Concrete Developments in Cementing Technology
Jean Marc Boisnault
Dominique Guillot
Montrouge, France
Abderrahim Bourahla
Timothy Tirlia
Anadarko Algeria Company
Hassi Messaoud, Algeria
Perhaps the most difficult borehole fluid to handle, cement is critical to the
performance and life of a well. Optimal slurry properties for placement of
standard oilfield cements typically do not coincide with optimal mechanical
properties of set cement necessary for long-term zonal isolation. New
technology optimizes both slurry and set-cement properties simultaneously.
Trevor Dahl
PanCanadian Petroleum Ltd.
Calgary, Alberta, Canada
Chris Holmes
A.M. Raiturkar
Petroleum Development Oman
Muscat, Sultanate of Oman
Pierre Maroy
Clamart, France
Charles Moffett
Hunt Petroleum Corporation
Jena, Louisiana, USA
Genaro Pérez Mejía
Ignacio Ramírez Martínez
Petróleos Mexicanos
Villahermosa, Mexico
Philippe Revil
Houston, Texas, USA
Robert Roemer
Aberdeen, Scotland
For help in preparation of this article, thanks to Andrew
Acock and Kevin England, Dowell, Houston, Texas, USA;
Tyler Bittner, Walter Chmilowski and Mike Roy, Dowell,
Calgary, Alberta, Canada; Leo Burdylo, Oilfield Services,
Sugar Land, Texas; Erling Prado-Velarde, Dowell,
Villahermosa, Mexico; Tarek Ramadan, Dowell, Muscat,
Sultanate of Oman; and Eugene Toukam, Dowell, Hassi
Messaoud, Algeria.
CemCADE, CemCRETE, DeepCRETE, DensCRETE, DESC
(Design and Evaluation Services for Clients), FLAC (fluidloss additives for cement), GASBLOK, LiteCRETE,
SqueezeCRETE, USI (UltraSonic Imager) and Variable
Density are marks of Schlumberger. Ping-Pong is a mark
of Parker Brothers, Inc.
16
Since a flawlessly cemented wellbore protects
the conduit that links reservoir fluids to the surface where they are used, high-quality oilfield
cement is an essential ingredient in any successful well. The quality and integrity of a cement job
can determine how long a well remains stable
and productive without requiring repair. In addition to promoting ongoing operational safety and
success, today’s cements must also be designed
with cost savings and challenging operating environments in mind. Environmental protection is a
greater concern than ever, especially protection
of shallow aquifers during and after drilling. A
good primary cement job is essential because
remedial cementing (squeezing) is difficult to
accomplish and provides only temporary, local
zonal isolation—it is preferable to do the job correctly the first time. Overcoming the trade-off
between cement slurry properties, including
rheology, fluid loss, pumpability and thickening
time, and mechanical properties of set cement,
such as compressive strength, porosity and permeability, is a major challenge.
Traditional Cementing Approaches
There are several fundamental purposes for placing cement in oil and gas wells. Cement is used
to support the casing. In addition, it hydraulically
isolates the various formations the well penetrates, thereby protecting aquifers and preventing fluid flow from high-pressure to low-pressure
formations, which might result in a loss of hydrocarbon production or excessive water production.
Cement guards against fluid broaching to the surface, which could lead to a catastrophic blowout.
Cement also protects the casing from corrosion
by chemically aggressive brines.
In the past, the least expensive material and
technology—typically displacing drilling fluids
by pumping Portland cement behind casing—
were acceptable in all but the most difficult
cases. Portland cement mixes easily with water
to produce a slurry that is readily pumpable and
can be placed anywhere within hydrostatic pressure constraints of a wellbore. Prepared at the
recommended water-to-cement ratio, Portland
cement fulfills the most important objective,
hydraulically isolating the formations. Furthermore, Portland cement is readily available worldwide and is inexpensive.
Oilfield Review
The usual method for placing a slurry in a well
during primary cementing operations consists of
pumping a series of fluids down the casing while
the fluid already in the well—the drilling mud—
flows out the casing-formation annulus to surface. The first fluid pumped is usually a preflush
or spacer, or both, that separates the drilling fluid
from the cement slurry. The spacer must be compatible with both the drilling fluid and the slurry,
yet keep those fluids apart to preclude contamination of the slurry by drilling fluid. Such contamination degrades the quality of the set
cement. This is followed by as many as four
slurries. The preflush-spacer-cement series must
displace from the annulus all fluids ahead of it to
prevent development of mud channels within the
cement sheath.1 Such channels allow formation
fluid migration. The presence of mud can also
negatively affect set-cement properties, for
1. Bonett A and Pafitis D: “Getting to the Root of Gas
Migration,” Oilfield Review 8, no. 1 (Spring 1996): 36-49.
2. Aldred W, Cook J, Bern P, Carpenter B, Hutchinson M,
Lovell J, Rezmer-Cooper I and Leder P: “Using Downhole
Annular Pressure Measurements to Improve Drilling
Performance,” Oilfield Review 10, no. 4 (Winter
1998): 40-55.
Spring 1999
example by inducing shrinkage cracks, reducing
compressive strength or increasing permeability.
A mechanical plug is then launched into the casing and displaced to the bottom of the well by
another fluid, typically the drilling fluid needed to
drill the next section of hole. At the end of the
operation, the cement occupies the annular
space between the casing and the penetrated
formation from the bottom of the hole up to the
desired level.
During the cementing operation, the critical
goal is to maintain the pressure in the annulus
between the pore and fracture pressures of the
penetrated formations at all times and all depths
throughout the openhole interval.2 If the annular
pressure becomes lower than the formation pore
pressure, fluids can flow into the annulus and
lead to a potentially catastrophic situation, a
blowout. At the other extreme, if the annular
pressure becomes higher than the formation fracture pressure, then annular fluids can split the
surrounding rock, damaging the borehole and
escaping into the formation.
