Pushing the Limits of Formation

Evaluation While Drilling

Through a few case studies this article demonstrates how new

logging-while-drilling measurements are being used to open
frontiers and evaluate formations as soon as they are encountered.

Tim J. Bourgeois Logging-while-drilling (LWD) technology became Enhancing drilling safety and efficiency—
Ken Bramlett available a mere ten years ago. At that time, the Measurements while drilling provide real-time
Pete Craig tools fulfilled the primary purpose of their data on drillstring mechanics, fluid dynamics and
Shell Deepwater Production, Inc. design, which was to aid in correlation. Within a petrophysics for assessing pore pressure and
New Orleans, Louisiana, USA couple of years, the industry had found six main wellbore stability and for drilling program and
applications for these tools—applications that completion strategies (see “Using Downhole
Darrel Cannon remain key today: Annular Pressure Measurements to Improve
Kyel Hodenfield
Formation evaluation—Real-time correlation Drilling Performance,” page 40 ).
John Lovell
and evaluation allow coring and casing point Geosteering—By comparing real-time log
Sugar Land, Texas, USA
selection. Logging before extensive invasion responses to an expected model, the wellbore
Ray Harkins occurs may reveal hydrocarbon zones that can be trajectory is modified, thereby placing the well in
Ian Pigram saturated with borehole fluid by the time wire- the most productive portion of a pay zone.
ARCO British Limited line logs are run. As the real-time nature of LWD information
Guildford, Surrey, England Multiple-pass logging—Comparison logs began to be fully exploited, the early emphasis
made at different times can help distinguish pay on correlation in the late 1980s gave way to dom-
For help in preparation of this article, thanks to
Dave Bergt, Schlumberger Oilfield Services, Sugar Land,
from water zones, locate fluid contacts and iden- inance by geosteering and well-placement appli-
Texas, USA; Ted Bornemann, Bill Carpenter, Frank Shray tify true formation resistivity (Rt). Permeable cations. Availability of LWD data permitted safe
and Rachel Strickland, Anadrill, Sugar Land, Texas;
Joseph Chiaramonte and Darwin Ellis, Schlumberger-Doll
zones may be identified from time-lapse filtrate and efficient drilling of exotic trajectories and
Research, Ridgefield, Connecticut, USA; Craig Kienitz, movement. extended-reach and multilateral wells that were
Anadrill, The Hague, The Netherlands; Martin Lüling,
Schlumberger Riboud Product Center, Clamart, France;
Insurance logging—Logs obtained while unimaginable ten years ago (see “Key Issues in
Dave Maggs, Anadrill, New Orleans, Louisiana, USA; and drilling provide contingency data in case the well Multilateral Technology,” page 14 ). These wells
David Robertson, Forest Oil, Denver, Colorado, USA.
is lost or when conditions create boreholes that frequently make headlines in industry journals
ADN (Azimuthal Density Neutron), ARC5 (Array Resistivity
Compensated), ARC675, CDR (Compensated Dual yield poor-quality wireline logs. when technological advances contribute to
Resistivity), ELAN (Elemental Log Analysis), FMI (Fullbore Cost reduction—Running wireline tools in breaking existing directional drilling records.1
Formation MicroImager), GeoVISION675, PowerPulse, RAB
(Restivity-at-the-Bit), TLC (Tough Logging Conditions), high-deviation wells requires conveyance by
1. Allen F, Tooms P, Conran G, Lesso B and Van de Slijke P:
VISION475, VISION675, VISION First Look and VISION Telemetry drillpipe. In some cases, these wells can be “Extended-Reach Drilling: Breaking the 10-km Barrier,”
Protocol are marks of Schlumberger. Oilfield Review 9, no. 4 (Winter 1997): 32-47.
logged with LWD tools, either while or immedi-
ately after drilling, saving rig time offshore or in
wells otherwise needing the TLC Tough Logging
Conditions system.

