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Reservoir-triggered seismicity commonly occurs as a result of reservoir impoundment. In particular, the Nova Ponte reservoir triggered a series of earthquakes, including the 1998 M4.0 earthquake, which represents the second-largest earthquake triggered by reservoir impoundment in Brazil. The earthquake occurred after prolonged seismic activity following reservoir impoundment starting in 1993. After more than two decades, the mechanisms governing these earthquakes and their relation with the spatiotemporal evolution of the seismic events are still poorly understood. Here, we explain the causal mechanisms of the two largest earthquakes: an initial response M3.5 in 1995 and the delayed M4.0 in 1998. To this end, we numerically simulate the poromechanical subsurface response to reservoir impoundment using a 3D model that includes three geological layers down to 10 km depth. From the proposed potential nodal planes of the 1995 M3.5 earthquake, we show that the earthquake has most likely occurred on a vertical, E–W-oriented strike-slip fault with a reverse-displacement component. Deviatoric stresses generated by the water column loading on the surface, superimposed by undrained pore pressure enhancement in deep low-permeability layers can explain the fault reactivation. We find that for the 1998 M4.0 earthquake to occur, conductive flow pathways with permeability as high as 6.6·10−15 m2 should exist to transmit pore pressure to a deep critically oriented fault. Our analysis raises the importance of accounting for coupled poromechanical mechanisms controlling fault stability, hydromechanical properties of different rock layers, and realistic shape of the reservoir to accurately assess the potential for reservoir-triggered seismicity. We conclude that reliable forecasting models require accurate subsurface characterization before reservoir filling to enable managing the associated reservoir-triggered seismicity. [ABSTRACT FROM AUTHOR]

Liquid nitrogen (LN2) fracturing is a kind of non-aqueous fracturing technology, which is expected to provide a new and efficient way for coalbed methane (CBM) development. The mechanical properties of coal under LN2 freezing are very important for studying the mechanism of LN2 fracturing. However, most of the current research is limited to studying mechanical properties of rocks after being frozen by LN2 and returned to room temperature. In this paper, the effect of LN2 freezing on the mechanical properties of coal was studied. Uniaxial strength tests and Brazil tests were carried out for dry and water-saturated coal samples with different types and bedding directions. In addition, standard electron microscopy (standard SEM) and cryo-electron microscopy (Cryo-SEM) were used to compare the fracture morphology of coal samples at room temperature and LN2 temperature. The results showed that LN2 freezing can damage and improve the mechanical properties of coal simultaneously. The strength of saturated coal under freezing is higher than that of dry coal, and the filling of ice can enhance the mechanical strength of coal. In addition, the mechanical properties of coal with higher porosity are enhanced more than that of coal with lower porosity under LN2 freezing. The main findings of this study are the keys to the research of LN2 fracturing mechanisms in CBM reservoirs. [ABSTRACT FROM AUTHOR]

In this paper, different types of rocks exploited in a quarry located in the Limeira region (São Paulo State, Brazil) were studied to investigate its main petrographic characteristics, physical–mechanical (apparent specific weigh ρa, apparent porosity ηa, water absorption αa and uniaxial compressive strength UCS) and thermoanalytical properties, whose curves of each of the rocks were obtained from thermogravimetry (TG), differential thermal analysis and thermomechanical analysis (TMA). Four main petrographic types were evaluated: an aphanitic rock (APH), a diabase (DIA) and two monzodiorites with distinct granulations designated as MOMM and MONMC. In terms of mineralogical composition APH is the only rock that presents volcanic glass, while for the other rocks the main mineralogy was always associated with plagioclases, clinopyroxenes and opaque minerals. The results of the physical and mechanical characterization indicate the good aptitude of these materials to be used as aggregates in engineering works. The obtained values of apparent porosity (ηa) were less than 1% for all rocks. The value of resistance rates for all samples showed that the APH has the larger value, which is attributed to the grain size of the rock. The TG curves of each of the four samples contain small mass losses between the initial temperature and around 105 °C, which attributed to the water absorption. Besides, the APH sample had a mass loss attributed to the carbonate compound. The TMA curves showed that form DIA and MOMM are similar between them, while the APH differ from both. Further, the MONMC has not presented an appreciable thermal expansion. [ABSTRACT FROM AUTHOR]

