ABC Analysis

Behind Casing
X

Evaluate,
reevaluate, and
monitor reservoirs
through casing
Applications The value of analysis behind casing Multiple services to satisfy
■ Evaluation of bypassed pay Since logging its first openhole well in multiple objectives
■ Evaluation of old wells with 1927, Schlumberger has used advanced ABC services can provide comprehensive
new measurements technology to acquire essential reservoir formation evaluation under most condi-
data for the oil and gas industry. Today tions. Because they are a suite of services
■ Reevaluation of existing fields
we can make quality formation evalua- rather than a single platform, measure-
■ Primary formation evaluation tion measurements in cased holes. We ments can be chosen based on objectives,
in cased wells offer ABC* analysis behind casing type of formation, type of completion,
■ Data to complement logging- services to satisfy three primary industry borehole environment, lithology, reservoir
while-drilling (LWD) data requirements: dynamics, and the availability of primary
■ Alternative to openhole data Obtaining essential well log data under evaluation data.
acquisition under difficult any conditions—If the well being drilled Analysis behind casing lets you search
well conditions has, or is expected to have, hole stability for new zones and identify bypassed zones
problems, operators may want to case the after casing is set. These innovative serv-
■ Evaluation of wells drilled
well as soon as it is drilled. Formation ices measure porosity, resistivity, lithology,
with casing
evaluation, which until recently had to shale content, fluid saturations, and pres-
■ Reservoir monitoring be performed exclusively in open hole, sure; and they enable you to recover for-
■ Evaluation of fluid contact can now be performed in cased hole. mation fluid samples from cased holes.
movement, saturation and Finding and evaluating bypassed pay—
pressure changes, and deple- Large amounts of bypassed hydrocarbons Determination of water saturations
tion and injection profiles exist in old wells. It is considerably more in many formation conditions
cost effective, and often more environmen- Accurate determinations of water satura-
Benefits tally friendly, to explore for those hidden tions can be made in a wide range of for-
hydrocarbons in old wells rather than to mation porosities, water salinities, and
■ State-of-the-art formation evalu-
drill new wells. formation resistivities.
ation measurements in old wells
Optimizing reservoir management— Formation resistivity through casing—
■ Data acquisition when openhole Formation evaluation measurements made The CHFR-Plus* Cased Hole Formation
logging conditions are difficult in representative old wells in a reservoir, Resistivity tool makes direct deep-reading
■ Risk minimization through whether one time or in a time-lapse mode, formation resistivity measurements
planned cased hole logging can greatly assist in efficient management through casing and cement. The concept
■ Cost minimization through of the reservoir. of measuring resistivity through casing
planned cased hole logging ABC services do not end with optimal is not new, but it is only recent break-
data acquisition. Rather, data are pro- throughs in downhole electronics and
■ Data for time-lapse analysis;
cessed and interpreted to provide a total electrode design that have made these
most measurements are the
solution for efficient operations, enhanced challenging measurements possible. Now
same type as primary openhole
production, and extension of the economic the same basic measurements can be com-
measurements
life of an asset. pared in open and cased holes, thereby
■ Reduction of risk associated
eliminating the errors caused by compar-
with asset purchase
The ABC suite of services can help extend the ing different types of measurements.
economic life of a producing asset. Pulsed neutron measurements—The
Features RSTPro* Reservoir Saturation Tool makes
■ Choice of deep-reading resis- both formation sigma and carbon/oxygen
tivity, carbon-oxygen, and for- ratio (COR) measurements. In formations
mation sigma measurements with high formation-water salinity, the
for saturation determination sigma measurement has been used for
■ Accurate measurements of several decades to determine saturations.
formation porosity, density, Now, the COR measurement is providing
and acoustic properties an accurate means to evaluate formation
water saturation in moderate- to high-
■ Reliable lithology identifica-
porosity formations.
tion, shale volume computa-
The correct measurement technique
tion, and elemental analysis
depends on formation properties, bore-
■ Measurement of reservoir hole environment, and well completion
pressures and formation details, as described in the table on the
permeability through casing next page.
■ Reservoir fluid sampling Time-lapse formation-water saturation
through casing measurements can be used to monitor
■ Comprehensive answer prod- the performance of a well or reservoir
uct for quick decision making over time.
Tool Selection Guide for Determining Water Saturation
RSTPro Tool CHFR Tool Remarks
COR Sigma
Formation
Low porosity (< 15 p.u.) ■ ■ ■ Limits on maximum measurable true resistivity (Rt ).
Moderate porosity and low salinity ■ ■ ■ Limits on maximum measurable Rt .
(< 20,000 ppm)
Moderate porosity and moderate salinity ■ ■ ■