The first factor affecting annular pressure
during drilling or cementing operations is the
density of the fluids, which exert hydrostatic
pressure on the exposed formations. The second
factor, fluid rheology, governs the frictional pressures during placement. Though density is a
parameter that can be controlled easily during
the design and operation phases, the actual
rheology of a fluid is more difficult to control or
modify. Once these properties have been
designed properly for a given operation, such as
with CemCADE cementing design and evaluation
software or other simulators, it is important that
they be maintained within reasonable tolerances during the entire placement operation.
The cement slurry must be stable—solid particles that are denser than the water in which they
are suspended must not separate from the liquid
during either static or dynamic conditions. The
17
> Particle-size optimization. A slurry made from particles of a single size (left) contains larger water-filled spaces than a slurry made from an optimized blend
of several particle sizes (right). The smallest particles fill the spaces between larger particles and function much like lubricating ball bearings.
slurry must not lose excessive interstitial water
to the formation when the pressure in the annulus is higher than in the formation. Excessive
fluid loss from a slurry can increase the viscosity,
which might result in incomplete placement of
the slurry and bridging of the annulus, and can
also lead to volume reduction in the cement, producing channels or other defects.3 Finally, the
slurry should not thicken or set prematurely
during placement.
Performance of conventional cement slurries
ultimately is a function of many variables, including the amount and types of solids, water, chemical additives, temperature and pressure.
Weighting agents increase density; extenders
decrease it. Dispersants control rheology by
breaking larger particles into smaller ones, which
can reduce viscosity. Stability is either intrinsic to
the design, or improved by using free water control or solid-suspending agents (antisettling
agents). Fluid-loss control is achieved by adding
FLAC fluid-loss additives for cement. Retarders or
accelerators control thickening time. Clearly,
chemical additives define the performance of
Portland cement slurries.
Once in place, the cement slurry should set
quickly and develop adequate strength to minimize the time spent waiting on cement (WOC) so
that the operator can proceed with the next
phase of well construction as soon as possible.
3. Bonett A and Pafitis D, reference 1: 38.
4. For more on primary cementing: Fraser L, Stanger B,
Griffin T, Jabri M, Sones G, Steelman M and Valkó P:
“Seamless Fluids Programs: A Key to Better Well Construction,” Oilfield Review 8, no. 1 (Spring 1996): 42-56.
18
Limitations of Conventional
Cementing Technology
Good slurry and set-cement properties are mutually exclusive in many conventional cementing
situations. For example, standard high-density
cements, while necessary for well control in
high-pressure drilling, are difficult to pump and
prone to sedimentation as weighting agents settle out of suspension. Low-density slurries with
proportionately higher liquid volumes develop
compressive strength slowly and attain low final
compressive strengths, limiting their value when
cementing production casing. Although chemical
additives are crucial to successful cementing
operations, the ultimate performance of conventional cement systems is dominated by the
water-to-cement ratio.
The optimal water-to-cement ratio is about
44% by weight for a low-viscosity, stable slurry
of API (American Petroleum Institute) Class G
cement, one of the most commonly used Portland
cements in the oil field. This gives a density of
around 15.8 lbm/gal [1900 kg/m3]. Higher densities can be reached by either decreasing the
water-to-solid ratio or increasing the density of
the solid blend at a given water-to-solid ratio.
When the water-to-cement ratio is close to the
optimal value, the better choice is to reduce the
amount of water; but this quickly leads to
unpumpable or unmixable slurries. At that point,
the only option is to add weighting materials to
the cement, normally high-density minerals such
as barite, hematite (the most common weighting
agent) or ilmenite. The densities of these minerals are 35 to 43 lbm/gal [4200 to 5200 kg/m3],
whereas the density of Portland cement is about
27 lbm/gal [3200 kg/m3].
To achieve lower densities, the methods are
reversed: either increase the water-to-solid ratio
or add lightweight aggregates. Another possible
option is foaming the slurry with gas—usually
nitrogen or air. When the water-to-cement ratio
approaches the optimal value, the simplest
approach is to add more water to the slurry, but
this jeopardizes its stability, reduces the strength
of the set cement and increases porosity and permeability. To rectify stability problems, interstitial water can be viscosified using colloidal clays
(bentonite or attapulgite), sodium silicates or
hydrosoluble polymers. However, these cement
systems exhibit higher porosity and permeability
once set, which often precludes their use in critical casing strings. Another technique consists of
blending Portland cement with lighter solid materials such as diatomaceous earth, perlite, fly ash,
fumed silica, blast furnace slag or hollow microspheres. This method works only in relatively
narrow density ranges where the water-to-solid
ratio is maintained above a given threshold for
the slurry to be mixable and pumpable.
A further problem with standard cement
systems is that remediation of unsatisfactory primary cement jobs is difficult. Squeeze cementing,
even when performed satisfactorily, merely provides a temporary patch. Conventional cements
are difficult to place in small defects, such as
partially plugged perforations and damaged casing, because of their relatively large particle
sizes and poor injectability.
Oilfield Review
Operators seek cementing materials that not
only are easier to place the first time, but also
offer the best long-term performance. Cements
that achieve compressive strength earlier
reduce waiting time and increase efficiency.
Because drilling a well is typically the culmination of months or even years of intensive effort,
including the acquisition and interpretation of
seismic data and planning well construction, it
is critical to achieve 100% cementing success
at the outset.4
Concrete Improvement
Typically, cements are weighted without consideration for the particle sizes of the ingredients
(primarily cement and weighting agents). As the
required density increases, conventional additives alone quickly lead to either an unpumpable
or unmixable slurry if the solid-to-liquid ratio is
too high, or to a system that does not contain
enough cement to develop a reasonable strength.