Coquinas are a sedimentary rock composed mainly of shells and their fragments with a carbonate or siliciclastic matrix. These rocks exhibit a high range of porosity and permeability values that are related to depositional and diagenetic processes. A series of coquina reservoirs were discovered in the Campos and Santos Basins in the eastern margin of Brazil and the understanding of the petrophysical properties of these rocks is essential for reservoir production. In this work, samples from the Morro do Chaves Formation (Sergipe–Alagoas Basin) and the Itapema Formation (Santos Basin) were selected to identify the porosity vs permeability relationship according to the biofabric characteristics and porosity control. The biofabric textures where classified into four groups according to taphonomic patterns and diagenetic modification: (i) well-sorted and preserved biofabric, (ii) well-sorted and altered biofabric, (iii) unsorted and preserved biofabric, and (iv) unsorted and altered biofabric. The porosity control varies in all groups from depositional to diagenetic with distinct pore types, pore sizes, and pore connectivity that are located in different global hydraulic elements. The analysis of the pore space was carried out through laboratorial measurements (gas injection) and by images from high-resolution computed tomography from plug samples, resulting in a pore network model where values of pore radius, pore throat radius, and coordination number were obtained. The best values of porosity and permeability are related to well-sorted altered biofabrics, whereas the lowest values, either with unsorted biofabrics, with low pore connectivity, or biofabrics that had the porosity reduced by compaction and cementation. [ABSTRACT FROM AUTHOR]

Geomechanical classifications of rock masses are of great interest and importance for the design of civil and mining works, as slopes, open pits, and underground excavations stability are directly related to the mechanical properties of the rock mass classes. Some of the most common geomechanical rock mass classifications currently used are the RMR (Rock Mass Rating), the Q system, and the GSI (Geological Strength Index). Recently (2011), it was presented a new approach for RMR in which the author had pointed an erroneous understanding by some of the RMR users related to the weights adopted for some parameters. Due to the great difficulty found by some professionals to classify highly weathered rock masses by using RMR89, various authors have proposed a correction of for RMR89 values by considering an adjustment index. On this context, the present work has the aim of verifying, for several rock masses from Quadrilátero Ferrífero, Minas Gerais, Brazil, the effectiveness of these three proposed adjustments on RMR89. The results has shown that the corrections proposed better represent the weathered rock masses usually found on open pit on Quadrilátero Ferrífero. [ABSTRACT FROM AUTHOR]

Most subsurface rock models consider salt bodies as homogenous masses and set to evaporite minerals constant values of elastic properties such as density, compressional and shear velocities, Young's modulus and Poisson's ratio. Lithological analyzes of outcrops and wells indicate that evaporites present large vertical and lateral heterogeneities. Recent studies have demonstrated that the knowledge of the elastic properties of rock salt can substantially benefit velocity modeling, seismic imaging, reservoir geomechanical analysis, prediction of wellbore stability and calculation of optimum fluid weight. However, few papers document the spatial estimation of elastic properties for salt formations. We propose an approach to characterize the internal structures of the salt bodies in the Santos Basin, offshore Brazil, using well and 3D seismic data. It starts with a rock-physics analysis at well-log scale, which reveals that the elastic properties of rock salt are highly correlated. Once the relationships of the elastic properties are established, we explore the functionality of the empirical equations as property predictors. We demonstrate that the elastic properties can be empirically estimated from compressional velocity. The comparison between the estimated and the measured well logs validates the estimation. Then, we perform the seismic inversion to generate the spatial distribution of the acoustic impedance. We derive compressional and shear velocities, density, Young's modulus and Poisson's ratio volumes by applying the empirical equations to the acoustic impedance volume. The blind well points out that, through this workflow, the seismic data can successfully predict the elastic properties of the salt formation in undrilled portions. • Studies of in-situ elastic behavior of evaporites. • Formulation of new empirical equations as property predictors. • Compaction has null influence on the elastic properties of rock salt. • Combination of seismic inversion and rock physics for quantitative characterization. • Review of multidisciplinary applications of elastic properties of rock salt. [ABSTRACT FROM AUTHOR]

Three-dimensional distribution of karstic zones and estimated porosity were obtained in fractured carbonate units of the Salitre Formation (Brazil) from GPR data. Low-amplitude (shadow) GPR zones could be associated with mineral dissolution halos that developed around fractures and bedding planes. GPR volumes show a 3D mazelike geometry of the vertical karstified carbonate zones, similar to the set of passages controlled by ~ N–S and E–W oriented fracture systems, observed on the surface in unmanned aerial vehicle images. Measurements in plug samples indicate that the mineral dissolution caused bulk density and dielectric permittivity decline and incipient increase in porosity from 0.95 to 1.01%. These changes in the rock properties affect the EM impedance contrasts between carbonate bedding, causing loss of the reflected GPR signal. The correlation between dielectric permittivity and porosity could be analyzed using the complex refractive index model (CRIM), which is used to convert the relative dielectric permittivity into porosity. The CRIM porosities are somewhat lower than plug sample porosities due to different estimation methodologies and measurement scales. However, the fact that porosity is slightly higher in the dissolved carbonates was maintained. In addition, GPR amplitude and frequency instantaneous attributes were used to estimate spatial variability in porosity throughout the whole GPR data volume. The crossplot of these two GPR attributes show that altered zones generally cluster in narrow ranges of instantaneous GPR amplitude and in broad range of instantaneous GPR frequency. Assuming empirical correlation between physical properties and GPR attributes two predicted porosity volumes were established for the karstified carbonates in the studied area. [ABSTRACT FROM AUTHOR]