Winter 1998 29
 Many of the LWD innovations that have Ten years ago, the available LWD measure- ogy to provide formation evaluation and drilling
helped directional drillers master the art and sci- ments were gamma ray, neutron porosity, litho- measurements in 5 3⁄4- to 6 3⁄4-in. holes. In addition
ence of geosteering are also advancing the cause density, photoelectric effect and phase-shift and to direction, inclination and toolface, the
of assessing reservoir quality while drilling. attenuation resistivities.3 In the interim, techno- VISION475 tool makes a neutron porosity mea-
Forward modeling routines have been developed logical advancements have vastly improved and surement, azimuthal readings of lithodensity, Pe
that allow real-time comparison between pre- enhanced these basic measurements and even and gamma ray, and records 2-MHz phase-shift
dicted and observed logs, helping drillers stay in added new formation evaluation measurements and attenuation resistivities at up to ten depths
the pay.2 This modeling capability also lets inter- not previously available in the logging industry. of investigation.
preters evaluate LWD data for petrophysical and First came azimuthal or quadrant measurements Deciphering phase-shift measurements with
fluid properties and for geologic structure. such as the quadrant density and photoelectric multiple depths of investigation for resistivity
Oil company interpreters are becoming more factor (Pe) on the ADN Azimuthal Density Neutron interpretation has become common practice in
familiar with while-drilling measurements, tool and the quadrant gamma ray and real-time the industry. However, the inclusion of attenua-
understanding their departure from wireline- resistivities on the RAB Resistivity-at-the-Bit tool. tion resistivity measurements with multiple
style logs, and trusting them. Operators are also Then came quantitative images with multiple- depths of investigation has brought additional
demanding more measurements for more hole depth resistivity images from the RAB tool and value to the petrophysicist. Although acquired
sizes, and as a result, a broader range of services density images from the VISION475 system. The with the same transmitter-receiver spacing, the
is being offered. The more comprehensive reser- addition of multiple depths of investigation to the attenuation measurement has a greater depth of
voir assessment that is now possible makes LWD azimuthal data has created new opportunities to investigation than the corresponding phase-shift
formation evaluation results valuable not only for complete the formation evaluation picture. measurement. These complementary measure-
wellsite decisions, but also for longer term reser- The comprehensive Schlumberger VISION475 ments offer an opportunity to understand more
voir planning and development—as wireline log- system (the nominal outer diameter of the tool is about the fluid and resistivity characteristics of
ging results have been all along. 4.75 inches) encompasses the enhanced technol- the formation. For example, comparison of atten-

> Possible oil-water contact

>

on phase-shift resistivity.
The gamma ray (GR) in
track 1 shows sand from
7740 to 8020 ft, and the
phase-shift resistivity in
track 2 indicates the zone
above 7920 has high
resistivity—a possible pay
zone above the oil-water
contact. Attenuation
resistivity in track 3 shows
the possible oil layer to be to
a zone of resistive invasion,
and not worth completing.

30 Oilfield Review
 > Borehole compensation for accurate VISION475 multidepth measurements. Without borehole compensation and correction (top), spikes and
separations in the curves of the phase-shift resistivity measurements cannot be interpreted reliably. With correction (bottom), high-resolution
data and curve separations can be identified and interpreted.

