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Sixth EAGE Shale Workshop
- Conference date: April-May 28-01, 2019
- Location: Bordeaux, France
- Published: 28 April 2019
67 results
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Large Scale Climate Teleconnections Driving Marine Black Shale Formation Across the Jurassic Boreal Seaway (KFC): A Geochemistry-Modelling Perspective
Authors T. Wagner, E. Atar, A. Aplin, O. Dellwig, V. Lamoureux-Var, T.L. Leith, D. Lunt, C. März and B. SchnetgerSummaryExpanded successions of marine black shale provide important proxy records to test climatic teleconnections across ocean basins and paleo-latitudes. The causal and generic relationships controlling the timing, composition, and internal variability of marine organic carbon (OC)-rich shale over large distances and during variable global climate states are, however, far from understood but relevant for hydrocarbon generation potential. Despite detailed documentation of the processes operating in individual ocean settings and under specific climatic conditions, it remains a challenge to present a unifying concept that explains the generic and far field interconnections between climate zones and ocean basins at multiple temporal and spatial scales. Progress towards such a unifying model has been made for the Cretaceous Atlantic providing an important step forward to simulate the location and internal heterogeneity of black shale formation more reliably, especially for areas where limited field data exist due to harsh or remote conditions, such as the Mesozoic Arctic. In this study we combine new, high resolution geochemical and sedimentological records from the Jurassic boreal seaway, where widespread deposition of the Jurassic Kimmeridge Clay Formation (KFC) is well documented with large scale climate and biogeochemical modelling, placing the seaway into a global climatic context.
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Benchmark Study of Undrained Triaxial Testing of Opalinus Clay: Results and Implications for Robust Testing
Authors S.B. Giger, A. Minardi, A. Ferrari, L. Laloui, R.T. Ewy, R. Stankovic, J. Stenebråten, M. Soldal, E. Pimentel and H. BaumgartnerSummaryWe have performed a comparative study of undrained triaxial testing with five different laboratories, to explore the reproducibility of test results. Opalinus Clay was sourced as testing material from a borehole at the Mont Terri URL. Cores were vacuum-sealed, and resin impregnated immediately after recovery. Systematic determination of basic properties such as water content, grain density and bulk mineralogy of specimens after testing assisted in diagnostic test evaluation. A detailed testing protocol was requested to avoid specimen damage during initial loading («swelling») and to verify specimen saturation. A balanced pore fluid was used for testing, and a consolidation phase was performed to reach specific target effective stress levels prior to the shear phase. One laboratory deviated from these protocols, as it did not use an external pore fluid. Instead, specimens were brought to variable saturation levels in a desiccator prior to assembling them into the rig. For specimens with almost identical basic properties, the test results were indeed found to be in very good agreement, despite the different procedures applied. Differences in test results can be attributed to material heterogeneity. The study provides compelling evidence that robust triaxial testing can be achieved with shales.
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Secular Change in Mineral Composition of A Mesoproterozoic Black Shale Predating the Advent of Terrestrial Life and Soil-Formed Clay Minerals
Authors M. Rafiei and M. KennedySummaryA key transition in the history of the Earth's biosphere is from the lifeless terrestrial surfaces to the biologically mediated weathering environment of soils. Detrital clay minerals formed in soils that are ultimately deposited in continental margin sediments comprise >60% of the sedimentary record in the Phanerozoic. The Mesoproterozoic Velkerri Formation provides a contrasting record of continental margin shale before the advent of terrestrial life and soils. Micro X-Ray mineral mapping of black shale intervals within the Velkerri Formation shows that while clay minerals and clay sized grains are of similar abundance, petrographic relationships showing intergrowth of illite-smectite and quartz identify an authigenic and not detrital origin for this fraction that controls the porosity, permeability and brittleness. The detrital minerals that are present include large (>60 µm) mica flakes, feldspar and lesser quartz grains indicating limited chemical weathering. Organic carbon is dominated by laminated matlike laminae as well as a significant pyrobitumen fraction found in fractures and shelter porosity. Other authigenic phases include pyrite, apatite (early diagenesis), and kaolinite (late stage diagenesis). The lack of maturity in the detrital mineral assemblage is consistent with the absence of soil chemical weathering influence during physical sediment production. Key rock properties in the Velkerri Formation such as organic carbon enrichment, cementation and porosity are thus dominated by diagenetic processes that modified a distinctly non-uniformitarian initial suite of detrital grains.
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Cap Rock Evaluation of Central North Sea Shales, Through Log-Derived Poisson's Ratio and Young's Modulus
Authors J.A. Hansen, J.R. Johnson and N.H. MondolSummaryWe present an evaluation of shale dominated cap rocks relevant for Middle Jurassic sandstone reservoirs in the Central North Sea, based on well log data from the Norwegian Continental Shelf. Previously established indicators for brittleness and seal quality, E (Young's modulus) and ν (Poisson's ratio), are utilized in the analysis. Similar ductile to fairly ductile behaviour is found in different formations for five analysed wells, of which two are oil discoveries, one contains only oil shows, and two are dry. Cap rocks in the discovery wells are comparatively most brittle, compared to a published E–ν template. Uplift of ~500 m in one of the discovery wells is not found to have compromised the sealing capability. We also investigate how organic content influence an organic-rich shale interval in terms of cap rock properties by using kerogen substitution and comparing to the other more organic-lean shales, which does not support a direct correlation between TOC and ductility. Finally, we consider how observed properties of different shales relate to different mineralogical composition.
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Silica Diagenesis in the Marcellus Shale: A Trace Element and Hyperspectral Cathodoluminescence Study
Authors C. Delle Piane, C. MacRae, N. Wilson, A. Torpy and D.N. DewhurstSummaryQuartz is one of the major constitutive minerals of sedimentary rocks and its abundance in shales is thought to have a significant control on their mechanical properties as well as on the efficiency of hydraulic fracturing stimulation. While mineralogy based estimations of the strength and deformation behaviour (brittle or ductile) of shales are commonly used in practical applications, it should be noted that they do not account for parameters like phase distribution and cementation that also affect mechanical and elastic properties. Quartz in shales and can occur in at least two forms: i) as a detrital component and ii) as an authigenic mineral precipitated during diagenesis. In this study, we present high resolution scanning electron microscopy (SEM)-cathodo-luminescence (CL) imaging of authigenic quartz combined with Electron Probe Micro-Analysis (EPMA) trace-element quantification at sub-micron spatial resolution in samples from the Marcellus Shale of NE Pennsylvania, USA. For the first time the geochemistry of authigenic quartz in shales is presented and discussed in relation to the silica source and conditions of precipitation.
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Volcanic and Hydrothermal Signs in the Rocks of Black Shale Formations
Authors A. Gavrilov and M. TugarovaSummaryBlack shales of the Bazhenov Formation are characterized by a high heterogeneity of the composition — a complex combination of carbonate, siliceous, clay, and organic phases, which alter laterally and in the section. Ash beds occur in the lower unit (Tithonian stage, Late Jurassic). Based on a detailed description of the core and complex analytical data (petrographic analysis, scanning electron microscopy with energy dispersive spectrometry, X-ray fluorescence and X-ray phase analyzes, ICP-MS, etc.) the composition, structure and genetic features of ash layers are given. The paper also discusses the carbonate microbialites of the Bazhenov Formation and the impact of hydrothermal processes on their genesis. Volcanogenic and hydrothermal phenomena could have a fundamental impact on the water transparency and other environmental factors, affect the marine biota and its mass extinction and finaly could lead to the formation of stratas with highly increased content of organic matter. Jurassic- Cretaceous global geological processes in the Western Siberia and in adjacent areas confirm the suggested assumptions.
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Geomechanical Analysis of Maturation for the Draupne Shale, Offshore Norway
Authors J. Johnson, J. Hansen, F. Renard and N. MondolSummaryUnderstanding maturation of source rock is increasingly of interest in both conventional and unconventional plays. Shale diagenesis and hydrocarbon generation in shales has a direct relationship with the evolution of the mechanics of maturation. The Draupne Formation, a world class source rock, which stretches over a broad range of depths and maturity, provides the ideal candidate to study the interplay between maturation and geomechanical parameters. Wireline logs and Rock-Eval data from eight wells were used to analyse how seismic waves and rock strength interplays with the source rock shales. Results reveal that the Draupne shale behaves typically in terms of maturation, however the relationship between maturation and geomechanics counters the common trend for shale. There are a wide variety of factors that could impact the geomechanical trends, including but not limited to, lithology, mineralogy, pressure, temperature, fluid content, diagenesis, compaction, fracture density, and organic content. This paper identifies key relationships between geomechanical parameters and a number of these factors, while identifying further work that could be carried out in other areas. This further highlights the importance of in-depth play fairway analysis and presents questions that require answers for successful exploration and exploitation of hydrocarbon.
