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Characterization of Fracture System of a Shale Aquifer Using Azimuthal Resistivity Survey: A Case History From CAS Campus, Ebonyi State University, Nigeria.
Utom, A.U. (Ebonyi State University, Abakaliki, Nigeria.) | Odoh, Ben I. (Ebonyi State University, Abakaliki, Nigeria.) | Ogala, F. (Ebonyi State University, Abakaliki, Nigeria.)
Exploration for groundwater at the College of Agricultural Sciences (CAS) campus of Ebonyi State University in Abakaliki Capital Territory, Nigeria becomes critical where secondary features such as fractures and faults control groundwater movement and occurrence. Intermittent water shortages have developed since the boreholes (resulting from the commonly used Schlumberger array) at the campus have been put to use after well completion. To study the groundwater conditions in details, we used Azimuthal Resistivity Survey to characterize the shale fractures at the campus as part of the research project of merit award to Ebonyi State University Geophysical Society by the SEG Foundation on "potable groundwater exploration in Abakaliki shale Formation". Measured apparent resistivities changed with the orientation of the fractures. Graphical interpretation of the survey data indicates that the fractures trend generally in the northwestsoutheast direction at the depths of 28.3, 40.0 and 50.0 m with the coefficient of anisotropy ranging from 1.36 to 1.55 with the higher anisotropy factor indicating area of higher permeability. The applied square array is more sensitive to a given rock anisotropy than the more commonly used Schlumberger and Wenner arrays.Generally, an additional advantage of the array technique used is that it requires about 65 percent less surface area than an equivalent survey using a Schlumberger or Wenner array.
Abakaliki, anisotropy, apparent resistivity, array, azimuthal resistivity survey, campus, complex reservoir, drilling data acquisition, drilling measurement, Ebonyi State University, fracture, fracture system, hydraulic fracturing, knowledge management, logging while drilling, LWD, management and information, Nigeria, Reservoir Characterization, reservoir description and dynamics, resistivity, shale, structural geology, Upstream Oil & Gas, well completion
Reservoir seismic applications require offset domain seismic data to be converted into angles. While it is straightforward to convert offsets to angles under the isotropic assumptions, such a transformation is complex in the presence of anisotropy. Correct interpretation of angle dependent reflection amplitudes requires offset domain data to be corrected for the non-hyperbolic anisotropic moveout and distinction between the phase and the group angles. Some of these fundamental issues relating to the angle dependent P-wave seismic reflections for transversely isotropic elastic medium with a vertical symmetry axis (VTI medium) are reviewed here with illustrations.
amplitude, angle, angle gather, anisotropy, domain, equation, Green River shale, group, group angle, NMO, reflection, Reservoir Characterization, reservoir description and dynamics, seg las vegas, seismic processing and interpretation, slowness, Thomsen, transversely isotropic, Upstream Oil & Gas, vector, VTI
Oilfield Places:
- North America > United States > Wyoming > Green River Basin (0.98)
- North America > United States > Utah > Green River Basin (0.98)
SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
Elastic anisotropy of shale is mainly controlled by the intrinsic anisotropy of individual clay minerals as well as by the textural alignment of grains, pores, and fractures. One of the major challenges in predicting the elastic anisotropy of shales, while using rock physics models, is that the elastic properties of rock-forming clay minerals are poorly known. Since it is impossible to find single and large enough clay crystals for acoustic measurements and ab initio calculations are still incomplete, few data exist on the elastic moduli of clay minerals.
In an attempt to derive the intrinsic anisotropy of pure clay minerals, we present laboratory measurements of compressional and shear wave anisotropy in compacted clay powders at different porosities. In the present work, we focus on the anisotropy of montmorillonitic clays. We used a cold-press method by applying uniaxial compaction in order to obtain compacted mineral aggregates. Different degrees of compaction enable us to obtain samples with variable porosities and crystallite alignments. We measure ultrasonic P- and S- wave velocities along the beddingnormal and the parallel directions. The textural orientation of compacted clay aggregates is found to be controlled by compaction. We obtain the orientation distribution of the clay minerals using synchrotron X-ray diffraction.