uation and phase-shift resistivities provides a tion, which indicates little to no invasion. A sim- water saturation. If this zone had been com-
diagnostic method for differentiating between ple resistivity index calculation yields 38% water pleted, a significant investment would have pro-
borehole fluid invasion and formation anisotropy, saturation, making the potential oil layer a candi- duced only water. The extra information brought
a technique discussed later in this article date for testing. by the deeper reading attenuation measurements
In one case from a Forest Oil well in the Gulf However, the attenuation resistivities that are avoided the cost of an unnecessary completion.
of Mexico, the while-drilling gamma ray (GR) simultaneously recorded by the VISION475 tool Extracting meaningful information from the
indicated a sand from 7740 to 8020 ft and the appear to contain evidence to the contrary. These two-receiver, five-transmitter tool configuration to
phase-shift resistivities identified a possible oil- deeper reading resistivities show significant sep- probe five depths of investigation each for phase-
water contact at 7920 ft (previous page). The five aration, with the deepest measurement—an shift and attenuation resistivity requires careful
phase-shift resistivities, each with a different approximately 30-in. [75-cm] depth of investiga- borehole compensation and borehole correction
depth of investigation, have very little separa- tion from the 34-in. receiver-transmitter spac- of the measurements. Without borehole correc-
2. Bonner S, Burgess T, Clark B, Decker D, Orban J,
ing—recording the lowest resistivity of about 0.4 tion, washouts together with conductive mud can
Prevedel B, Lüling M and White J: “Measurements at the ohm-m. This profile indicates resistive invasion, masquerade as invaded or anisotropic zones.
Bit: A New Generation of MWD Tools,” Oilfield Review 5,
no. 2/3 (April/July 1993): 44-54.
which might be expected for wells drilled with Borehole rugosity can cause spikes, or resistivity
Bonner S, Fredette M, Lovell J, Montaron B, Rosthal R, oil-base mud, but this was water-base mud with horns that may be misinterpreted as laminated
Tabanou J, Wu P, Clark B, Mills R and Williams R: a resistivity, Rm, of 0.1 ohm-m. However, forma- formations (above). Borehole compensation is
“Resistivity While Drilling—Images from the String,”
Oilfield Review 8, no. 1 (Spring 1996): 4-19. tion water resistivity, Rw, in this zone is approxi- necessary because it significantly reduces the
Allen D, Dennis B, Edwards J, Franklin S, Livingson J, mately 0.03 ohm-m, causing the resistive effects of borehole rugosity and precisely cancels
Kirkwood A, White J, Lehtonen L, Lyon B, Prilliman J and
Simms G: “Modeling Logs for Horizontal Well Planning
invasion profile. When formation resistivity is measurement errors caused by gain and phase-
and Evaluation,” Oilfield Review 7, no. 4 (Winter 1995): computed by inversion processing that takes shift differences in the receivers’ electronics,
47-63.
invasion into account, the zone shows 100% which typically vary with temperature.
3. Bonner S, Clark B, Holenka J, Voisin B, Dusang J,
Hansen R, White J and Walsgrove T: “Logging While
Drilling: A Three-Year Perspective,” Oilfield Review 4,
no. 3 (July 1992): 4-21.

Winter 1998 31
> Formation evaluation while drilling in the Gulf of Mexico. In this highly deviated well the GR and ROP in track 1, phase-shift and attenuation resistivities in
tracks 2, 3 and 4, and average density and neutron data in tracks 3 and 5 indicate a homogeneous pay formation. Phase-shift resistivity curve separation
suggests conductive invasion, but this is not confirmed by attenuation resistivities; resistivity anisotropy is responsible. Quadrant displays of density, on top,
bottom, left and right of the borehole, are in tracks 6 and 7.

A series of logs from a Shell deep-water pro- shift curves exhibit a conductive invasion profile If this were truly a conductive invasion profile,
ject in the Gulf of Mexico demonstrates the with the deepest spacing at 34-in. measuring the as the phase-shift measurement indicates, the
impact of adding still more LWD measurements highest resistivity, about 4 ohm-m. Resistivity deepest attenuation curves would show higher
to the interpretation. In this highly deviated well, processing to compensate for the invasion resistivity than the deepest phase-shift curves.
the standard GR, rate of penetration (ROP), effects would correct Rt to above 4 ohm-m. However, all the attenuation outputs read a
phase-shift resistivity and average density and This is the limit of information available from lower resistivity than even the shallowest phase-
neutron data indicate a homogeneous formation a conventional “triple combo” be it LWD or a shift curve. This is an example of resistivity
in this potential pay zone (above). The well was wireline system, and it appears to give a anisotropy—a difference in resistivity value
drilled with high-salinity drill-in fluid, and phase- respectable interpretation of the reservoir, but depending on the direction in which the mea-
the VISION475 system provides more information
and sheds new light on the reservoir interval.