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Detailed Characterization of Namurian Shale in the Campine and Wallonian Basin, Belgium
Authors W. Wei and R. SwennenSummaryThe Namurian Chokier Formation of late Mississippian has been studied in two shallow boreholes in southern Belgium which addresses to its shale gas potential. To help with the global assessment of this shale gas succession, the research focuses on the detailed characterization of the Chokier mudstones in Belgium which distribute in the Campine Basin and Wallonian Basin. After the description of cores and quarries from 6 sampling locations, the samples have been analysed with X-ray diffraction (XRD), microscopy, cold cathodoluminescence for petrographical and mineralogical study. It reveals that Chokier Formation is composed of successive mudstones in the Campine Basin while mudstones with more interbedded sandstones in the Wallonian Basin. The mudstones consist of the mixture of clay minerals (including kaolinite, illite, chlorite), quartz, feldspars, mica (including muscovite, biotite, phlogopite), carbonates (including calcite, dolomite, siderite, ankerite), sulfides (mainly pyrite), organic matter and amorphous material. In consideration of variable sedimentary structures and relative proportions of clay minerals, carbonates, quartz, feldspar and mica, 3 lithologies and 8 lithofacies have been classified in the Chokier mudstones. The deposition setting of Chokier Formation changes from marine, prodelta to delta front from the Campine Basin in northern Belgium to the Wallonian Basin in southern Belgium.
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Characterisation of the Bowland Shale Porosity Using N2 and CO2 Adsorption
Authors H. Ansari, M. Trusler, G. Maitland, C. Delle Piane and R. PiniSummaryThe heterogeneous nature of shales arising from their sheer abundance in the natural world, presents a unique challenge to the field of characterisation. Shales vary significantly in terms of their age, composition and pore systems so the wide-range applicability of a study on just one shale is questionable. This uncertainty can be reduced through the use of analogous materials, such as pure carbons and clay minerals, which can make shale characterisation much more predictive. In this work, we characterise the Bowland shale in the UK. Three samples, taken at different depths, and of varying composition in terms of organic content and clay minerals, were studied. Characterisation was achieved using low pressure adsorption on N2 at 77K and CO2 at 273K. The results were complemented with the use of the same technique performed on pure components such as mesoporous carbon (representing the organic matter) and clay minerals. The pure material results are used to infer the independent contribution of the constituents to shale characteristics and can be used to build artificial isotherms. Results show that the shale composition is a key indicator of the pore space in shale and therefore adsorption capacity and gas storage potential.
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Compaction Trends of Elastic Properties and Permeability in Shales
Authors R. Beloborodov, M. Pervukhina and M. LebedevSummary% We present methodology that allows measuring vertical hydraulic permeability and a full stiffness tensor of transversely isotropic shale samples simultaneously during mechanical compaction experiments. In this study we investigated artificial shales composed of quartz, smectite, and kaolinite. For the first time, compaction trends of these properties are obtained, at a 15--40~\% porosity range. The hydraulic permeability values of these shales exhibit exponential compaction trends and lay within the bounds of compaction trends found in natural shales. Strong correlations between the hydraulic permeability and elastic properties of shales are observed.
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Microstructural Investigation of Mudrock Seals Using Nanometer-Scale Resolution Techniques
Authors A. Rezaeyan, V. Pipich, P. Bertier, T. Seemann, L. Leu, N. Kampman, A. Feoktystov, L. Barnsley and A. BuschSummarySmall angle neutron scattering (SANS) and nitrogen low-pressure adsorption (LPS) have been used to characterise the pore structure of two organic lean mudrocks, Opalinus Clay, Mont Terri, Switzerland and Carmel Claystone, Utah. This was done in order to obtain a better understanding of H2 and CO2 transport, reaction and sorption related to radioactive waste disposal and carbon storage, respectively. The pore structure information derived by SANS and LPS are comparable and the results have revealed a vast heterogeneity from 2 nm to 2 µm, which can be related to the high clay contents. Due to the high clay contents, pores smaller than 10 nm constitute a large fraction of total porosity (25–30 %) and most of specific surface area (up to 80 %) in the sample mudrocks. Accordingly, these interplays contribute to a pore network of few-to-several nano-Darcy permeability in which pore size dependent transport mechanisms can vary from high sorptive diffusional fluid flow in small pores to low sorptive slip flow regime at progressively larger pores.
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Engineering Parameters of Draupne Shale - Fracture Characterization and Integration with Mechanical Data
Authors H. Smith, B. Bohloli, E. Skurtveit and N.H. MondolSummaryFracture characteristics are important as they provide information about the mechanical integrity of its host rock. This study addresses the properties of fractured surfaces of Upper Jurassic organic-rich Shale (Draupne Formation) cored from an exploration well (16/8-3 S) in the Central North Sea. The characterization consists of two steps: i) petrographic studies of the fractured material at micro- and macroscale compared with the mechanical data, and ii) mapping of surfaces with photogrammetric method before and after the samples were sheared in a direct shear test (DST) to determine frictional properties. The preliminary results reveal complex structures, with the natural fractures appearing more diverse than the artificially reactivated surfaces produced from the direct shear test, which may affirm the importance of factors as e.g. temperature, displacement, slip rate and number of reactivations. Moreover, these small-scale fracture planes seem to link up with adjacent fractures to accumulate displacement over a wider area. Little attention has been paid to the details of fractures in shale, despite their importance in the energy industry. Therefore, objective of this study is to provide detailed descriptions of fracture surfaces in shale and the consequences this might induce.
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Impact of Mineralogical Composition on Geotechnical Properties of Opalinus Clay
Authors E. Crisci, A. Ferrari, S. Giger and L. LalouiSummaryOpalinus Clay is the selected host geomaterial for the construction of underground geological disposals for radioactive waste in Switzerland. The formation has been studied in several sites, i.e. the Mont Terri laboratory, and several boreholes in the northeastern part of the Country, and several lithostratigraphic sub-units were identified. In this work, a hydro-mechanical testing campaign was performed on samples from a recently cored shallow borehole (< 70), and the results were compared with previous testing campaigns. The mineralogical compositions of the specimens were analysed and correlated with the geotechnical properties and the hydro-mechanical behaviour. It is shown that the mineralogy (and in particular the clay-mineral content) plays a predominant role on the specimen compaction properties. On the other hand, the burial history is manifested in the porosity of the geomaterial, that is considerably lower for the samples sourced at greater depth with a similar composition.
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Impact of the Degree of Saturation on the Mechanical Behaviour of the Whitby Mudstone
Authors L. Douma, J. Dautriat, J. Sarout, D. Dewhurst and A. BarnhoornSummaryMudstones are of great interest in the petroleum industry, since they can act as a source rocks, reservoirs and seals. Understanding the mechanical behaviour of these clay-rich rocks is crucial for successful hydrocarbon exploration and production. This study examines the impact of water saturation on the mechanical behaviour of the Whitby Mudstone. Triaxial compression tests are performed on mudstone samples with different water saturations at effective confining pressures of 5 MPa, 15 MPa, 25 MPa, and 40 MPa. The results show that the degree of saturation significantly affects the rock strength and static elastic properties (Young's modulus and Poisson's ratio). The mechanical properties obtained from partially-saturated mudstones should be treated with care when extrapolating them to in situ conditions.
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Geomechanical Characterisation of Posidonia Shale Using Nanomechanical Testing
Authors M. Goodarzi, M. Rouainia, A. Aplin and P. CubillasSummaryThe effective exploitation of unconventional shale hydrocarbon resources requires the robust geomechanical characterisation of the reservoir. However, there is still a paucity of mechanical data on well-characterised shale, partly due to the technical and economic difficulties in obtaining high quality core for laboratory testing. Here, we present the results of a study which combines PeakForce Quantitative Nanomechanical Mapping and nanoidentation testing on three shales from the Posidonia of NW Germany, covering a range of thermal maturities. PF-QNM is used to determine the mechanical properties of individual organic, clay mineral and other phases, including their anisotropy. Estimates of bulk elastic and viscoelastic parameters are determined through use of high load instrumented indentation testing. These data form the building blocks for the estimation of the geomechanical properties of shales at larger scales, as a function of mineralogy and organic matter type and content.
