bulk density, porosity, permeability, Rock Quality Index (RQI), Normalized Porosity Index (NPI), Flow Zone Indicator (FZI), compressive strength, etc. with their relationships indicate that Barail sandstone owing characters to become a good petroleum reservoir. The rock samples consisted mainly of quartz with an insignificant amount of rock fragments and plagioclase feldspar and are categorized as sub-arkose to sub-litharenite. The rock samples also contains lithic (andesine, microcline, muscovite, biotite, etc.) of granitic and gneissic fabric and some volcanic product like aguite, albite, andesine, garnet, spinel and ulvo-spinel indicating the source of nearby orogeny. The euhedral to subhedral shape of the quartz grain in a porphyritic texture, moderately sorted with a smaller amount of clay minerals indicating the moderately mature rock type. The iron oxide border around the quartz grain also indicates that the Barail sandstone was deposited under dry climatic condition.

Texture, mineralogy and geochemistry of Teri sediments from the Kuthiraimozhi deposit, Southern Tamilnadu, India: implications on provenance, weathering and palaeoclimate

The study examines about the red sand dune deposit locally designated as teri deposits; it is an omnipresent geomorphologic feature present in the coastal region of Thoothukudi and Ramanathapuram districts of Tamil Nadu, India. One of the inland teri sand dune outcrops is located around the Kuthiraimozhi village of Thoothukudi district in Tamil Nadu, India. Textural, mineralogical and geochemical studies were carried out in the teri sediments and its compact sandstone outcrops. The sediments are moderately sorted to well-sorted and finely skewed nature which indicates that fluvio-marine depositional environment. Geochemical analysis results of major, trace and rare earth elements for teri deposits help to predict the provenance, weathering status, depositional environment and climate. The geochemical study reveals that the sediments were derived from marine and non-marine sources. Teri sediments are geochemically classified as lithic arenite or wacke. Petrography and X-ray diffraction analysis reveal the predominance of quartz and feldspars along with the accessory minerals like ilmenite, rutile, garnet, magnetite, hematite, zircon, diopside, hypersthenes and biotite. Mineralogical observation illustrates that the teri sediments have originated from the weathering of felsic and mafic source rocks. The Chemical Index of Alteration (CIA) values of sediments represent moderate to the high status of weathering conditions in the source area. The depositional environment indicates that the sediments are fine-grained with high maturity index. Despite that the sediments are formed by fluvio-marine sources, the reddening character in the teri deposits is due to oxidation and leaching of iron-bearing minerals by percolating surface water from high rainfall and groundwater fluctuation of the aquifer under arid and semi-arid climate conditions.

Statistical Analyses and Geotechnical Evaluation of Nubia Sandstone, Golden Triangle Area, Egypt

Great efforts had been made to use indirect non-destructive tests in the geotechnical evaluation of rocks, especially sandstones, employing different empirical equations. However, most of these equations have been derived from hard and compacted sandstones data; therefore, the focus of this research is on weak and weakly compacted sandstones, aiming firstly to obtain empirical equations for estimating their characteristics, secondly to demonstrate and visualize the correlations between the studied variables, and finally to cluster the studied samples based on their characteristics. To attain these aims, twenty oriented block samples were collected from Nubia sandstone, central Eastern Desert, Golden Triangle area, Egypt. These samples were prepared and tested according to standard test methods, including uniaxial compressive strength (UCS), Brazilian tensile strength (BTS), Schmidt rebound number (SRN), porosity (n), bulk density (ρ), and ultrasonic P-wave velocity (UPV). The loss on ignition (LOI) was also employed as a physicochemical test for classifying the studied samples and indicating pores filling materials. The results revealed that these sandstones are characterized mainly by high n, low ρ, and low UPV values and these give an indication of weakly compacted and weakly cemented sandstone with shallow burial diagenetic conditions. Based on UCS and elastic modulus values, these sandstones are mainly classified as very low strength and highly yielding rocks. The results of regression analysis show satisfactory correlations between physical and mechanical characteristics, indicating the suitability of obtained empirical equations to deduce these properties. Principal component analysis revealed that the LOI, BTS, SRN, and USC have a positive correlation to each other and weakly correlated with ρ and UPV, which positively correlated to each other and negatively correlated to n. The results of agglomerative hierarchical clustering revealed that the studied samples can group into three main clusters depending on their USC, LOI, and n values.