32 Oilfield Review
 surement is made (see the first case study from 7700 to 7740 ft for another
example of anisotropy, page 30 ).4
In vertical wells penetrating horizontal layers with no invasion, 2-MHz
tools measure horizontal resistivity, Rh. This is taken as equivalent to Rt, the
resistivity input to most formulae derived to predict fluid saturation, and so
serves as the reference, or threshold, by which formations are judged to con-
tain pay or not. At other angles, for example, in highly deviated and horizon-
tal wells passing through horizontal layers, 2-MHz tools respond to some
combination of vertical and horizontal resistivities. Vertical resistivity (Rv), or
resistivity perpendicular to bedding, is always at least as much as, and usu-
ally more than, horizontal resistivity—sometimes reaching a 10 to 1 ratio (see
“Anisotropy and Invasion,” next page ).
In the case at hand, the phase-shift curves are each reading a different
combination of horizontal and vertical resistivity, depending on the transmit-
ter-receiver spacing. Formation resistivity, Rt, is not greater than 4 ohm-m, as
would have been calculated by a radial-invasion resistivity inversion program.
An anisotropy inversion program can be used to calculate Rh and Rv, and then
Rh is used in water saturation calculations to derive Sw.
Density curves from the VISION475 log also provide more information than
previous-generation LWD density tools, which combine weighted averages of
density from all around the borehole. The density and Pe measurements of
the VISION475 tool are recorded in 16 oriented sectors. These can be displayed
either as an image, or presented as four quadrants—top, bottom, right and
left—as the drillstring rotates (below).
For a first view, the bottom and average densities can be compared for
consistency (previous page). This log was recorded in a highly deviated well,
so the bottom-quadrant density, in closer contact with the borehole, should
give the best quantitative data. In this interval, not only does the bottom-
quadrant density disagree with the average density, but it also occasionally
measures a lower bulk density. This appears strange because assuming the
bottom of the tool is in contact with the formation also implies that the top
of the tool is not. When that occurs, the mud density, which here is less than
that of the formation, should influence the top-quadrant reading and as a
result, the average density would tend to be lower.
Taking the next step in evaluating this well, all four quadrant densities are
presented with photoelectric factor and bulk density correction for each quad-
rant (left). The right and left density quadrants agree well throughout the
entire interval. The top and bottom quadrant densities not only disagree, but
cross each other. A threaded borehole, borehole breakout, or a combination
of heavy mud and hole conditions could explain this unusual response. The
response could also be due to a position change of the borehole assembly in
4. Anderson B, Bryant I, Lüling M, Spies B and Helbig K: “Oilfield Anisotropy: Its Origins and
Electrical Characteristics,” Oilfield Review 6, no. 4 (October 1994): 48-56.
Lüling MG, Rosthal RA and Shray F: “Processing and Modeling 2-MHz Resistivity Tools in
Dipping, Laminated, Anisotropic Formations,” Transactions of the SPWLA 35th Annual
Logging Symposium, Tulsa, Oklahoma, USA, June 19-22, 1994, paper QQ.

< What quadrant densities mean. Density mea-

>

surements are gathered into four quadrants—

top, bottom, right and left—as the drillstring
rotates. If the VISION475 tool cuts across a layer
boundary with a near full-gauge stabilizer (left),
all quadrant measurements are quantitative.
The view is along the tool axis, which in this
case is also in the plane of the layer boundary.
Quadrant density measurements readily define
this boundary, providing a distinct bulk density
in each layer. Without a stabilizer (right) the tool
tends to lie on the bottom side of the hole, giving
better borehole contact and so better quality
to the bottom quadrant measurement. The top
quadrant measurement would be qualitative
due to the standoff distance.

