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GCUsing
3-D Outcrop Laserscans for Fracture Analysis*
By
Steve Ahlgren1 and Jim Holmlund2
Search and Discovery Article #40099 (2003)
*Adapted for online presentation from the Geophysical Corner column in AAPG Explorer July, 2002, entitled “Outcrop Scans Give New View,” prepared by the authors. Appreciation is expressed to the authors, to R. Randy Ray, Chairman of the AAPG Geophysical Integration Committee, and to Larry Nation, AAPG Communications Director, for their support of this online version.
1Midland Valley Exploration, Glasgow, UK ([email protected])
2Geo-Map Inc., Tucson, Arizona
General Statement
Understanding natural fracture systems may be difficult
using
limited borehole, production, or seismic data. When available, fracture
data from analog outcrops provide additional insight necessary for effective
exploration and production in fractured reservoirs. Surficial fracture data are
often collected
using
hands-on, time-tested techniques such as:
-
Scanline analysis, which includes recording the attitude and location of each fracture intersecting a measuring tape at the base of an analog outcrop.
-
Cell mapping, which is performed by spatially dividing the survey area into cells and measuring gross orientations of primary fracture sets within each cell.
Although widely
utilized, these inherently two-dimensional techniques may be biased or provide
an incomplete assessment of fracture systems -- so we address these challenges
by using
a new fracture analysis methodology based on high-resolution laserscan
technology. This technology is successfully being used for a wide variety of
technical and mapping applications, and also has been successfully applied in
the petroleum industry (see example of similar airborne technology in the
February, 2002, EXPLORER, p. 6-9), but on a much larger scale.
|
The
fine-scale laser scanner is tripod mounted, laptop-controlled and
reasonably portable (Figure
1). The
For large
areas or regional analysis, multiple point clouds may be collected and
merged into a single scene during post-processing. Prior to utilizing
the scan for geological analysis, the unconnected points must be
triangulated to produce a three-dimensional convex hull, which is then
visualized and analyzed as desired (Figure
2). Processing also includes registering the data within a UTM
Fracture
detection is best performed on relatively high-quality laserscan data
free from noise and obstructions, such as rockfall, trees, and shrubs.
Fractures are extracted from the laserscan data In addition to simple orientation and location information, the fracture data are also automatically divided into related populations, and descriptive statistics are collected for each of these populations. These data are then used to synthesize three-dimensional fracture models with the same statistical footprint as fractures measured in the field (Figure 4). The fracture models may be used in myriad ways, for example, populating a structural model of the fractured reservoir with a realistic, three-dimensional fracture network.
· The laserscanning method is the first truly three-dimensional technique for collecting fracture information over broad outcrops. · The method has numerous advantages over traditional methods including consistent measurement accuracy, processing speed and reduced sampling bias. · From a safety standpoint, the scanner also is favorable to other techniques because the operator can stand over 100 meters away from the scanned outcrop. · Models created with the laserscanner not only provide an important conceptual framework for the geoscientist or engineer working to understand a fracture reservoir but also contribute to structural modeling, well planning, and stress analysis. · Furthermore, the models may be used not only in petroleum geosciences, but also in mining exploration/production, geotechnical assessment and high-precision surveying/mapping. |