Moving in the
right direction
The increasing shift to high-angle and
horizontal drilling has precipitated the
industry’s quest for more comprehensive
and higher quality, real-time logging-whiledrilling (LWD) data, particularly in the
carbonate reservoirs of the Middle East.
Knowing whether a boundary is being
entered from above or below takes away a
lot of the guesswork for drillers and
geologists, and the ability to react
immediately is of great value to the
geosteering engineer.
Recent technological advances have added
an ‘azimuthal’ capability to resistivity and
gamma ray LWD tools, enabling real-time
measurement in a number of equally spaced
sectors around the borehole. Increased
sensitivity allows detection of features that
would have remained unseen using
conventional methods.
In this article, Ahmad Madjidi explains the
characteristics of these new methods and
examines their application in the field.
he coming decades will see
significant increases in the worldwide
demand for oil and gas from carbonate
reservoirs. In particular, gas is
increasingly important to the world's
energy balance. Since about 70% of the
Middle East’s oil and gas reservoirs are
carbonates, the region stands to benefit
more than most from advances in logging
while drilling (LWD), formation
evaluation while drilling (FEWD) and
geosteering technologies.
Advances in LWD technology have
already improved the quality and
quantity of the real-time data used for
the evaluation of hydrocarbon-bearing,
carbonate reservoirs across the Middle
East region.
LWD technology was introduced in the
1980s when measurement was limited to
natural gamma rays and the resistivity of
the formation. There were also
limitations in the bandwidth available for
transmitting data to the surface in real
time, and concerns over data validity in
unpredictable drilling environments such
as heterogeneous carbonate sequences.
The shift to high-angle and horizontal
boreholes accelerated the industry’s
requirement for real-time acquisition of
information for geosteering and
petrophysical and geological evaluation.
Today’s advanced LWD tools acquire
data from a number of equally spaced
sectors around the borehole as the the
bottomhole assembly (BHA) revolves.
Encountering features such as bed
boundaries, faults and borehole
enlargement in deviated wells results
in differing ‘top’ and ‘bottom’
measurements. These data indicate, in
real time, whether the tool is entering a
formation from above or below, and
allow analysts to estimate dip values.
The extra sensitivity of azimuthal
measurement often detects features that
may go unnoticed using conventional,
circumferentially averaged methods,
which give ‘smeared’ density outputs at
bed boundaries.
There are two types of LWD tool with
azimuthal capability. The RAB*
Resistivity-at-the-Bit tool collects
T
Number 2, 2001
48
Middle East Reservoir Review
formation resistivity from 56 sectors that
will produce a relatively detailed borehole
image. The ADN* Azimuthal Density
Neutron tool acquires formation bulk
density and photoelectric factor (PeF)
measurements at high sampling rates
from 16 sectors around the borehole.
For the purposes of explaining
geosteering, this article concentrates on
the use of the ADN tool. The more
subtle applications of the ADN and the
RAB tools, when used in formation
evaluation while drilling, are discussed
in a separate article – Cracking the
carbonate code page 56.
Azimuthal data acquisition
Real-time azimuthal data that are
transmitted to the surface are referred
to as quadrant data. Figure 4.1 shows
the orientation of the ADN tool in a
nonvertical borehole, and how the
borehole is divided into up, right, down
and left quadrants. In addition to the
real-time data for geosteering, density
and PeF data from the tool’s 16
segments are stored in the BHA for
processing and production of density
and photoelectric images when the tool
is retrieved to surface.
Field example
In one Middle East gas-bearing
reservoir, the objective was to drill
horizontally approximately 1500 ft in the
reservoir, following a step-down profile
with about half the length of the
borehole drilled 15 ft from the top of the
reservoir and the remainder 15 ft from
the bottom. The reservoir is a clean,
gas-bearing limestone with a field-wide
extent deposited between dense
limestone units.
The LWD method was selected to
provide geosteering and petrophysical
evaluation. The usual practice is to land
the well nearly horizontal and case it off
with a 7-in. liner in the dense limestone
above the reservoir. A 6-in. hole is then
drilled following the plan. Real-time data
are monitored at the wellsite and at the
office, and then a decision is made on
whether or not to revise the projected
plan. The density and neutron
measurement data are the most
important for geosteering in this type of
reservoir. The gamma-ray curve is usually
flat, except over the dense limestone
interval, and the resistivity of the porous,
gas-bearing section is very similar to the
dense layers above and below.
