The presence of vertically oriented fractures and/or unequal horizontal stresses has created an azimuthally anisotropic earth, in which shear‐wave (SH) data collected along the principal axes of the anisotropy display time and reflection amplitude anomalies. Amoco shot two crossing shear‐wave (SH) lines that were approximately parallel to the orthogonal principal axes of the azimuthal anisotropy. At the tie point, these crossing SH lines display a time‐variant mis‐tie. The tie point also displays reflection‐coefficient anomalies, attributable to azimuthally dependent shear‐wave velocities. Field mapping documented a set of fractures striking N69E which are approximately parallel to the line that exhibited greater traveltimes. Time‐variant mis‐ties and reflection coefficient anomalies are two of the seismic responses theoretically expected of an azimuthally anisotropic earth, i.e., one in which the shear‐wave velocity depends upon the polarization azimuth of the shear wave.
Reflection P- and S-wave data were used in an investigation to determine the relative merits and strengths of these two data sets to characterize a naturally fractured gas reservoir in the Tertiary Upper Green River formation. The objective is to evaluate the viability of P-wave seismic to detect the presence of gas‐filled fractures, estimate fracture density and orientation, and compare the results with estimates obtained from the S-wave data. The P-wave response to vertical fractures must be evaluated at different source‐receiver azimuths (travelpaths) relative to fracture strike. Two perpendicular lines of multicomponent reflection data were acquired approximately parallel and normal to the dominant strike of Upper Green River fractures as obtained from outcrop, core analysis, and borehole image logs. The P-wave amplitude response is extracted from prestack amplitude variation with offset (AVO) analysis, which is compared to isotropic‐model AVO responses of gas sand versus brine sand in the Upper Green River. A nine‐component vertical seismic profile (VSP) was also obtained for calibration of S-wave reflections with P-wave reflections, and support of reflection S-wave results. The direction of the fast (S1) shear‐wave component from the reflection data and the VSP coincides with the northwest orientation of Upper Green River fractures, and the direction of maximum horizontal in‐situ stress as determined from borehole ellipticity logs. Significant differences were observed in the P-wave AVO gradient measured parallel and perpendicular to the orientation of Upper Green River fractures. Positive AVO gradients were associated with gas‐producing fractured intervals for propagation normal to fractures. AVO gradients measured normal to fractures at known waterwet zones were near zero or negative. A proportional relationship was observed between the azimuthal variation of the P-wave AVO gradient as measured at the tops of fractured intervals, and the fractional difference between the vertical traveltimes of split S-waves (the “S-wave anisotropy”) of the intervals.
This case history is one of three field projects funded by the US Department of Energy as part of its ongoing research effort aimed to expand current levels of drilling and production efficiency in naturally‐fractured tight‐gas reservoirs. The original stated goal for the 3-D P-wave seismic survey was to evaluate and map fracture azimuth and relative fracture density throughout a naturally‐fractured gas reservoir interval. At Rulison field, this interval is the Cretaceous Mesaverde, approximately 2500 ft (760 m) of lenticular sands, silts, and shales. Three‐dimensional full‐azimuth P-wave data were acquired for the evaluation of azimuthal anisotropy and the relationship of the anisotropy to commercial pay in the target interval. The methodology is based on the evaluation of two restricted‐azimuth orthogonal (source‐receiver azimuth) 3-D P-wave volumes aligned with the natural principal axes of the azimuthal anisotropy, as estimated from velocity analysis of multiazimuth prestack gathers. The Dix interval velocity, as well as the interval amplitude variation with offset (AVO) gradient, was calculated for both azimuths for the gas‐saturated Mesaverde interval. The two seismic attributes best correlated with commercial gas pay (at a 21-well control set) were (1) values greater than 4% azimuthal variation in the interval velocity ratio (source‐receiver azimuth N60E/N30W) of the target interval (the gas‐saturated Mesaverde), and (2) the sum of the interval AVO gradients (N60E + N30W). The sum of the interval AVO gradients is an attribute sensitive to the presence of gas, but not diagnostic of an azimuthal variation in the amplitude. The two‐azimuth interval velocity anisotropy mapped over the survey area suggests spatial variations in the orientation of the maximum horizontal stress field and the open (to flow) fracture system.
Gas production in Bluebell-Altamont Field in northeasternUtah is from numerous sandstones and carbonates in the Tertiary upper Green River Formation at depths of approximately 6500-8500 ft. The local geologic setting is nearly flatlying sediments with no faulting observed at target depths. The field has a very gentle anticlinal closure (less than 50 ft). Matrix porosity and permeability in the reservoir rocks are generally low, such that fracturing yields substantially higher production rates. Consequently, seismic detection of fracturing is a potentially important aid to field development.A Department of Energy project, supported by Pennzoil Corporation, investigated how seismic data could effectively characterize such reservoirs. Both P-and S-wave data were used because they respond in different ways to aligned vertical gas-filled fractures. The P-wave response at Bluebell-Altamont is an azimuthal variation of AVO signature which depends on source-receiver azimuth relative to fracture strike.S-waves are considered more reliable than P-waves as indicators of the stiff and compliant directions of rock for two main reasons. First, traveltimes of S-waves with two different Figure 1. Location of seismic lines 1 and 2 and relevant wells. Rose diagram (bottom left) shows fracture azimuths from cores and outcrop, maximum horizontal stress direction from borehole elongation data and the trend of gilsonite dikes, and first arriving shear-wave azimuth from VSPs and reflected S-wave seismic. Open fracture set N30W-N40W controls the seismic anisotropy.polarizations can be measured along virtually the same travel path whereas P-wave traveltime measurements must be compared along different azimuths. This necessarily involves differences in the travel paths and therefore introduces potential heterogeneities. This is especially important when the target of investigation is vertical fractures (because the P-wave response is limited to relatively long offsets). Second, S-wave polarization is determined by the "natural coordinates" of the rock which are, by definition, the rock's stiff and compliant directions; thus S-wave polarization is directly related to dominant open fracture azimuth. P-waves also contain the relevant information but it must be extracted in prestack analyses such as AVO. In this project, both ninecomponent VSPs and S-wave reflection data are used to support our P-wave AVO results. Local seismic calibration, using a nine-component VSP, is essential for this type of work. Moreover, shear-wave data are also important for showing matrix porosity developments in high velocity units (shearwave velocity decreases with increase in porosity).Direct evidence of vertical fractures from cores, and fractures mapped on outcrop were used to orient the nine-component seismic lines (Figure 1). Two joint and fracture sets were identified in surface mapping, one striking N22W-N32W and one striking N60E-N77E. Maximum horizontal stress was estimated as N40W-N45W from borehole ellipticity observed in four-arm caliper logs in two a...
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