ABC Analysis Behind Casing – slb.com

 

 

ABC Analysis
Behind Casing

X

Evaluate,

reevaluate, and

monitor reservoirs

through casing

Applications

■ Evaluation of bypassed pay

■ Evaluation of old wells with

new measurements

■ Reevaluation of existing fields

■ Primary formation evaluation

in cased wells

■ Data to complement logging-
while-drilling (LWD) data

■ Alternative to openhole data
acquisition under difficult
well conditions

■ Evaluation of wells drilled

with casing

■ Reservoir monitoring

■ Evaluation of fluid contact
movement, saturation and
pressure changes, and deple-
tion and injection profiles

Benefits

■ State-of-the-art formation evalu-
ation measurements in old wells

■ Data acquisition when openhole
logging conditions are difficult

■ Risk minimization through
planned cased hole logging

■ Cost minimization through
planned cased hole logging

■ Data for time-lapse analysis;
most measurements are the
same type as primary openhole
measurements

■ Reduction of risk associated

with asset purchase

Features

■ Choice of deep-reading resis-
tivity, carbon-oxygen, and for-
mation sigma measurements
for saturation determination

■ Accurate measurements of
formation porosity, density,
and acoustic properties

■ Reliable lithology identifica-
tion, shale volume computa-
tion, and elemental analysis

■ Measurement of reservoir
pressures and formation
permeability through casing

■ Reservoir fluid sampling

through casing

■ Comprehensive answer prod-
uct for quick decision making

The value of analysis behind casing

Since logging its first openhole well in
1927, Schlumberger has used advanced
technology to acquire essential reservoir
data for the oil and gas industry. Today
we can make quality formation evalua-
tion measurements in cased holes. We
offer ABC* analysis behind casing
services to satisfy three primary industry
requirements:

Obtaining essential well log data under
any conditions—If the well being drilled
has, or is expected to have, hole stability
problems, operators may want to case the
well as soon as it is drilled. Formation
evaluation, which until recently had to
be performed exclusively in open hole,
can now be performed in cased hole.

Finding and evaluating bypassed pay—
Large amounts of bypassed hydrocarbons
exist in old wells. It is considerably more
cost effective, and often more environmen-
tally friendly, to explore for those hidden
hydrocarbons in old wells rather than to
drill new wells.

Optimizing reservoir management—
Formation evaluation measurements made
in representative old wells in a reservoir,
whether one time or in a time-lapse mode,
can greatly assist in efficient management
of the reservoir.

ABC services do not end with optimal

data acquisition. Rather, data are pro-
cessed and interpreted to provide a total
solution for efficient operations, enhanced
production, and extension of the economic
life of an asset.

The ABC suite of services can help extend the
economic life of a producing asset.

Multiple services to satisfy
multiple objectives

ABC services can provide comprehensive
formation evaluation under most condi-
tions. Because they are a suite of services
rather than a single platform, measure-
ments can be chosen based on objectives,
type of formation, type of completion,
borehole environment, lithology, reservoir
dynamics, and the availability of primary
evaluation data.

Analysis behind casing lets you search
for new zones and identify bypassed zones
after casing is set. These innovative serv-
ices measure porosity, resistivity, lithology,
shale content, fluid saturations, and pres-
sure; and they enable you to recover for-
mation fluid samples from cased holes.

Determination of water saturations
in many formation conditions

Accurate determinations of water satura-
tions can be made in a wide range of for-
mation porosities, water salinities, and
formation resistivities.

Formation resistivity through casing—

The CHFR-Plus* Cased Hole Formation
Resistivity tool makes direct deep-reading
formation resistivity measurements
through casing and cement. The concept
of measuring resistivity through casing
is not new, but it is only recent break-
throughs in downhole electronics and
electrode design that have made these
challenging measurements possible. Now
the same basic measurements can be com-
pared in open and cased holes, thereby
eliminating the errors caused by compar-
ing different types of measurements.

Pulsed neutron measurements—The
RSTPro* Reservoir Saturation Tool makes
both formation sigma and carbon/oxygen
ratio (COR) measurements. In formations
with high formation-water salinity, the
sigma measurement has been used for
several decades to determine saturations.
Now, the COR measurement is providing
an accurate means to evaluate formation
water saturation in moderate- to high-
porosity formations.

The correct measurement technique
depends on formation properties, bore-
hole environment, and well completion
details, as described in the table on the
next page.

Time-lapse formation-water saturation

measurements can be used to monitor
the performance of a well or reservoir
over time.

Tool Selection Guide for Determining Water Saturation

RSTPro Tool

CHFR Tool

Remarks

COR

Sigma

Formation

Low porosity (< 15 p.u.) Moderate porosity and low salinity (< 20,000 ppm) Moderate porosity and moderate salinity High porosity (> 30 p.u.) and high salinity
Variable water resistivity (flood)

Very low water saturation

Completion

Casing collars

Run through small tubing
Log inside tubing

Heavy casing
Dual casing

Alloy or chrome casing
Fiberglass casing

Borehole

Dry microannulus
Gas-cut cement
Washed-out holes

Flowing wells
Fluid contacts in hole
Near-wellbore effects

Deviated wells
Acid effect
Perforations
Lithology
Scale

Limits on maximum measurable true resistivity (Rt).
Limits on maximum measurable Rt .

