What Do P-Wave Velocities Tell Us About the Critical Zone?

Fractures in Earth's critical zone influence groundwater flow and storage and promote chemical weathering. Fractured materials are difficult to characterize on large spatial scales because they contain fractures that span a range of sizes, have complex spatial distributions, and are often inaccessible. Therefore, geophysical characterizations of the critical zone depend on the scale of measurements and on the response of the medium to impulses at that scale. Using P-wave velocities collected at two scales, we show that seismic velocities in the fractured bedrock layer of the critical zone are scale-dependent. The smaller-scale velocities, derived from sonic logs with a dominant wavelength of ~0.3 m, show substantial vertical and lateral heterogeneity in the fractured rock, with sonic velocities varying by 2,000 m/s over short lateral distances (~20 m), indicating strong spatial variations in fracture density. In contrast, the larger-scale velocities, derived from seismic refraction surveys with a dominant wavelength of ~50 m, are notably slower than the sonic velocities (a difference of ~3,000 m/s) and lack lateral heterogeneity. We show that this discrepancy is a consequence of contrasting measurement scales between the two methods; in other words, the contrast is not an artifact but rather information—the signature of a fractured medium (weathered/fractured bedrock) when probed at vastly different scales. We explore the sample volumes of each measurement and show that surface refraction velocities provide reliable estimates of critical zone thickness but are relatively insensitive to lateral changes in fracture density at scales of a few tens of meters. At depth, converging refraction and sonic velocities likely indicate the top of unweathered bedrock, indicative of material with similar fracture density across scales.

Comparing the results of lineament analysis with isotope geochemistry data

This article presents the results of a geochemical survey carried out in the southwestern part of the Siberian platform, within the Sayan-Yenisei (Angara) syneclise (a superorder Riphean-Middle Paleozoic structure). The object of research was hydrocarbon gases contained in the subsoil rocks (clays). The subsoil samples were taken from the bottom of boreholes (40 mm in diameter) made with an electric drill. The sampling depth was 0.6–1 m. Further laboratory studies included chromatographic and isotope analysis. Lineament analysis of the digital elevation model was carried out as a complementary study. One of the lineament analysis results was a lineament density map, which reflects the permeability (macro-fracture density) of the sedimentary cover. This allowed a comparison of the macro-fracture density with the gas content and isotopic composition. The study revealed that gases with a high content of heavy isotopes tend to gather in the low permeability areas. This can be explained by the fact that the gases disperse quickly within fractured zones, and deep gases should be expected only in the areas with strong cap rocks, i.e. in the areas with low macrofracture density where stable hydrocarbon deposits have already formed. Keywords: hydrocarbons; geochemical survey; isotope geochemistry; lineament analysis.

Outcrop Analogues to Fractured Reservoirs, UAE

Fractures were not the focus of reservoir studies in Abu Dhabi for the last decades, although its importance in enhancing production, as the general understanding considering fractures are not contributing to production. The fractured carbonate outcrops provide useful analogue observations, data and concepts to support subsurface hydrocarbon reservoir characterization from well and seismic data. The fracture orientation, size, porosity, length, spacing, crosscutting relationships, fracture density versus lithology and bed thickness and connectivity are difficult to measure directly from subsurface well and core data. The understanding of fracture formation and distribution and their effects on fluid flow has been greatly improved by the use of outcrop analogue data through the current work. This paper address the fracture geometry, kinematics and mechanical properties based on outcrops matching Abu Dhabi subsurface reservoir analogues. Integrating outcrop data with fracture orientation and fracture density from core and borehole image data, and seismic capturing fractures characteristics. The outcrop analogues constrain the uncertainty and developing new concepts in characterizing the interplay of rock matrix and fracture networks relevant to fluid flow and hydrocarbon recovery. Analysing the fractures with fracture lengths, aperture, spacing per each interval and relate them to the tectonic event are extracted strictly in the reservoir section. The results showing developed highly dipping shear fractures with short length, small spacing and bimodal aperture distribution that related to fracture orientation. Fracture porosity is dependent on size and controlled by lithology, bed thickness, paleostress and rock composition. Understanding fractures and their behaviour will optimize production greatly and they create exploration targets in otherwise tight reservoir zones, including under-explored sections.