Laboratory experiment to investigate permeability change in tight sandstone samples in case of water-based formation damage

In the current study, an investigation possibility is introduced regarding the appearance of the so-called formation damage phenomenon in case of overbalanced drilling of compacted sandstones with water-based mud and the fracturing of the layers with water-based fracturing fluid. The results presented in the study are part of a series of experiments.

Experimental research on stress-dependent permeability and porosity of compact sandstone with different moisture saturations

In this paper, compact sandstone with different moisture saturation from 0% to 73.60% was prepared. Subsequently, gas permeability and porosity of the prepared rock specimens under different pore pressure during confining pressure loading and unloading was measured. The experimental results indicate that, for rock specimen with moisture saturation 0%, 9.63%, 22.10%, 33.60%, and 42.30%, gas permeability rises with the increase of pore pressure due to Klinkenberg Effect, while for rock specimens with moisture saturation 54.36% and 73.60%, the opposite phenomenon was observed because the flow rate of gas has the dominant influences on the gas permeability than that of Klinkenberg Effect. The quadratic equation has better performance in describing the Klinkenberg Effect than that of Klinkenberg equation. Klinkenberg Effect enhances with the increase of moisture saturation and confining pressure, respectively, due to the decrease of equivalent pore radius for gas flow. With the rise of moisture saturation, the decreasing rate of intrinsic permeability under low moisture saturation is low than that under high moisture saturation. The intrinsic permeability of rock specimens with high moisture saturation is more sensitive to loading and unloading than that with low moisture saturation. The relationship between intrinsic permeability/porosity of rock specimens and confining pressure can be described correctly by adopting the exponential function model and power function model, respectively. Additionally, the gradual increase in moisture content in rock specimens revealed significant decrease on intrinsic permeability. This was attributed to clay swelling which contributed to blockage of seepage paths.

Developed correlations between sound wave velocity and porosity, permeability and mechanical properties of sandstone core samples

In the evaluation of a petroleum reserve, it is necessary to accurately determine certain petrophysical properties such as porosity and permeability of the reservoir rocks and rock mechanical properties. Petrophysical properties are key factors in the reservoir description and geomechanical properties are determining parameters in drilling operations and in stimulation and hydraulic fracturing jobs and also the development plans for given reservoir. It is more convenient to use homogenous rock samples with nearly constant initial permeability; however, obtaining such cores is very difficult. In this paper a simulated natural and homogeneous compacted sandstone rock with known physical and petrophysical properties were used. Physical properties of reservoir rocks include pore size distribution; grain size, cementing material concentration, and confining pressure affect rock porosity and permeability. Sound wave velocity was measured using an ultra sound tool on different sandstone core samples. Good correlations have been developed between sound wave velocity (VP and VS) the petrophysical properties mainly porosity and permeability. Significant correlations have been found between seismic wave velocity (VP, VS and VP/VS) and mechanical parameters namely Young’s modulus, shear modulus, bulk modulus and Poison’s ratio for sandstone core samples. This study has been carried out on dry core samples and core samples with different water saturations and results showed that there are changes in the correlation between seismic wave velocity and mechanical and petrophysical properties as function of water saturation change from dry rock to water wet rock.

Cosmic muon flux measurement and tunnel overburden structure imaging

We present a cosmic ray muon radiography experiment for measuring the muon flux and imaging the tunnel overburden structures in Changshu, China. The device used in this study is a tracking detector based on the plastic scintillator with SiPM technology, which can be conveniently operated in field works. The compact system with sensitive area of 6400 cm2 can measure the angular distribution of cosmic muons. It's able to image the overburden density length from the surface of overburden to the detector along the muon tracks. The open sky muon flux measurement outside the tunnel has a good agreement with the modified Gassier Formula model. The distributional pattens of muon flux at three positions inside the tunnel are very similar to that of open sky. Assuming the average density of overburden compact sandstone is 2.65 g/cm3, the overburden thickness can be obtained from the density length derived from the difference of muon flux inside and outside the tunnel. Moreover, for known penetrated lengths (i.e., topography of overburden), the density anomalies of the overburden can also been obtained. This study suggests a potential application for imaging and detecting subsurface structures in civil engineering, tunnels or caverns with the cosmic ray muon telescope.