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Integration of Multi-Scale 3D Imaging of Porosity in Shales
More LessSummaryIn this research the Haynesville-Bossier Shale play (USA), which is a clay-bearing siliceous, organic-rich, gas-mature shale play, was characterized over pore diameters ranging 2 nm to 3000 nm. Three advanced imaging techniques were utilized correlatively, including the application of Xe+ plasma focused ion beam scanning electron microscopy (plasma FIB or PFIB), complemented by the Ga+FIB method which is now frequently used to characterise porosity and organic/inorganic phases, together with transmission electron microscope tomography of the nano-scale pores. This is the first study to comprehensively develop pore geometric and network models in 3D based on experimental characterisation over these critical length scales
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Dominance of Wind Blown Minerals in Black Shales, Connecting Continental Climate, Fe Fertilization and Mineral Ballasting
Authors M. Kennedy and L. MayerSummaryThe origin of organic carbon-rich (black) shales is commonly attributed to changes in oceanographic conditions such as anoxia or heightened biological productivity. Here we use sub-micron in situ X-Ray mineral mapping at the grain scale to identify sedimentological and diagenetic processes controlling deposition that are not evident in traditional bulk approaches. Lack of sorting, abundance of angular feldspar and other less stable mineral phases, and absence of evidence of winnowing all imply pelagic deposition of detrital minerals of aeolian origin. Using modern values of iron content in dust, the fraction of bioavailable iron, and the range of Fe:C ratios needed by phytoplankton, we calculate that the amount of organic matter associated with dust mineral particles could have been stimulated by the bioavailable iron in that dust. The anoxic bottom waters would efficiently preserve this high ratio of organic matter to minerals. We propose that aeolian deposition is a fundamental and overlooked component of black shale formation, accounting for minerals that are associated with nutrient influx and assist with ballasting of resultant blooms, and stimulate anoxia in the centre of basins. Such processes would be tightly coupled with changes in continental climate and provenance.
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Claystone Porosity and Mechanical Behavior vs. Geologic Burial Stress
By R. EwySummaryA dataset was assembled resulting from studies on ~35 preserved claystone cores from different locations around the world and burial depths from <200m to ~3700m. Combining all data together, within a small amount of scatter there appears to be a single common trend for bulk density (or porosity) vs. depth (or vs. effective stress), irrespective of location, clay type or the exact clay content, as long as the formation is clay-supported. High-pressure mercury injection porosimetry shows that the modal pore size reduces in a systematic way with increasing burial stress. This loss of porosity and closer packing of the clay aggregates, perhaps combined with geologic time and/or temperature, causes mechanical properties (e.g. stiffness, strength) to also change in a systematic way. Whether or not a claystone behaves in brittle vs. a ductile manner generally depends on the value of confining stress relative to the maximum past burial stress, with young high-porosity claystones behaving quite similar to soil mechanics critical-state theory and deeper, low-porosity claystones deviating from several aspects of this theory and having an apparent maximum past stress higher than actual. In some cases there is evidence of a clay-type effect (smectite vs. kaolinite) on mechanical behavior.
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Outcrop to Core Comparison of Natural Fractures in the Vaca Muerta Formation, Argentina
Authors E. Ukar, R.G. Lopez, J.F.W. Gale and R. MancedaSummaryCharacterizing permeability-controlling fracture attributes is essential for well planning and stimulation design, especially in unconventional reservoirs. Outcrops allow to measure fracture attributes that are below seismic resolution or cannot be readily sampled by core, such as fracture height and length. For outcrops to be useful analogs to the subsurface, however, fractures in outcrop must be representative of those in the buried reservoir rocks. Here we show that fractures in Vaca Muerta Formation outcrops in the Agrio fold-and-thrust belt have similar characteristics to those in the subsurface reservoirs in the Neuquén basin. Because of this close correspondence between fractures in core and in the Loncopué outcrops, fracture attributes (height, length, aperture, intensity, spacing) measured in outcrop may be used as a proxy to populate DFN models. Most fractures are infilled by blocky and/or fibrous calcite cement and densities are low, therefore present-day fluid flow through natural fractures is likely to be limited but likelihood of reactivation of preexisting natural fractures by hydraulic fractures is potentially high.
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Characterization of Quartz in Wufeng-Longmaxi Shale: Implications for Sedimentology and Shale Gas Exploration
Authors L. Ke, L. Ganzer, C. Sattler and W. XuanSummaryExisting exploration and research about Wufeng—Longmaxi shale gas reservoir show that the content of quartz is generally more than 50% and the American shale gas reservoir also share the same characteristics. The objective of this paper is to further these three question: what is the source of quartz? Why is quartz enriched? How does quartz affect the shale gas reservoir? In this study, the mineralogical analysis was conducted on a total of 202 samples collected from Longmaxi Formation. The samples were used for methane sorption analysis, the XRD, Microscope, SEM, TOC analysis. According to the final results, Wufeng—Longmaxi shale the quartz can be divided into 3 types according to their sources: Biogenic source quartz, terrigenous source quartz, Recrystallization quartz. Different sources of quartz have different effects on shale reservoirs: Biogenic quartz containing an abundance of organic matter and quartz critical to form high quality source rocks and shale reservoirs, Terrigenous quartz impedes organic matter accumulation but makes the rock brittle, Recrystallization of quartz is negative to the physical reservoir properties. Defining quartz sources should be a new key factor for the Exploration and Evaluation of shale gas reservoirs.
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Early & Late Diagenetic Mineral Development within the Upper Jurassic Haynesville-Bossier Shale, USA
More LessSummaryDespite recent advances, diagenetic processes in fine-grained sediments remain relatively poorly understood. Key questions still to be resolved include how mineral cements develop in low porosity and permeability systems and the extent of element mobility during the diagenesis of fine-grained sediments. This study uses petrographic, mineralogical and geochemical data from the Haynesville-Bossier Shale identify early and late diagenetic mineral development. On the basis of grainsize and mineralogy five facies are designated: (i) silica-rich argillaceous mudstones, (ii) argillaceous siliceous mudstones, (iii) mixed siliceous mudstones, (iv) mixed mudstones and (v) cemented mudstones. The diagenetic development of the Haynesville-Bossier Shale can be divided in to early and late diagenesis. Ferroan and non-ferroan dolomite, framboidal pyrite, and microfossil pore-filling kaolinite cements all formed during early diagenesis. Late diagenetic cements include microcrystalline calcite, euhedral pyrite, replacive and displacive chlorite, calcite-replacive albite, and replacive and/or displacive quartz. Late diagenetic cements would have required a wide range of geochemically mobile elements, (bed scale); illitisation of smectite during late diagenesis is a potential source for these elements. The work impacts porosity and permeability evolution and fracture susceptibility in unconventional shales.
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Mineral Diagenetic Control On Hydrocarbon Expulsion in Organic-Rich Mudrocks, Bakken Formation, North Dakota, U.S.A.
Authors K. Milliken and T. ZhangSummaryAn indicator of expulsion efficiency obtained from gases extracted from mudstone correlates with the degree of microquartz cementation. Cemented samples have stronger retention of hydrocarbons. The strongest degrees of expulsion are observed in samples that lack cements and have diagenesis dominated by compaction. This suggests that mudrock compaction remains an active process in the zone hydrocarbon generation.
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The Elastic Anisotropy of the Whitby Mudstone Formation at Varying Water Saturations
Authors L. Douma, J. Dautriat, J. Sarout, D. Dewhurst and A. BarnhoornSummaryMudstones are characterized by their tight matrix and are therefore of interest in various industries, including the petroleum and underground repository industry. These clay-rich rocks often show dynamic elastic anisotropy, which causes significant problems in geophysical interpretations. In addition, pore water has a significant effect on the bulk properties of mudstones. However, the degree of saturation is often not reported in the literature. This study investigates the impact of water saturation on the elastic anisotropy of the Whitby Mudstone. Four core plugs with different water saturations were deformed until failure and tested ultrasonically at effective confining pressure conditions of 25 MPa. P-wave and S-wave velocities were monitored along the symmetry axis, across the core diameter, and at ~49˚ to the horizontal bedding plane to calculate the full elastic tensor and subsequently the Thomsen anisotropy parameters. The degree of saturation highly affects the rock strength and static elastic properties and leads to significant changes in the elastic anisotropy parameters.
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Experimental Assessment of the Impact of Partial Saturation On the Mechanical Properties of Gas Shales
Authors A. Minardi, A. Ferrari, R. Ewy and L. LalouiSummaryThe impact of partial water saturation on the mechanical behaviour of clays and claystones has been widely proved. This work aims at demonstrating that also for gas shales partial water saturation has a significant influence on their stiffness and strength properties although they have different characteristics compared to other clayey materials. Samples extracted from two different unconventional shale gas reservoirs are tested. An experimental methodology and a testing device are developed to perform uniaxial compressive tests and unloading-reloading cycles at different total suction values. The control of total suction with the vapour equilibrium technique is used to equalize specimens to different water saturation conditions and assess the consequent impact on mechanical properties. The obtained outcomes clearly demonstrate that partial water saturation cannot be neglected to characterize the mechanical properties of gas shales. A reduction of uniaxial compressive strength of 22 % is observed in the total suction range between 150 and 10 MPa. Regarding the elastic stiffness, a decrease up to 50% is exhibited when the material is wetted from 150 MPa to 0 MPa of total suction.