Formation 1
Bulk density
detector
Up
Left
Right
Down
Formation 2
Figure 4.1: The ADN
tool with respect to
the borehole,
indicating the
position of the bulk
density detector
during measurement
of formation 1. At
this point in time, the
upper quadrant
density (ROBU) gives
the formation 1 bulk
density. Bulk density
and photoelectric
factor are measured
16 times in every
BHA revolution and
are each stored in 16
equally spaced
sectors around the
borehole
-0.75
DRHB
(g/cm3)
0.25
Moved gas
50
0
200
TVD
(ft)
GR
(gAPI)
ROP
(ft/hr)
0
1
100
1
MD
1 : 350
0 ft 1
P10H
(ohm-m)
P22H
(ohm-m)
P34H
(ohm-m)
100 45
100 1.95
100 1.95
TNPH
(ft3/ft3)
ROBU
(g/cm3)
ROBB
(g/cm3)
-15
2.95
2.95
0000
0500
1000
Figure 4.2: Real-time LWD data. In addition to tool face, direction and inclination, eight log curves
are sent to the surface via MWD mud-pulse telemetry
Real-time data
A section of ADN data from the same
borehole is shown in Figure 4.3. It covers
the bottom part of the upper zone (a
thin, low-porosity layer) and the top part
of the lower zone. The separation
between the TNPH and ROBB
measurements clearly indicates the
presence of a gas-bearing, high-porosity
interval from the top of the display to
around 660 ft, followed by a reduction in
porosity and a much-reduced, neutrondensity separation from 670 to 710 ft.
Note that the ROBB detects the change
first, with measured values increasing at
around 10 ft earlier than the ROBU. This
indicates that the dense layer is being
penetrated from the top. As the tool exits
this thin, dense layer (around 1 ft thick)
at a depth of 720 ft, and enters the lower,
low-porosity, gas-bearing zone, once
again the ROBB sees the change first.
Thin layers of this size are not usually
detected using conventional logs in
vertical holes.
Number 2, 2001
Real-time log data for the subject well
are shown in Figure 4.2. The data
transmission frame was chosen to
transmit real-time direction and
inclination (D&I), tool face, up- and
bottom-quadrant densities (ROBU and
ROBB), thermal-neutron porosity
(TNPH), bottom-quadrant density
correction (DRHB), gamma ray (GR),
and three out of the ten resistivities.
These data are sent to surface in real
time via measurement-while-drilling,
mud-pulse telemetry.
While LWD data are usually sampled
and recorded in tool memory every
10 sec, with such a transmission frame
in real time, the update rate for the
eight log curves is every 70 sec, D&I
every 35 sec and tool face every 140 sec.
With a drilling penetration rate
averaging 50 ft/hr, and mud-pulse
telemetry rate of 1.5 bits per second, a
previously unequalled real-time
sampling rate of around one sample for
every foot was achieved.
Bed boundary effect on
azimuthal data
Middle East Reservoir Review
49
PEB
( )
4
24
ROBB
1.95
(g/cm3)
ROBU
2.95
1.95
(g/cm3)
2.95
TNPH
TVD
50
(ft)
0.45
0
(ft3/ft3)
0
(gAPI)
ROP
500
(ft/h)
-0.15
DRHB
GR
-0.25
100
MD
1 : 800
0
ft
(g/cm3)
0.25
RPM
500
(c/min)
0
Figure 4.3: LWD data
across the thin layer
crossing. During
intersection of the
dense layer from the
top of the layer (around
675ft), the dense layer
is detected by the
bottom density first.
During intersection of
the dense layer from
the bottom of the layer
(around 925ft) it is
detected by the top
density first
700
800
900
In this case, the anomaly shows that
the lower zone is being entered, so
drillers can adjust the drilling course
upwards. After drilling nearly 150 ft in
the upper part of the lower reservoir,
the borehole intersects the dense layer
again, but this time from the bottom.
The ROBU sees the density changes
nearly 20 ft before the ROBB when
entering and exiting the dense layer.
Knowing whether a boundary is
being entered from the top or from the
bottom takes away a lot of the
guesswork for drillers and geologists. It
is of great value to the geosteering
engineer in deciding whether to go up
or down when an unplanned formation
is encountered. The wellbore is more
Number 2, 2001
50
Middle East Reservoir Review
likely to stay in the target reservoir
and this can significantly reduce
drilling time.