Monitoring the strata behavior in the Destressed Zone of a shallow Indian longwall panel with hard sandstone cover using Mine-Microseismicity and Borehole Televiewer data

Massive coal extraction from the longwall panel results in the formation large void space (goaf), which is followed by elevation in the stress level of the overlying strata. The increasing stress leads to rock displacements in form of deformation, fracturing and caving of the roof behind the powered supports for regaining the equilibrium state. These rock displacements divided the longwall roof into three zones known as Caved Zone (CZ), Fractured Zone (FZ) and Continuous Deformation Zone (CD). Previous studies have shown that the overlying strata in the Destressed Zone behaves violently in presence of hard rock (such as compact sandstone, shale etc.), which is a major problem faced in the Indian underground mines. Therefore, the present study intends to monitor the roof behavior of a shallow longwall mine (working depth: 68-74m; mining height: 2–3m) of Central India, which is overlain by a thick sandstone bed (average thickness: 37-48m) using six months (Oct’01-Mar’02) mine-induced Microseismicity containing event coordinates (in 3D plane) and event parameters (such as Magnitude, Energy and Apparent Stress) along with Borehole Televiewer (BHTV) data. The paper discusses the results of some important studies, which are very essential for enhancing the safety in the mining area, such as: [a] The height of Destressed Zone was calculated through multiple approaches (empirical relations, mining height and bulking factor) and, found to be around eighteen times the mining height (hc+hf~18h), [b] Microseismicity study of the longwall roof showed that events corresponding to Fractured Zone had the lowest b-value (b=0.3301) and highest occurrence frequency (a=2.3178), [c] Borehole Televiewer (BHTV) data showed the presence of massive fractures and water seepage level in Fractured Zone at depth of about 40-50m, [d] The integrated approach of b-value, Energy-Magnitude (E-m) and Gutenberg-Richter Frequency-Magnitude relation showed that events corresponding Fractured Zone had the highest magnitude (+1.2) and energy (106.6J), and [e] The behavior of microseismic event properties (such as Event Counts, Energy, Apparent Stress and Magnitude) was also studied with respect to mine geology, and abnormal changes were seen when the roof strata was encountered with hard rock (mainly coarse-grained sandstone with RQD>60%) and fault (FF`). The study successfully helped towards obtaining several useful results related to dynamic roof displacement and failures, which very much enhanced the safety in the longwall mines.

Analysis of filtration and electrical properties anisotropy of terrigenous reservoir rocks (for DDB axial zone reservoirs)

The paper focuses on the filtration and electrical anisotropy coefficients and relationship between vertical and horizontal permeability in sandstone reservoir rocks. Field case study of DDB reservoir rocks. Petrophysical properties and parameters are estimated from core and log data from a Moscovian and Serpukhovian stages of Dnipro-Donetsk Basin (West-Shebelynka area well 701-Bis and South-Kolomak area well 31). Routine core analysis included estimation of absolute permeability, open porosity, irreducible water saturation and electrical resistivity (on dry and saturated by mineralized solution) of 40 core samples along two orthogonal directions. Shale fraction is estimated using well logging data in wells which are analyzed. The authors report that reservoir rocks are represented by compacted poor-porous (φ <10 %), low permeable (k<1mD) laminated sandstone with different ratios of clay minerals (Vsh from 0,03 to 0,7) and high volume of micaceous minerals (in some cases 20-30 %). Research theory. One of the main objectives of the work is to develop empirical correlation between vertical permeability and other capacitive and filtration properties for compacted sandstone reservoirs. A modified Kozeny-Carman equation and the concept of hydraulic average radius form the basis for the technique. Results. Coefficients of the anisotropy of gas permeability (IA) and electrical resistivity (λ) are defined based on the results of petrophysical studies. The experiments proved that IA lies in a range from 0,49 to 5 and λ from 0,77 to 1,06. Permeability and electrical resistivity anisotropy in most cases have horizontal distribution. It has been shown that in West-Shebelynka area sample №1 (depth 4933 m) there is probably no fluids flow in vertical direction and in samples №№3 and 15 fractures have the vertical orientation. We have also found that the values of electrical and filtration anisotropy for all samples of South-Kolomak area are similar, this characterized the unidirectionality in their filtration properties, as well as the fact that the motion of the fluid flow mainly in the horizontal direction. In the studied rocks the degree of anisotropy has been concluded to depend on the volume of clay and micaceous minerals, their stratification, fractures, density, and their orientation. New correlation between vertical permeability, horizontal permeability and effective porosity are developed for Late Carboniferous DDB intervals that are analyzed.

