American Oil and Gas Reporter - September 2018 - 90

FIGURE 2
Bed Counting Method to Track Stratigraphic Position
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200

Gamma (API)

1,000' MD

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Synthetic Data Between Wells
When improved imaging visibility is
combined with 3-D geomodeling incorporating on-the-fly correlation and updating,
a higher level of correlation confidence
can be achieved. Geomodeling permits the
creation of synthetic petrophysical data by
propagating data from all nearby wells,
accounting for thickening and thinning according to the geometry of surfaces within
the model. These synthetic properties can
be projected through the model and compared with actual data. Correlation adjusts
surface depths and thicknesses, resulting
in a corresponding change of character in
the synthetic curves, resulting in validation
of the geosteering interpretation.
A correctly interpreted correlation is reflected in high character coherence between
the actual and synthetic curves behind the
bit. Surface deformation is propagated ahead
of the bit with a corresponding change in
projected synthetic petrophysical curves.
This permits greater understanding of expected properties ahead of the bit to mitigate
unnecessary steering changes based on misinterpretations, and allows smoother and
more gradual steering adjustments to remain
in the target zone while mitigating increased
wellbore tortuosity.
This continuous updating with new data
and correlations results in a high-quality
geomodel that can be used in laterally
continuous bedding to model the azimuthal
gamma response along a projected well
path. As real-time gamma image data are
logged, a close match between the synthetic
azimuthal gamma image and measured
azimuthal image is a sign of an accurate
stratigraphic position interpretation.
This projection method reduces unnecessary course corrections and increases
correlation confidence. To achieve an ac90 THE AMERICAN OIL & GAS REPORTER

curate synthetic image, a 3-D model is required to account for bed thickening/thinning
and topographical changes. A close match
between both images gives the geologist a
high level of confidence in the interpretation
and the accuracy of the geomodel. When
encountering unexpected laterally discontinuous bedding, the 3-D model provides
greater confidence in maintaining trajectory
until correlatable laterally continuous and
stratified features are encountered again.
Since azimuthal gamma image features
are a result of wellbore trajectory, bed
dip angle and wellbore direction, to accurately generate synthetic images that
honor measured images, utilizing a continuous inclination-generated survey results
in a more accurate match. The relationship
between bed dip, wellbore survey and
gamma image features are presented in
Figure 3, which demonstrates discrepancies
between a 90-foot survey position (left)
and continuous survey position gamma

image (right) for the same dip pick. In
this case, the continuous survey shows a
2.1-degree difference in bed dip compared
with the 90-foot survey calculation.
The lack of wellbore inclination characterization is one of the largest contributing
factors to true vertical depth measurement
errors. This error presents itself in bed
dip calculations when dip picking azimuthal
gamma image features, as well as across
correlation cells in geosteering interpretations. A lack of TVD characterization
can lead to an inability to understand
local bed dips, as well as give bed boundaries the appearance of being jagged or
having shifting surface angles with every
new segment/correlation cell. Proper wellbore inclination characterization with continuous inclination results in more realistic
bed dip calculations and removes much
of the undulation often seen in geosteering
interpretation cross-sections.
Laterally Discontinuous Zones
Higher-energy depositional environments
often result in chaotic bedding, with little
lateral continuity or stratification. In these
environments, logging data characteristics
exude little coherency from well to well,
making correlation difficult and inaccurate.
In marine depositional environments, distal
and proximal depositions can be in direct
stratigraphic contact with one another. As
a result, small stratigraphic movements of
the wellbore-whether from trajectory
change, pinching out or faulting-can result
in encountering lithology with very different
depositional bedding characteristics.
When using a total gamma measurement only for geosteering, the transition
into a zone of laterally discontinuous
bedding may not become apparent until
the logging data cannot be correlated
against the type log with any degree of
certainly. At this point, correlation confi-

FIGURE 3
Bed Dip Calculation Utilizing 90-foot Surveys
Versus Continuous Inclination

American Oil and Gas Reporter - September 2018

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