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The Zafra de Záncara anticline (also known as the El Hito anticline), located in the Loranca Cenozoic Basin (part of the Tagus Basin, Central Spain), had been studied by several oil companies during the late 1960s and early 1970s. In 2009, within the 'Plan for selection and characterization of suitable structures of CO2 geological storage', this anticline was selected as a potential CO2 storage site. A preliminary three‐dimensional geological model, based on five geological cross sections that were constrained with the interpretation of the available seismic profiles (that are rather old and do not have very good quality), was created. With the aim of improving the geological knowledge of the Zafra de Záncara anticline and helping to investigate the suitability of a nearby anticline, namely La Rambla, as another structural closure that might make it a possible CO2 storage site, a local gravity survey (1 station every km2) was carried out in the area, seven new geological cross sections, based on these existing seismic profiles and field geology, were build, and a new three‐dimensional geological model that included both anticlines, improved through three‐dimensional stochastic gravity inversion, was constructed. The densities needed for the geological formations of the model come from the analysis of rock samples, logging data from El Hito‐1 drillhole and petrophysical information from Instituto Geológico y Minero de España database. The inversion has improved the knowledge about the geometry of the anticlines' traps and seals as well as the geometry of the basement relief and the structural relationship between basement and cover. [ABSTRACT FROM AUTHOR]

Analysing S‐wave splitting has become a routine step in processing multicomponent data. Typically, this analysis leads to determining the principal directions of a transversely isotropic medium with a horizontal symmetry axis, which is assumed to be responsible for azimuthal anisotropy, and to the time delays between the fast and slow S‐waves. These parameters are commonly estimated layer‐by‐layer from the top. Errors in layer stripping occurring in shallow layers might propagate to deeper layers. We propose a method for S‐wave splitting analysis and compensation that consists of inverting interval values of splitting intensity to obtain a model of anisotropic parameters that vary with time and/or depth. Splitting intensity is a robust attribute with respect to structural variations and is commutative, which means that it can be summed along a ray (or throughout a sensitivity kernel volume) and can be linearly related to anisotropic perturbations at depth. Therefore, it is possible to estimate anisotropic properties within a geological formation (e.g. the reservoir) by analysing the differences of splitting intensity measured at the top and at the bottom of the layer. This allows us to avoid layer stripping, in particular, for shallow layers where anisotropic parameters are difficult to estimate due to poor coverage, and it makes S‐wave splitting analysis simpler to apply. We demonstrate this method on synthetic and real data. Because the splitting intensity attribute shows usefulness in S‐wave splitting analysis in transversely isotropic media, we extend the splitting intensity theory to lower symmetry classes. It enables the characterization of tilted transversely isotropic and tilted orthorhombic media, opening new opportunities for anisotropic model building. [ABSTRACT FROM AUTHOR]

ABSTRACT Pore throat plugging of porous rock by fine particles causes formation damage, and thus has attracted attention in various areas such as petroleum engineering, hydrology and geothermal energy production. Despite significant efforts, the detailed pore-scale mechanisms leading to formation damage and the associated permeability reduction are not well understood. We thus investigated plugging mechanisms and characteristics with a combination of ex situ (i.e., coreflooding measurements and scanning electron microscopy imaging) and in situ (i.e., nuclear magnetic resonance and μCT) methods, with a particular focus on the effect of wettability. The corefloods indicated that permeability drops rapidly when fines are injected; mechanistically thin pore throats are plugged first, followed by filling of adjacent pore bodies with the fine material (as evidenced by the nuclear magnetic resonance and μCT experiments, which can measure the pore size distribution evolution with fines injection). Furthermore, it is clear that wettability plays a major role: if fines and rock wettability are identical, plugging is significantly accelerated; wettability also controls the 3D distribution of the fines in the pore space. Furthermore we note that the deposited fines were tightly packed, apparently due to strong adhesion forces. [ABSTRACT FROM AUTHOR]

ABSTRACT The technical and economic success of a CO2 geological storage project requires the preservation of the site injectivity and integrity properties over its lifetime. Unlike conventional hydrocarbon gas injection, CO2 injection may imply geochemical reactions between acidified pore fluids and target reservoir formations, leading to modifications of their poromechanical properties. To date, the chemical effects on the host rock mechanical behaviour are not satisfactorily taken into account in site-scale numerical models of CO2 injection, mainly due to a lack of quantitative data. The present experimental work aims at characterizing the evolution of carbonate poromechanical properties induced by acid alteration. Unlike standard experimental approaches, the implemented alteration method induces a homogeneous dissolution pattern, which ensures reliable poromechanical measurements on altered samples. These well-controlled alteration conditions allow a proper interpretation of the test results through the macroscopic continuous approach of poromechanics. Petrophysical, geomechanical, and petroacoustic properties of outcrop carbonate samples have been measured for different levels of alteration to mimic long-term exposure to reactive brine. The obtained experimental data show clear trends of chemically induced mechanical weakening. Nuclear magnetic resonance measurements and microscanner imaging performed before and after alteration have provided complementary insights into the alteration effects at the microscopic scale. [ABSTRACT FROM AUTHOR]