Increasing anisotropy of the clay assemblages corresponds to an increase in the preferred orientation of the clay minerals. The combined usage of P- and S- anisotropy measurements with orientation distributions allows us to better constrain the inversion of clay mineral moduli. Our work provides laboratory data on elastic anisotropy of pure clay minerals while linking them to the variation of clay orientation distribution with porosity.
anisotropy, clay, clay mineral, compaction, distribution, elastic anisotropy, elastic property, Epsilon, exponential, formation evaluation, geophysics, intrinsic anisotropy, log analysis, mineral, model, montmorillonite, orientation, porosity, Reservoir Characterization, reservoir description and dynamics, sample, seismic processing and interpretation, shale, stiffness, Thomsen, Upstream Oil & Gas, well logging
An inversion procedure is described wherein microseismic data recorded by a network of three component geophones are assumed to be represented as the sum of a compressional (P) and one or two shear (S) arrivals. The inversion operates in the frequency-space domain and includes a linear inversion for source waveforms and a nonlinear inversion for model properties or source locations. The linear inversion effectively reverses time using a ray trace Green function to recover the source-time functions. For the nonlinear inversion two waveform fitting functionals are constructed; one captures moveout and polarization information through a reconstructed data misfit, another captures information from arrival time differences through a spectral coherence functional. The two may be scaled and summed to form a joint X2 misfit which may be combined with soft prior information in a Bayesian posterior. This is then maximized using global search techniques. Model calibration is accomplished by inverting waveform data from known locations (e.g. perforation shots) for anisotropy and optionally for model smoothness and Q. Micro-earthquake event locations are determined by inverting waveform data given the calibrated model.
Since the procedure involves fitting waveforms, time picking is not required. The beam-forming property of the receiver array and the complete polarization vector are used to enhance the signal to noise ratio of arrivals. The presence of a P arrival is not necessary to determine a location. The algorithm implementation uses layered VTI models, includes losses due to spreading, transmission and Q and handles an arbitrary distribution of receivers (e.g. from horizontal or multiple wells or surface locations). The inversion permits automated, objective data analysis with quantified uncertainties in estimated unknowns.
amplitude, anisotropy, approach, arrival, Artificial Intelligence, frequency, function, information, inversion, least-squares time reversal, location, model, objective function, posterior, receiver, Reservoir Characterization, reservoir description and dynamics, seismic processing and interpretation, source, source function, source location, Upstream Oil & Gas, waveform, waveform fitting
SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
Technology: Information Technology > Artificial Intelligence > Representation & Reasoning > Search (0.34)
Summary
Shear-wave seismic acquired at Rulison Field, Piceance basin, Colorado in 2003 exhibits evidence of faults and natural fractures from reflection discontinuity and shearwave splitting analysis. Rulison Field is a thick unconventional natural gas reservoir producing from the fluvial tight gas sandstones of the Late Cretaceous Williams Fork Formation.
Fault interpretations made from multicomponent seismic data clearly show near vertical faults in the lower reservoir Cameo Coal interval that strike in a north-northwest direction. The shear-wave (s-wave) seismic shows better evidence of faults propagating upward through the main reservoir interval than the p-wave. Borehole image logs confirmed these faults. These faults splay upward into the reservoir as flower structures that create fault zones and control natural fracturing within the reservoir. Natural fractures were observed from s-wave splitting. Since the reservoir is more than a wavelength thick, s-wave splitting calculations were done on seismic volumes as opposed to the traditional horizon-based approach. This process resulted in s-wave splitting volumes that correlate with image log fracture interpretations and anisotropy logs from cross-dipole sonic logs. These volumes show spatial and vertical variations in the degree of s-wave splitting that are geologically and can ultimately be used to optimize well locations and drilling efficiency.
anisotropy, anisotropy log, Colorado, difference, fault, high eur well, high impedance anisotropy, impedance, impedance anisotropy, multicomponent seismic, natural fracture, natural fracture identification, research, Reservoir Characterization, reservoir description and dynamics, Rulison Field, s-wave splitting, seismic processing and interpretation, similarity, Upstream Oil & Gas, well
Oilfield Places:
SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
Ultrasonic velocities, densities and porosities for a set of coal and surrounding silty coal, silty shale, and shaly coal samples were measured in laboratory. The pressure effect, temperature effect, and saturation effect on velocities and anisotropy were evaluated. The results were compared and correlated with the well log data and discrepancies were analyzed. The potential influences of the coal seams to neighboring gas or oil reservoir seismic response were discussed.