Geochemical and geological characterization of marine–continental transitional shales from Longtan Formation in Yangtze area, South China

Abstract Although previous research has provided useful insights into unconventional shale gas resources in recent years, the geochemical and geological characteristics of marine–continental transitional shale have not been studied systematically. During the Late Permian, Longtan Formation shales were widely deposited in the Yangtze area of South China. These shales clearly indicate the need to deepen our understanding of marine–continental transitional shales. This study describes the various characteristics of black shales from the Longtan Formation in this area on the basis of a field investigation and relevant laboratory analyses. Longtan shale reservoirs, with a thickness and burial depth range of 20–650 and 800–2400 m, respectively, are usually interbedded with coal and compact sandstone reservoirs due to their special sedimentary environment. Geochemical analysis of the samples of Longtan shales from seven wells in the Yangtze area indicates the presence of high organic matters (OMs), with a total organic carbon content in the range of 0.85%–35.7% (average of 7.33%) but low hydrocarbon genetic potential (S2) with a value below 3 mg HC/g rock. The analysis of carbon isotopes and organic macerals demonstrates that the investigated samples mainly contain type III kerogen. Data from gas chromatography and gas chromatography-mass spectrometry analyses illustrate a weakly oxidizing to a weakly reducing environment, with most organic materials originating from algal-bacterial organisms and terrestrial plants. The terrestrial plants contribute significantly to the OM. The Longtan shales are thermally over-matured, and they have entered the dry-gas generation window according to Ro, production index data, and biomarker maturity ratios. X-ray diffraction analyses show that the content of brittle minerals of the Longtan shales is generally lower than that of marine shales in North America. The Longtan shales show a high clay content averaging 60.32 wt% and low calcite mineral content averaging 3.73 wt%. Field emission scanning electron microscopy shows that various types of nanometer-to micrometer-scale pores, including interparticle and intraparticle pores, OM pores, and microfractures, are well developed in the shales. Generally, high OM abundance, thermal evolution degree, clay mineral content, and pore space (porosity of 0.56%–10.6%) positively influence shale gas content (1.0%–3.8%). However, high clay content and variable reservoir thickness hinder the successful production of shale gas.

Numerical study of the impact mechanism of decoupling charge on blasting-enhanced permeability in low-permeability sandstones