A new system, CemCRETE technology, is concrete-based slurry technology to optimize slurry
performance during placement while ensuring a
high set-cement quality. By adjusting the particle-size distribution (PSD) of the different solids,
this technique uses more solid particles in a
given slurry volume while keeping slurry rheology
reasonably low. This allows slurries with densities as high as 24 lbm/gal [2900 kg/m3] to be
used to cement critical casing strings in wells
with high pressure gradients.
Because many traditional cement slurries
have single-size particles, they can be visualized as a box full of Ping-Pong balls (previous
page). Between each ball, there are large airfilled voids. In a real slurry, the void space
would be filled with water rather than air. In a
high-performance slurry with engineered PSD
optimization, particles of three or more different
sizes are carefully selected. A box of Ping-Pong
balls with green peas and grains of sand filling
the voids is crudely analogous to a trimodal PSD
CemCRETE system.
By adjusting the PSD of the solids in the
blend, CemCRETE technology increases the
solids per unit volume of slurry above that
of Portland cement slurries. This increases
compressive strength and reduces porosity and
permeability by achieving a higher packing
volume fraction (PVF) independent of slurry
density. Packing volume fraction is defined as
the ratio of the sum of the absolute volumes
of all particles in the dry blend divided by the
bulk volume of the dry-blend components. Higher
PVF values generally indicate better set-cement
properties. For example, hexagonal packing of
identical spheres results in a PVF of 0.74, but
random packing of the same spheres achieves
a PVF of 0.64. The packing volume fraction of
14
0.20
Mix water needed, gal/sack
12
0.15
Permeability, mD
an optimized dry blend is increased by using
a trimodal PSD, which in turn decreases setcement permeability (below left).
Because the remaining fluid content is used
more efficiently, CemCRETE technology usually
requires lower concentrations of most chemical
additives compared to traditional approaches.
Gas-migration technology is more easily applied
because of the lower water-to-solid ratio and
because of the lower permeability and porosity
of the cement slurry during the transition from
liquid to solid as the cement sets. The 35 to 45%
porosity, or water content, of the new highperformance slurries is significantly lower than
the average 55 to 75% porosity for standard
slurries (below right).
In contrast to conventional Portland cement,
state-of-the-art cements contain a specific blend
of particles engineered for each specific slurry
density. The PVF of the optimized blends commonly exceeds 0.80. The high solids content
results in stable systems that disconnect the
slurry density from rheology, require few additives and are easy to mix and place in operations
that are as simple as ordinary jobs yet require no
specialized equipment. These systems exhibit
low porosity and permeability once set, even
for slurry densities as low as 10 lbm/gal
[1200 kg/m3]. More simply stated, physics succeeds where chemistry often fails.
0.10
0.05
77%
porosity
10
8
6
59%
porosity
4
40%
porosity
2
0
0
Extended
lightweight
cement
15.8-lbm/gal
Class G
cement
CemCRETE
cement
> Set-cement permeability. Permeabilities to gas
of set conventional and extended lightweight
cements can be as high as 0.20 mD. (“Extended”
lightweight cements have high porosities, typically 75%, because the slurry density is lowered
by increasing the water-to-cement ratio.) The
granulometric optimization of CemCRETE blends
results in set-cement permeability below 0.05 mD.
Spring 1999
12.5-lbm/gal
Extended
lightweight
cement
15.8-lbm/gal
Class G
cement
10- to 24lbm/gal
CemCRETE
cement
> Slurry porosity. High water content, or porosity,
of a cement slurry improves its pumpability, but
can lead to sedimentation in the slurry and
higher permeability and lower compressive
strength once the cement sets. Conventional
slurry porosities range from 55 to 75% or more,
whereas CemCRETE slurry porosities are typically 35 to 45%. Sized particles in the optimized
blend ensure high strength in the set cement and
good slurry rheology despite low water content.
19
500
250
450
400
350
Plastic viscosity, cp
Plastic viscosity, cp
200
150
100
300
250
200
150
100
50
50
0
30
40
50
0
30
60
Solid volume fraction, %
40
50
Solid volume fraction, %
Conventional slurry (20% fluid loss)
CemCRETE slurry (20% fluid loss)
Monomodal silica suspension in 0.15 M NaCl, PVF 0.5
60
Conventional slurry
CemCRETE slurry
Trimodal silica suspension in 0.15 M NaCl, PVF 0.8
> Plastic viscosity of silica suspensions. A dry blend consisting of a
monomodal particle-size distribution produces a high-viscosity slurry
even at a relatively low solids content. The blend with the trimodal
particle-size distribution, typical of CemCRETE technology, achieves
better slurry properties and contains more solids per unit volume.
> Fluid-loss effects. As slurries lose fluids to permeable formations, plastic
viscosity tends to increase. Compared with optimized slurries, conventional
Portland cement slurries tend to suffer greater increases in plastic viscosity
per unit of fluid loss. The bottom two curves show the difference in viscosity
between an optimized blend and a standard blend. The top two curves show
the increase in viscosity after both slurries have lost 20% of their fluid.
Optimized blends suffer less viscosification per unit of fluid loss.
