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Considerable attention has been paid to the development of unconventional reservoir fields due to the availability of such resources worldwide. The heterogeneities throughout the pore space structure in a rock and their effects on the occurrence of gas and oil can be quantitatively analysed using fractal theory. This study focuses on the investigation of the fractal dimension ( FD ) in shale and tight gas sandstones (TGS) from Brazilian oil and gas fields using 3-D X-ray nano- and micro-computed tomography. Obtaining the broad size distribution of pore space structures in unconventional reservoir rocks using 3-D images is challenging due to the multiscale characteristics of the rocks. Thus, sample images were acquired with two voxel size/resolutions (0.064 μm and from 0.46 to 1.20 μm). Very small pores (ganglia), main pore networks and organic matter grains were observed as segmented structures within sample sets for which the FD was assessed and quantified. Box-counting (BC) and pore size distribution (PSD) approaches were applied. The average FD values ranged from 1.78 to 2.59 in shale and from 2.36 to 2.46 in TGS. Correlations of the FD with the experimentally acquired specific surface area, porosity and permeability related petrophysical properties, relevant to the quantitative modelling of gas transport in unconventional reservoirs, were established. The pore scale morphologies described by the FD from 3-D X-ray CT images help on the foundation for further research in shale/tight gas sandstones. [ABSTRACT FROM AUTHOR]

This work focuses on the characterization of the overall pore size distribution in tight gas sandstones originating from distinct Brazilian fields, as well as fluid flow circulation described by single-phase permeability. The former was achieved by the combination of nitrogen adsorption and mercury intrusion techniques and the latter estimated by empirical relations validated by the pulse-decay technique. The nature of nitrogen permeability change was investigated on tight gas core samples under a varying effective stress of 2–40 MPa, while the gas slippage was verified shifting mean pore pressures from 8.62 to 2.76 MPa in the laboratory pulse-decay. The best agreements between the estimated and measured permeabilities (from 0.00033 to 0.016 mD and from 0.0004 to 0.034 mD) reveal that obtained overall pore structure’ parameters play significant role in the results which were also supported by Field Emission Scanning Electron Microscopy (FESEM) and X-Ray Diffraction (XRD) analysis. [ABSTRACT FROM AUTHOR]

Abstract: Porous microstructure parameters of seal rock samples originating from different depths within Brazilian geological formations were correlated to empirical models which predict the intrinsic permeability. Mercury Injection Capillary Pressure (MICP) and Nitrogen Gas Adsorption (N2GA) were applied in combination as complementary techniques; MICP to obtain the porosity values and the size distribution of meso- and macropores, and N2GA associated with the Brunauer, Emmett and Teller (BET) theory to determine the specific surface area (S o ). The Barret, Joyner and Hallenda (BJH) theory was applied to find the size distribution of the micro- and mesopores. The combination of the MICP and N2GA curves showed that the samples analyzed present a polymodal pore size distribution (PSD) and a total porosity ranging from 0.33 % to 10.44 %. The S o values measured by N2GA were higher than those calculated by MICP, due to the majority of the samples having a mean pore size of 20–1000 Å. The intrinsic permeability could also be predicted applying the measured parameters, S o , PSD curves and total porosity in the Carman–Kozeny and Series–Parallel models. The ranges of permeability values obtained were 4.09 × 10−24–4.96 × 10−21 m2 and 9.48 × 10−27–9.14 × 10−22 m2, respectively. These results were compared with values reported in the related literature and those obtained for four samples submitted to pressure pulse decay permeability (PDP) tests. [Copyright &y& Elsevier]

Microstructural parameter evaluation of reservoir rocks is of great importance to petroleum production companies. In this connection, X-ray computed microtomography (μ-CT) has proven to be a quite useful method for the assessment of rocks, as it provides important microstructural parameters, such as porosity, permeability, pore size distribution and porous phase of the sample. X-ray computed microtomography is a non-destructive technique that enables the reuse of samples already measured and also yields 2-D cross-sectional images of the sample as well as volume rendering. This technique offers an additional advantage, as it does not require sample preparation, of reducing the measurement time, which is approximately one to three hours, depending on the spatial resolution used. Although this technique is extensively used, accuracy verification of measurements is hard to obtain because the existing calibrated samples (phantoms) have large volumes and are assessed in medical CT scanners with millimeter spatial resolution. Accordingly, this study aims to determine the accuracy of an X-ray computed microtomography system using a Skyscan 1172 X-ray microtomograph. To accomplish this investigation, it was used a nylon thread set with known appropriate diameter inserted into a glass tube. The results for porosity size and phase distribution by X-ray microtomography were very close to the geometrically calculated values. The geometrically calculated porosity and the porosity determined by the methodology using the μ-CT was 33.4 ± 3.4% and 31.0 ± 0.3%, respectively. The outcome of this investigation was excellent. It was also observed a small variability in the results along all 401 sections of the analyzed image. Minimum and maximum porosity values between the cross sections were 30.9% and 31.1%, respectively. A 3-D image representing the actual structure of the sample was also rendered from the 2-D images. [ABSTRACT FROM AUTHOR]