CHFR tool could work, but cement effect becomes significant at low Rt /Rcem.
CHFR tool can identify change from original reservoir saturation but not
quantitatively.
Limits on maximum measurable Rt .

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.

RSTPro tool will give answer if the fluid effect between tubing and
casing can be corrected.
40-lbm/ft limit for CHFR signal-to-noise ratio.
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.
Electrode scratching may induce corrosion.
Induction logging is another option.

Dry microannulus may affect CHFR current flow.
Gas-cut cement may affect CHFR current flow.
Sigma provides a deeper measurement than COR, so it is less affected by
washouts unless the washout is comparable to the depth of investigation.
Borehole fluid properties must be known or measured in situ.

Sigma measurement is more robust than COR measurement when near-
wellbore effects are significant.

Hydrogen chloride may affect sigma measurement.
Invasion may affect COR measurement.

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

ABC services allow evaluation of forma-
tion petrophysical properties such as
formation density, porosity, and acoustic
properties in cased wells. This is even
more significant in wells where primary
evaluation data were lost, were of poor
quality, or were never acquired.

In old wells, an operator may want to
reevaluate the formation with measure-
ments that were unavailable at the time
the well was drilled. ABC services allow
the latest formation evaluation technology
to be applied in wells that were drilled
decades ago. It is no longer necessary to
drill new wells in existing fields solely
for the purpose of data gathering.

Formation porosity—The CHFP*
Cased Hole Formation Porosity service
makes accurate formation porosity and
sigma measurements in wells that are
cased. The CHFP measurement, based
on an electronic neutron source instead
of a chemical source, uses borehole
shielding and focusing to obtain poros-
ity measurements that are affected only
minimally by borehole environment,
casing standoff, and formation charac-
teristics such as lithology and salinity.
The CNL* Compensated Neutron Log
has traditionally been run as a porosity
indicator in cased wells. Even though it
provides a good estimation of formation
porosity in most conditions, the unfo-
cused nature of the CNL log does not

allow correction for environmental
effects, such as thickness of casing and
cement, nor effects due to positions
of the tool and casing in the borehole.
When the highest possible accuracy is
desired, the CHFP service is the meas-
urement of choice.

Formation density—The CHFD*
Cased Hole Formation Density service
makes accurate formation density meas-
urements in cased wells. The CHFD
service, which is based on a chemical
gamma ray source and a three-detector
measurement system, makes measure-
ments in a wide range of casing and
borehole sizes. The three-detector sys-
tem makes a density measurement cor-
rected for casing and cement thickness.

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Formation acoustics—The DSI*
Dipole Shear Sonic Imager, now coupled
with the BestDT* automated sonic
waveform processing for best slowness,
provides accurate formation compres-
sional and shear slowness measurements
in cased wells. The BestDT processing
is based on optimally designed frequency
filters and advanced signal processing.
These significantly attenuate casing
arrivals, as seen on the log at right.

Lithology—In many complex litholo-
gies and clays, a better understanding
of the matrix is necessary to produce
a credible fluid analysis. SpectroLith
lithology processing of spectra from
neutron-induced gamma ray spectro-
scopy tools is a quantitative, mineral-
based lithology interpretation derived
from single-tool elemental yields. A
modified geochemical oxides-closure
model transforms capture yields into
elemental concentrations.

An exclusive core database was used

to develop the lithology interpretation
that converts concentrations to fractions
of clay, carbonate, and framework quartz.
The SpectroLith service is available with
the RSTPro service, which makes a
below-tubing and behind-tubing meas-
urement of capture elemental yields.

Reservoir fluid identification
and dynamics

The CHDT* Cased Hole Dynamics Tester
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.

The innovative CHDT tool seals
against the casing and uses a flexible
drill shaft to penetrate through the
casing and cement into the formation.
It eliminates the use of explosives
altogether. Downhole sensors measure
formation pressure, pressure transients,
and formation fluid resistivity.

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XX60

XX70

XX80

XX90

XX00

XX10

Tools centered in the casing, low-frequency firing, and optimized transmitter-receiver tool spacing give ABC
casing-optimized processing (right) better coherence and continuity than standard processing (left).

0

0

0

Gamma Ray
(gAPI)

50

ChC
( )

ChS
( )

1

1

40

MD
1 : 120
ft

SPR4.MPS.016.BDT [A193

SPR4.MPS.016.BDT [A193

(us/ft)

220

40

(us/ft)

220

Combining the CHDT tool with various
modules of the MDT* Modular Formation
Dynamics Tester enables enhanced fluid
identification, contamination monitoring,
and high-quality sampling. After all
measurements and samples have been
taken, the tool inserts a corrosion-
resistant metal plug into the hole drilled
in the casing, thereby preserving casing
integrity and eliminating the need for
costly repair procedures.