High porosity (> 30 p.u.) and high salinity ■ ■ ■ CHFR tool could work, but cement effect becomes significant at low Rt /Rcem.
Variable water resistivity (flood) ■ ■ ■ CHFR tool can identify change from original reservoir saturation but not
quantitatively.
Very low water saturation ■ ■ ■ Limits on maximum measurable Rt .
Completion
Casing collars ■ ■ ■ CHFR tool may lose data over 4 to 6 ft. RSTPro tool COR mode will give
good answers after SpectroLith* processing quantifies the iron content.
Run through small tubing ■ ■ ■

Log inside tubing ■ ■ ■ RSTPro tool will give answer if the fluid effect between tubing and
casing can be corrected.
Heavy casing ■ ■ ■ 40-lbm/ft limit for CHFR signal-to-noise ratio.
Dual casing ■ ■ ■ RSTPro tool will give answer if the fluid/formation/cement effect
between tubing and casing can be corrected. In COR mode,
characterization may be needed.
Alloy or chrome casing ■ ■ ■ Electrode scratching may induce corrosion.
Fiberglass casing ■ ■ ■ Induction logging is another option.
Borehole
Dry microannulus ■ ■ ■ Dry microannulus may affect CHFR current flow.
Gas-cut cement ■ ■ ■ Gas-cut cement may affect CHFR current flow.
Washed-out holes ■ ■ ■ Sigma provides a deeper measurement than COR, so it is less affected by
washouts unless the washout is comparable to the depth of investigation.
Flowing wells ■ ■ ■ Borehole fluid properties must be known or measured in situ.
Fluid contacts in hole ■ ■ ■

Near-wellbore effects ■ ■ ■ Sigma measurement is more robust than COR measurement when near-
wellbore effects are significant.
Deviated wells ■ ■ ■

Acid effect ■ ■ ■ Hydrogen chloride may affect sigma measurement.

Perforations ■ ■ ■ Invasion may affect COR measurement.
Lithology ■ ■ ■

Scale ■ ■ ■ CHFR tool relies on good electrical contact between electrodes and casing.
Casing must be clean.
■ Not recommended except with expert advice. ■ Use as recommended in remarks. ■ Recommended.

