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), but on a much larger scale.
The fine-scale
laser scanner is tripod mounted, laptop-controlled and reasonably
portable (figure 1). The system collects
three-dimensional data by measuring the elapsed time between emitting
and detecting laser pulses to determine the distance between the
scanner and the scanned surface, much like radar or sonar.
The unit
measures approximately 1,000 points per second, with maximum expected
error of about five millimeters measured along the scanner axis.
A single
collection of points produced by the scanner typically comprises
750,000 to 1.25 million points and is termed a point cloud. Each
point is composed of a three-dimensional location and a measured
intensity value, which is dependent on surface roughness, moisture,
etc.
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
coordinate system, smoothing to reduce noise and decimating to reduce
point density.
Decimating
is especially important when processing larger datasets, as it helps
to reduce processing time.
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 using an
automatic feature detection algorithm, which is controlled by user-supplied
parameters such as minimum patch size and desired patch quality.
After collection, patches are exported with orientation and location
for analysis and visualization on stereonets, rose diagrams and
within three-dimensional structural models (figure
3).
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.
In conclusion,
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.