Application of Wide-Azimuth Anisotropy Inversion Predicting Carbonate Fracture Intensity and Direction

Fractures in carbonate rock are both storing reservoirs and migrating channels for oil and gas, so such fractures are one of the key targets for oil exploration and development. Traditional fracture prediction methods by seismic data include ant tracking cube, coherence cube and other seismic attributes. Fractures predicted by these methods are less accurate. This paper introduces a wide-azimuth anisotropic inversion method to effectively predict the fracture density and direction in carbonates. a wide-azimuth seismic anisotropy inversion workflow is established to predict the fractures in carbonates, and consequently the fractured zones in the target layer. The key steps include: (1) carry out quality control and optimization of wide-azimuth seismic gathers; (2) conduct pre-stack simultaneous inversion of pre-stack seismic data at partial sub-offsets and sub-azimuths to obtain the Vp/Vs of the azimuths; (3) use Azimuthal Fourier Coefficient to calculate the anisotropic gradient and direction. Based on the anisotropic intensity and direction and elastic parameters in the study area, the density and direction of fractures are obtained. The prediction results show that in the study area, nearly SN-striking fractures are developed, which are chiefly tectonic fractures, and consistent with the imaging logging results. It has been proved that the method is reasonable and feasible, and the accuracy of fracture prediction is improved.

ANALISIS CITRA SATELIT LANDSAT 8 DAN DEMNAS UNTUK IDENTIFIKASI PROSPEK PANAS BUMI DI KABUPATEN ACEH TENGAH, PROVINSI ACEH

Gabungan beberapa analisis pada citra satelit Landsat dan Digital Elevation Model Nasional (DEMNAS) dapat dipergunakan untuk mengidentifikasi indikasi area prospek panas bumi. Analisis dilakukan di Kabupaten Aceh Tengah yang diawali dari informasi keberadaan mata air panas pada peta geologi regional lembar Takengon. Metoda penginderaan jauh seperti metoda Fault and Fracture Density (FFD) dan interpretasi circular feature diterapkan pada citra DEMNAS. Sedangkan metoda Land Surface Temperature (LST) dan Direct Principal Component Analysis (DPCA) diterapkan pada citra Landsat 8. Kenampakan circular feature, anomali LST dan indikator adanya mineral ubahan bersuhu tinggi, dapat digunakan untuk memperkirakan keberadaan sumber panas. Sedangkan penerapan FFD digunakan untuk memperoleh indikator adanya zona dengan permeabilitas tinggi yang diperlukan dalam sistem panas bumi. Hasil penelitian menunjukkan bahwa indikasi sumber panas diperkirakan berada pada komplek vulkanik Gunung Telege yang berada di daerah Kecamatan Atu Lintang. Hal ini diperlihatkan dengan adanya circular feature dan anomali LST yang terdapat di daerah tersebut. Penerapan metoda FFD mengindikasikan adanya zona outflow yang berada di sekitar manifestasi mata air panas yang terletak di sebelah barat laut Gunung Telege. Sedangkan dari hasil penerapan metoda DPCA sulit untuk diinterpretasi dikarenakan belum adanya pemisahan yang tegas antara indikator zona argilik lanjut dan zona propilitik dari hasil DPCA tersebut. Hal ini kemungkinan disebabkan adanya nilai pencampuran antar beberapa indikasi mineral dalam satu piksel yang sama. Secara umum, penggunaan metoda penginderaan jauh di Kabupaten Aceh Tengah dapat membantu untuk memberikan petunjuk awal adanya kemungkinan sistem panas bumi di daerah tersebut

Natural Fractures Characteristics of the Carboniferous Volcanic Reservoir in Northwestern Margin of Junggar Basin