To confirm this behavior, INFORM*
Integrated Forward Modeling was used to
simulate the azimuthal density log
response across the interval shown in
Figure 4.2. A 1-ft-thick, dense layer
between the upper and lower reservoir,
the planned formation dip, and actual
survey data of the drilled horizontal hole
were used.
The match between the predicted and
the actual LWD (Figure 4.4) confirms
the formation porosity model, formation
dip and the thin, dense-layer properties.
Taking into account the borehole
inclination, the borehole depth distance
between the up and bottom density
readings, and the borehole size, an
apparent formation dip can be computed
at the wellsite using a simple equation.
Figure 4.5 is a schematic drawing of a
borehole with an inclination of I that
intersects a plane dipping at angle (a),
with borehole depth distance between
the up and bottom density of DD. The
effective borehole diameter is De. The
angle (a) can be computed by:
a = Arc tan (De/DD) + I - 90
This approach gives a quick estimate
of apparent dip in real time. This can be
useful when the structural dips are
unpredictable and poorly defined.
Section cross section – 1: N 137 E - RHOB formation property
Figure 4.4: INFORM
modeling to compare
the LWD log data
with the modeled
LWD data during
crossing the thin,
dense layer gave
fairly good
agreement. The
position of a fault
with a very small
throw (3 to 4 ft) as
indicated by top and
bottom density log
curves around 700ft
drift, is also shown.
This agreement
confirms the
formation dip, the
thickness of the
dense layer, and the
location of the fault
2.8
Pseudowell
Bottom
2.66
Sysdrill-1
2.52
ROBB.LW
2.38
ROBU.MOD
2.24
ROBU.MOD
2.1
ROBU.LWD
ROBB.MOD
2.8
ROBU.MOD
Top
2.66
2.52
2.38
2.24
2.1
Modeled ADN
2.8
2.66
2.52
2.38
2.24
2.1
420
TVD, ft
443
466
489
2.17
2.28
512
2.44
2.71
535
355
704
1053
1402
1751
2100
15
Drift along the section, ft
Figure 4.5: Estimating
the apparent formation
dip using the depth
distance between up
and bottom density
curves, borehole
inclination and borehole
effective diameter
22.5
33.5
44.5
RHOB
Borehole
Dipping plane
Ι
90
α (apparent dip)
90 - Ι
90 - Ι
De
Number 2, 2001
Borehole depth distance between
bottom and top density readings (∆D)
β = α + (90 - Ι)
β = Arc tan (De/∆D)
α = Arc tan (De/∆D) + Ι - 90
Middle East Reservoir Review
51
DRHB
-0.75
Figure 4.6: Real-time
data across the
interval where the
borehole seems to be
intersected by a fault.
ROBB and ROBU
show a nearly
simultaneous
increase around
X50 ft, followed by a
reduction in their
values
RPM .DF [A816507]
500
(c/min)
P10H
0
1
GR
0
(gAPI)
ROP
200
(ft/hr)
(ohm-m)
1
MD
1 : 500
0
1
ft
(ohm-m)
(ft3/ft3)
-15
ROBU
100 1.95
P34H
(ohm-m-
0.25
TNPH
100 45
P22H
150
(g/cm3)
(g/cm3)
2.95
ROBB
100 1.95
(g/cm3)
2.95
X00
X50
X000
Neutron changes
Bulk density changes
TNPH .LWD
Figure 4.7:
Comparison of LWD
and LWT logs
Shallow resistivity changes
P10H .LWD
1
(ohm-m)
100
0.45
(ft3/ft3)
TNPH .LWT
-0.15
0.45
(ft3/ft3)
ROBB .LWD
-0.15
1.95
(g/cm3)
2.95
ROBB .LWT
P10H .LWT
1
(ohm-m)
100
1.95
X20
X40
X60
X80
Number 2, 2001
52
Middle East Reservoir Review
(g/cm3)
2.95
Fault finding
The plan for the second half of this
horizontal borehole was to drill 15 ft
from the base of the reservoir once the
step-down had been accomplished.
During this step-down, the lower zone
was unexpectedly penetrated.
Geologists and petrophysicists analyzed
the LWD logs in more detail to
understand this structural anomaly.