Top
section
density
17.8
lbm/gal
17.5
lbm/gal
19.3
lbm/gal
18.7
lbm/gal
20.9
lbm/gal
Middle
section
density
18
lbm/gal
18
lbm/gal
19.5
lbm/gal
19.5
lbm/gal
21
lbm/gal
Bottom
section
density
18.1
lbm/gal
18.7
lbm/gal
19.6
lbm/gal
20.7
lbm/gal
21.2
lbm/gal
DensCRETE
cement
Conventional
cement
DensCRETE
cement
Conventional
cement
DensCRETE
cement
> Sedimentation and segregation. In the BP settling test, a column of set cement cured under controlled pressure and
temperature is cut into sections and the density of each cylindrical section is measured. High-density conventional
cements tend to show greater vertical density variation because the weighting agent tends to settle out of suspension
as the cement sets. DensCRETE cements, or high-density CemCRETE cements, show little variation in density from top
to bottom because the network of particles and associated reduced water content inhibit sedimentation or segregation
of the heaviest particles. Each column represents a different cement type and density, with density variation measured
in the top, middle (where the designed density is most likely to be found) and bottom sections of the column.
20
Oilfield Review
6000
Compressive strength, psi
The rheology of CemCRETE slurry is decoupled from its density (previous page, top left).
These water-reduced slurries have constant viscosities even at high densities, low gel strengths
and are easy to place. Low water content diminishes sedimentation (previous page, bottom), or
separation of liquid and solids during cementing,
yielding higher compressive strength and lower
permeability (previous page, top right). The specially engineered particle sizes allow easy mixing
and pumping because the smallest particles act
like ball bearings to provide lubricity for the
larger solids in the slurry. The compressive
strength of set CemCRETE slurries, whether of
high or low density, develops faster and reaches
higher levels than conventional cements (right)
because of the low water content.
CemCRETE technology benefits not only primary cementing applications, but also remediation. Particle-size optimization inhibits premature
dehydration of the slurry and the associated friction-pressure increase that commonly prevents
any remedial slurry from achieving deep penetration. Water-reduced primary cements have a
lower incidence of costly remediation than
Portland cements.
Additional benefits are that CemCRETE technology does not require specialized equipment or
personnel, and while never desirable, mixing
errors are better tolerated in the new slurries
than in Portland cement. Optimized dry blends
may be mixed with fresh water, seawater or salt
water. Optimized slurries can include conven-
5000
4000
3000
2000
1000
0
0
2
4
Time, hr
8
16
24
18-lbm/gal DensCRETE slurry
12-lbm/gal LiteCRETE slurry
15.8-lbm/gal conventional slurry
> Compressive-strength development. CemCRETE slurries, both low-density LiteCRETE
cement and high-density DensCRETE cement, develop compressive strength earlier
and reach higher levels than conventional cement slurries. Rapid compressive-strength
development reduces waiting-on-cement time and speeds well construction.
tional defoamers, accelerators, dispersants,
retarders, fluid-loss control additives, right-angle
set (RAS) additives and GASBLOK gas migration
control cement technology. In fact, the combination of specialized gas-migration control additives, low bulk shrinkage and rapid strength
development of optimized cements is breaking
new ground in gas-migration control. Clearly, as
exemplified in the case histories that follow,
advanced cementing technology can be tailored
to specific needs by changing components of
the dry blend.
Specialized Applications
There are four broad applications of CemCRETE
technology, encompassing low-density, highdensity, remedial and deep-water cementing
situations. LiteCRETE slurry systems have low
densities and are ideal for cementing weak formations or eliminating a casing string or a risky
multiple-stage operation (below). LiteCRETE slurries of 9.7 to 13 lbm/gal [1166 to 1563 kg/m3]
perform comparably to ordinary 15.8-lbm/gal
[1900 kg/m3] slurries. Optimized lightweight
cement develops compressive strength earlier
LiteCRETE Cementing Production Liner
LiteCRETE Cement Plugs
LiteCRETE Stage-Operation Replacement
Zone 1
2
Filler
slurry
Weak zone
Tail
slurry
1
Conventional cement
LiteCRETE cement
Conventional cement
Zone 2
Zone 3
LiteCRETE cement
Conventional cement
LiteCRETE cement
> New approaches to common problems. LiteCRETE cement (left) can replace stage-cementing operations, saving rig time and avoiding a complex, more
expensive operation. Here, the two-stage cementing operation on the left has a weak zone that is eliminated in the single-stage LiteCRETE operation on the
right. For cementing production liners (center) or casing across a weak or depleted zone, high-quality cement is placed across the primary pay zone as a
tail slurry at the bottom of the well. Shallower formations, isolated with lower-quality filler slurry, cannot be completed without additional cementing work.
LiteCRETE cement can be placed throughout the entire annulus so that any zone may be completed without additional cementing work, such as block
squeezes. Placing a higher density cement plug in a lightweight fluid (right) can lead to instability as the fluids intermix. Cement placement is improved by
matching low fluid densities with LiteCRETE slurries, which prevents fluid contamination and degradation of set-cement properties.
Spring 1999
21
DensCRETE Applications
> Clean perforating. While conventional cements can shatter
during perforating, CemCRETE cement remains intact after
perforating. The perforation diameter is 0.4 in.
than conventional cement, reducing WOC time.
In addition, this type of slurry is more stable than
low-density Portland cement slurries because of
its low water content. It is strong enough to be
perforated cleanly and withstands fracturing and
stimulation treatments (above left).5
DensCRETE technology offers better rheology at high density, adjustable density at the
wellsite and improved well control during
cementing (above right). High-density, waterreduced cement is useful for whipstock plugs
and high-pressure cementing operations, for situations where the fracture and pore pressure
margin is narrow, and for grouting (injection
of cement to consolidate seabed sediments or
injection of high-strength cement between
pipes such as the legs of offshore platforms). A
high-performance, high-density slurry of 17 to
24 lbm/gal [2040 to 2900 kg/m3] has a lower
equivalent circulating density than that of a conventional high-density cement slurry, allowing
placement even when the window between pore
pressure and fracture pressure is tight and conventional high-density slurries are inadequate.