Research on the regional fracture’s development is important for reservoir fracturing. This paper takes the Carboniferous volcanic reservoir in the northwestern margin of Junggar Basin as the research object. Based on understanding the regional tectonic faults and geological characteristics, the parameter characteristics of natural fractures are analyzed using imaging logging data, and natural fractures distribution characteristics are compared with regional faults and in-situ stresses, as well as the pattern of natural fractures formation is revealed. The results indicated that: (1) The Carboniferous in the northwestern margin of Junggar Basin area mainly develops 3 NE-trending reverse faults. The reservoir type is pore-fracture dual media type, with an average porosity of 7.64% and an average permeability of 1.16mD, which belongs to the medium-porosity and ultra-low permeability reservoir; (2) Reservoir fractures are generally well developed. High-conductivity fractures and high-resistance fractures coexist, but high-conductivity fractures are the main ones. The fracture width is between 0.053 and 0.23 mm, and the fracture density is between 0.5 and 1.68 strips/m. The length is between 0.54-1.88m, the fracture porosity is between 3.4×10-5-41×10-5, and the dominant fracture trend is mainly NE50°-NE80°; (3) The direction of the maximum horizontal in-situ stress of the reservoir is mainly NE30°-NE60°, in the direction of NEE, it differs from fracture strike by 10°-50°, and roughly the same as the strike of the three reverse faults.

A Bio-Based Render for Insulating Agglomerated Cork Panels

Natural and bio-based thermal insulation materials play an important role in the lifecycle impact of buildings due to their influence on the amount of energy used in indoor temperature control and the environmental impact of building debris. Among bio-based materials, cork is widespread in the Mediterranean region and is one of the bio-based materials that is most frequently used as thermal insulation for buildings. A particular problem is the protection of the cork-agglomerated panels from external stress and adverse weather conditions; in fact, cork granulates are soft and, consequently, cork panels could be damaged by being hit or by excessive sun radiation. In this study, an innovative external coat for cork-agglomerated panels made of a blending composite of beeswax and rosin (colophony) is proposed. The performance of this composite, using different amounts of elements, was analysed to discover which mix led to the best performance. The mix of 50% beeswax and 50% rosin exhibited the best performance out of all the mixes. This blend demonstrated the best elongation and the lowest fracture density, characteristics that determine the durability of the coating. A performance comparison was carried out between cork panel samples coated with lime render and beeswax–rosin coating. The coating of beeswax and resin highlighted a detachment value about 3.5 times higher than the lime plaster applied on the side of the cork.

Factors influencing gas well productivity in fractured tight sandstone reservoir

There are many factors which influence the absolute open flow potential (AOFP) of gas well. One of them is the angle between maximum principal stress direction and natural fracture strike in gas reservoir. In order to find out how the angle influences the AOFP of gas well. A lot of data related to gas well productivity of 14 wells located in gas reservoir T were collected and collated. Influential intensity of each factor on the AOFP before and after reservoir modification was investigated through grey relation analysis method. Results indicated that the AOFP of gas well before and after reservoir modification was governed by 10 factors. The five central factors influencing the initial AOFP are natural fracture density, porosity, permeability, elevation of geological top surface, and gas saturation, respectively. The five central factors influencing the AOFP of hydraulically fractured gas well are porosity, gas saturation, elevation of geological top surface, minimum principal stress, and permeability, respectively. Angle between maximum principal stress direction and natural fracture strike was not the central factor influencing gas well productivity. Reservoir modification can greatly improve gas well productivity in fractured tight sandstone reservoir. Natural fracture density was the strongest influencing factor of the initial AOFP. Minimum principal stress was one of the central factors influencing the AOFP of hydraulically fractured gas well. Research results can be used to guide well deployment and gas productivity investment projects of fractured tight sandstone reservoir.