Winter 1998 33
 Anisotropy and Invasion

Many resistivity logs exhibit a combination of Two keys are used to distinguish conductive Resistivity responses were modeled for an
anisotropy, invasion and shoulder (adjacent) invasion from anisotropy. The first is comparison anisotropic formaton whose anisotropy changes
bed effects, and each effect must be taken into of phase-shift to attenuation measurements: with invasion (below right). The virgin forma-
account to deduce true formation resistivity. although phase-shift resistivities indicate a con- tion anisotropy ratio, Rvt/Rht, is 6.8, but once
Resistivity anisotropy can be caused by layer- ductive invasion profile, corresponding attenua- invaded, the anisotropy ratio falls to 1.25—
ing, lithology or fluid content. It is typically tion outputs measure lower resistivity. If nearly isotropic. This change will have different
expressed as the ratio of vertical to horizontal conductive invasion were causing the phase-shift effects on the phase-shift and attenuation resis-
resistivity, Rv/Rh, and its effects on tool response curve separation, the deeper reading attenuation tivity responses, depending on their depth of
can be understood by modeling. The standard outputs would measure a higher apparent resis- investigation. For invasion diameters less than
response of 2-MHz tools in vertical wells pene- tivity than the phase-shift curves. This is an 15 in., the anisotropy effect dominates.
trating horizontal layers with no invasion is important use of attenuation measurements. The Anisotropy is recognizable by separated phase-
taken as the reference, and tool response to lay- second key is revealed in the modeled exam- shift curves reading higher than attenuation
ers at other relative angles can be computed ple—the curves are uniformly separated when curves. At invasion diameters greater than 50
(below left). viewed on a logarithmic scale. Uniform separa- inches, the effects of invasion rule curve separa-
In this model the formation consists of a sand tion is less common with invasion. tion. Measuring phase-shift and attenuation
interbedded with an equal amount of shale for If anisotropy can be identified in sands, it is resistivities before extensive invasion is there-
an anisotropy ratio Rv/Rh of 6.7. As the relative usually an indication of hydrocarbons. However, fore crucial when anisotropy is present.
angle increases, the apparent resistivity mea- in anisotropic formations that are hydrocarbon-
1. Klein JD, Martin PR and Allen DF: “The Petrophysics of
sured by both phase shift and attenuation bearing, deep invasion could hide the Electrically Anisotropic Reservoirs,” Transactions of the
increases. Above 45 degrees, the effect is anisotropic response if Rmf and Rw are similar.1 SPWLA 36th Annual Logging Symposium, Paris, France,
June 26-29, 1995, paper HH.
greater on the longer spacings; for example, the To understand the effect of invasion on an
2. Anderson B, Druskin V, Habashy T, Lee P, Lüling M,
phase-shift 34-in. curve measures a higher anisotropic formation, a state-of-the-art 3D Barber T, Grove G, Lovell J, Rosthal R, Tabanou J,
apparent resistivity than the phase-shift 28-in. finite-difference code was developed that com- Kennedy D and Shen L: “New Dimensions in Modeling
Resistivity,” Oilfield Review 9, no. 1 (Spring 1997): 40-56.
curve. The curve order resembles a conductive putes phase-shift and attenuation responses
invasion profile, and therefore may be misinter- with increasing diameter of invasion.2
preted. Anisotropy can cause resistivities mea-
sured in high-angle wells to be deceptively high.

> Effects of anisotropy on phase-shift and attenuation resistivities. Anisotropy > Effect of invasion on an anisotropic formation. In this formation, which is
becomes evident as the relative angle between bedding and tool axis increases. anisotropic before invasion, but less so after, modeling shows that for invasion
The curves resemble those seen in a conductive invasion profile except that with diameters less than 15 in., the anisotropy is still interpretable from phase-shift
anisotropy, phase-shift curves read more resistive than attenuation. and attenuation resistivity curves. After invasion diameters surpass 50 in., the
effects of invasion mask the anisotropy of the virgin formation.

34 Oilfield Review
the wellbore caused by changes in wellbore incli- tivity, neutron and average density measurements Invasion, Dip and Gas
nation. But no interpretation guesses are neces- may not always be sufficient for complete forma- The previous examples show how LWD logs
sary because the VISION475 system clearly tion evaluation. In this case, all the standard mea- improve formation evaluation in deviated oil
provides the answer with density image data. surements pointed to a homogeneous zone. wells with invaded zones, anisotropic layers and
The density images reveal the detail of reser- Clearly the revelation of a laminated sand-shale thin dipping beds. Determining accurate values
voir configuration—a series of thin sands and sequence can have an impact on the appraisal of of porosity and water saturation in gas wells
shales dipping at varying angles relative to the reservoir quality and its subsequent drainage. under these conditions, however, has been a
borehole (below left). These VISION475 images Second, techniques that assume maximum den- special problem that only recently is seeing
provide an easy and efficient means of interpret- sity to be the correct density would greatly under- some resolution.
ing complex data. The first track image is color- estimate porosity and distort the true reservoir In vertical wells, depth of invasion of mud fil-
scaled to represent measured quantitative character. This new information is valuable not trate into a formation depends on many factors,
density variations, while in the second track the only to drillers in real time, but also to well plan- including mud properties and lithology, porosity
variations have been enhanced by changing the ners who may need to change future drilling tra- and absolute and relative permeability of the for-
color scale to bring out detail. jectories, to completion engineers for effecting mation. In the simplest case of a vertical hole in
Throughout this interval, the azimuthal density more efficient completions, to reservoir engineers a homogeneous permeable formation, the inva-
imaging was the only measurement to flag the for modeling and simulating production and to sion profile is radially symmetric. But when
subtle sand-shale layering. The lessons learned geologists for calculating structural dip.5 impermeable or dipping layers, or both, are
are twofold: first, the standard suite of GR, resis- encountered, the volume invaded by borehole
fluid takes on a new shape (below right).
RHOB Image The invasion front becomes even more dis-
Measured