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Mineralogy and Mechanical Properties of Mudrocks Before and After Stepwise Ageing in sCO2
Authors D. Dewhurst, M. Raven, S. Salwani Bt Md Shah, S. Syareena Bt Md Ali, A. Giwelli and S. FirnsSummaryTwo mudrock seals from a site being evaluated for geological storage of CO2 were tested to understand their geomechanical properties, mineralogical composition and geochemical make up before and after immersion in super-critical CO2 (sCO2) at high pressure and temperature (150°C and 4200 psi) representing close to in situ conditions. Physico-chemical properties such as cation exchange capacity and specific surface area were also evaluated. Samples for mineralogy, geochemistry and physico-chemical properties were tested stepwise from their initial state through to 1, 4 and 6 months immersion in sCO2. Samples for geomechanical testing were compared in their initial preserved state and after ageing for 6 months in sCO2. No change in composition was noted for either mineralogy or major element geochemistry and the physico-chemical properties measured were also unchanged. However, geomechanical properties did change, with compressive strength, tensile strength, friction coefficient and elastic stiffness all increasing after exposure to sCO2. As there was no change in mineralogy or geochemistry, it is most likely that the loss of water during exposure to sCO2 resulted in the strength increase observed in these mudrocks.
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Measuring the Anisotropic Dynamic and Static Moduli of the Duvernay Shale
Authors L. Shen, D.R. Schmitt, W. Li and X. ChenSummaryWe measure the anisotropic static and dynamic moduli of a calcareous shale from the Duvernay unconventional hydrocarbon reservoir. The full set of the five stiffness, following the assumed Transverse Isotropy for earth materials, needed to describe the complete elastic behaviour of the test sample are obtained by measuring the acoustic wave velocities on a sample cut into the shape of prism allowing Piezoelectric wafers to be attached to its surface and receive/emit waves in different angles. Measurements are taken at different confining pressures up to 180 MPa. Static loading tests are performed on the cylindrical plugs cored at different angles using the Hoek Cell. We observed that Young's modulus at the direction parallel to the direction of symmetry axis (E3) is highly pressure-dependent for both dynamic (20 – 30 GPa) and static (12 – 37 GPa) measurements. Lesser pressure dependence is observed for the dynamic (34–36 GPa) and static (~26 GPa) E1,2 . Static and dynamic Poisson's ratios demonstrate pressure dependence too. Generally speaking, the static moduli are more prone to pressure change compared with their dynamic counterparts.
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Assessment of stress dependent elastic and permeability anisotropy using small cubic shale samples
Authors J. Dautriat, M. Cauchefert, Y. Kovalyshen, J. Sarout, A. Giwelli and D.N. DewhurstSummaryAssuming transverse anisotropy, elastic properties and anisotropy of rock formations at depth are usually determined in the laboratory using a cylindrical rock sample, preferably plugged orthogonal to the symmetry plane. Such anisotropy is usually caused by bedding or laminations when present. On the other hand, permeability anisotropy evaluation requires a pair of companion samples cored along and orthogonal to the bedding plane. Shaly cores recovered from the well often experience drying, desiccation and stress-relief fracturing when not properly preserved, which makes it difficult to sub-sample standard-size cylindrical plugs. Therefore, we developed a new laboratory approach to test relatively small cubic rock samples without modifying existing pressure vessel equipment designed for standard cylindrical samples. This approach is tested on the Opalinus Clay and validated by comparing the elastic anisotropy results, in term of Thomsen's parameters, obtained on a parent specimen and three cubes extracted from it. We demonstrate that elastic anisotropy parameters inferred from the cubes are accurate as long as a small, homogeneous and representative samples of the formation can be recovered. If this condition is fulfilled, permeability anisotropy can also be successfully determined from small cubic samples using a dedicated permeameter designed for tight and low permeability cylindrical rock samples.
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Chemo-Mechanical Coupling in Fine-Grained Soils and Sedimentary Rocks
Authors I. Bourg, F. Carrillo, X. Shen and T. UnderwoodSummaryFine-grained sedimentary rocks such as shale or mudstone are ubiquitous in sedimentary basins and play important roles as caprocks, host rocks, or source rocks in many energy technologies including carbon capture and storage, nuclear waste storage, and hydrocarbon extraction. Fine-grained soils are equally ubiquitous and play important roles in soil carbon storage and food security. Accurate predictive models of the hydrologic properties of these media remain elusive, however, because of the significant experimental challenges posed by their low mechanical strength, ultra-low permeability, and sensitivity to geochemical and geomechanical alteration. An even greater challenge is that nanoscale interactions between clay particles give rise to strong couplings between the chemistry, mechanics, and hydrology of these media. Our research aims to gain fundamental insight into these interactions and couplings by using computational fluid dynamics, discrete element model, brownian dynamics, and atomistic-level simulations of water-clay-salt-(organic) systems.
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Determination of Thermal Conductivity Variations within Oil Shale Reservoirs Using Integration of Thermal Core Logging and Standard Well Logging Data
Authors A. Shakirov, E. Chekhonin, E. Popov, R. Romushkevich and Y. PopovSummaryContinuous vertical variations of thermal conductivity were registered using optical scanning technique at 847 full-size core samples of the oil field located in West-Siberia. Target interval is highly heterogeneous and anisotropic oil-source rock – the Bazhenov formation. Thermal core logging and well logging data were integrated for shale reservoirs thermal conductivity prediction. Correlation analysis between neutron porosity and thermal conductivity was performed. The vertical resolution of the corresponding logging tool was considered. The obtained regression equation was used for thermal conductivity prediction. Mean absolute percentage error is 8 %. For shale oil thermal conductivity prediction volumetric mineralogical model from standard well logs was constructed. Using different mixing rules including the Liechtenecker equation vertical variations of thermal conductivity were predicted. Mean absolute percentage error is 6.6 %.
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Ekofisk Time-Lapse Seismic Monitoring of Injection in Shale
More LessSummaryCutting re-injection operations in overburden shale formations at the Ekofisk field, has started since 1997. High fidelity time-lapse seismic data (Life of Field Seismic, LoFS) are a critical tool to monitor development of pressure around the injected area. Time-strain data in particular can provide robust pressure and fracture development information. The data also efficiently assist in quantitative pressure estimation around the injected interval. This study has implications for unconventional shale reservoirs where time-lapse seismic are employed.
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Yield characteristics of caprocks: a criticial state mechanics approach
Authors C.C. Graham, J.F. Harrington, R.C. Cuss and K. DanielsSummaryThe yield behaviour of both clay soils and reservoir materials can effectively be assessed using critical state mechanics. Nevertheless, this approach has been relatively unused, in relation to caprock materials and the shape of the yield envelope is poorly-constrained in comparison. In this study, we present experimental data on the hydromechanical behaviour of caprocks and their associated critical state yield envelopes. The degree of uncertainty in the form of the yield surface is considered, before the likely consequences of various stress path scenarios is examined. The potential for permeability enhancement versus reduction is also considered and the influence of stress history is found to be key. This approach demonstrates the potential for yield to be reached during some depletion scenarios, though this deformation is often compactive and permeability-reducing in nature. As such, the critical state approach represents an effective tool for assessing performance of caprocks under a range of potential stress paths.
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Improving Anisotropic Velocity Model Calibration in Microseismic Data From the Woodford Shale
Authors P.J. Usher and J. HuangSummaryThis study demonstrates a workflow of anisotropic velocity calibration for microseismic monitoring. We search for a layered P and S velocity, along with Thomsen parameters, that minimises travel time residuals with our calibration shots. Due to the limited calibration shots, the travel time residual was minimised for a set of early microseismic events from each stage. The velocity models varied from isotropic, to constant non-zero Thomsen parameters, layered Thomsen parameters that are proportional to 1/VP and finally to a general layered vertical transversely isotropic velocity model. We showed that different levels of model complexity effect event location in the Woodford Shale. With increasing complexity, the event locations become more accurate, cluster around the injection points, and have less artefacts and systematic biases. The calibrated anisotropic model can later be used to characterise the fractures and mineral content of the shale and thus add value to microseismic monitoring. This process can be automated to help remove biases, to quantify velocity model error, and as step towards joint inversion for layered anisotropic velocity and event locations.
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Analysis of Cretaceous Shales in the Rovuma Basin, Offshore Mozambique
Authors O. Nhabanga, P. Ringrose and R. HoltSummaryAcoustic log data are important and have been widely used in the petroleum industry to detect fractures in the formation, the fluids and to check the quality of the cementation process in the wellbore. In present study, we used the acoustic log data to detect the overpressured Cretaceous shale units, by contrasting P- and S-waves, under mechanical and chemical compactions.
