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GCS-
Wave
Analysis of Fracture Systems*
By
Bob A. Hardage1 and Michael V. DeAngelo1
Search and Discovery Article #40227 (2006)
Posted December 6, 2006
*Adapted from the Geophysical Corner columns, prepared by the authors, in AAPG Explorer, October and November, 2006. Title of column in October, Part 1 here, is the same as that given above; title of column in November, Part 2 here, is “S-Waves and Fractured Reservoirs.” Editor of Geophysical Corner is Bob A. Hardage. Managing Editor of AAPG Explorer is Vern Stefanic; Larry Nation is Communications Director.
1Bureau of Economic Geology, Austin, Texas ([email protected] )
Most rocks are
anisotropic, meaning that their elastic properties are different when measured
in different directions. For example, elastic moduli measured perpendicular to
bedding differ from elastic moduli measured parallel to bedding – and moduli
measured parallel to elongated and aligned grains differ from moduli measured
perpendicular to that grain axis. Because elastic moduli affect seismic
propagation
velocity, seismic
wave
modes react to rock anisotropy by exhibiting
direction-dependent velocity, which in turn creates direction-dependent
reflectivity. Repeated tests by numerous people have shown shear (S) waves have
greater sensitivity to rock anisotropy than do compressional (P) waves.
Slowly the
important role of S-waves for evaluating fracture systems, one of the most
common types of rock anisotropy, is moving from the research arena into actual
use across fracture prospects. Examples of S-wave
technology being used to
determine fracture orientation have been published by Gaiser (2004) and Gaiser
and Van Dok (2005), for example. It seems timely to introduce one more example
.
Part1uGeneral StatementuFigures 1 & 2uExampleuConclusionuCommentuAcknowledgmentuReferencesPart 2uGeneral statementuFigure 3uExampleuLocal differenceuLocal variationsuProofuAcknowledgment
Part1uGeneral StatementuFigures 1 & 2uExampleuConclusionuCommentuAcknowledgmentuReferencesPart 2uGeneral statementuFigure 3uExampleuLocal differenceuLocal variationsuProofuAcknowledgment
Part1uGeneral StatementuFigures 1 & 2uExampleuConclusionuCommentuAcknowledgmentuReferencesPart 2uGeneral statementuFigure 3uExampleuLocal differenceuLocal variationsuProofuAcknowledgment
Part1uGeneral StatementuFigures 1 & 2uExampleuConclusionuCommentuAcknowledgmentuReferencesPart 2uGeneral statementuFigure 3uExampleuLocal differenceuLocal variationsuProofuAcknowledgment
Part1uGeneral StatementuFigures 1 & 2uExampleuConclusionuCommentuAcknowledgmentuReferencesPart 2uGeneral statementuFigure 3uExampleuLocal differenceuLocal variationsuProofuAcknowledgment
Part1uGeneral StatementuFigures 1 & 2uExampleuConclusionuCommentuAcknowledgmentuReferencesPart 2uGeneral statementuFigure 3uExampleuLocal differenceuLocal variationsuProofuAcknowledgment
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The prospect considered here involves two fractured carbonate intervals at a depth of a little more than 1800 meters (6000 feet). A small 5.75-km2 (2.25-mi2) three-component 3-D seismic survey (3C3D) was acquired to determine whether PP (compressional) and PS (converted-S) data could be used to determine fracture orientation for optimal positioning of a horizontal well. Figure 1 shows a PP and PS azimuth-dependent data analysis done in a superbin near the center of this survey. At this superbin location, common-azimuth gathers of PP and PS data extending from 0 to 2000-meter offsets were made in narrow, overlapping, 20-degree azimuth corridors. In each of these azimuth corridors, the far-offset traces were excellent quality and were summed to make a single trace showing arrival times and amplitudes of the reflection waveforms from two fracture target intervals A and B. To aid in visually assessing the character of these summed traces, each trace is repeated three times inside its azimuth corridor in the display format used in Figure 1.
Inspection of these azimuth-dependent data shows two important facts: · PS waves arrive earliest in the azimuth corridor centered 50° east of north (the fast-S mode, S1) and latest in an azimuth direction 140° east of north (the slow-S mode, S2). · PS waves exhibit a greater variation in arrival times and amplitudes than do their companion PP waves. For example, PP reflectivity from interval A is practically constant in all azimuth directions, whereas PS reflectivity varies significantly with azimuth. Likewise, PP arrival time of event A changes by only 4 ms between azimuth directions 50° and 140°, but PS arrival times change by almost 50 ms, an order of magnitude greater than the variation in PP arrival times.
