Predicting the azimuth of natural fractures and in situ horizontal stress: A case study from the Sichuan Basin, China
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Abstract:
The azimuth of fractures and in situ horizontal stress are important factors in planning horizontal wells and hydraulic fracturing for unconventional resource plays. The azimuth of natural fractures can be directly obtained by analyzing image logs. The azimuth of the maximum horizontal stress [Formula: see text] can be predicted by analyzing the induced fractures on image logs. The clustering of microseismic events also can be used to predict the azimuth of in situ maximum horizontal stress. However, the azimuth of natural fractures and the in situ maximum horizontal stress obtained from image logs and microseismic events are limited to the wellbore locations. Wide-azimuth seismic data provide an alternative way to predict the azimuth of natural fractures and maximum in situ horizontal stress if the seismic attributes are properly calibrated with interpretations from well logs and microseismic data. To predict the azimuth of natural fractures and in situ maximum horizontal stress, we have focused our analysis on correlating the seismic attributes computed from pre- and poststack seismic data with the interpreted azimuth obtained from image logs and microseismic data. The application indicates that the strike of the most-positive principal curvature [Formula: see text] can be used as an indicator for the azimuth of natural fractures within our study area. The azimuthal anisotropy of the dominant frequency component of offset vector title seismic data can be used to predict the azimuth of maximum in situ horizontal stress within our study area that is located in the southern region of the Sichuan Basin, China. The predicted azimuths provide important information for the subsequent well planning and hydraulic fracturing.Keywords:
Microseism
Observation of azimuthal shear wave anisotropy can be useful for characterisation of fractures or stress field. Shear wave anisotropy is often estimated by measuring splitting of individual shear-wave events on VSP data; however this method may become un
Shear wave splitting
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Summary We describe a new-generation laterolog tool, the azimuthal resistivity imager (ARI). The tool makes deep azimuthal resistivity measurements around the borehole with higher vertical resolution than the Dual Laterolog (DLL). Twelve azimuthal electrodes are incorporated into the DLL electrode array to provide 12 deep, oriented resistivity measurements while retaining the standard deep and shallow laterolog measurements. To allow full correction of the azimuthal resistivities for borehole effect, a shallow auxiliary measurement is incorporated on the azimuthal array. Although the full-coverage azimuthal resistivity image has much lower spatial resolution than borehole microelectrical images, it complements these because of its lower sensitivity to shallow features. Evaluation of heterogeneous and fractured formations, and computation of fracture dip are applications of the tool's imaging capabilities and are discussed and illustrated with log examples. Other log examples cover thin-bed response and corrections for Groningen and borehole effects, including that for eccentering of the tool in the borehole.
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SummaryOnshore 3D seismic data is often acquired with rich azimuth coverage by utilising orthogonal shooting (or cross-spread) layouts. This means that azimuthal anisotropy, if present, can be readily observed and its effects on the data can be quantified and accounted for as part of the processing and imaging flow.In this example from the Canning Basin, the data appeared initially to be exhibiting the effects of azimuthal anisotropy. However, with careful handling and honouring of the azimuths of the acquired data during the PreSDM velocity model building, it was possible to correct for most of the variation simply with a laterally variable velocity model, without the need to invoke the presence of azimuthal anisotropy.
Variation (astronomy)
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The rapid and precise determination of the azimuth information, purely based on the satellite positioning technology of global positioning system (GPS), is tested and evaluated by this paper. The main consideration of using GPS for the azimuth determination is expected to replace the traditional astronomical azimuth measurement, which is believed to be time-consuming and weather-dependent. A GPS approach is simply based on setting up the GPS receivers at the two ends of a baseline, recording and processing the phase observable, and converting its coordinate solution into an azimuth with the inverse geodetic formulas. This type of azimuth obtained by the GPS static solution has been assessed to be well consistent with the astronomical azimuth by a level of better than ±1″. The GPS kinematic mode of azimuth value, however, is biased from the standard value for a RMS error of ±9″. The correction of the deflection of the vertical, theoretically required by the geodetic and astronomical azimuth conversion, is also implemented and found to be only effective at the observation site where the ηtanφ value is higher than the estimation accuracy of the η value provided by a gravimetric geoid model.