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Characterization of Shale-Gas Reservoir Based On An Anisotropic AVO Inversion and Quantitative Seismic Interpretation
More LessSummaryA well log penetrating the Lower Silurain shale (S1l) in the south of the Sichuan Basin shows that this shale formation represents strong VTI (vertically transverse isotropy) properties. Anisotropy has significant influence on the seismic amplitude variation with offset (AVO) response, so AVO analysis and inversion ignoring the influence of anisotropy can be inaccurate. In this parper, an anisotropic AVO inversion is introduced based on a modified approximation of the PP-wave reflection coefficient for VTI media. Three attributes including the acoustic impedance (attribute A), the shear modulus weighted by an anelliptical anisotropy parameter (attribute B), and the approximated horizontal P-wave velocity (attribute C) can be simultaneously inverted by using a Bayesian-framework-based inversion scheme. A statistical rock-physics workflow is then applied to the inverted attributes for quantitative seismic interpretation. The field-data application on a lower Silurian-age shale-gas reservoir shows the feasibility of both approaches. The shale formation can be discriminated from surrounding formations by using the combination of the attributes A and C, and the gas-bearing shale can be identified through the combination of the attributes A and B.
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Time-Lapse Analysis on Repeated Well Logs: A Focus On Shales
Authors A. Jaramillo, M.D. Mangriotis and C. MacBethSummaryShales play a significant role controlling fluid flow and seismic-wave propagation due to their low permeability and anisotropic structure. Field observations indicate that during production, pore pressure reduction in the reservoir generates strain deformation in the overburden that varies with lithology and stress path. 4D seismic is a primary source of spatial information on these geomechanical effects. For this purpose, a physical model that links the velocity perturbations to the vertical strain via a factor R is widely used. This study aims at computing R factors from a repeated well log analysis. The combination of seismic with petrophysics is key to understand the physics of the lithology-dependent R. We performed the analysis on two Jurassic shales from Central North Sea to calculate R factors in the overburden shales based on the existing pore-related model proposed by Hatchell and Bourne (2005) . The petrophysical analysis of repeated wireline logs has shown that R-factors of around 5 are appropriate for both the Heather and Kimmeridge clay based on an HBR model using log-derived time-lapse porosity values. This range is in agreement with most published overburden R-values from seismic data, and can be explained by porosity deformation.
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AVO Modelling Considering Various Caprock and Reservoir Scenarios for Potential CO2 Storage in Smeaheie Area, Northern North Sea
Authors M. Fawad and N.H. MondolSummaryUnlike a hydrocarbon accumulation where we are sure of a working reservoir and the caprock integrity, the CO2 storage has many unknowns before the gas is injected and placed there. To reduce this risk it is imperative to model all the possible scenarios before taking a major decision for CO2 storage. To evaluate subsurface reservoirs and caprocks for CO2 sequestration, the CO2 plume movement while injection and subsequent changes in the elastic properties at the reservoir-caprock interface are important. The AVO method might provide us with a tool to detect the position and level of saturation of CO2 in a reservoir while and after injection. The upper Jurassic Sognefjord Formation is a potential CO2 storage formation overlain by the Heather and Draupne Formations considered to be the cap rocks in the Smeaheie area within the northern North Sea. In this study we considered two different reservoir-caprock cases with five different saturation, pressure and temperature scenarios for each case to check the sensitivity of the AVO method in this area. These findings will help understanding the change in elastic properties at the reservoir-caprock interface as a function of CO2 saturation, facilitating detection of its migration and possible phase changes.
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Phase and Group Velocities in TI and ORT Media
By A. StovasSummaryPhase and group velocity approximations are important to characterize the anisotroopic matherial. We develop simple and very accurate velocity approximation with simple parameter estimation. Approximations are tested for TI and ORT models.
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Estimation of Unconventional Reservoir Matrix Permeability and Pore Volume Using Rate-Transient Analysis Techniques: Method Refinements
Authors C. Clarkson, A. Vahedian, A. Ghanizadeh and C. SongSummaryA core analysis procedure based on rate-transient analysis theory has been successfully tested in the lab. Two permeability estimates and one pore volume estimate are possible using this technique, with the results being achieved faster than conventional testing (e.g. pulse-decay test). Refinements in one permeability estimation method, based on the distance of investigation, were illustrated. The combination of the rate-normalized pseudopressure derivative, used to identify the end of linear flow, and a new expression for DOI, yielded permeability values very close to the pulse-decay method.
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Laboratory- and Simulation-Based CO2 Storage Resource Estimates for the Bakken Unconventional Tight Oil Formation
Authors J. Sorensen, S. Hawthorne, N. Azzolina, B. Kurz and V. HerdegenSummaryThe Bakken Formation in North America includes two organic rich shale units that serve as oil source rocks with an intermediate tight, silty carbonate unit that serves as the target reservoir. The juxtaposition of a need to improve Bakken oil productivity with a desire to manage carbon dioxide (CO2) emissions from power plants has led to an interest in the injection of CO2 for enhanced oil recovery (EOR) and associated CO2 storage. Characterization methods, including micro-x-ray computer tomography imaging, field emission and focused ion beam scanning electron microscopy, and CO2 sorption isotherm measurement, were applied to quantify the petrophysical factors that control CO2 interactions and oil mobility in the Bakken. Laboratory experiments investigated the ability of CO2 to permeate Bakken rocks and mobilize oil. Modeling was performed to history match experiments, and simulate injection scenarios. These efforts resulted in the development of a refined method for estimating the potential CO2 storage resource of the Bakken. Application of the refined method indicates that the range of storage resource estimates for the Bakken shale units is 5.8 to 26.3 kg CO2/m3 rock. Estimates of storage resource for the nonshale Bakken member ranged from 1.9 to 12.4 kg CO2/m3 rock.
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Detailed profiling organic carbon content of oil shales with thermal core logging technique
Authors E. Popov, Y. Popov, R. Romushkevich, M. Spasennykh and E. KozlovaSummaryThe development of a thermal core logging technique as a field-scale geophysical technique presented new possibilities for multiphysical testing of shales, e.g. preparation of representative databases of the thermal properties for basin modeling and hydrodynamic modeling for thermal methods of EOR, and practical application of correlations between shale thermal conductivity and total organic carbon. The technique of determination of oil shale total organic carbon from the thermal core logging data was developed that provides detailed continuous high-resolution profiling total organic carbon of oil shales along wells. The technique was studied and tested on more than 11 500 oil shale samples from Bazhenov and Domanic Formations when more than 1250 shale samples were also tested for total organic carbon content with Rock-Eval pyrolysis. The data on shale total organic carbon obtained from the measurements with pyrolysis allow to calibrate and control the results of total organic carbon determinations from the thermal core logging. Continuous profiling total organic carbon from continuous thermal core logging provides reasonable selection of shale samples for future investigations and are basis for basin modeling and calculation of reserves
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Breakout Stabilization in Shales - Evidence and Imaging from Large Scale Laboratory Simulation
SummaryThe paper intends to provide an insight into a large scale experimental campaign incorporating numerical simulations to develop understanding and to, refine the shale failure geometry stabilization mechanism and criteria. The paper also discusses the technical details, considerations and observations of the large scale mechanical rock testing campaign (including triaxial and polyaxial block test) which were carried out on 2 shales. The results of the induced onset of breakout and stabilized breakout geometry were laser scanned, measured and compared with the 3D finite element numerical simulation predictions. The outcome of the research is applied in well design considerations, predicts wellbore stability in 3D and offers valuable insights to the well design team. The results provide visualization of the extent and magnitude of the mechanical wellbore damage whilst drilling in shale formations.
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Fluid Flow Pathways in Shales: Damage Induced Permeability Change
Authors M. Houben, J. Van Eeden and S. HangxSummaryShale reservoir potential is largely determined by the connected pore network in the rock and the connection between the pore network and the naturally present or mechanically induced fracture network. These together determine the total permeability of the rock. We have used a number of different techniques to investigate the microstructure and permeability of Early Jurassic Shales from the UK (Whitby mudstone) when intact and with a mechanical induced fracture network. Permeability changes in the shales due to mechanically induced fractures are not straightforward, depending on the bedding orientation with respect to fluid flow and bedding orientation with respect to induced fractures permeability of the samples either increase by orders of magnitude, increase slightly or are very similar before and after fracturing.
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Organic Hosted Porosity in the Wufeng-Longmaxi Shale: A Combined Electron Microscopy and Neutron Scattering Approach
Authors C. Delle Piane, H. Ansari, W. Rickard, M. Saunders, J. Mata and D.N. DewhurstSummaryThe upper Ordovician Wufeng shale and lower Silurian Longmaxi shale are part of the Fuling shale gas play located in the south-eastern part of the Sichuan Basin, southern China, representing the first commercial shale gas production project outside North America. We studied the occurrence of porosity at micro- and nano-scale in samples of contrasting organic richness from the post-mature part of the Wufeng-Longmaxi gas play. Using a combination of high resolution scanning and transmission electron microscopy and small angle neutron scattering we highlight the impact of different types of organic matter (primary versus migrated) on the development of organic matter (OM)-hosted porosity. The results indicate that the overall porosity in the samples is proportional to the organic richness, although the nanoscale imaging revealed that OM-hosted porosity is preferentially present in the migrated bitumen and not in the primary detrital particles. Distinguishing between primary and migrated OM is therefore important for understanding the creation of an interconnected network of OM during hydrocarbon migration. This may have an important control on the estimation of gas in place and the transport properties of the shale.