Slurry density can be adjusted by as much as
1 lbm/gal at the last minute on location without
perturbing other slurry properties. DensCRETE
slurries usually develop compressive strengths
well in excess of 5000 psi [34.5 MPa] and can
reach 20,000 psi [138 MPa] in especially
demanding applications.
For remediation of faulty cement jobs and
for water control, SqueezeCRETE technology
offers a new solution for wellbore repairs, such
< Cementing high-pressure
formations. In high-pressure
wells with narrow pore-fracture
pressure windows, the friction
pressure increase in a tight
annulus during cementing can
fracture the formation (left),
leading to improper zonal
isolation. DensCRETE slurries
have lower viscosity, allowing
slurry placement throughout
the annulus. In deviated holes,
standard high-density slurries
are prone to sedimentation as
hematite particles settle on
the low side of the wellbore and
do not contribute to the total
hydrostatic pressure (right).
This instability can lead to
serious well control problems.
as casing leaks, liner top leaks, old partially
plugged perforations, channels behind casing,
leaking stage tools, fractures or even squeezing
a gravel pack (below). A SqueezeCRETE slurry
system applies the new technology at the
microscale for injection into very small gaps or
fractures in primary cements and casing.
Optimized slurries with specially engineered
particle-size distributions penetrate deeply not
only because of the small particle sizes of the
blend, but also because their improved resistance to dehydration reduces viscosification during placement. The improved injectability that
results from fine-sized particles is key to success
in remediation. In addition to high injectability,
SqueezeCRETE cement has high compressive
strength and low permeability. Strength makes
SqueezeCRETE Applications
Microannulus
squeeze
Casing leak
repair
Repair of channel
behind casing
Top of liner
squeeze
Old perforation
squeeze
Squeeze through
gravel pack
> Remediation success. Perhaps the most versatile application of CemCRETE technology, SqueezeCRETE slurries penetrate more effectively than other
cement slurries. SqueezeCRETE slurries repair small microannuli and leaks in casing, channels in cement and liner tops. They can also isolate old, partially
plugged perforations and even be placed through gravel packs.
22
Oilfield Review
Spring 1999
Injection point
Injection point
SqueezeCRETE slurry
Standard microcement slurry
> Improved penetration of remedial cement. Squeeze cementing materials were injected through
the valve on the left side of the 120-micron slots shown in the photographs. As indicated by the black
arrows below the slots, the SqueezeCRETE slurry (left) achieved deeper penetration into the narrow
slot than the conventional microcement slurry (right), which lost more water earlier, viscosified and
plugged the left side of the slot. Improved penetration reflects lower fluid loss and reduced viscosification, allowing SqueezeCRETE slurry to better repair tiny wellbore defects.
1.30
40
35
1.25
30
1.20
25
T/E
Cement solubility in mud acid after 4 hr, wt %
SqueezeCRETE cement an appropriate material
to plug wells upon abandonment, although it is
more commonly applied to remediate wellbore
problems that cannot be repaired with typical
cementing materials.
SqueezeCRETE technology succeeds where
standard gels used for water-control applications might fail, including remediation of crossflow behind casing and as a tail behind
conventional gel treatments. When water crossflow behind the casing is diagnosed, the path
through the primary cement sheath might not
yet be large enough to place ordinary squeeze
slurries. On the other hand, the path may already
be so large that a standard gel used for watercontrol applications cannot perform correctly
or withstand the differential pressure once the
well returns to production. The advanced slurry
experiences a lower viscosity increase for the
same volume of fluid loss than conventional
squeeze cements. Its enhanced fluid-loss control
properties, commonly better than those of
drilling fluids, greatly improve slurry penetration
properties: it can penetrate 120 micron slots
more than 10 times farther than well-dispersed
squeeze slurries (top right).
Engineered slurry for squeeze applications is
placed after deep penetration through the channel and set like ordinary primary cement. In this
manner, SqueezeCRETE technology restores the
integrity of the cement sheath and provides competent zonal isolation.
An alternative to foamed cement, DeepCRETE
technology, has been developed for deepwater
wells. Foamed cement—cement plus nitrogen
or air—requires specialized equipment and a
cementing team trained in its use (as well as
availability of nitrogen when air is not used),
which might be logistically challenging and
costly on some offshore rigs and platforms.
DeepCRETE cement develops strength faster,
even at temperatures as low as 39°F [4°C], so
WOC time is reduced when rig costs are calculated by the minute, such as in deepwater areas.
No specialized equipment clutters up limited
floor space. LiteCRETE slurry systems can also
substitute for foamed cement.
Traditionally, cement jobs were planned by
identifying the application of the cement and the
total hydrostatic limitations on the placed
cement column. The liquid slurry density was
inferred from the physical properties necessary
for the set cement. A major change precipitated
by new cementing technology is that the initial
planning step is to decide the slurry density first
20
15
1.15
1.10
10
1.05
5
0
1.00
Extended
lightweight
cement
15.8-lbm/gal
Class G
cement
CemCRETE
cement
> Resistance to acid attack. Better zonal isolation
is inherent in all CemCRETE systems because of
their improved resistance to aggressive, corrosive fluids, as demonstrated in laboratory tests
on cement solubility by acid or brine. This property makes LiteCRETE systems particularly valuable for geothermal applications or when acid
stimulation is planned, since low density and
resistance to corrosive fluids are of paramount
importance in those situations.
and then the slurry porosity. From that, the specific gravity of the dry blend is calculated and a
blend designed according to the job parameters.6
CemCRETE technology results in cement
properties that ensure long-lasting zonal isolation. Its strong resistance to corrosion from acid
stimulations and formation fluids is enhanced by
its low permeability (above left). Its mechanical
integrity is high, even in workover, perforating
and other specialized applications (above right).
Oilfield cement must withstand corrosion
and CemCRETE cements provide good sulfate
resistance when designed for that purpose.
