Heloise B. Lynn

Correlation between P-wave AVOA and S-wave traveltime anisotropy in a naturally fractured gas reservoir

A reservoir characterization study of Upper Green River gas reservoirs in the Bluebell‐Altamont Field, in the Uinta Basin, Utah was completed in August 1995 for the U.S. Department of Energy. The study included acquisition of two crossing 2-D, 9-C seismic lines, 9-C VSP, and evaluation of the P-wave and S-wave seismic responses to the fractured reservoirs. Lynn et al. (TLE, August 1995) reported P-wave amplitude variations with offset and azimuth (AVOA) at seismic reflectors associated with fractured Upper Green River gas reservoirs in this field, and supporting observations of shear‐wave birefringence in multicomponent VSPs and surface reflection seismic. This paper documents the proportional relationship observed between the difference in the P-wave AVO gradients with azimuth and the magnitude of the S-wave birefringence, estimated from the two multicomponent reflection seismic lines, evaluated at the line intersection.

Reflection shear-wave data collected near the principal axes of azimuthal anisotropy

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.

Azimuthal anisotropy in P-wave 3-D (multiazimuth) data

A naturally‐fractured gas reservoir in the Wind River Basin, Wyoming, is the site of a Department of Energy‐sponsored project to optimize seismic techniques for characterizing natural fractures (density, orientation). The target zone is the Lower Fort Union (LFU, depth 5400–10 000 ft, two‐way time ∼1.3–∼2 s), a low porosity, low permeability section in which fractures are necessary for commercial gas production. Flow rates incompatible with matrix porosity and permeability are observed.

P-wave and S-wave azimuthal anisotropy at a naturally fractured gas reservoir, Bluebell‐Altamont Field, Utah

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...

3D/4C Emilio Azimuth processing and anisotropy analysis in a fractured carbonate reservoir

In recent years multicomponent 3D seismic data have demonstrated their usefulness for characterizing fractured reservoirs. Many theoretical and field studies have shown that variation of attributes (such as velocity, amplitude, and frequency of P-wave data acquired along different source to receiver paths), can be used as an indicator of azimuthal anisotropy. Lateral heterogeneity encountered by the different azimuth source-receiver raypaths could give rise to azimuthal variations in traveltime and/or transmission characteristics, and thus masquerade as azimuthal anisotropy. To remove this ambiguity, mode-converted split shear waves can be employed. S-wave splitting across a fractured medium is a well studied and understood phenomenon that exploits the traveltime delay between the fast and the slow S-waves, and their polarization azimuths to infer fracture properties.

The winds of change Anisotropic rocks—their preferred direction of fluid flow and their associated seismic signatures—Part 1

Editors note: This article is the first of two parts that together form an abbreviated version of the Fall 2004 SEG/AAPG Distinguished Lecture. A PowerPoint presentation of this material is posted on the SEG Web site. “The winds of change” refer to much more than just anisotropic rocks whose seismic anisotropy provides insight into horizontal permeability anisotropy. The way we view our planet is changing. The planet is now being studied as the interaction of four interlocking systems—the hydro-sphere, the atmosphere, the lithosphere, and the biosphere—as all absorb energy from our sun and change. The planet and all its subsystems, like geology or climate, are explicitly evaluated as interdependent and interacting within the context of the local community. The heterogeneity at the surface of our planet and the recycling of compounds and elements (water, nitrogen, sulfur, etc.) enable our planet to support life. My earth science career has spanned two revolutions: the plate-tectonic revolution during the 1970s, and todays profound reevaluation of “how our planet works” and our roles as planet-dwellers. The rocks that are the primary focus of most of our studies are best understood as, firstly, the result of the interactions of the climate, the water conditions, the depositional setting, the biosphere, and the provenance of the sediments, and secondly, the lithification and geologic history they have undergone. The biosphere shows up in our rocks both as fossils (a key means of dating rocks!) and as liquid or solid organic remains. We understand any given stratigraphic facies in the context of its spatial neighbors—its community—and its plate-tectonic setting. The climate includes the actions of gravity, sunlight, and atmosphere, but it is affected by where the land masses are and how they are linked together. Oil industry geoscientists are uniquely placed, via access to important data and sophisticated tools, …

Velocity and attenuation anisotropy: Implication of seismic fracture characterizations

Recent observations from several walkaround, walkaway, and 3D VSPs from various parts of the world have shown that anisotropy symmetry directions (or fracture orientations) estimated from traveltime or velocity anisotropy do not necessarily agree with symmetry directions from amplitude or attenuation analysis. We have also consistently found that attenuation anisotropy in reservoir intervals is generally stronger than in overburdens. We argue in this article that, instead of reconciling the differences between various attributes, we should try to understand the mechanisms and use the difference in velocity and attenuation anisotropy which may contain additional information about subsurface fracture systems.

Use of anisotropy in P-wave and S-wave data for fracture characterization in a naturally fractured gas reservoir

Gas production in Bluebell‐Altamont Field in northeastern Utah 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 flat‐lying 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.

Relationship of P-wave seismic attributes, azimuthal anisotropy, and commercial gas pay in 3-D P-wave multiazimuth data, Rulison Field, Piceance Basin, Colorado

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 ...

Field measurements of azimuthal anisotropy; first 60 meters, San Francisco Bay area, CA, and estimation of the horizontal stresses' ratio from V s1 /V s2

Vp,Vs, Q) of near‐surface strata. The azimuthal anisotropy revealed in these field experiments is the result of a combination of circumstances: unequal horizontal stresses (the data were collected near the center of the San Andreas and associated fault systems), fabric anisotropy introduced by the depositional agent, and stress‐aligned fluid‐filled microcracks, cracks, or pore spaces.