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Quantitative Analysis of the Pore Structure of Premature-To-Postmature Organic Rich Mudrocks Using Small Angle Neutron Scattering
Authors A. Rezaeyan, V. Pipich, P. Bertier, T. Seemann, L. Leu, N. Kampman, A. Feoktystov, L. Barnsley and A. BuschSummaryThe pore structure of organic rich mudrocks is associated with both inorganic and organic constituents. The contribution of organic matter to the pore structure has been investigated on Posidonia and Bossier Shale samples having different organic carbon content and thermal maturity. Development and distribution of organic matter pores were studied by using small angle neutron scattering technique at a broad pore scale size investigation, from 2 nm to 2 µm. The pore structure of the mudrocks studied is highly complicated at which total pore volume and specific surface area are not significantly affected by thermal maturation, however, the maturity attribute contributes to different pore size distribution on meso- and macro-pores. Thermal maturation is likely to be the factor of amalgamating small organic matter pores into larger pores in overmature organic rich mudrocks, potentially causing an increase in pore volume at macroscale pores. Although not considerably, the increased macroporosity can enhance the permeability of pore network for viscous gas flow in organic rich mudrocks.
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Resolving the Pore Structure and Sorption Properties of Methane in Mudrocks - A Small Angle Neutron Scattering Study
Authors T. Seemann, P. Bertier, N. Maes, A. Rezaeyan, V. Pipich, L. Barnsley, A. Busch and V. CnuddeSummarySmall angle neutron scattering has been combined with supercritical methane sorption resulting in a collection of isothermal scattering curves for Opalinus Clay, Posidonia Shale and Eagleford Shale. Scattering data have been analyzed with respect to sorbed phase behavior for which a two- and three-phase model have been used. The two-phase model clearly indicates the formation of a sorbed phase of which properties like density and volume fraction change with pore size. Application of the three-phase model yields sorbed phase densities higher than the bulk density of methane as predicted by the equation of state provided by NIST. Current work focuses on implementing localized density calculations in order to quantify the effect of pore size on the sorbed phase properties.
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SAXS and WAXS Microscopy Applied to Mudrocks: A New Method for Systematic Multiscale Studies of Porosity, Pore Orientation and Mineralogy
Authors L. Leu, P. Bertier, A. Georgiadis, A. Busch, A. Diaz, J. Klaver, J. Schmatz, V. Lutz-Bueno, J. Ihli, H. Ott and M. BluntSummaryWe apply scanning SAXS and WAXS microscopy to different mudrock samples. The method characterizes the microstructure in terms of porosity and preferential pore alignment of small pores 6 −202 nm size. These small features are experimentally challenging to resolve for statistically relevant sample volumes with state of the art characterization techniques, such as imaging methods. A key novelty in this study is the quantification of the mineralogy and mineral phase content from the WAXS measurements. Thus, a detailed quantification and comparison of important microstructural parameters is achieved. The method is used in a raster scanning mode, where thousands of consecutive measurements are performed, with a high micrometric spatial resolution, over mm sized sample areas. Therefore, simultaneously the variation of the microstructure is resolved on the pore and lamina scale. We propose to use scanning SAXS-WAXS microcopy in future studies for investigations of the systematic relationships between mineralogy and the pore network.
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Improved Visualization of Heterogeneity within Shales: Colour Contoured Maps Constructed from Large Area High-Resolution SEM Montages
Authors J. Buckman and A. BuschSummaryLarge area high-resolution scanning electron microscopy montages are often taken of shales to characterise variability in composition. Although resolution can be superb in such montages, details can be hard to ascertain from the full images, and observations made from single high-resolution tiles are difficult to place within the context of the whole sample. This paper presents a new method that utilizes the individual images collected from such montages, to produce contoured coloured maps to better visualise heterogeneous distributions within shales. The method repurposes previously collected backscattered electron images, through batch image analysis processing, to quickly extract numerical data and produce coloured maps of a range of important parameters. Examples of maps for mean gray value, pyrite, quartz, calcite, porosity and permeability distribution are illustrated and discussed. Such maps allow variability in distribution to be graphically displayed in a fashion that clearly displays any heterogeneity at the millimetre to centimetre scale, as well as numerically defining any variation present.
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On the Evidence of Multiscale Anisotropy of Shales
By L. DurantiSummaryA heuristic and empirical model of elastic anisotropy of shales derived from experimental studies is applied to real geophysical conditions in the attempt to predict elastic constants and anisotropy of shales. Results are then compared to other expressions of elastic properties and anisotropy obtained at different scales. In general, the overall approach is found to be valuable, however there is evidence that further tuning of the initial model, mostly linked to the impact of temperature on elastic and anisotropic properties, will likely reduce the misfit between predictions and real data.
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Insights into Fractures and Fabric of Shales from Microseismicity
By A.F. BairdSummaryShales typically have low permeabilities due to their fine grain size, unless fractures are present to provide fluid conduits. Both aligned fractures and the preferred orientation of phyllosilicate minerals in shales are effective mechanisms to produce seismic anisotropy. Microseismicity recorded during hydraulic fracture stimulation can be used to provide excellent estimates of anisotropy through the observation of shear wave splitting. Here I illustrate through examples how these measurements can be used to infer the development and evolution of vertical fracture parameters during stimulation as well as provide improved estimates of the in situ rock fabric. A key challenge is untangling intrinsic anisotropy due to mineral alignment from extrinsic anisotropy due to cracks and pores aligned parallel to the dominate petrofabric of the shales. This may be improved by incorporating petrofabric and microstructural analyses of shale samples from the reservoir to better constrain the orientations and proportions of the constituent minerals.
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Rapid Workflow for Quantitative Interpretation of Shale Mineralogical Composition From Seismic Data
Authors R. Beloborodov, M. Pervukhina, J. Hauser, M. Josh, V. Shulakova, D. Chagalov, M.B. Clennell, G. Ward and M. WaughSummaryHere we propose a workflow for the quantitative interpretation of clay mineral composition in shales that includes a combination of statistical learning methods and seismic inversion. The workflow is validated on a case study from the Northern Carnarvon Basin, the North West Shelf of Australia, where the thick smectite rich shales seal the source rock. To overcome the issues of rock physics modelling for shales we establish the relationship between mineral composition and elastic properties of shales by applying clustering and regression analysis to available petrophysical and laboratory data. Consequently, Amplitude Variation with Offset/Angle (AVO) inversion is applied to seismic data to estimate elastic properties of shales using linearised Zoeppritz reflectivity equations for anisotropic medium. Finally, the smectite probability maps are obtained by translating the elastic properties into smectite content using the regression relationship established earlier.
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Modelling Advective Gas Flow in a Compact Clay: Application and Assessment of Different Numerical Approaches
Authors J. Harrington, E. Tamayo-Mas, H. Shao, E.E. Dagher, J. Lee, K. Kim, S.H. Lai, N. Chittenden, Y. Wang, I.P. Damians and S. OlivellaSummaryIn a repository for radioactive waste, corrosion of metallic materials leads to the formation of hydrogen and other gases. If the gas production rate exceeds the gas diffusion rate within the pores of the surrounding material, a discrete gas phase forms. Gas pressure will then continue to accumulate until its pressure becomes sufficient to exceed the entry pressure of the surrounding material.
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Borehole Imaging of Natural Fractures and Stress Indicators in the Jurassic Carbonate Shale Plays, Eastern Saudi Arabia
By M.S. AmeenSummaryAn operational geomechanics study has been underway to facilitate the unconventional exploration and appraisal of the Jurassic carbonate shale (mudstone) pay zones of Eastern Saudi Arabia. Part of the study involved the multi-scale imaging of natural fractures and stress indicators. Natural fractures are mainly of microscopic and mesocopic scale (individual fracture length ≤ several ft). No major faults occur in any of the tens of imaged long reach horizontal wells, and the imaged and cored vertical wells. Therefore, the natural fractures facilitate hydrofracturing. The absence of major fault zones or fracture clusters reduces the risk of pressure and fluid thieves during hydrofracturing, and minimizes potential damage associated with fracture hits. The natural fractures and current day stress regime is persistent regionally, and has been actively driven by the Late Cretaceous to present Arabian Plate tectonics. The current day stresses within the pay zones are characteristic of thrust to strike slip tectonics with the maximum horizontal in-situ stress trending ENE-WSW parallel to the current day Zagros stresses. These regional stress and fracture patterns facilitate well planning, completion and stimulation design with horizontal wells designed perpendicular to the maximum horizontal in situ stress.