Claystone formations in Germany: what we (don't) know about them and how we can change this

Deep geological formations are considered for safe long-term disposal of high-level radioactive waste. Such a repository would be requested to prevent radionuclides from entering the biosphere for a period of 1 million years (StandAG, 2017). Consequently, a holistic characterization including lithological, mineralogical, geochemical, hydrological, structural and geomechanical properties of any potential repository-hosting rock formation is required. Nine claystone formations have been identified as “sub-areas” within the German site-selection procedure (BGE, 2020). The area covered by these formations comprises about half of the total area considered as being qualified for further exploration. However, despite its relevance to act as a geological barrier for, e.g. hydrocarbons or radionuclides, the characterization of clay-rich formations at depths exceeding 300 m in Germany has attained substantially less attention than economically more relevant units hosted by, e.g. sandstones or rock salt, which have been intensively explored. The BGR project BASTION aims at contributing to characterizing these claystone formations and emphasizes properties relevant to host a repository for nuclear waste. Investigations comprise (micro)structural/petrographic, mineralogical, geochemical, geophysical, hydraulic and thermomechanical analyses. In project phase I (2013–2019), claystones deposited in Northern Germany during the Lower Cretaceous were studied. These rocks belong to the fourth largest sub-area hosting claystones. Two of the main foci were to explore variations in lithology, mineralogy and geochemistry, and to identify deformation mechanisms (natural and artificial) by microstructural analyses. Although rocks appeared to be quite homogeneous on the 10–100 m scale, the results revealed distinct structural and sedimentary heterogeneities on the meter scale affecting fracture density. Another sub-area located in Southern Germany hosts the Opalinus Clay Formation (OPA). This up to 150 m thick claystone formation was deposited during the Middle Jurassic (Franz and Nitsch, 2009). Owing to its self-sealing capacity and ability to retain fluids, it is supposed to host the nuclear waste repository of Switzerland (Bossart et al., 2017). The OPA is quite well understood in terms of its lithology and (bio)stratigraphy, and there have been mineralogical, hydrological and petrophysical analyses, mostly documented in university theses a few decades old and sometimes difficult to access. However, it is questionable to what extent these investigations reflect the situation at depths relevant for the site-selection procedure. Well-documented data on the OPA and its properties relevant for nuclear waste disposal are available via the Swiss site-selection procedure (Bossart et al., 2017). However, as there remain substantial questions regarding the nature of the German portions of the OPA (e.g. spatial distribution of lithology, mineralogy, microstructures) at depths greater than a few decameters, it is unclear to what degree insights obtained in the Swiss site-selection procedure also account for Germany. Therefore, phase II of BASTION, which began in 2020, aims to use the multidisciplinary approach developed during phase I to characterize properties of the OPA relevant for the save long-term disposal of nuclear waste by identifying and quantifying structural and rheological heterogeneities. This will constitute important input for numerical models in any long-term safety assessment.

Analyzing the Impacts of Meshing and Grid Alignment in Dual-Porosity Dual-Permeability Upscaling

Summary The discrete fracture network (DFN) model is widely used to simulate and represent the complex fractures occurring over multiple length scales. However, computational constraints often necessitate that these DFN models be upscaled into a dual-porositydual-permeability (DPDK) model and discretized over a corner-point grid system, which is still commonly implemented in many commercial simulation packages. Many analytical upscaling techniques are applicable, provided that the fracture density is high, but this condition generally does not hold in most unconventional reservoir settings. A particular undesirable outcome is that connectivity between neighboring fracture cells could be erroneously removed if the fracture plane connecting the two cells is not aligned along the meshing direction. In this work, we propose a novel scheme to detect such misalignments and to adjust the DPDK fracture parameters locally, such that the proper fracture connectivity can be restored. A search subroutine is implemented to identify any diagonally adjacent cells of which the connectivity has been erroneously removed during the upscaling step. A correction scheme is implemented to facilitate a local adjustment to the shape factors in the vicinity of these two cells while ensuring the local fracture intensity remains unaffected. The results are assessed in terms of the stimulated reservoir volume calculations, and the sensitivity to fracture intensity is analyzed. The method is tested on a set of tight oil models constructed based on the Bakken Formation. Simulation results of the corrected, upscaled models are closer to those of DFN simulations. There is a noticeable improvement in the production after restoring the connectivity between those previously disconnected cells. The difference is most significant in cases with medium DFN density, where more fracture cells become disconnected after upscaling (this is also when most analytical upscaling techniques are no longer valid); in some 2D cases, up to a 22% difference in cumulative production is recorded. Ignoring the impacts of mesh discretization could result in an unintended reduction in the simulated fracture connectivity and a considerable underestimation of the cumulative production.