Reservoir petrophysical evaluation Formation porosity—The CHFP* allow correction for environmental
ABC services allow evaluation of forma- Cased Hole Formation Porosity service effects, such as thickness of casing and
tion petrophysical properties such as makes accurate formation porosity and cement, nor effects due to positions
formation density, porosity, and acoustic sigma measurements in wells that are of the tool and casing in the borehole.
properties in cased wells. This is even cased. The CHFP measurement, based When the highest possible accuracy is
more significant in wells where primary on an electronic neutron source instead desired, the CHFP service is the meas-
evaluation data were lost, were of poor of a chemical source, uses borehole urement of choice.
quality, or were never acquired. shielding and focusing to obtain poros- Formation density—The CHFD*
In old wells, an operator may want to ity measurements that are affected only Cased Hole Formation Density service
reevaluate the formation with measure- minimally by borehole environment, makes accurate formation density meas-
ments that were unavailable at the time casing standoff, and formation charac- urements in cased wells. The CHFD
the well was drilled. ABC services allow teristics such as lithology and salinity. service, which is based on a chemical
the latest formation evaluation technology The CNL* Compensated Neutron Log gamma ray source and a three-detector
to be applied in wells that were drilled has traditionally been run as a porosity measurement system, makes measure-
decades ago. It is no longer necessary to indicator in cased wells. Even though it ments in a wide range of casing and
drill new wells in existing fields solely provides a good estimation of formation borehole sizes. The three-detector sys-
for the purpose of data gathering. porosity in most conditions, the unfo- tem makes a density measurement cor-
cused nature of the CNL log does not rected for casing and cement thickness.
 Formation acoustics—The DSI*
Tools centered in the casing, low-frequency firing, and optimized transmitter-receiver tool spacing give ABC
Dipole Shear Sonic Imager, now coupled casing-optimized processing (right) better coherence and continuity than standard processing (left).
with the BestDT* automated sonic
waveform processing for best slowness,
provides accurate formation compres-
Gamma Ray
sional and shear slowness measurements 0 (gAPI) 50
in cased wells. The BestDT processing ChC
is based on optimally designed frequency 0 ( ) 1
filters and advanced signal processing. MD ChS
SPR4.MPS.016.BDT [A193 SPR4.MPS.016.BDT [A193
1 : 120
These significantly attenuate casing ft 0 ( ) 1 40 (us/ft) 220 40 (us/ft) 220
arrivals, as seen on the log at right.
Lithology—In many complex litholo-
gies and clays, a better understanding XX50
of the matrix is necessary to produce
a credible fluid analysis. SpectroLith
lithology processing of spectra from
neutron-induced gamma ray spectro- XX60
scopy tools is a quantitative, mineral-
based lithology interpretation derived
from single-tool elemental yields. A
XX70
modified geochemical oxides-closure
model transforms capture yields into
elemental concentrations.
An exclusive core database was used
XX80
to develop the lithology interpretation
that converts concentrations to fractions
of clay, carbonate, and framework quartz.
The SpectroLith service is available with XX90
the RSTPro service, which makes a
below-tubing and behind-tubing meas-
urement of capture elemental yields.
XX00
Reservoir fluid identification
and dynamics
The CHDT* Cased Hole Dynamics Tester
XX10
provides a technique for determining
formation pressures in old or new cased
wells, and it enables efficient, cost-
effective fluid sampling without the
inherent risks of standard sampling
techniques. Combining the CHDT tool with various Oil and gas companies can use this
The innovative CHDT tool seals modules of the MDT* Modular Formation technology to identify zones with
against the casing and uses a flexible Dynamics Tester enables enhanced fluid bypassed hydrocarbons and to monitor
drill shaft to penetrate through the identification, contamination monitoring, depletion of reservoirs, effectiveness
casing and cement into the formation. and high-quality sampling. After all of water or gas injection, and changes
It eliminates the use of explosives measurements and samples have been in fluid contacts.
altogether. Downhole sensors measure taken, the tool inserts a corrosion-
formation pressure, pressure transients, resistant metal plug into the hole drilled
and formation fluid resistivity. in the casing, thereby preserving casing
integrity and eliminating the need for
costly repair procedures.
ABC measurements save $40,000
CHFR and CHDT services were run to acquire data in the Elkton and Rock Creek formations that could
in perforating and testing costs not be obtained in open hole. CHFR data overlaid deep-reading openhole resistivity, and the CHDT tester
The lowest section of a Canadian well, acquired formation pressures and fluid samples, allowing the well to be completed successfully in the
Rock Creek formation.
through the Elkton formation, could not
be logged in open hole because of unsta-
ble hole conditions. Further, the upper Resistivity Decision Track
Cement
Depth
Map
section of the well, through the Rock
Hydrostatic Pressure
Creek formation, had not been fully 4050.0 (psi) 4550.0
evaluated in open hole. The operator, Formation Pressure
4050.0 (psi) 4550.0
Big Horn Resources, decided to evaluate TNPH (OH)
both formations fully in cased hole using 0.45 (V/V) -0.15
ABC services. Bit Size CHFR Resistivity TNPH (CH)
6.0 (in.) 16.0 0.2 (ohm-m) 2000 0.45 (V/V) -0.15
The Elkton formation was evaluated Caliper AIT-H 10 in. Investigation RHOZ (OH)
using CHFR, CNL, and DSI services. In 6.0 (in.) 16.0 0.2 (ohm-m) 2000 1.95 (g/cm3) 2.95
the Rock Creek formation, the same GR (CH) AIT-H 90 in. Investigation Casing
1:200 (m)
0.0 (gAPI) 150.0 0.2 (ohm-m) 2000 0.0 (in.) 20.0
data were acquired for comparison with
existing openhole data, and the CHDT
tester was run to determine reservoir
pressure and permeability. XX50
ABC logs and analyses confirmed the
Elkton formation was dry. In the Rock
Creek formation, CHFR data agreed with
openhole deep-resistivity measurements,
and cased hole DSI and CNL measure-
ments confirmed openhole porosity
measurements. The CHDT tester was
then used to make pressure measure-
ments to establish a fluid gradient and
permeabilities.
Based on ABC measurements of
resistivity and porosity, the operator
completed the well in the Rock Creek
formation. Accurate evaluation through
casing saved $40,000 in perforating and 1 213
testing costs. 1 321 XX75