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Polygonal Faults and Seal Integrity
More LessSummaryThis presentation reviews the impact polygonal faults exert on seal integrity. Often invoked as a class of seal bypass system, polygonal faults are embedded into many effective seals particularly in petroleum systems in which seal and reservoir were deposited in marine slope settings. Direct evidence for the role played by polygonal faults as conduits for fluid migration is sparse. However, there are numerous examples of seismic studies of leakage via polygonal fault networks. Two end member examples are reviewed here from the Ormen Lange Field, offshore Norway, and the Scarborough Field, NW Australia. In both cases, previously formed polygonal faults in the seal acted as fluid pathways during later leakage events, and this leakage occurred under radically different pressure and stress conditions than those prevailing during the formation and growth of the polygonal faults.
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Effect of the Orientation of Beddings on the Distribution of Stress around the Borehole in Anisotropic Shale Formation
Authors W. Li and D. SchmittSummaryThe far-field stresses will be concentrated near a borehole existing in any geological formations. If the magnitudes of the concentrated stresses are sufficiently large, the formation rocks near the borehole yields or even fails. In isotropic formations, the solution of the stress concentration near a borehole is solved in two-dimensions by Kirsch (1898) and three-dimensions by Hiramatsu and Oka (1962) (see also Schmitt et al., 2012). However, many formation rocks, for example shales, are elastically anisotropic due to their oriented mineralogical textures or micro-fractures at many scales. In anisotropic formations, a closed form of the solution is established by Lekhnitskij (1963) and Amadei (1983) . In isotropic cases, the magnitude of the concentrated σθθ on the contour of the borehole equals three times that of the far-field horizontal principal stress ( Kirsch, 1898 ). This conclusion is not suitable for anisotropic formations, such as shale formations. In this research, we study the effect of the orientation of beddings on the concentration of stress by a borehole in anisotropic shale formations.
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Forward Modeling of Evolution of Shale Properties in Geological Time
Authors I. Bekkevold and T. PetersenSummaryThe overburden is an essential part of the petroleum system. It provides the reservoir seal and host a significant part of the production facilities, and reservoir depletion might trigger severe deformations in the overburden compromising both seal and infrastructure integrity. In addition, the initial in-situ conditions in the reservoir (geo-stresses, pore pressure, fractures etc.) are result of the weight of the overlaying sediments. However, since the dawn of the O&G Industry, the main focus of the predictive, modelling and data acquisition efforts was the reservoir, thus, there is a massive lack of data (core samples, logs etc.) and significant gaps in our understanding of the medium and its reaction to our E&P activities.
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Domanik of the Russian Timan-Pechora Province - Structure and Development Prospects
Authors E. Grokhotov and O. PrischepaSummaryThe distribution area and conditions for sedimentation of the Domanik in the Timan-Pechora province are described. The structural features of shale strata and the degree of Domanic catagenesis are given. The method of mastering the Domanik in the Timan-Pechora province is determined.
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Characterization of the Organic Matter in the Bazhenov Formation Deposits From Northern and Central Parts of Western Siberia
Authors E. Leushina, E. Kozlova, A. Voropaev, A. Yurchenko, N. Bogdanovich and M. SpasennykhSummaryThe object of the study is the organic matter from the Bazhenov Formation. Rock-Eval pyrolysis, GC×GC-MS analysis of core extracts, IRMS of gases and kerogen in the Bazhenov Formation shale rocks were performed to characterize the organic matter from Northern and Central parts of Western Siberia. The studies revealed that variations in quality of the BF organic matter in central and peripheral regions are related to differences in both the thermal maturity and sedimentation conditions. More oxidative burial environment in South-Eastern and Northern regions resulted in lower generation potential of the Bazhenov Formation deposits when compared to in Central parts of Western Siberia.
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Marine Mudstone Attributes at Core, Well Log and Seismic Scales: Implications for Characterizing Fluid Flow in Thick Subsurface Shale Successions
By R. JonkSummaryThe recent increase in high-fidelity subsurface data (core, logs and seismic) from unconventional plays has shown that mudstone succesions are much more heterogeneous with regard to flow properties than typically assumed. Much of the flow heterogeneity is ultimately controlled by stratigraphic variations and dictated by the extremes of the lithofacies, which are often volumetrically insignificant, and therefore easily overlooked in a sampling and rock characterization strategy. Recognition of the flow extremes in cores and outcrop, appropriate upscaling to wireline log and seismic attributes, will significantly improve our abilities to predict patterns of fluid flow and retention within overall shale successions. This paper will demonstrate the nature of this variability and how to sample and upscale it from rock properties through seismic attributes in a number of subsurface case studies.
Fine-grained, clay-mineral rich sedimentary rocks deposited in the marine realm are the most common sedimentary rock type preserved in the stratigraphic record. Traditionally, marine mudstones were interpreted to be deposited by gravitational settling through the water column. The implications were that depositional conditions at the seafloor were quiet and steady, resulting in the interpretation of rather homogeneous deposits with fairly uniform attributes.
Within the petroleum industry, the study of mudstones was historically limited to two main disciplines: source rock and seal evaluation. Seal evaluation focused on defining the interface between reservoirs and overlying mudstones (seals). Typically, mechanical and capillary attributes of a limited set of samples are related to the bulk properties of the sealing interval. Some attempts were made to relate sealing attributes to bulk sequence stratigraphic packages ( Dawson & Almon, 2002 ), but limited sample measurements were still related directly to the bulk properties of thick, laterally extensive intervals.
Source evaluation focused on placing specific geologic periods in the context of regional or global anoxic events. With the engrained notion that mudstones recorded steady and quiet depositional conditions, heterogeneity in mudstone attributes with regard to preservation of organic matter are most easily explained with bottom water anoxia. Research on outcrops and well logs by Exxon workers in the late eighties and early nineties ( Creaney & Passey., 1990 ; Bohacs, 1998 ) demonstrated degrees of variability that could be tied to predictive stratigraphic frameworks (and hence, varying depositional conditions as fundamental controls on source rock preservation), but this work did not lead to a significant relook at how mudstone sequences were evaluated for rock and flow properties.
This attitude changed significantly during the rise of shale petroleum plays (“unconventional plays”) in the early 2000s. Suddenly, petroleum geologists were asked for detailed reservoir descriptions and predictions for shale sequences, much as they were used to doing for carbonate and sandstone reservoir intervals. This lead to a realization that (a) flow attributes of mudstone successions varied greatly at all scales, and (b) the industry was poorly equipped to provide meaningful characterizations and predictions for this variability. The necessity for reservoir characterization of mudstone successions, combined with a wealth of high-fidelity subsurface data (core, logs and seismic) coming available, has led to a significant improvement in our ability to characterize mudstone successions ( Passey et al., 2010 ), appreciation of the link between depositional processes and diagenesis ( Macquaker et al., 2014 ) and use modern oceanographic datasets to better understand the controls on transport and deposition of mudstones.
A number of the learnings from the unconventional revolution should be integrated back into evaluations of mudstone successions for applications related to the mechanical and capillary attributes important for understanding subsurface flow and retention of fluids. A key learning is the appreciation for very significant stratigraphic heterogeneity exerting a first-order control on subsurface fluid flow. Any characterization of subsurface shale successions has to include a robust sampling characterization within the stratigraphic framework. In addition, relating a sample measurement to critical and bulk properties of larger units (“upscaling”) has to be done within the understanding of the stratigraphic framework. Here, we will demonstrate examples of this process with a number of subsurface case studies.
Figure 1 shows an example of a detailed stratigraphic model of the Cretaceous Mowry Shale Formation (a mudstone succession containing source rocks, seals and hydrocarbon accumulations) populated with capillary properties to simulate the flow and retention of immiscible fluids (oil and gas) within a water-wet medium. The model was constructed using targeted sampling of identified lithofacies from outcrop and core, and using stratigraphic models to populate the succession with the appropriate lithofacies using upscaling of the attributes to wireline log properties. A key observation from this work shows the fundamental control of ash beds in controlling the flow and retention of hydrocarbon fluids within the succession. While volumetrically insignificant (and easily overlooked in a sampling strategy), these intervals with high capillary threshold pressures dominate the overall migration and retention patterns of hydrocarbon fluids within the succession.
While unconventional with regard to the required drilling and stimulation technology, study of numerous “shale gas” plays demonstrated that hydrocarbon liquids had migrated into certain mudstone reservoir facies and been retained through capillary mechanisms in subtle stratigraphic traps. Subsequent burial of these oil reservoirs led to secondary cracking of oil in conventional pore spaces and the development of solid pyrobitumen filled with gas. An important consequence of these observations is that certain mudstone facies possess pore geometries that allow for trapping of oil at relatively high oil saturations and relatively low capillary pressures. As such, while traditionally viewed as source rocks, seals and “unconventional” reservoirs, many of these mudstone successions contain attributes akin to conventional reservoirs with regard to the potential to retain significant amounts of hydrocarbon fluids through displacement of water in conventional pore systems.
