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.

Anisotropy analysis for optimized gas production in Coalbed Methane reservoir, Bokaro field, India

The efficient production of Coalbed Methane (CBM) gas is facing challenges due to the larger dewatering period from fracture connectivity to the aquifer zone. Also, commingled production from well makes it more difficult to identify the coal seam-wise problem. Therefore, prior knowledge of sub-surface fractures in coal seams is necessary to execute an accurate simulation model for planning hydraulic fracturing treatment. This paper highlights the studies in Bokaro CBM reservoir to mitigate challenges in few wells by characterizing anisotropy, determining fast shear wave polarization angle, maximum horizontal stress direction, fracture orientation, and analysis of low resistivity signature. Both the fast shear wave polarization angle and fracture orientation in resistivity image are observed in the same direction (N26°-35°E) in coal. The fast and slow shear slowness versus frequency plot concludes stress-induced anisotropy resulting from fractures that are supported by resistivity image and drilling core. Processing of the resistivity image log shows the maximum horizontal stress is along NE-SW direction, as identified from drilling-induced fractures. The observation of low resistivity signature with resistivity ranging from 0.4 to 0.8 ohm-m in few wells confirms the presence of conducting minerals such as siderite and pyrite from the x-ray diffraction studies of sidewall core. The present work guides in making production, drilling, and hydraulic fracturing design strategies to better understand the fluid propagation for optimized CBM production and will also help in future geomechanical studies.

Geometry and topology of fractures and faults affecting anticlines in the Zagros fold-and-thrust belt: a multiscale approach

<p>Faults and fractures play a key role in the permeability of the upper crust. Since anticlines represent very common structural traps for fluids, geometrical (i.e., orientation, length distribution) and topological (i.e., cross-cutting and abutting relationships, intensity) characterization of their fracture network is crucial to assess the migration and accumulation of fluids for CO<sub>2</sub> sequestration or hydrocarbon exploitation purposes. For this reason, many previous studies focused on anticlines worldwide, and in particular on the Zagros fold-and-thrust belt where they represent the outcropping analogs of oil fields in SW Iran.</p><p>The Zagros fold-and-thrust belt involve sediments of the pre-collisional Arabian plate passive margin, arranged in folds elongated in a NW-SE direction and tectonic transport toward SW. The belt is dissected by N-S dextral strike slip transfer faults reactivating former rift-related normal faults. Most of the studies on fracturing in the Zagros belt are based on fracture orientation data collected mainly in the field, or alternatively coming from satellite images, and deal with the origin of fracture sets (fold-related or not). Although two of the classical fold-related sets, oriented roughly parallel and perpendicular to fold axis (i.e., NW-SE and NE-SW striking respectively) can be generally recognized everywhere in the belt, other fracture orientation (e.g., N-S and E-W striking) are locally predominant and there is still no consensus on the nature of all fracture sets. For example, the role of the strike-slip reactivation of N-S and E-W striking inherited faults on fracture set distribution is still not clear.</p><p>In this study we leverage on high quality Bing Maps satellite images of the Zagros anticlines and on scanlines performed in the field to provide a multiscale investigation of geometry and topology of the fracture network affecting three anticlines, namely Sim, Kuh-e-Asmari, and Kuh-e-Sarbalesh. The three anticlines have similar dimensions and are variably affected by ~N-S striking dextral strike slip tectonic lineaments. In particular, Kuh-e-Asmari and Sim anticlines are located ~10km far from the Izeh and Sabz-Pushan faults respectively, whilst the Kuh-e-Sarbalesh anticline is characterized by an evident drag in map view against the Kazerun fault.</p><p>We manually interpreted the fracture network on satellite images at different scales (1:100 to 1:100.000), producing fracture maps with resolution ranging from 10m to 1km. Each fracture map was then analyzed using the NetworkGT plugin in QGIS. In particular, we were able to identify fracture sets, their spatial distribution and, were possible, the topology of the fracture network. In this framework, scanlines performed in the field represent punctual observations at furtherly higher resolution (~1 cm). Following the same procedure for the three anticlines enables us to test the role of N-S faults on fracture set distribution at various scales.</p><p>With such a multiscale approach we provide a “big picture” that can help to shed light on the nature and distribution of the various fracture sets in the anticlines of the Zagros belt. Moreover, fracture sets identified at different scales in this study can be used to better interpret previous and future fracture data collected in the field.</p>

Reliability analysis of seismic attribute in the detection of fault-karst

In the Tarim Basin, various irregular fractured-vuggy reservoirs have developed along with the main faults. These reservoirs are geologically defined as carbonate fault karst. In the past few years, seismic attributes have been widely used for the identification and evaluation of fault karst. However, there has been less reliability analysis regarding their usage. Imaging using the theoretical fault-karst velocity model can reflect the shapes and distributions of fractures and vugs, whereas imaging using the background velocity can simulate seismic data in real cases. We have adopted an approach based on typical fault-karst theoretical forward modeling to evaluate the reliability of seismic attributes in practical applications. First, we extract various attributes from the images using the theoretical velocity and the background velocity using similarity estimation between them to optimize the sensitive attributes. The analysis result indicates that the instantaneous phase, variance, amplitude gradient, coherence, and texture entropy are more suitable to characterize the anomalies of fractures and vugs with prediction accuracy of 71.7%. Because fracture orientation and density are the key parameters for quantifying the differences between the two images, taking coherence as an example, we extract the fracture traces through circular scanlines and circular windows based on the optimized attributes. The coincidence rate between the predicted fracture density and the known model reaches 83%, and that between the predicted fracture orientation and the known model is greater than 95%. With this remarkable coincidence, we can conclude that optimized seismic attributes are reliable for characterizing fractured-vuggy reservoirs.

IDENTIFIKASI ORIENTASI REKAHAN MIKRO AREA PANAS BUMI MONTE AMIATA BERDASARKAN ANALISIS STUDI SHEAR WAVE SPLITTING

Shear Wave Splitting is an application of seismic wave to analyse the anisotropy level of a certain medium. Generally, shear wave propagation through a rock formation will be polarized (φ) into two parts especially when the medium structures are different, such as fracture. The polarized shear wave which is perpendicular to fracture will propagate slower than the wave that propagates parallel to the fracture. The delay time (δt) of both wave is proportional with the fracture intensity along the wave propagation from the source to the station. The description regarding fracture orientation can be obtained by analysing both Shear Wave Splitting parameters (φ and δt), and this information is adequately important in geothermal exploration or exploitation phase at Mt. Amiata. Based on the result of this research, the micro earthquake source is focused on the east to the south area and spread along 3 earthquake stations. The existence of micro earthquake source is mainly focused at the depth of 1 to 4 km. In addition, the polarization direction of each earthquake station at the geological map shows a dominant fracture orientation consistently at NW-SE. All of the three stations also show that the polarization direction is integrated to the local fault existence in the subsurface. Furthermore, the research shows that the high intensity fracture distribution occurred at MCIV station area in the southern part of research location. Meanwhile, the low intensity fracture distribution occurred at ARCI and SACS station area in the western and the eastern part of research location. The high value of fracture intensity accompanied by the high amount of structure intensity, strengthen the prediction of the high anisotropy existence which potentially tends to the high permeability presence at the area.Keywords: shear wave splitting, anisotropy, fracture, geothermal, polarization direction, fracture intensity.