DEVI Deviation torted in a gas zone, because the borehole fluid is

Depth, ft

2.05 g/cm3 2.45 Dynamic RHOB Image degrees so much heavier than the formation gas. Invasion
0 90
Gamma Ray begins radially, but with time the heavier phase
1:240 0 API100 g/cm3 True Apparent
Dip Dip 5. Bornemann E, Bourgeois T, Bramlett K, Hodenfield K and
ROP Maggs D: “The Application and Accuracy of Geological
ft/hr Image Orientation Image Orientation
Information from a Logging-While-Drilling Density Tool,”
200 0 U R B L U U R B L U 0 degrees 90 Transactions of the SPWLA 39th Annual Logging
Symposium, Keystone, Colorado, USA, May 26-29, 1998,
XX200 paper L.

Wellbore

Filtrate

XX250

Impermeable
layer

XX300
W
ell
bo
re

Slumped
XX350 filtrate

Impermeable
> Density image from the VISION475 system. Measured densities appear in track 1, and layer
are redisplayed to highlight detail in track 2. Structural dips (green dots in track 3) are
hand picked using the same process as for wireline FMI Fullbore Formation Micro-
Imager data and relative dip to the borehole is calculated (blue dots). These images
are also useful for calculating a sand count or determining the net-to-gross sand ratio. > Invasion and slumping filtrate. A radial
invasion front becomes distorted in the
presence of a horizontal layer (top) or dipping
impermeable layer (bottom).

Winter 1998 35
slumps in the down-dip direction. The rate of determination from nuclear tools requires that What is needed is a way to quantify the vol-
slumping depends on the vertical permeability of the effects of gas be removed. For this, knowl- ume of gas radially and azimuthally over the
each zone: the higher the vertical permeability, edge of the gas volume and both radial and same region of formation that the density and
the more rapid the slumping. In addition, perme- azimuthal location is needed. Further complicat- neutron logs investigate. Then those volumes are
ability anisotropy will distort slump geometry. In ing the problem, the neutron and density mea- used to apply appropriate gas corrections to the
formations with permeability on the order of one surements respond differently to the radial and nuclear tools for final computation of porosity.
darcy, strong azimuthal variations in invasion azimuthal location of gas. The neutron tool reads Radial and azimuthal gas quantification is
have been observed less than an hour after the deeper and is sensitive to any gas near the bore- accomplished by analyzing while-drilling quad-
bit penetrates the formation. hole, relatively independent of the azimuthal rant resistivity data acquired as the RAB tool
In gas zones, such variations can make quan- location of the gas. The density tool reads shal- rotates in the borehole. The RAB tool investi-
titative porosity interpretation from nuclear tools lower and is sensitive only to the gas in front of gates a region similar to that probed by nuclear
an even greater challenge than usual. Porosity its detectors. tools, and has five depths of investigation. By

RAB Resistivity Porosity

RAB VISION
Shallow Button Down Density Up Resistivity Density
Depth, ft

Shallow Button Up Density Down

Bottom

Bottom
Deep Button Up Neutron

Top

Top

Top
0.1 ohm-m 100 40 p.u. 0

X050

X100

X150

X200
CDR

> Images of invasion slump. Density and RAB images show filtrate slumping, but not always down. Track 1 displays resistivities:
three from the RAB tool and attenuation resistivity from the CDR Compensated Dual Resistivity tool. Additional filtrate detected by
the resistivity in the down direction compared to the up direction is shaded. Similarly for the porosity curves in track 2, the left
curves are from the up and down quadrants of the density tool and the right curve is from the neutron tool. Track 3 contains the RAB
image, with white most resistive, and track 4 shows the density image with dark as the most dense, and lighter colors as less dense.