12-lbm/gal
CemCRETE
cement
15.8-lbm/gal
Class G
cement
18-lbm/gal
CemCRETE
cement
> Cement integrity. The mechanical integrity of
cement, or its ability to withstand stresses from
perforating, hydraulic fracturing and other operations, is critical for long-term zonal isolation. The
ratio of the tensile strength (T) and Young’s modulus (E) is one indicator of the relative performance
of different cements. The higher T/E of CemCRETE
cements reflects their superior integrity.
5. For more on high-performance, lightweight cement
slurries: Moulin E, Revil P and Jain B: “Using Concrete
Technology to Improve the Performance of Lightweight
Cements,” paper SPE/IADC 39276, presented at the
SPE/IADC Middle East Drilling Technology Conference,
Bahrain, November 23-25, 1997.
Revil P and Jain B: “A New Approach to Designing HighPerformance Lightweight Cement Slurries for Improved
Zonal Isolation in Challenging Situations,” paper
IADC/SPE 47830, presented at the IADC/SPE Asia Pacific
Drilling Technology Conference, Jakarta, Indonesia,
September 7-9, 1998.
Sumartha I and Martinez R. JA: “Application of a
New Technique for Lightweight Cement Formulation to
Improve Cement Placement in Campeche Bay Area,”
paper SPE 39889, presented at the SPE International
Petroleum Conference and Exhibition, Villahermosa,
Mexico, March 3-5, 1998.
6. Moulin E et al, reference 5.
23
257 CemCRETE jobs worldwide in 1998
> Locations of CemCRETE operations. The size of each circle is proportional to the number of jobs in
the area. During 1998, more than 250 cementing operations using CemCRETE technology demonstrated
the versatility of optimized cement blends in a variety of critical casing operations. Stage-operation
replacement has been the most significant application to date.
Typical Casing Program
Fresh water
LiteCRETE Casing Program
Fresh water
The stage tool created a
weakness in the 9 5/8-in.
casing, requiring 7-in.
casing to surface
Low fracture
gradient
Low fracture
gradient
9 5/8-in. intermediate casing
cemented in two stages to
cover freshwater zone with
low-permeability cement
9 5/8-in. casing
cemented in one
stage with
LiteCRETE slurry
7-in. full production
string
7-in. production liner
replaces the full string
due to the elimination
of the stage tool
> Elimination of stage-cementing operations. In the Hassi Berkine field, Algeria, LiteCRETE technology meets multiple operational challenges: protection
of freshwater supplies, high strength with low density and reduced cost and
risk. Senonian carbonate and evaporite rocks must be isolated from underlying Albian sandstone, a freshwater aquifer. Oil production comes from deeper
Cambrian sandstones. By eliminating stage-cementing operations, a 7-in. production string to the surface can be replaced by a 7-in. production liner.
24
Also, their low permeability inhibits water percolation into the cement, slowing corrosion (see
bottom left figure, page 19 ). Destructive events,
such as repeated freeze-thaw cycling, tectonic
activity, production-induced subsidence and
thermal expansion during production and tests
prior to abandonment of wells, can impact
cement integrity.
Protection of shallow aquifers is an ongoing
concern, so regulatory requirements for cement
performance, such as in well abandonments, are
becoming stricter in many areas. Recently,
prudent operators have recognized that surface
casing should be cemented as carefully as
production liners. New high-performance oilfield
cements have greater reliability than traditional
cements, even in extreme conditions, so using the
best technology available might help operators
meet stricter environmental protection standards.
During 1998, more than 250 CemCRETE jobs
were carried out in 20 countries (left). LiteCRETE,
DensCRETE and SqueezeCRETE technologies
have been used in most cases, although
DeepCRETE technology, introduced at the end of
1998, is also gaining popularity.
Elimination of Stage-Cementing Operations
In the Hassi Berkine field in the Ghadames basin
of Algeria, Anadarko Algeria Company uses
LiteCRETE cement to avoid stage-cementing
operations and better protect the supply of fresh
water coming from the overpressured Albian
sandstone. The Albian aquifer overlies oilproducing Cambrian sandstones and underlies
salty Senonian carbonate and evaporite rocks.
Additional geologic complications include the
weakness of certain formations below the Albian
that are prone to lost circulation during drilling
and the potential for flowing salt. The previous
approach had been to set a stage tool below
the Albian, cement the lower zones, and then
isolate the Albian in the second stage of cementing operations.
Stage cementing resulted in higher costs than
a single-stage operation and suboptimal zonal
isolation that often required remedial cementing.
After careful consideration of the risks and
rewards of different approaches, Anadarko chose
a solution proposed by Dowell engineers—
single-stage cementing using a LiteCRETE slurry.
Key factors that make this preferable to conventional cementing include rapid setting time, high
compressive strength, low set-cement porosity
and permeability that result in better zonal isolation and superior resistance to corrosive formation fluids (left).
Oilfield Review
Fluid compensated CBL amplitude (CBLF)
0
Gamma Ray
(GR)
0
API
100
Caliper 1
(DCD CALI1)
10
in.
20
10
Bit size (BS)
in.