acoustic property, anisotropy, bedding, coal, coal formation, coal sample, coal seam, core, effect, las vegas, porosity, Reservoir Characterization, reservoir description and dynamics, sample, saturation, seismic processing and interpretation, shale, Upstream Oil & Gas, water, water saturation, Wave, well
SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
Near-surface velocities could vary with azimuth, impacting seismic data processing and interpretation. In this study, we developed a methodology to investigate the variations of near-surface velocities with azimuth, using 3D turning-ray tomography. The input data are the first arrivals selected from pre-defined azimuth sectors in terms of shot-receiverpair directions. The output velocities from tomography correspond to the selected azimuth sectors. A near-surface tomography study based on seismic data from a shallow heavy-oil reservoir in Canada has suggested that the observed azimuthal traveltime variations are not necessarily related to azimuthal (HTI) anisotropy induced by the stress field or fractures. It could also be caused by the nearsurface heterogeneity or acquisition footprint. Near-surface complexity could masquerade as anisotropy. Potentially this can influence statics and prestack imaging.
acquisition footprint, anisotropy, azimuth, azimuth sector, azimuthal, azimuthal near-surface, azimuthal turning-ray, Masquerade, near-surface complexity, near-surface heterogeneity, predefined azimuth sector, Reservoir Characterization, reservoir description and dynamics, sector, seg las vegas, seismic processing and interpretation, slowness, tomography, topography, traveltime, Upstream Oil & Gas, variation
SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
Summary
Oriented cores of Barnett shale were studied in detail with the help of ultrasonic and X-ray imaging methods. The velocities of compressional wave (VP) and two shear waves (VSh and VSv) were measured on 56 samples. As a result, we found a relation between the X-ray images and specific features in behavior of elastic wave velocities. The average splitting of shear waves observed for the Barnett shale cores at room conditions is 30%.
anisotropy, attenuated x-ray beam, Barnett shale, Barnett Shale Core, core, device, direction, elastic wave, formation evaluation, intensity, log analysis, method, mineralogical composition, porosity, Reservoir Characterization, reservoir description and dynamics, sample, seismic processing and interpretation, shear wave, Upstream Oil & Gas, Wave, well logging, x-ray image, x-ray imaging
2D parabolic Radon filtering is a widely used method for multiple attenuation. However, for dense and wideazimuth gathers that have azimuthal anisotropy effects this approach can have problems. Because of the variation of the curvature of the events with azimuth, the bin gathers can not be processed in one go but must rather be split into sub-collections where the azimuth either has little variation or varies smoothly. We instead propose herein to take into account the azimuthal anisotropy by incorporating an elliptical model for the variations of the curvature with azimuth, to define a 3D parabolic Radon filtering. This is a more natural way of processing dense wide-azimuth gathers by honoring their actual 3D geometry.
algorithm, anisotropy, azimuth, azimuth sector, azimuthal, azimuthal anisotropy, curvature, elliptical model, gather, geometry, high resolution, las vegas, model space, parabolic radon, Radon domain, Reservoir Characterization, reservoir description and dynamics, sector, seismic processing and interpretation, trace, Upstream Oil & Gas, variation
SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (0.31)
Relating seismic wave velocity and maturity in organic-rich shale is still a fundamental issue in the study of organicrich rocks. We found that the inception of the maturity peak -expressed in terms of vitrinite reflectance- (Ro % = 0.65) separates the pressure-dependent anisotropic behavior of organich-rich rocks in two domains: for maturities less than 0.65 (from immature to peak mature rocks), rocks exhibit a low pressure-sensitivity (velocity and anisotropy) but increasing magnitude of anisotropy with increasing Ro%; for maturities greater than 0.65, rocks show an higher sensitivity of velocity to pressure and decreasing magnitude of anisotropy. To start understanding the role of maturation processes and how the spatial arrangement of kerogen could control the elastic properties of these rocks, we complemented traditional rock-physics measurements with the analysis of images obtained using confocal laser scanning microscopy. This latter represents a suitable technique to image organic matter yielding relevant inputs for rock-physics computational analysis.
anisotropic thomsen, anisotropy, distribution, fluorescence, function, image, kerogen, maceral, Magnitude, maturation, maturity, Microstructure, organic-rich shale, Reservoir Characterization, reservoir description and dynamics, rock, sample, seismic processing and interpretation, spatial arrangement, structural geology, Upstream Oil & Gas, vitrinite, vitrinite reflectance, wavelength
SPE Disciplines:
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