36 Oilfield Review
 Depth of Invasion Porosity

Density Up
Density Down
RAB Up

Depth, ft
0 in. 30 Quadrant Corrected Effective Porosity
RAB Down Neutron
0 in. 30 40 p.u. 0

X050

X100

X150

X200

Core porosity

> Porosity computed from corrected neutron- and density-while-drilling data. Track 1 displays the diameter of invasion, DI,
used to calculate corrections. The area between DI calculated from the up- and down-RAB measurements is shaded. Track 2
contains porosities from up- and down-quadrant density measurements, neutron measurements and core (red dots). The
effective porosity computed after corrections (orange curve) compares favorably with core measurements.

using readings from all around the borehole, response function of the density tool has been Finally Rxo, Rt, invasion factors for density and
three of those depths of measurement can be quantified and is relatively independent of the neutron, bulk density, neutron porosity, plus
partitioned azimuthally into 56 segments. From fluids involved. The neutron radial response func- appropriate parameters are entered in ELAN
these three measurements, three quantities can tion has been elusive, but Ellis and Chiaramonte Elemental Log Analysis software to solve for
be solved for—the diameter of invasion, DI; of Schlumberger-Doll Research, Ridgefield, porosity and water saturation.
invaded zone resistivity, Rxo ; and true formation Connecticut, USA have recently completed a This method was tested by partners ARCO
resistivity, Rt —in any or all of the 56 azimuthal modeling code to allow the response to be calcu- and Enterprise on a North Sea gas well deviated
segments. Rxo and Rt are assumed to be constant lated under all conditions. Their modeling shows about 40° encountering formations with 70°
around the hole; only DI varies. The determina- that the neutron responds to the gas closest to apparent dip. Images from the ADN and RAB
tion of Rt is most robust from the direction with the borehole. Therefore the DI needed to correct tools plot the location of the higher density,
minimum DI, and Rxo is most robust from the the neutron is the minimum DI computed around lower resistivity mud filtrate, which did not
direction with maximum DI. the wellbore. In the typical slumping-filtrate always slump straight down (previous page). The
Correcting the LWD density and neutron tools case, the closest gas usually is at the top of the diameter of invasion is plotted, and the com-
requires an appropriate radial response function hole or possibly on the sides, but definitely not at puted porosity displayed for comparison with
and appropriate DI; both are different for each the bottom. The DI for the density correction is core measurements (above).
tool. The qualitative response of density and neu- the one computed in the direction the density 7. Sherman H and Locke S: “Depth of Investigation of
tron tools has long been understood.7 The radial sensor is pointing. Neutron and Density Sondes for 35-Percent-Porosity Sand,”
Transactions of the SPWLA 16th Annual Logging
Symposium, New Orleans, Louisiana, USA, 1975, paper Q.

Winter 1998 37
 Gamma Ray Density, Bottom
0 API 100 22-in. Phase-Shift 1.65 g/cm3 2.65

Depth, ft
Resistivity
ROP5 Neutron Porosity
100 ft/hr 0 1 ohm-m 100 0.6 ft3/ft3 0

XX500

XX550

Real-time data revealing tight treaks.

>

While drilling in the Ram Powell field,

Gulf of Mexico, tight streaks were
encountered unexpectedly. These XX600
events show up as high-density, low-
porosity interfaces in track 3. Rate of
penetration (track 1) dropped signifi-
cantly each time, and resistivity
increased (track 2).

Getting a First Look quickly interpreting logs in highly deviated wells These tight streaks were of concern, as they
High-quality images provide valuable input to the and making drilling decisions. In this real-time might influence production, perhaps necessitat-
interpretation process. Geological information, interpretation for the Shell deep-water Ram ing a change in wellbore trajectory. They first
such as laminations, location of the wellbore Powell field in the Gulf of Mexico, the horizontal were assumed to be depositional features lying
with respect to bed boundaries and the apparent wellbore is drilled in a clean sand sheet deposit parallel to bedding. However, careful examina-
dip magnitude and direction of bedding planes, is nearly parallel to bedding. During drilling, several tion of the signature of the events on the density
essential for interpreting log responses in highly tight or hard features were encountered unex- and neutron curves reveals that these are vertical
deviated wells. Images quickly reveal whether pectedly (above). Rate of penetration dropped interfaces. If the hard streaks were parallel to
the bit is drilling down into or up through bedding significantly, and resistivity and bulk density bedding planes, the tool would encounter the
planes—critical for geosteering a well and refin- increased while neutron porosity approached boundary more gradually and the measurement
ing geological interpretations. zero p.u. These tight streaks were a surprise, transition from reservoir to tight streak would
The VISION First Look display is a wellsite because two vertical wells drilled in this area occur over some distance. These transitions are
answer product that combines images with the had not encountered such a feature. quite abrupt, indicating a high-angle boundary.
complete VISION dataset to provide a format for