20
MV
Transit time (TT)
Cement map with
400
µsec
200
impedance
Transit time (Sliding Gate) (TTSL)
classification
0
MV
50
0.0000
0.3000
Predicted Amplitude for 100% BI from
1.9000
Dowell cement data (DCD PA 100 BI)
2.0909
Bonded from
USBI to LHF2
Gas from
LHF2 to USGI
Liquid from
USGI to USBI
2.2818
2.4727
2.6836
2.8545
3.0454
3.2364
3.4273
3.6182
3.8091
4.0000
0
MV
50
Predicted Amplitude for 80% BI from
Dowell cement data (DCD PA 80 BI)
0
MV
50
Min
5
200
CCL (CCLU)
-35
The cost savings associated with the singlestage operation and decreased need for remedial
cementing were also compelling. A typical single-stage operation in this area can save almost
a full day of rig time and decrease costs of fluid
contamination that might occur during the first
stage of cementing. Additional savings stem
from the low incidence of remedial work, which
typically requires two days of rig time as well as
additional cementing costs. The elimination of
the stage tool removes a known weak point from
the 95⁄8-in. casing string, making it possible to
replace a full 7-in. production casing to surface
with a 7-in. production liner, saving on tubular
and cementing costs as well as rig time (above).7
Spring 1999
50
Amplitude
Max
Sonic_VDL_Curve (VDL)
µsec
1200
In the United Arab Emirates, Abu Dhabi
Company for Onshore Oil Operations (ADCO)
has performed similar successful single-stage
LiteCRETE cementing operations.8
Ongoing collaboration between engineers
from Dowell and Schlumberger Wireline &
Testing has improved interpretation of bond logs
of lightweight cementing systems. In the past,
acoustic properties were incorrectly related to
compressive strengths of cement, resulting in a
false expectation of similar log responses
between 15.8-lbm/gal Portland and LiteCRETE
systems. The new systems have compressive
strengths as high as 15.8-lbm/gal Portland
< Evaluation of LiteCRETE cement using bond
logs. The USI UltraSonic Imager (USI) log,
cement bond log (CBL) and Variable Density log
(VDL) from a well in Algeria give information
related to the presence of a 10.85-lbm/gal
[1.33-kg/l] LiteCRETE cement behind 95⁄8-in.
casing. In the first track (from left to right), the
green gamma ray curve shows minor lithology
variation with depth; the black curve indicates
bit size and the red curve hole size (as uploaded
from the CemCADE software). The bond index is
denoted from 100% to 0% in track 2, with yellow
indicating cement behind the casing. The
cement map in the third track is a circumferential representation of the material present
behind the casing. The cement map was
generated by rescaling USI UltraSonic Imager
data from the default (0 to 8 MRayl) to a scale
of 0 to 4 MRayl to better fit the lower acoustic
impedance of LiteCRETE cement, which
averages 3 to 4 MRayl. Dark areas, equivalent
to 4 MRayl here, indicate excellent cement bond
to the casing. The fourth track displays classic
cement bond log information, including amplitude (solid purple), transit time (blue and red
dotted) and casing collar locations (black).
Additionally, the orange and green solid lines
represent the expected amplitude for 100%
and 80% bond (as predicted by the CemCADE
simulator). The amplitude values are higher
for LiteCRETE cement than for standard,
heavier cements, which typically have greater
attenuation. Finally, the Variable Density cement
bond log (VDL) in track 5 provides information
about the quality of the cement-formation bond
by displaying a color-coded traveltime trace at
every depth. The relatively low color contrast
(low amplitudes) at early times indicates weak
casing arrivals, which is to be expected for a
good bond between the casing and a relatively
low acoustic impedance cement. (A high
acoustic impedance cement under the same
circumstances would give lower amplitudes
and weaker casing arrivals, if any.) The higher
color contrast (high amplitudes) at later times
represents arrivals from the formation, whose
velocity varies with lithology, and correlates
roughly with lithology indicated in the
gamma ray log.
cements, but their acoustic impedances are
between 15.8-lbm/gal cements and ordinary
lightweight cements. LiteCRETE systems display
a lower acoustic impedance contrast with
drilling fluids, producing a different log
response, so log interpretation for these systems
is not as straightforward.
7. For more on single-stage cementing operations in
Algeria: Toukam E: “New Cement Improves Costs,
Operations In Northern Africa,” Petroleum Engineer
International 72, no. 3 (March 1999): 23-29.
8. Mukhalalaty T, Al Suwaidi A and Shaheen M: “Increasing
Well Life Cycle by Eliminating the Multistage Cementer
and Utilizing a Light-Weight, High-Performance Slurry,”
paper 53283, presented at the SPE Middle East Oil Show,
Bahrain, February 20-23, 1999.
25
Optimized CemCRETE Plug for Sidetracking
2878 m
8 5/8-in., 24-lbm/ft surface
casing at 1712 ft
Cement plug
4150 m
Cementing Low-Pressure Zones
Weak zone
4160 m
5 1/2-in., 15.5-lbm/ft
production casing
at 3150 ft
> High-performance lightweight slurry. Optimized,
low-density blends are used for whipstock plugs
and liner cements in depleted reservoirs with
low fracture gradients. In this example, PEMEX
decided to sidetrack to reach a better part of
the reservoir. By using CemCRETE technology,
PEMEX has improved its success ratio for
kickoff plugs and minimized WOC time.
> Cementing in a low-pressure gradient. The
use of conventional cements in the Olla field
typically required two or three block squeezes
after each primary cementing operation. Using
LiteCRETE slurry systems on five wells improved
zonal isolation without block squeezes. It also
makes it possible to complete shallower zones
without additional cementing work. In this
example, the LiteCRETE slurry column could
be placed high enough in the annulus to cover
the weak zone.
Whipstock Plugs and Liner Cementing
In Mexico, Petróleos Mexicanos (PEMEX) has
used LiteCRETE cement for whipstock plugs and
liner cementing. PEMEX initially used the
lightweight optimized blend for whipstock plugs
to kick off deviated wells past irretrievable fish.
The success ratio of kickoff plugs has been
improved greatly by using the new technology in
a low-density environment. The matched densities of the drilling fluids and cement slurries prevented swapping and mixing of fluids during
placement and ensured development of the
required compressive strength.