For Brazil, a country frequented by droughts and whose rural inhabitants largely depend on groundwater, reliance on isotope for its monitoring, though accurate, is expensive and limited in spatial coverage. We exploit total water storage (TWS) derived from Gravity Recovery and Climate Experiment (GRACE) satellites to analyse spatial-temporal groundwater changes in relation to geological characteristics. Large-scale groundwater changes are estimated using GRACE-derived TWS and altimetry observations in addition to GLDAS and WGHM model outputs. Additionally, TRMM precipitation data are used to infer impacts of climate variability on groundwater fluctuations. The results indicate that climate variability mainly controls groundwater change trends while geological properties control change rates, spatial distribution, and storage capacity. Granular rocks in the Amazon and Guarani aquifers are found to influence larger storage capability, higher permeability ( >10 -4 m/s) and faster response to rainfall (1 to 3 months' lag) compared to fractured rocks (permeability <10 -7 m/s and lags > 3 months) found only in Bambui aquifer. Groundwater in the Amazon region is found to rely not only on precipitation but also on inflow from other regions. Areas beyond the northern and southern Amazon basin depict a ‘dam-like’ pattern, with high inflow and slow outflow rates (recharge slope > 0.75, discharge slope < 0.45). This is due to two impermeable rock layer-like ‘walls' (permeability <10 -8 m/s) along the northern and southern Alter do Chão aquifer that help retain groundwater. The largest groundwater storage capacity in Brazil is the Amazon aquifer (with annual amplitudes of > 30 cm). Amazon's groundwater declined between 2002 and 2008 due to below normal precipitation (wet seasons lasted for about 36 to 47% of the time). The Guarani aquifer and adjacent coastline areas rank second in terms of storage capacity, while the northeast and southeast coastal regions indicate the smallest storage capacity due to lack of rainfall (annual average is rainfall <10 cm). [ABSTRACT FROM AUTHOR]

Hydrothermal silicification and deformation bands influence the physical properties of porous siliciclastic rocks. However, the impacts generated by the coexistence of these two processes on fault zone flow properties, such as porosity and permeability reductions, are still debatable. We integrated structural, geomechanical, and petrophysical data to investigate the influence of hydrothermal silicification on the physical properties of a fault zone. The fault affects Precambrian crystalline rocks and porous sandstones in the Rio do Peixe Basin, northeastern Brazil. The results indicate that quartz cementation is confined to a halo along the fault. Silicification decreases away from the fault zone toward the basin, which we subdivided into three main zones: (1) nonsilicified sandstone – Zone I, (2) moderately silicified sandstone – Zone II, and (3) intensely silicified sandstone – Zone III. The elongated geometry, the thickness of the silicified body, the proximity to the fault, and the high silica cement concentration indicate an external silica source. Nevertheless, we also propose internal sources related to feldspar dissolution. The primary porosity is rarely preserved in Zone III, while the secondary porosity is filled with silica cement. The primary sandstone porosity is well-preserved in Zone II, while many dissolved feldspar grains are not filled with cement. The petrographic and petrophysical analysis (cataclastic matrix amount) indicates that cement precipitation also occurred inside the deformation bands. Silicification increased the uniaxial compressive strength (UCS) by one order of magnitude in the fault damage zone relative to the protolith. On the other hand, the porosity and permeability in the silicified fault zone exhibit a reduction of two and four orders of magnitude, respectively, relative to the undeformed and nonsilicified protolith. This study indicates that silicification along a fault zone has crucial implications for the reservoir properties of siliciclastic rocks, strongly reducing the fluid flow properties and increasing the UCS values. [Display omitted] • We recorded hydrothermal silicification in a deformation band fault zone at the Rio do Peixe Basin, Brazil • We assessed how hydrothermal silicification affects the physical properties of arkosic sandstone in a fault zone • Hydrothermal silicification splits the fault zone into non-silicified, moderately silicified, and intensely silicified zones • Percolation of hydrothermal fluid and precipitation of silica cement occur within deformation bands and adjacent host rocks • Silicification increases UCS by 65% and decreases porosity-permeability by up to 2-4 orders of magnitude in fault zones [ABSTRACT FROM AUTHOR]