More than seventy percent of sandstone-type uranium mine occur in the compact sandstone reservoir with extremely low permeability in China, there is no doubt that it has seriously restricted the utilization of uranium resource. Thus, improving the permeability of sandstone reservoir is the key factor for enhancing the uranium deposits recovery. Obviously, blasting fracturing technique is one of the most promising options for enhancing the permeability of sandstone reservoir. This paper emphatically studies the influence rules of decoupling charge on the improvement of permeability in sandstone reservoir through combining two-dimensional particle flow code (PFC2D) with Universal Distinct Element Code (UDEC). Firstly, the discrete fracture network (DFN) induced by blasting are simulated by PFC2D under the condition of six decoupling coefficient of charges. Secondly, isolate fractures and the fracture clusters that are not connected to the borehole are neglected because of their invalidation for the migration of uranium mine dissolved in leaching liquid from the sandstone reservoir into the borehole, while the fracture clusters that are beneficial for uranium deposits recovery are exported from the PFC2D and imported into the UDEC to establish the numerical model for seepage analysis. Finally, through simulating the flow of leaching liquid in the useful fracture clusters, the average permeability coefficient at different distance to the center of borehole are derived from Darcy law. It is significant to note that the action from the explosive stress wave and the detonation gas are considered simultaneously when the initiation and propagation of fractures induced by blasting are simulated in PFC2D. From the studies in this paper, it is concluded that the effective area for uranium mine recovery increase first and then decrease with the increase of decoupling coefficients, which indicates that the effective distribution area do not increase monotonically with the increase of decoupling coefficient and there must be an optimal decoupling coefficient for enhancing the permeability of sandstone reservoir.

Laboratory investigations of inert gas flow behaviors in compact sandstone

The seepage evolution behavior of compact rock is significant for the stability and safety of many engineering applications. In this research, both hydrostatic and triaxial compression tests were conducted on compact sandstone using an inert gas, namely argon. A triaxial compression test with a water permeability measurement was carried out to study the difference between the gas permeability and water permeability evolutions during the complete stress–strain process. Based on the experimental data, the hydrostatic stress-dependent gas permeability was discussed firstly. A second-order function was proposed to predict and explain the gas slippage effect. The mechanical properties and crack development of the sandstone samples were discussed to better understand the permeability evolution with crack growth during the complete stress–strain process. The results show that the gas permeability evolution can be divided into five stages according to the different crack growth stages. Then, the permeability changes in the crack closure stress $$ \sigma_{\text{cc}} $$, crack initiation stress $$ \sigma_{\text{ci}} $$, crack damage stress $$ \sigma_{\text{cd}} $$ and peak stress $$ \sigma_{\text{p}} $$ with confining pressures were analyzed. Finally, we found that the difference between the corrected gas permeability and water permeability can be attributed to the interaction between the water and sandstone grains.

POST- ALPINE LATE PLIOCENE – MIDDLE PLEISTOCENE UPLIFTED MARINE SEQUENCES IN ZAKYNTHOS ISLAND

Post-orogenic marine sequences crop out in two areas of Zakynthos Island: a) In Gerakas peninsula at the southeastern edge of the island and b) Along the coastal zone of central northern Zakynthos from Alikanas in the west to Zakynthos town in the east. Detailed stratigraphic analysis of Gerakas area has shown the existence of three formations separated by two unconformities: 1) Gerakas Fm Q1 comprising marls of Late Pliocene–Early Pleistocene age (2.8 – 0.9 Ma). 2) Kalogeras Fm Q2 comprising littoral sandstones at the base (Q2a), alternations of sandstones and marls in the middle (Q2b) and marls (e.g. Porto Roma) in the upper part (Q2c). The age of this formation is middle Pleistocene (0,8 – 0,5 Ma). 3) Agios Nikolaos Fm Q3 comprising littoral sandstones. Stratigraphic correlations between the two aforementioned areas have been made and equivalent stratigraphic formations, slightly differentiated in the central northern Zakynthos, have been identified. The upper unconformity is well pronounced in the area of Cape Gaidaros whereas the lower unconformity is often masked by debris accumulated along the base of the cliffs produced by the compact sandstones of Q2a above the soft marls of the underlying Q1. A remarkable increase in the thickness of the middle member Q2b is observed in the northern part with respect to the equivalent Kalogeras Fm in Gerakas area. In contrast, the thickness of the upper member (Q2c) is highly reduced in the limited northern outcrops compared to those of the southeastern exposure. The Plio-Quaternary post-orogenic marine deposits in both regions represent the creation of the accommodation space formed by the westernmost extensional structure of the Hellenic arc. The two unconformities reflect sea-level changes during the Quaternary within an overall tectonic uplift of Zakynthos Island at the front of the Hellenic arc and trench system, while the entire post-orogenic sequence reflects the intense palaeoclimatic fluctuations of the Quaternary.