Oil and gas companies can use this

technology to identify zones with
bypassed hydrocarbons and to monitor
depletion of reservoirs, effectiveness
of water or gas injection, and changes
in fluid contacts.

ABC measurements save $40,000
in perforating and testing costs

The lowest section of a Canadian well,
through the Elkton formation, could not
be logged in open hole because of unsta-
ble hole conditions. Further, the upper
section of the well, through the Rock
Creek formation, had not been fully
evaluated in open hole. The operator,
Big Horn Resources, decided to evaluate
both formations fully in cased hole using
ABC services.

The Elkton formation was evaluated
using CHFR, CNL, and DSI services. In
the Rock Creek formation, the same
data were acquired for comparison with
existing openhole data, and the CHDT
tester was run to determine reservoir
pressure and permeability.

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
testing costs.

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
an Alberta Energy Corporation well that
had been logged in open hole. Open and
cased hole data matched closely.

Two wells logged subsequently
using ABC services have shown equally
good results.

CHFR and CHDT services were run to acquire data in the Elkton and Rock Creek formations that could
not be obtained in open hole. CHFR data overlaid deep-reading openhole resistivity, and the CHDT tester
acquired formation pressures and fluid samples, allowing the well to be completed successfully in the
Rock Creek formation.

Resistivity

Decision Track

Cement
Map

Depth

6.0

6.0

0.0

Bit Size
(in.)
Caliper
(in.)
GR (CH)
(gAPI)

0.2

0.2

0.2

CHFR Resistivity
(ohm-m)
AIT-H 10 in. Investigation
(ohm-m)

2000

2000

AIT-H 90 in. Investigation
(ohm-m)

2000

16.0

16.0

150.0

4050.0

4050.0

0.45

0.45

1.95

0.0

Hydrostatic Pressure
(psi)
Formation Pressure
(psi)
TNPH (OH)
(V/V)
TNPH (CH)
(V/V)
RHOZ (OH)
(g/cm3)
Casing
(in.)

4550.0

4550.0

-0.15

-0.15

2.95

20.0

1:200 (m)

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1

1

321

321

XX75

Density, porosity, and sonic data acquired in this cased well in Ecuador closely match openhole data even
though they were acquired through 7-in. casing in a 9 3⁄4-in. borehole.

0

0

6

GR Open Hole
(gAPI)
GR Cased Hole
(gAPI)
HCAL
(in.)

150

150

16

1.65

0.6

140

RHOZ Open Hole
(g/cm3)
NPOR Open Hole
(V/V)
DT Open Hole
(us/ft)

1.65

0.6

140

2.65

0

40

CHFD Cased Hole
(g/cm3)
CHFP Cased Hole
(V/V)
DTCO Cased Hole
(us/ft)

2.65

0

40

XX50
MD
1:200 ft
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XX80

XX90

XX00

Rapid, trouble-free data collection
in a high-risk environment

Snorre, a mature field in the Tampen area
of the Norwegian North Sea, presents
a challenging environment for well con-
struction. Oil is produced from a variety
of geologically complex reservoir units
through horizontal production wells.
Hydrocarbon recovery is maximized
through water-alternating-gas (WAG)
injection, creating a number of different
pressure regimes and further adding
to the complexity of producing the field.
Accurate reservoir pressure informa-
tion is crucial for reservoir modeling
to enable Norsk Hydro to manage the
field effectively.

Openhole logs had been acquired using
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.
Acquiring pressure data quickly and
accurately was critical. Borehole condi-
tions are known to deteriorate rapidly,
sometimes to the degree that casing
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.
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-
tor, pressure measurements were made,
and fluid samples were recovered from
all the reservoirs.

Norsk Hydro was able to obtain
valuable reservoir data, while virtually
eliminating the risk associated with
conventional openhole formation test-
ing. Particularly important were the
pore pressure data on which the com-
pletion-fluid weight was based. Without
actual pressure measurements, the
completion-fluid weight would have
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
would not have flowed, production
would have been delayed, and the well
would have required an intervention
for stimulation operations.

ABC services were run in this North Sea well offshore Norway to complement the initial LWD petrophysical
evaluation after conventional openhole formation testing was deemed high risk. The ABC answer product
illustrates LWD, CHDT, and USI data in a composite display.

Casing Condition Cement Map

Formation

CHDT Pressures

Well
Sketch

Depth

Hydrocarbon

Sand

Bound Water

Shale Solids

-1000.0
-500.0
0.3
2.6
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0

Effective Porosity

Hydrostatic Pressure

1.0

(V/V)

0.0

250.0

(bar)

400.0

Bonded

Cement Map

Clay Volume

Formation Pressure

0.0

(V/V)

1.0

250.0

(bar)

400.0

4 (in.) 5

1:1000 (m)

Internal
Radius
Average
(IRAV)

4 (in.) 5

External
Radius
Average
(ERAV)

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Pocket

www.slb.com/oilfield

FE_03_002_0

July 2003

©Schlumberger

*Mark of Schlumberger