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Microseisms are ground vibrations caused largely by ocean gravity waves. Multiple spatially separate noise sources may be coincidentally active. A method for source separation and individual wavefield retrieval of microseisms using a single pair of seismic stations is introduced, and a method of back azimuth estimation assuming Rayleigh‐wave arrivals of microseisms is described. These methods are combined to separate and locate sources of microseisms in a synthetic model and then applied to field microseismic recordings from Ireland in the Northeast Atlantic. It is shown that source separation is an important step prior to location for both accurate microseism locations and microseisms wavefield studies.
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Three series of simultaneous wave and microseism records are examined. They give a clear indication that bands of microseismic waves from different sources can be distinguished by submitting seismograph records to frequency analysis. The agreement between the results of analysis and the theoretical expectation from the prevailing meteorological conditions appears to justify the assumption that microseismic waves of different periods travel independently. Under the simple meteorological conditions that have been studied, each band of microseismic activity can be identified with a band of sea waves of twice its period. The existence of this two to one ratio between the period of waves and microseisms affords some confirmation of the theory that microseisms are produced in a region of interference between similar wave trains travelling in opposite directions either near the coast or in deep water.
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In general, microseisms are a nuisance.It is n9w quite widely agreed that microseisms are channel waves, similar in nature to the Lg and Rg waves.It is not at all unusual to observe microseisms which have traveled over distances of continental proportions.In order to determine the direction from which the microseisms come and to investigate in more detail the characteristics of microseismic disturbances, we established a tri-partite system on Mount Palomar.The array consisted of three stations, located at the vertices of a triangle, which was about 2,500 feet on a side.Two Benloft strain seismographs and one Bentoff vertical pendulum seismograph were installed in a strain vault at one location.One vertical pendulum seismograph was installed at a second location, and two horizontal and one vertical pendulum seismographs were installed at a third location.Portions of the records which were obtained on two consecutive days are included in Figure 1.The direction of approach can be calculated from the displacement between the peaks on the three verticals.These observations indicated that the waves practically always come from the coast.The direction of approach generally lies between north-northwest and south-southwest.The horizontal pendulum traces in record (a) are parallel.This indicates'that the horizontal motion is in a northeast-southwest direction and vice versa.Everything on record (a) points to a Rayleigh type motion.Shear waves predominate on record (b).The fact that the horizontal pendulums are opposed to each other indicates that the motion is in a northwest-southeast direction.The horizontal component of displacement is, therefore, perpendicular to the direction of propagation.The sum of strain components is practically zero, as it should be for a horizontally polarized shear wave.The vertical pendulums show some stoa1!motions.This is to be expected, since some Rayleigh type motion is always present.Both of these waves are character~ ized by six-second periods, but usually the Rayleigh type motion predominates.Among the many other types of microseisms which are observed are the twosecond microseisms with wavelengths between about one-third and one-half the wavelength of the six-second waves and practically the same velocity.These waves originate near the coast--probably on the continental shelf.it is generally believed that they are generated by turbulent air masses over the continental shelf.The horizontal pendulum and the horizontal strain seismographs show practically only the six-second waves and very little of the shorter waves.We were puzzled by the [act that here we had a wave which traveled with the Rayleigh wave velocity but possessed mainly a vertical component.Fortunately, Dr. Press could give us the probable solution.Drs.
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N-06 AZIMUTHAL ANISOTROPY—MORE THAN JUST THEORY S. RONEN M. JARVIS and A. PROBERT 1 WesternGeco Schlumberger House Gatwick RH6 0NZ UK Abstract The effect of azimuthal anisotropy on reflection seismic data is significant to the extent that the imaging of such data is often poor if azimuthal isotropy is assumed. An analysis of azimuthal anisotropy can be based on the sign flipping of the transverse component of wide azimuth multicomponent data. Following the analysis the data are imaged properly by taking the anisotropy into account. We have applied such processing to a wide-azimuth 3-D multicomponent survey from the North Sea
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Seismic data moveout on a full azimuth land 3D dataset revealed strong spatially variant azimuthal anisotropy. It was interpreted as a result of short-wavelength heterogeneity and true azimuthal anisotropy for the interval velocity. Iterative depth-velo
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