Soil carbonate accumulation plays an important role in C sequestration, accounting for approximately 40% of the C stored in soils worldwide. Under humid tropical climate these carbonates are rapidly dissolved, intensifying CO 2 emissions. Despite this, little is known about the weathering of carbonates in areas with complex geology. This study aimed to evaluate the influence of the parent material on soil attributes and weathering processes under humid tropical climate conditions along toposequences formed by dolomitic marbles and calc-silicate rocks interspersed with garnet-biotite gneiss. Samples and soils profiles were collected on sites of both parental materials and at different points of a toposequence, namely: upper third, middle third and lower third of a slope. The profiles were analyzed for their morphological, physical, chemical and mineralogical attributes. The attributes of soils formed from metamorphic sedimentary rocks were more influenced by geological processes than pedological ones. It was found that the nature of the parent material was the main responsible for the attributes of the profiles, especially the strongly alkaline pH, being greater than 8.0 in most soil horizons, and the high contents of exchangeable Ca2+ and CaCO 3 equivalent, with maximum values of 40.6 cmol c kg−1 and 763 g kg−1, respectively. As for mineralogical composition, it was verified the occurrence of minerals such as talc, vermiculite and Fe and Ti oxides, with metamorphic processes and the addition of hydrothermal fluids being the main responsible for the presence of these minerals. The supply of high contents of basic cations, with maximum values of 61.4 cmol c kg−1, high Si/Al ratio (reaching values close to 5.0) and the high contents of CaCO 3 conditioning the strongly alkaline pH hinder the dissolution of silicates, even those with low resistance to weathering, despite the hot and humid climate conditions. The action of weathering processes in humid tropical climate is more marked in acidic materials, being responsible for the in situ formation of kaolinite from the alteration of mica and feldspar. • Soil mineralogy is influciated for metamorphic processes and hydrothermal fluids. • Despite the tropical climate, the decarbonation process is slow. • Weathering processes in humid tropical climate is more marked in acidic materials. • The parent material affects soil attributes more than relief. [ABSTRACT FROM AUTHOR]

In the daily practice of Formation Evaluation for porosity calculation considering the shale volume is common the adoption of simple hypotheses about the physical properties of the reservoir rock constituents. In this paper, it is argued that an accurate estimate of clay mineral physical properties in the reservoir rock are essential for a realistic porosity calculation, once it may be different from the physical properties of clay minerals present in nearby shale layers. Geologically, the assumption of similarity between the clay mineral physical properties in the reservoir rock and adjacent shales properties means to presume a depositional continuity in sedimentary process and disregarded the innumerous postpositional processes involved in the sedimentary basin. By applying the angular competitive neural network to the Density-Neutron method to identify the clay mineral in the reservoir rock based on all identifiable clay minerals in the oil field, it is shown that a particular clay mineral signals a characteristic angular pattern in the Density-Neutron cross-plot. The methodology is presented with synthetic data and evaluated with actual well logs and core analysis from a borehole drilled in the Namorado oil field, Campos basin, Brazil. • A geologically coherent method to study petrophysical properties of reservoir rock. • The angular competitive neural network is applied to determine the predominant clay mineral in the reservoir rock. • The identified clay mineral in the reservoir is proposed to a realistic porosity estimate. [ABSTRACT FROM AUTHOR]

One of the most significant problems in the characterization and recovery of Aptian carbonate reservoirs, especially the Brazilian pre-salt, has been the lack of precise estimates of permeability. The heterogeneity of the permeability of the carbonate reservoirs occurs owed great different of the pore form occurred mainly by diagenetic processes. In this study, propose a joint and integrated methodology to estimate the permeability in the reservoir. To achieve this goal, porosity-permeability core data, image logs, applying the Rock Types concepts, analysis of the results of well log Nuclear Magnetic Resonance (NMR), modeling the well logs resistivity (laterolog and induction), as well as estimating the specific surface of the rock using images of section 2D from the pugs, to quantitatively estimate the permeability of the reservoir based on the petrophysical properties of the rocks. In the study it was possible to identify different pore systems distributed in eight Hydraulic Flow Units (HFU) determined from the pore groove radii. The Nuclear Magnetic Resonance (NMR) log it served to separate area of the spectrum corresponding to the small pores from the area corresponding to the large pores. The resistivity logs were analyzed with the purpose of estimating the direction (vertical, horizontal and dipping) and thickness of the fractures, which were also modeled with the purpose of identifying the invasion of the drilling fluid. The specific surface area was obtained by image processing algorithms. The results showed an acceptable precision of this methodology to estimate the permeability in carbonate reservoirs that have in their composition fragments of stromatolites and associated bioclastics, found partially or totally dolomitized. • The Hydraulic Flow Units with the highest velocity zones coincide with the fractured. • Dissolution improves reservoir quality in both spherulites and stromatolites. • The presence of fractures does not affect the quality of the reservoir studied. • The digital image analysis allows estimation of the petrophysical parameters. • The radial resistivity image identified the drilling mud in the fracture zones. [ABSTRACT FROM AUTHOR]