ABSTRACT In this study, we derived accurate and high-resolution attenuation profiles using spectral ratio, centroid frequency shift, and seismic interferometry methods. We utilized high-quality vertical seismic profiling and sonic waveform data acquired in a carbonate reservoir located in Abu Dhabi, United Arab Emirates. The scattering profile of vertical-seismic-profiling data contributes significantly to wave attenuation that can be explained by high heterogeneity of the carbonate rocks. The scattering profile also correlates well with the reservoir lithology and fractured zones imaged by the Formation MicroImager. A tar mat zone occurs within the lower part of Arab D reservoir. This zone corresponds with a decrease in scattering attenuation. The tar mat may have filled the pores and made this zone less heterogeneous. Therefore, a decrease in scattering attenuation can be considered a potential parameter for tar mat detection. After removing the scattering effect, nonphysical negative intrinsic attenuation values still exist at certain depths. The most probable explanation for this is the three-dimensional scattering effect, which is not taken into account in this paper, and short-period upgoing waves. Seismic interferometry is less sensitive to the remaining scattered upgoing wave, which is why seismic interferometry method shows fewer negative values than the spectral ratio and centroid frequency shift methods. Compared with vertical-seismic-profiling attenuation, scattering attenuation estimated from sonic waveforms recorded in the reservoir zones is insignificant, and the intrinsic attenuation is almost equivalent to the total attenuation. We attribute this underestimation of the scattering attenuation to the sparse spatial sampling of the sonic logging data at 0.1524 m, which is not sufficient to appropriately estimate the scattering effect in heterogeneous media. The cross-plots between sonic attenuation and various petrophysical properties show slight dependence between the sonic attenuation and neutron porosity and resistivity in the reservoir zones. However, we can highlight from these plots two zones belonging to the Arab reservoirs. The lower zone corresponds to Arab D reservoir and displays higher sonic intrinsic attenuation than the upper zone (Arab A-C reservoirs) due to higher oil saturation. This highlights the sensitivity of the intrinsic attenuation to the oil saturation. [ABSTRACT FROM AUTHOR]

ABSTRACT A detailed magnetotelluric survey was conducted in 2013 in the Sehqanat oil field, southwestern Iran to map the geoelectrical structures of the sedimentary Zagros zone, particularly the boundary between the Gachsaran Formation acting as cap rock and the Asmari Formation as the reservoir. According to the electrical well logs, a large resistivity contrast exists between the two formations. The Gachsaran Formation is formed by tens to hundreds of metres of evaporites and it is highly conductive (ca. 1 Ωm-10 Ωm), and the Asmari Formation consists of dense carbonates, which are considerably more resistive (more than 100 Ωm). Broadband magnetotelluric data were collected along five southwest-northeast directed parallel lines with more than 600 stations crossing the main geological trend. Although dimensionality and strike analysis of the magnetotelluric transfer functions showed that overall they satisfied local 2D conditions, there were also strong 3D conditions found in some of the sites. Therefore, in order to obtain a more reliable image of the resistivity distribution in the Sehqanat oil field, in addition to standard 2D inversion, we investigated to what extent 3D inversion of the data was feasible and what improvements in the resistivity image could be obtained. The 2D inversion models using the determinant average of the impedance tensor depict the main resistivity structures well, whereas the estimated 3D model shows significantly more details although problems were encountered in fitting the data with the latter. Both approaches resolved the Gachsaran-Asmari transition from high conductivity to moderate conductivity. The well-known Sehqanat anticline could also be delineated throughout the 2D and 3D resistivity models as a resistive dome-shaped body in the middle parts of the magnetotelluric profiles. [ABSTRACT FROM AUTHOR]

ABSTRACT In seismic interpretation and seismic data analysis, it is of critical importance to effectively identify certain geologic formations from very large seismic data sets. In particular, the problem of salt characterization from seismic data can lead to important savings in time during the interpretation process if solved efficiently and in an automatic manner. In this work, we present a novel numerical approach that is able to automatically segmenting or identifying salt structures from a post-stack seismic data set with a minimum intervention from the interpreter. The proposed methodology is based on the recent theory of sparse representation and consists in three major steps: first, a supervised learning assisted by the user which is performed only once, second a segmentation process via unconstrained ℓ1 optimization, and finally a post-processing step based on signal separation. Furthermore, since the second step only depends upon local information at each time, the whole process greatly benefits from parallel computing platforms. We conduct numerical experiments in a synthetic 3D seismic data set demonstrating the viability of our method. More specifically, we found that the proposed approach matches up to 98.53% with respect to the corresponding 3D velocity model available in advance. Finally, in appendixes A and B, we present a convergence analysis providing theoretical guarantees for the proposed method. [ABSTRACT FROM AUTHOR]

ABSTRACT We show how to estimate the fluid permeability changes due to accumulated biopolymer within the pore space of a granular material using laboratory measurements of overall permeability, together with various well-known quantitative measures (e.g., porosity, specific surface area, and formation factor) of the granular medium microstructure. The main focus of the paper is on mutual validation of existing theory and a synthesis of new experimental results. We find that the theory and data are in good agreement within normal experimental uncertainties. We also establish quantitative empirical relationships between seismic and/or acoustic attenuation and overall permeability for these same systems. [ABSTRACT FROM AUTHOR]