American Oil and Gas Reporter - November 2016 - 58

SpecialReport: MWD/LWD Technology
the current loop, a charge buildup at the
boundary acts as a virtual secondary
transmitter, and causes increased resistivity
in the direction of the high-resistivity
layer.
The azimuthal density borehole imaging
sensor yields dip and structural information,
and the wellbore's position relative to the
stratigraphy can be interpreted from the
density image. The azimuthal laterolog
sensor delivers resistivity images at the

fine vertical resolution necessary to make
accurate net-pay calculations, multiple
depth-of-investigation resistivity measurements, and at-bit resistivity measurements.
The detailed images of structural and
stratigraphic features are key in accurately
determining dip and fracture orientation.
Figure 2 shows an azimuthal laterolog
image interpretation for the West Delta
73 Columbo well.
Before drilling, a comprehensive

FIGURE 2
Azimuthal Laterolog Resistivity Dip Analysis for Columbo Well

.9,420'
.9,430'
.9,440'

prewell model was developed using petrophysical data from offset wells. It included
seismic grids of the reservoir and a well
path that intersected the sand target interval
at varying angles of incidence, as well as
the planned trajectory. The geological
model was updated before the landing
section, during the casing break, and
before starting the lateral to include the
most recent LWD data, surveys and well
plans before rerunning the possible scenarios for the upcoming section.
This approach provided a sense of
how logs and images from the various
LWD tools would react in different drilling
and geological scenarios, and assured the
tool would provide the necessary data
(indicators of approaching boundary, for
example) to accurately determine wellbore
positioning in the target formations.

.9,450'

Building angle
penetrating beds
from below.
Note induced
fractures appear
only at the top as the
deviation angle is
increasing

.9,460'
.9,470'
.9,480'
.9,490'
.9,500'
.9,510'
.9,520'
.9,530'
.9,540'
.9,550'
.9,560'
.9,570'
.9,580'
.9,590'
.9,600'
.9,610'

FIGURE 3
Geosteering Prewell Model for Hawkeye Well
100
10
1

0.1
200
100
0
9,280

TVD (ft)

9,300

9,320

9,340

9,360

40
10,200

10,400

58 THE AMERICAN OIL & GAS REPORTER

MD (ft)

10,600

100 140
Gamma Ray

9,300

9,300

9,310

9,310

9,320

9,320

9,330

9,330

9,340

9,340

9,350

9,350

9,360

10,800

0.1

1
24
Resist Deep

9,360

11,000

Landing The Laterals
Landing the lateral sections was critical
to success at West Delta 73. If the target
zone came in higher than expected, the
well could be landed near or below the
target zone base, and in some cases, near
the OWC. If the target zone was deeper
than expected, the well could be landed
above the target zone, and a long distance
could be drilled before dropping angle
and entering the reservoir. Questionable
formation tops added landing unpredictability.
While landing the wells, geosteering
began approximately 1,000 feet measured
depth before the planned landing. Upper
formation tops were monitored, and the
landing total vertical depth was adjusted
if differences from the expected depth
occurred. As the target reservoir was approached, inclination was held at 87-88
degrees until the target zone was confirmed
with gamma ray at the bit.
After a successful landing, the next
goal was to remain within the sand over
the length of a 1,000-foot lateral. Reservoir
variations within the thin oil columns in
the F, G and H series sands can reduce
the true vertical thickness of sweet spots
to as little as four feet. Sweet spots within
the sands generally show higher resistivity
properties than the formation above. Although gamma ray measurements help
ensure that the wellbore remains within
the target sand, resistivity measurements
are vital to confirming the BHA stays in
the sweet spot.
After a few wells were drilled, it was
decided to begin the lateral section at least
five feet TVD below the reservoir top as a
contingency plan for unexpected changes
American Oil and Gas Reporter - November 2016

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