Real-time LWD data (Figure 4.6)
show a very small displacement (around
1 ft) between the ROBB and ROBU. The
density increase that they show (more
than 0.1 g/cm3 at around a depth of
948 ft) suggests that there is a thin,
dense layer at very high angle to the
wellbore (in the case of bedding,
depending on the borehole inclination,
this distance would be much greater, as
can be seen in Figure 4.9). Moreover, a
decrease of around 0.1 g/cm3 in the
ROBB suggests an increase in porosity
after 950 ft. The response of the neutron
porosity to this density decrease is
minimal, as it is affected by the
presence of gas and lacks sufficient
resolution to detect minor geological
variations such as this. The slight drop
in resistivity also indicates an increase
in porosity.
Comparing the LWD with the logging
while tripping (LWT) data (Figure 4.7), it
appears from the neutron-density
separation and shallow, resistivityseparation profile that there is a change in
reservoir characteristics starting at the
position of the fault.
Density and PeF images across the
interval around the fault are shown in
Figure 4.8. The borehole circumference is
divided into 16 parts and each part is
being examined separately. This allows
analysis of the borehole in all directions,
permitting analysts to determine whether
the anomalies are random, uniform or
local. As with acoustic and electrical
imaging, the azimuthal data variation
along the borehole wall is converted to
color variation. The intensity of the color
and its variation indicate different
densities on each side of the fault, uniform
density around the borehole and a lighter
color, hence denser formation, at the fault
plane itself.
ADN_RHOB .RA
1 : 18.8
TOH
linear scaling
RHOB
Borehole
tadpoles
50
100
Deg.
0
Cond. (g/cm3) Resist.
Orientation
U R B
L U
ROBU
1 2
2.4
( )
(g/cm3)
MD
1 : 400
ft
ROBB
2
ADN_PeF .RA
2.4
St. Im.
Apparent dip
Top of true dip
Fault dip view
(Sinusoid)
Orientation top of hole
(g/cm3 )
1 : 18.8
Fault dip view
Linear scaling
PeF
0
Deg.
90
Low
()
High
900
950
Figure 4.8: Density and PeF images across
the interval where the borehole is
intersected by a fault
Number 2, 2001
The throw of the fault was estimated
by INFORM (Figure 4.4) using two tie-in
points, borehole intersection with the
reservoir, and azimuthal density
response across the thin, dense layer.
Since the photoelectric effect is
sensitive to lithology changes, and the
reservoir is almost pure limestone, no
variation in the PeF image is expected.
These analyses suggest a sealing fault
with a throw of around 3 to 4 ft
intersecting the borehole at 945 ft.
Middle East Reservoir Review
53
True-dip computation
Dips are computed from the density
image by identifying the bed boundaries
on the image and picking a sinusoidal line
on the density contrast using an
interactive workstation. A density
contrast of at least 0.1g/cm3 is needed for
good definition of the bed boundaries.
The density image across an interval
from 350 to 450 ft is shown in Figure 4.9.
The dense layer (density contrast around
0.1 g/cm3) appears white on the image
and can be clearly differentiated from the
rest of the rock sequence. The dip
calculated for this feature is 1–2º with a
dip azimuth of 310º.
To evaluate geological dip accurately,
the uncertainties in both measurement
and processing must be considered.
Density and PeF images inherently lack
the vertical and lateral resolution of
microresistivity images and only identify
large features. As with other imaging
devices, the correlation error can be large
if the pick is not done properly, the most
sensitive factor being the dip azimuth.
LWD integrated
answer product
A wide plot was designed to get
maximum benefit from LWD data. This
integrated display (Figure 4.10) was
helpful in reconciling the LWD data and
the proposed geological model, especially
visualization of the azimuthal density
response with respect to the boundaries.
Conclusion
The case study demonstrates the
successful application of an azimuthal
density LWD log for geosteering, the
computation of apparent and true dip,
and the identification of structural
features such as faults.
Conventional LWD density
measurements, where the average
formation bulk density is measured, can
mask or smear many formation and
borehole properties, such as bed
boundaries, entrance to and exit from
a formation, thin beds, formation
heterogeneities and borehole
enlargement. The ADN, with its ability
Number 2, 2001
54
Middle East Reservoir Review
ADN_RHOB .RA
1 : 18.8
TOH
Linear scaling
RHOB
Borehole
Cond.
tadpoles
U
MD
1 : 400
ft
ROBU
2
(g/cm3)
2.4
Resist.
