American Oil and Gas Reporter - September 2018 - 89

SpecialReport: Reservoir Modeling & EUR

Approach Optimizes Well Geosteering

Laterally Continuous Bedding
In areas with laterally continuous
strata, the gamma character in the type
well typically will be present at the
landing point and throughout the wellbore's lateral length. This type of bedding
is ideal for geosteering since a proper interpretation will overlay the type log with
little deviation. While these cases are
ideal for geosteering, they are still subject
to ambiguity when utilizing a 90-foot
survey and only total gamma data.
Figure 1 demonstrates how geosteering
interpretations can be ambiguous and
highly subjective as a result of the position
of 90-foot surveys, as well as a lack of
up or down stratigraphic movement indications from the total gamma measure-

ment. The type log at right shows a 15foot vertical profile of a zone. The interpretation at the top is based on 90-foot
surveys and total gamma data. In contrast,
the lower interpretation uses continuous
inclination and azimuthal gamma imaging.
The different stratigraphic position as indicated by the numbering is a result of
the definitive direction indication from
azimuthal gamma, and the continuous
inclination curve bringing visibility to
wellbore inclinations not characterized
by the 90-foot surveys.
Figure 1 shows only 1,000 feet of measured depth in a lateral. One can imagine
how many incorrect interpretations may
be made over the course of a 5,000-10,000
foot lateral simply because of data ambiguity
if the proper measurements for accurate
wellbore placement are not utilized.
In laterally continuous zones without
faulting or strata pinching, the bed counting
method (Figure 2) can be performed to
track stratigraphic positioning relative to
the landing point. This is a powerful
method for validating stratigraphic positioning without actively performing a
geosteering correlation. However, the
method is not effective for landing a
curve because of its downward-cutting
trajectory.

FIGURE 1
Incorrect (Top) and Correct (Bottom) Geosteering Interpretations
With Type Log (Right)
1,000' Measured Depth
Survey Inclination
85°
95°

Type Log
Gamma Ray
150

Gamma Ray

15' TVD

150

Continuous Inclination
85°
95°

Interpretation w/90' Surveys
and Total Gamma

1,000' Measured Depth

Total Gamma Ray
90' Surveys

Gamma Ray
150

Cont. Inclination

HOUSTON-Geosteering in horizontal
wells involves correlating logging data
to a type log from a nearby offset well to
characterize the zone of interest. Correlating against a type log requires the bedding thickness and gamma character of
the target well to be close to that of the
type log, which can be offset by miles in
some cases. If not, the resulting geosteering
interpretation will have unreasonable bed
dips that do not accurately reflect the
target lithology being drilled, leaving the
geosteering geologist running multiple
simultaneous interpretations in the hope
that one will start to make sense as new
data come in during drilling.
Azimuthal gamma imaging, in conjunction with continuous inclination measurements, brings clarity to stratigraphic
position, bed dip, faults and zones where
bedding is laterally discontinuous or completely absent. In many of these situations,
geosteering cannot be performed with
traditional solutions. Through enhanced
visibility of the subsurface by azimuthal
gamma imaging, geological anomalies
can be identified quickly and handled in
the best possible manner.
Utilizing azimuthal gamma imaging
data to identify faults, laterally continuous/discontinuous bedding, stratified/unstratified zones, and high-energy sedimentary deposition improves interpretation
accuracy. Azimuthal gamma logging tools
measure the incoming gamma radiation
from all sides of the borehole and provide
a clear indication of whether a change in
total gamma resulted from stratigraphic
movements up or down in horizontal
drilling applications.
When a change in total gamma is not
associated with an up or down movement
through stratigraphy, it indicates faulting,
a depositional anomaly, heterogeneity, or
bedding that is not laterally continuous
or stratified. Because the fundamental
basis of geosteering is correlating to
marker beds that have lateral continuity,
in situations where lateral continuity is
absent, only a low level of interpretation
confidence can be achieved using traditional correlation methods.
To maximize the benefits of azimuthal
gamma imaging, a protocol has been developed to identify and handle challenging

geosteering situations using a modelbased 3-D geosteering solution rather
than conventional 2-D geosteering software. By integrating multiple well and
seismic surface inputs, this approach
gives the geosteering geologist the information to confidently resolve abrupt bed
dip changes, identify faults, identify areas
of lateral continuity/discontinuity, identify
stratified/unstratified zones and understand
formation-related directional drilling trajectory phenomena, etc.

Interpretation w/Continuous
Inclination and Azimuthal Gamma

By Christopher Viens
and Mark Tomlinson

SEPTEMBER 2018 89

American Oil and Gas Reporter - September 2018

Table of Contents for the Digital Edition of American Oil and Gas Reporter - September 2018