38 Oilfield Review
 Azimuth and
Deviation
0 90
Rw-corrected Bulk Volumes
TVD Attenuation Resistivities Bottom Density RHOB Image
0 ft 100 0 ohm-m 0.5 Effective Porosity ohm-m g/cm3
0.02 200 1.65 2.65 2.05 g/cm3 2.45
Clay
Resistivity Gamma Ray Phase Shift-Resistivities Neutron Porosity
Time After Bit Matrix Image orientation
0 hours 10 0 API 150 Bound Water 0.2 ohm-m 2000 0 ft3/ft3 60 U R B L U

XX200

Attenuation

Phase
Measured Depth, ft

Shift
10 in.
16 in.
22 in.
28 in.
34 in. BHA sliding

XX250

> VISION First Look wellsite images. Bulk volume analysis (track 2) and an Rw curve (track 1) corrected for
clay volume and type are computed and displayed. [Adapted from Cannon D: “Shales: An Alternate Source for
Water Resistivities,” Transactions of the SPWLA 36th Annual Logging Symposium, June 26-29, 1995, Paris, France,
paper LLL.] Phase-shift and attenuation resistivity curves are displayed in track 3, densities are in track 4.
Density images (track 5) of tight streaks show that these features are clearly not planar.

This interpretation of vertical boundaries The Future Vision Other measurements are making their way to
raised new concerns for the operator: Was the The ability to achieve better reservoir quality the 4.75-in. format, including downhole annular
reservoir compartmentalized? Were these assessments in real time has satisfied some, but pressure and bit inclination for precision trajec-
streaks mineralized fault planes? What is the not all, formation evaluation while-drilling needs. tory control.
vertical extent of these features? Should the Already operators are asking for these LWD mea- To keep pace with the introduction of new
wellbore trajectory be changed? The VISION First surements in more hole sizes, and this demand is measurements, interpretation experts are devis-
Look log, played back with recorded mode data, being met with the imminent introduction of the ing new techniques for getting the most from the
was able to answer these questions (above). VISION675 and GeoVISION675 systems for 8- to 12- new data. Programs for interpreting measure-
The density images displayed on the VISION in. holes. The VISION675 system will encompass ments in layers that are anisotropic, invaded, thin,
First Look log reveal the true nature of the the ARC675 Array Resistivity Compensated mea- dipping, or all of the above, are finding new chal-
“tight streaks.” The boundaries of the features surement, an enhanced PowerPulse MWD tool lenges when applied to time-lapse LWD data—
are not planar, but rather calcite-cemented nod- with the new VISION Telemetry Protocol system LWD logs acquired before and after bit changes
ules. The features are not continuous vertical and a new 6.75-in. VISION675 density-neutron or other delays in drilling. Researchers are devel-
planar events and will not have a large-scale tool. The VISION675 density-neutron tool extends oping methods for faster modeling and inversion
impact on production. the capabilities of the existing 6.75-in. ADN tool of tool responses in more complex geometries
The deeper reading attenuation resistivity by adding multisector density, Pe and caliper and more realistic formations. These efforts will
measurement confirms this interpretation. The images for both oil-base and water-base mud. A enhance our ability to perform formation evalua-
attenuation measurements are not influenced by related tool for geological imaging while drilling tion while drilling, and also will improve all other
the high-resistivity hard streaks to the degree will appear in the GeoVISION675 system, which LWD applications. —LS
that phase-shift measurements are, and there will contain a new-generation laterolog imaging
are no polarization horns, which indicates that tool in place of the ARC675 module.
these events do not extend far from the wellbore.

Winter 1998 39