(g/cm3)
Orientation
R B
L
U
Secondary dip view
Tadpoles
True dip
RPM
Secondary dip view
(Sinusoid)
Quality ]5,
200
Orientation top of hole
ROBB
0
( )
1 2
(g/cm3)
(c/min)
0
Quality ]15
2.4
0
Deg.
20 1
( )
0
350
400
450
Figure 4.9: Density image contrast indicating a
thin dense layer around borehole depth of 400 ft
to obtain density and photoelectric
measurements from 16 parts of the
borehole circumference in one revolution
of the BHA, addresses the shortcomings
of conventional LWD techniques.
Moreover, in addition to quadrant data, a
low-resolution borehole density and PeF
image can be produced. While they do not
contain as much geological information as
microresistivity surveys, these images can
reveal macroscopic geological features at
no extra cost to the drilling operation.
The LWD integrated display helps to
reconcile log information with the
geological model. LWD and LWT
comparison can reveal lateral
heterogeneity along the borehole and
INFORM modeling can play an
important role in certifying the
proposed geological model.
Dense thin bed
Upper zone
Dense layer top
70
MD
:250
ft
0
()
1
Borehole
tadpole
DRIF
(ft)
A28H
1
100
(ohm-m)
A22H
1
100
(ohm-m)
P28H
10
1000
(ohm-m)
P22H
10
1000
(ohm-m)
P16H
10
1000
(ohm-m)
P10H
10
1000
(ohm-m)
P34H
ROP
1000
0 10
200
(ohm-m)
(ft/hr)
GR
A34H
100
0
100 1
(ohm-m)
(gAPI)
OB .ADN_EC[
1 : 18.8
TOH
Linear scaling
RHOB
Cond. Resist.
ROBU
1.95
2.95
(g/cm3)
ROBL
1.95
2.95
(g/cm3)
ROBR
1.95
2.95
(g/cm3)
TNPH
0.45 -0.15
(ft3/ft3)
ROBB
1.95
2.95
(g/cm3)
(g/cm3)
Orientation
U R B L U
PEU
0
10
( )
PER
0
10
( )
PEL
0
10
( )
PEB
0
10
( )
DRHU
-0.25 0.25
(g/cm3)
BHA rot Boundary dip view
500
0
(Sinusoid)
(c/min)
Orientation top
DRHR
of hole
-0.25 0.25
(g/cm3)
Fault dip view
DRHL
(Sinusoid)
-0.25 0.25
(g/cm3)
Orientation top
DRHB
of hole
-0.25 0.25
3
(g/cm )
70
( )
Dense layer base
( )
-30
-30
Lower zone
Clay
Fault dip view
Tadpoles
True dip
Dense above
Formation
Dolomite
Quality ]5,
Dense below
Gas
Calcite
0
Quality ]15
Deg. 90
Fault dip view
Tadpoles
True dip
Quality ]5,
0
Quality ]15
Deg. 90
Well trajectory
70
1.95
1.95
70
(ft)
ROBU
(g/cm3)
ROBB
(g/cm3)
Sealing fault
( )
PSXO
-30 0.5 (ft3/ft3)
Porosity
0
VCLC
2.95
Moved gas
1
(ft3/ft3)
0
VDOL
2.95
Formation
0
PSW
-30 0.5 (ft3/ft3)
( )
1
VCL
0 0
(ft3/ft3)
1
0 1
( )
0
PHIE
Core image
Core image
0.5 (ft3/ft3)
BHA sliding
x 700
447.23
x 800
546.73
x 900
646.47
x 000
746.18
x 100
845.87
x 200
945.57
x 300
1045.3
x 400
1145.0
x 500
1244.3
x 600
1344.0
x 700
1443.8
x 800
1643.8
x 900
1643.7
x 000
1743.6
x 100
1843.6
x 200
1943.5
x 300
2043.5
Expanded
ADN image
Fault
throw around 4ft
BHA sliding
BHA sliding
BHA sliding
Pilot hole log
Number 2, 2001
Figure 4.10: Horizontal well integrated answer product
generated using structural dip and fault information, borehole
trajectory, and LWD raw and processed data
Middle East Reservoir Review
55