In certain cases, unstimulated flow tests suggest significant permeability networks existing within mudstone successions, even in the absence of obvious fracture networks or sandier interbeds. In order to match reservoir simulations, facies have to be populated with relatively high horizontal permeabilities ( Fig. 2 ). Microscopic studies reveal the presence of mm-scale horizontal bitumen-filled seams, which may have originated as mechanical flow networks during hydrocarbon migration, and continue to provide subtle, yet effective pathways for efficient migration of fluids through mudstone successions.
In summary, mudstone successions are much more heterogeneous with regard to flow properties than typically assumed. Much of the flow heterogeneity is ultimately controlled by stratigraphic variations and dictated by the extremes of the lithofacies, which are often volumetrically insignificant, and therefore easily overlooked in a sampling and rock characterization strategy. Recognition of the flow extremes in cores and outcrop, appropriate upscaling to wireline log and seismic attributes, will significantly improve our abilities to predict patterns of fluid flow and retention within overall shale successions.
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On the Definition of an Effective Stress for Shales
Authors A. Tuttolomondo, A. Ferrari and L. LalouiSummaryEnergy-related engineering applications involving shales pose ever-increasing challenges for the constitutive modelling of these geomaterials. The present work proposes a new definition of effective stress developed by using a thermodynamic approach associated with the concepts of continuum mechanics. The developed expression makes explicit the dependence of the effective stress on the chemical composition of the pore water. The paper shows that the proposed expression is able to improve the interpretation of unsaturated shear strength envelopes that would be achieved by using other formulations.
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Strain-Rate Dependent Pre-Failure, Failure and Post-Failure Behavior of Shale: Experiments and Modeling
Authors M. Gutierrez and Z. HouSummaryThe strain rate dependent mechanical behavior of shale was extensively characterized using triaxial compression tests carried out at different axal strain rates. Based on the experimental results, a constitutive model for shale under different rates of loading was proposed. The model is based on a combination of viscoelasticity and damage mechanics and is formulated to predict the brittle behavior of shales from the pre-peak stage, peak and post peak strain softening regimes. Shear failure and strain softening are attributed to damage due to the growth of fractures in the shale, and de-bonding and decohesion mechanisms responsible for the fracture evolution. Damage is described by a scalar variable D and is assumed to commence when the stress-strain behavior deviates from linear elasticity. It was found that damage evolution during shearing in shale can be adequately represented probabilistically using a Weibull probability distribution function based on the axial strain level. An empirical axial strain rate dependent Young's modulus, together with the damage evolution law, completes the viscoelastic damage model. The model is shown to adequately represent the complete stress-strain response of shale at different axial strain rates and to predict the axial strain rate dependent shear strength of shale.
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Novel Finite Element Approach on Evolution and Stabilization of Breakout in Shales
SummaryThis paper describes a novel approach to model breakout failure whereby cell removal is introduced to account for actual breakout geometry and stabilisation. Finite element modelling was conducted in several selected wells with good quality image log and caliper data to develop and subsequently validate the approach.
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Heating Induced Shale Barriers
Authors A. Bauer, X. Xie, J.F. Stenebråten, S. Bakheim, A. Lavrov, E. Fjær and T.G. KristiansenSummaryIn this work, it was investigated if heating of a cased borehole in a shale formation can induce plastic deformations that are sufficiently high to close the annulus between shale and casing and form a hydraulic barrier. This is of interest for plug and abandonment (P&A) of wells. Both laboratory experiments and numerical simulations have been carried out. Borehole heating experiments were performed with hollow-cylinder shale samples, using a novel experimental setup that allows for temperature and pore pressures measurements at different radial positions inside the shale samples. The results show that borehole heating results in a relatively large increase of pore pressure in the shale, inducing rock failure and annulus closure under certain temperature and stress conditions. A finite-element model was used for simulating the experimental data and obtaining relevant thermo-poro-elasto-plastic parameters. Those parameters were used in field-scale finite-element models to assess the possibility of forming shale barriers by borehole heating. The numerical models provide a good description of the experimental data; they indicate that under certain conditions heat induced displacement at the borehole wall is large enough to close the annulus around a casing.
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Hydration State and Interlayer Cation Type (Ca2+, Na+) Control CO2 Sorption Behavior of SWy-2 Montmorillonite
Authors R. Fink, P. Bertier, B. Krooss and P. WenigerSummaryCO2 uptake behavior of smectitic clays is less predictable compared to other geomaterials as molecules (especially H2O) and ions sorb in the interplayer space leading to a volumetric expansion. In the context of geologic sequestration of carbon dioxide, it is important to understand how this multi-component system reacts upon exposer of supercritical CO2. In this study, we performed high-pressure CO2 adsorption experiments on expandable clays (Na+ and Ca2+ exchanged SWy-2 Montmorillonite) at relevant conditions for CO2 sequestration. With increasing hydration from dry (0W) state to one-layer hydrate (1W) the CO2 excess sorption capacity sharply increases and then decreases again with hydration to two-layer hydrate (2W). The magnitude of this effect is strongly controlled by the interplayer cation type (Ca2+, Na+). We interpret that this behavior is closely linked to the structure and swelling behavior of these clays by CO2 adsorption. Dry montmorillonite hardly expands when exposed to CO2 whereas a strong shift in the mean basal spacing d001 between the hydrated clay layers upon exposure to CO2 is observed around the sub1W hydration state. This indicates that high-pressure CO2 adsorption in the presence of water is strongly influenced by the intercalation of CO2 into the expandable interplayers.
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Organic Matter of Bazhenov Formation: Chemical and Isotope Composition, Microstructure, Thermal Maturation, Generation Potential and the Effect on Rock Properties
Authors M. Spasennykh, A. Voropaev, E. Kozlova, E. Leushina, V. Polyakov, A. Vasiliev, N. Bogdanovich, Y. Popov and A. MaximovSummaryThe Bazhenov shale Formation is one of the largest source rock formation in the world. In this research the chemical and isotope compositions, the microstructure and other parameters of organic matter of the Bazhenov Formation from the Central, East, West and North parts of Western Siberia have been studied. A wide spectrum of analytical methods has been applied, including Rock-Eval pyrolysis, kinetics of kerogen thermal decomposition, electron-ion microscopy, X-ray micro-tomography, gas chromatography–mass spectrometry, solid state NMR and X-ray photoelectron spectroscopy, isotope measurements and other. It was shown that geochemical parameters of the organic matter of Bazhenov Formation, including kerogen, oil and gas chemical and isotope compositions, microstructure of kerogen and other considerably vary for different oilfields of Western Siberia depending on thermal maturity and other geological and lithological factors. Correlations between geochemical characteristics and petrophysical properties of rocks, including density, fluid saturation, geomechanical, thermal and other properties have been studied. Obtained results provide good opportunities for the development of advanced methods for shale reservoir characterization basing on logging and core analysis data
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Is the Uranium Content in Black Shale a Suitable Parameter to Assess the Alteration of Organic Matter and Petroleum Potential?
By H. SchulzSummaryThe simultaneous occurrence of high TOC and high uranium contents in black shale indicates that uranium is concentrated in the organic matter. Nevertheless, such uranium-OM correlations may be misleading as uranium-bearing minerals also occur in black shale. Thus, the spatial uranium distribution in black shale, but also the diagenetic timing of uranium fixation or possible later remobilization are critical factors, and control when and to what degree the OM may have been degraded due to the natural decay of high uranium contents. Previous studies only considered the bulk uranium content of samples as a parameter in estimating the degree of irradiation effects on the OM, and the sample sets were normally from different locations with different thermal maturities. Moreover, vertical and lateral variations in lithology and mineralogy as result of depositional environments as well as thermal maturation during burial are additional controls for different uranium contents and phase occurrences. In this contribution, published data from Schulz et al. (2019) will be presented about the approximately 4 m thick and thermally immature western Russian Koporie Formation (“Dictyonema” Shale) using an interdisciplinary analytical approach in order to answer questions about the source of uranium, its occurrence in the shale, both spatially and temporarily, and the consequences for the OM and the petroleum potential.
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Origins of Shale Gases Based on Revised Gas Genetic Diagrams
Authors A.V. Milkov and G. EtiopeSummaryWe collected a large dataset of gas samples recovered from shale formations around the world and interpreted the origin of shale gases using recently revised gas genetic diagrams. Gas in most shale plays has thermogenic origin. Some shale plays contain secondary microbial gas. Primary microbial gas is not common in shale play.
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Developing Unconventional Reservoirs Using Limited Natural Fractures Statistics – Challenges and Opportunities
By A. OuenesSummaryGiven the recognized importance of the natural fractures in the development of unconventional reservoirs, the sparse statistics created by the lack of cores and image logs requires practical engineering approaches and solutions. Among these solutions is the use of a continuous fracture model that uses a representative volume to describe the fracture density that can be estimated from seismic and surface drilling data. This approach leads to a quantitative use of these natural fractures and their interaction with hydraulic fracture using a robust geomechanical simulation able to predict microseismicty thus validating both the used natural fracture model and the geomechanical modelling approach.
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