Superposed events of rock exposure, fracturing, and the hydrological dynamics, are key elements that influence epigenic karstification in carbonate reservoirs. However, they are not fully identified in seismic or well data. One way forward is a multiscale characterization of the porous system to understand karst development, from the pore scale to large caves, confined to high-permeability layers that can reach the basin scale. The present study investigates superposed meteoric dissolution events in carbonate rocks by combining the analyses of depositional setting, diagenesis, stratigraphic architecture, tectonics, and hydrological zones. We performed analyses of 130 plugs collected in 11 outcrops using petrographic, petrophysical, mineralogical, and geochemical analyses. The study area is the Cretaceous Jandaíra Formation in the Potiguar Basin, Brazil, where we identified a regional subaerial unconformity-SU (sequence boundary-1). This surface is characterized by a change in facies and depositional system between dolomitic peritidal deposits (depositional sequence 1) and tight-cemented calcite subtidal deposits (depositional sequence 2). A concentration of caves occurs below the SU. We conclude that subaerial unconformities (vadose surfaces) can develop better reservoir qualities with superposed freshwater leaching and karstification. They control the lateral reservoir zonation, whereas faults and fractures provide vertical pore distribution within the vadose/phreatic hydrological zones. Water table fluctuations can develop multilevel caves and conduits along the epiphreatic zones, increasing the reservoir thickness, which reaches 50–100 m in the Potiguar Basin. We recognized two previous subaerial unconformities at the basin scale using well and seismic data. These findings may have important implications for assessing reservoir location and quality in large areas. [Display omitted] • We investigate high-permeability karst layers in carbonate units. • Caves systems concentrate below subaerial unconformities at the basin scale. • Early diagenesis affected peritidal deposits, with dolomitization and karstification. • Meteoric fluids cement the overlying subtidal deposits during burial diagenesis. • Fractures, unconformities, and hydrology control thickness-extension of reservoir. [ABSTRACT FROM AUTHOR]

Abstract: Binarization process plays an important role in structural characterization by means of micro-CT images analysis. The choice of the correct threshold to separate porous phase from the material is the procedure for binary images creation. Slight variations of the threshold level led to substantial variations in physical parameters determination. The aim of this work is to evaluate these variations based on manual threshold settings data of five operators. Porosity and pore size distribution of a sandstone sample were determined. [Copyright &y& Elsevier]

Micropaleontology is crucial branch of Earth Sciences, with a pivotal role in the success of the oil and gas industry over the decades. This sector of paleontology is based on the taxonomical description of microfossils, which encompass fossils with size variation from 0.001 mm to 1 m. Normally these microorganisms have a high rate on preservation, thus they are widely accepted as reliable evidence to infer about paleodepositional settings and paleoclimate change. Furthermore, microfossils are critical to understanding the evolution through time, being many of them recognized as index fossil and providing useful biozones to correlation. Since 1950, microfossils have been taxonomically described mainly by stereomicroscopes. However, as the technology progress, traditional ways to study these organisms are challenged and improved by nondestructive three-dimensional imaging techniques, as X-ray tomographic microscopy (SRXTM), three-dimensional X-ray microscopy (3D-XRM), the X-ray computed microtomography (microCT) and X-ray computed nanotomography (nanoCT). Recently, one of the most compelling research areas in micropaleontology is the desire to automatize and enhance the details of systematic classification. Thereby, an increasing number of researches have applied the high-resolution X-ray analysis aiming to improve the morphological, taxonomic and taphonomic examination. Most of them have detailed the enhancement provided by the technique when compared with the standard microscopes, and raised questions about the traditional characters used on the microfossil systematic taxonomy. Nevertheless, even with the surpassing detail on microfossil characterization, the application of microCT has been hampered by the costs and sometimes by the needs of specific computer skill. Thus, this research has evaluated the use of microCT as the technique to classify a random bulk of microfossil (comprising foraminifers, ostracods, radiolarians, gastropods and echinoderms) with no further software treatment. No standard microscope analysis was performed. Despite chemical composition of microfossils, most of the specimens morphology, especially internal structures, have been easily acquired and analyzed. 96% of the microfossils of the dataset were identified at least on genus level. Irrespective of the deepness of detail, when considering the overall taxonomic identification, the microCT seems to be effective as the standard microscope. Nonetheless, when problematic specimens are evaluated, the microCT seems to be a more reliable and practical tool than other methods as Scanning Electron Microscopy. Thus, the technique can be used solo or as a complementary method to the stereomicroscope. Additionally, the high-resolution has the potential to lead to the expected automatized micropaleontology, since they can provide numerous images in several planes. This may create a strong database necessary to machine learning and computer identification. • Random bulk of microfossils are evaluated through microCT. • No further computer treatment on microfossil image. • microCT effective method to microfossil identification. [ABSTRACT FROM AUTHOR]