ABC services prove a viable alternative

to openhole logging in risky wells
The Dorine field in Ecuador poses signif-
icant risk and expense to openhole log-
ging operations. Schlumberger acquired
ABC density, porosity, and sonic data for
Density, porosity, and sonic data acquired in this cased well in Ecuador closely match openhole data even
an Alberta Energy Corporation well that though they were acquired through 7-in. casing in a 9 3⁄4-in. borehole.
had been logged in open hole. Open and
cased hole data matched closely.
Two wells logged subsequently GR Open Hole RHOZ Open Hole CHFD Cased Hole
using ABC services have shown equally 0 (gAPI) 150 1.65 (g/cm3) 2.65 1.65 (g/cm3) 2.65
GR Cased Hole NPOR Open Hole CHFP Cased Hole
good results. 0 (gAPI) 150 0.6 (V/V) 0 0.6 (V/V) 0
XX50
HCAL MD DT Open Hole DTCO Cased Hole
6 (in.) 16 1:200 ft 140 (us/ft) 40 140 (us/ft) 40
XX60

XX70

XX80

XX90

XX00
Rapid, trouble-free data collection
ABC services were run in this North Sea well offshore Norway to complement the initial LWD petrophysical
in a high-risk environment evaluation after conventional openhole formation testing was deemed high risk. The ABC answer product
Snorre, a mature field in the Tampen area illustrates LWD, CHDT, and USI data in a composite display.
of the Norwegian North Sea, presents
a challenging environment for well con-
Well
struction. Oil is produced from a variety Casing Condition Cement Map Formation CHDT Pressures
Sketch
Depth

of geologically complex reservoir units -1000.0 Hydrocarbon

-500.0
through horizontal production wells. 0.3
2.6
Sand
Hydrocarbon recovery is maximized 3.0
3.5
4.0
through water-alternating-gas (WAG) 4.5 Bound Water
Internal
Radius
5.0
Average
injection, creating a number of different 5.5
6.0 (IRAV)
6.5 Shale Solids
pressure regimes and further adding 7.0
4 (in.) 5
7.5 Effective Porosity Hydrostatic Pressure External
to the complexity of producing the field. 8.0
1.0 (V/V) 0.0 250.0 (bar) 400.0 Radius
Average
Accurate reservoir pressure informa- Bonded Cement Map
Clay Volume Formation Pressure (ERAV)
1:1000 (m)
0.0 (V/V) 1.0 250.0 (bar) 400.0 4 (in.) 5
tion is crucial for reservoir modeling
to enable Norsk Hydro to manage the
field effectively.
Openhole logs had been acquired using XX50

memory logging while drilling. The risk of

conveying an openhole formation tester
on a TLC* Tough Logging Conditions
system was deemed to be too great. XX00

Acquiring pressure data quickly and

accurately was critical. Borehole condi-
tions are known to deteriorate rapidly,
sometimes to the degree that casing XX50

cannot be run successfully and zonal

isolation is suboptimal, so the well was
cased and cemented immediately after
the drillstring was pulled out of the hole. XX00

A USI* UltraSonic Imager was run to

evaluate the quality of the cement bond.
A CHDT tester was then conveyed in this
highly deviated well on a wireline trac- XX50

tor, pressure measurements were made,

and fluid samples were recovered from
all the reservoirs.
Norsk Hydro was able to obtain XX00

valuable reservoir data, while virtually

eliminating the risk associated with
conventional openhole formation test-
ing. Particularly important were the XX50

pore pressure data on which the com-

pletion-fluid weight was based. Without
actual pressure measurements, the
completion-fluid weight would have XX00

been based on the maximum pore pres-

sure prognosis for well control. If the
reservoir pressure had been consider-
ably lower than the prognosis, the well XX50

would not have flowed, production

would have been delayed, and the well
would have required an intervention
for stimulation operations. XX00

XX50
Pocket
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FE_03_002_0 ©Schlumberger
July 2003 *Mark of Schlumberger