American Oil and Gas Reporter - November 2016 - 56

SpecialReport: MWD/LWD Technology
The extensive prewell modeling of
different landing scenarios resulted in an
optimized bottom-hole assembly that
combined a point-the-bit rotary steerable
system with azimuthal gamma at-bit, inclination at-bit, azimuthal deep resistivity,
and azimuthal lithodensity logging-whiledrilling sensors. Sensors near the bit
quickly identified marker beds and provided an accurate map of the subsurface
so early changes could be made to the
well plan to ensure a successful landing.
Results were evaluated in real time
while wells were drilling to allow continual
improvements, including reconfiguring
the horizontal BHA by deploying sensors
nearer the bit to more quickly detect
lithology changes. Customizing the BHA
and azimuthal variables specific to geological targets for each well culminated
in successfully landing all 20 horizontal
wells in their optimal positions, and resulted in achieving more than 90 percent
net-to-gross "on-target" intersection with
reservoir targets.
West Delta 73 is bounded by a large
regional growth fault to the north and a
counter-regional fault to the south. The
normally-pressured hydrocarbon accumulation occurs at the crest of the rollover
anticline and between the bounding faults.
The productive sands were deposited
within north-to-south trending fluvial
valley deposits (low stand incised valley
blocky sands) and fluvial-deltaic system
deposits (high stand coarsening up sand

packages). Figure 1 shows the Pliocene
stratigraphic column.
Given the field's complexity, seismic
data are of limited use at West Delta 73.
Although the reservoir sands are sufficiently thick to be mapped confidently,
the level of detail needed to map structural
variations is impossible on the order of
10 feet, even with high-quality seismic.
Attempts had been made to "stretch"
seismic data to conform with well tops,
but even that could not provide the
required detail.
Variations in OWC and lateral heterogeneity in the sands make horizontal drilling
even more problematic. OWCs can vary
greatly across a small area, and cannot be
mapped consistently to prove the presence
of a fault or other geologic barrier.
Landing Lateral LWD Suites
With reservoir tops varying as much
as 33 feet in true vertical depth across
the field, it was critical for the landing
LWD suite to have sensors close to the
bit to allow immediate trajectory corrections. The solution was combining a 63⁄4inch gamma at-bit inclination sensor to
determine formation changes with electromagnetic wave resistivity and azimuthal
gamma ray sensors to update the geosteering model and modify the well plan in
real time with advanced 3-D geosteering
software.
At-bit azimuthal gamma ray and inclination measurements optimize well

FIGURE 1
West Delta 73 Pliocene Stratigraphic Column

5,000'

6,000'

7,000'

8,000'

9,000'

10,000'

56 THE AMERICAN OIL & GAS REPORTER

300'

600'

1,500'

3,000'

placement and provide immediate feedback on directional trends and formation
changes. Gamma measurements are acquired by four scintillation detectors
mounted 90 degrees apart near the bit,
enabling precise stops at the desired
casing point. Inclination readings contributed to a smooth landing, using the
deviation trends as a guide for unexpected
stringers, changes in dip angle, and other
formation-related characteristics.
The LWD package used at West Delta
73 combines formation evaluation and
drilling optimization data in one short
collar, integrating sensors to acquire electromagnetic wave resistivity, azimuthal
gamma ray, pressure-while-drilling, dynamic motion, and mud resistivity. The
electromagnetic wave resistivity sensor
provides compensated resistivity values
at multiple depths of investigation at the
landing point. The azimuthal gamma ray
sensor is the primary sensor for formation
evaluation.
Lateral heterogeneity with variable
OWCs and thin oil columns required
sensors in the lateral LWD suite capable
of providing early warnings of approaching
boundaries to correct well trajectories
and avoid unnecessarily exiting the target
zone. The 43⁄4-inch RSS was paired with
azimuthal deep propagation resistivity,
azimuthal density, and conventional gamma ray sensors in the lateral section to
maintain sweet spot trajectory. An azimuthal laterolog resistivity sensor also
was used in select wells when additional
stratigraphic/structural information was
needed.
The RSS provides a high degree of
steerability and full directional control
with continuous drill string rotation,
which is required with the azimuthal sensors to measure log responses with 100
percent borehole coverage. The azimuthal
deep propagation resistivity sensor provides resistivity and geosignal measurements (indicating boundaries and direction
to the approaching bed) for precise wellbore placement. The sensor acquires azimuthal data in 32 discrete directions at
multiple depths of investigation.
High-Resolution Images
The propagation resistivity sensor induces eddy currents that circle the collar
in a plane perpendicular to the tool axis.
The magnitude of the current is influenced
by the resistivity of the formation. When
the sensor is near a resistivity contrasting
layer with an angle that interferes with

American Oil and Gas Reporter - November 2016

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