Azimuth-dependent trace gathers like these
were created at many locations across the seismic image space, and the
azimuths in which PS reflection amplitudes from fracture intervals A and
B were maximum were determined at each analysis location to estimate
fracture orientation for each interval. A map of S- On the basis of this close correspondence
between FMI and S-
We conclude that application of S-
This particular horizontal well was not
placed in production – even though the well bore intersected a high
population of fractures trending perpendicular to the well axis –
because too many of the fractures were plugged with cement. That problem
sets the stage for a subsequent article, in which we will describe
S-
This research was funded by sponsors of the Exploration Geophysics Laboratory at the Bureau of Economic Geology.
Gaiser, James E., 2004, PS-
Gaiser, James E., and Richard R. Van Dok, 2005, Converted
Shear-
S-Waves and Fractured ReservoirsGeneral StatementIn Part 1, we show that fracture
orientation across fractured-reservoir intervals can be determined by
azimuth-based analyses of S- In Part 2, we return to the same 3C3D seismic data used in Part 1 and show how attributes determined from fast-S and slow-S data volumes allow patterns of relative fracture intensity to be determined in a qualitative, not quantitative, manner. In
Figure 1 we show that in a fractured medium, a converted-S wavefield
segregates into a fast-S mode and a slow-S mode, and that the azimuth
directions in which these fast-S and slow-S modes orient their polarized
displacement vectors differ by 90 degrees. Knowing the polarization
directions of these two S- (The procedures used to segregate S-
We show here in Figure 3 a vertical slice from the fast-S volume and the corresponding vertical slice from the slow-S volume. The two fractured carbonate intervals A and B are labeled on each display, as well as several horizons interpreted near these two reservoir intervals.
Differences between these fast-S and slow-S images include:
Some of these relative time-thickness changes are difficult to see by visual inspection of Figure 3, but numerical analyses of the isochron intervals between interpreted horizons show numerous examples of such behavior. Two locations where the time thickness of a reflection wavelet expands more in slow-S image space than in fast-S image space are labeled T1 and T2.
Local Difference: ReflectivityThe units bounding fracture intervals A and B have seismic impedances that are less than the impedances of fracture units A and B. This statement applies to most fractured targets and their bounding units. Fast-S and slow-S reflectivities across
targets A and B are controlled by the magnitude of the differences in
impedances across the top and bottom boundaries of A and B. When
fracture intensity and fracture openness increase locally, the
difference between slow-S and fast-S velocities increases. Fast-S
velocity changes little (usually not at all) when fracture intensity
increases, but slow-S velocity decreases and becomes closer to the
magnitude of the S- To define locations where relative fracture
intensity increases, we thus search the fast-S and slow-S volumes to
find coordinates where S- Local Variations: Interval-Time ThicknessWhen the slow-S interval-time between horizons aa and cc increases (Figure 3b).), two possible explanations are that (1) the thickness of reservoir A has increased or (2) reservoir A has a constant thickness, but slow-S velocity has lowered because of an increase in fracture intensity.
Other arguments may be proposed in different geological settings, but in this case, these two explanations were the most plausible. · Option 1 can be verified by measuring fast-S interval time between horizons aa and cc (Figure 3a). If the reservoir interval thickens, fast-S interval time should increase. · If fast-S interval time changes little, or not at all, then option 2 (increased fracture intensity) is accepted as the explanation for the increase in slow-S time thickness.
Two image coordinates where slow-S time thickness increases more than does fast-S time thickness are labeled T1 and T2. Increased fracture intensity is expected at each of these locations. Prove It!What we have demonstrated is that comparisons
of fast-S and slow-S reflectivities and time thicknesses across
fractured intervals allow locations of relative increases in fracture
intensity and openness to be identified. These S- Proving the validity of predictions of fracture intensity requires extensive calibration of fast-S and slow-S attributes with reliable fracture maps across prospects. Such investigations are ongoing and will be reported in time. For the present, we show you here the latest logic that seems to allow long-range, seismic definition of relative fracture intensity across multicomponent seismic image space.
This research was funded by sponsors of the Exploration Geophysics Laboratory at the Bureau of Economic Geology. |