In a field with a low fracture gradient in the
Villahermosa region, CemCRETE technology
proved to be the best answer for cementing
deep (4500- to 5000-m) [14,760- to 16,400-ft],
depleted, fractured, dolomitic Mesozoic carbonate reservoirs. Lightweight cement is employed
because the reservoirs have a low fracture
gradient.9 In one deviated well, PEMEX elected to
kick off in order to reach a better part of the
reservoir (above left). A special 15-lbm/gal optimized whipstock plug material designed for
PEMEX reached a compressive strength of
3750 psi [26 MPa] within eight hours and a final
compressive strength of 4203 psi [29 MPa] in
12.5 hours, allowing the sidetrack to be completed successfully.
Liner cementing has also been improved
through the use of new cementing technology.
Because of the low formation pressure and susceptibility to fracturing, a low-density slurry was
critical to success. In one case, an 11.1-lbm/gal
[1330-kg/m3] LiteCRETE slurry was used to
cement a 5-in. production liner from 13,399 ft
to 15,095 ft [4084 to 4600 m]. The cement
developed a compressive strength of 1200 psi
[8273 kPa] after eight hours. A cement bond log
confirmed a good seal between the liner cement
and formation.
The overall cost of using LiteCRETE technology, including service, products and rig time, is
lower than the cost of using traditional technology. PEMEX reduced the cost of rig time during
cementing by 30% because new lightweight slurries develop compressive strength rapidly. By
using optimized cement for kickoff plugs, PEMEX
saved 45% of the total operation cost compared
with the use of conventional cement, which commonly entailed repeating the cement plug. Also,
remedial squeeze operations have not been necessary. Conventional jobs commonly required one
or two squeezes.
26
Cementing Shallow, Low-Pressure Wells
Hunt Petroleum Corporation has used LiteCRETE
cement to complete five wells in the Olla field,
LaSalle Parish, Louisiana, USA. Shallow Wilcox
oil wells, with total depths of 3500 ft [1067 m]
and bottomhole static temperatures of 129°F
[54°C], have low bottomhole pressures and low
fracture gradients, so getting a column of cement
high enough in the annulus has proven difficult.
In the past, as many as three block squeezes per
well were performed to remediate poor primary
cement jobs in 51⁄2-in. casing (left).
The Wilcox reservoir in Olla field has a strong
waterdrive. Productive zones are completed by
perforating the top of the productive interval
above the oil-water contact. Offset wells commonly produce high volumes of water at water
cuts greater than 95%. The wells completed
with LiteCRETE cement produce at water cuts
less than 85% water, but, more importantly, the
total volume of water produced is significantly
reduced. Hunt Petroleum interprets the reduced
water production as verification of proper isolation of the producing zone from nearby zones
that contain 100% water. The additional water
production in the offset wells has been
attributed to water channeling from nearby
water zones; radioactive tracer injection logs
have verified this. None of the wells in which
Hunt Petroleum used LiteCRETE cement has
required remedial work.
Besides reducing the need for remedial work,
Hunt Petroleum has lowered total well costs on
Wilcox completions by avoiding the mechanical
risks associated with squeezing operations. Such
risks include the possibility of setting the cement
retainer incorrectly, drilling a hole in the casing
when drilling out the cement retainer, splitting
casing during the squeeze, cementing the workstring if cement sets up early, or fracturing into a
water zone. Because LiteCRETE cement columns
extend higher in the annulus, upper zones of the
Wilcox may be completed without additional
cementing to cover these zones, which generally
are not covered during conventional operations.
9. Pérez Mejía G, Ramírez Martínez I and Prado-Velarde E:
“Optimización de los Tapones de Desvío y Liners,
Utilizando un Sistema de Cemento de Baja Densidad y
Alta Resistencia a la Compresión (LBDARC), en la
Región Sur de Pemex, México,” presented at the XI
Congreso Latinoamericano de Perforación, Buenos
Aires, Argentina, October 25-29, 1998.
10. In the North Sea, a LiteCRETE blend remained on a supply boat for several days in bad weather. Nevertheless,
the blend did not segregate during its rough journey to
the wellsite.
Oilfield Review
Cementing High-Pressure Wells
High-pressure wells benefit from the use of
reduced-water cements. Petroleum Development
Oman (PDO) first adopted DensCRETE technology
to address numerous challenges in fields such as
the Al Noor and Sarmad fields of southern Oman.
While adjustments to the mud system and casing
program can reduce the cost and risk of drilling
operations, the use of new cementing technology
was the most important factor in improving operations for PDO.
In the southern Oman fields, PDO produces oil
from stringers of tight Cambrian Athel silicilyte
embedded in salt. The Athel reservoir, which is
also a world-class hydrocarbon source rock, is up
to 400 m [1312 ft] thick and contains 80 to 90%
microcrystalline silica, with an average porosity
of 22% and permeability below 0.05 mD. High
drawdown pressures are applied to produce oil
from such a tight reservoir, so it is crucial to
mechanically isolate the individual stringers of
reservoir rock.
Drilling and completing such wells successfully are challenging. At depths of 3500 to
4800 m [11,483 to 15,748 ft] and temperatures of
90°C [194°F], pressure control dictates a highdensity slurry. Segregation of the weighting
agent, hematite, from conventional dry blends
during transport across graded roads led to difficulty mixing and pumping slurries and up to
three hours of lost time to clean plugged
cementing lines. Displacing heavy muds with
high rheologies was inefficient. There was a narrow window between the formation pore pressure of 16.2 lbm/gal [1941 kg/m3] and formation
fracture pressure of 20.4 lbm/gal [2444 kg/m3],
as well as a low differential pressure between
the 17-lbm/gal [2037-kg/m3] mud system, 18.3lbm/gal [2193-kg/m3] spacer and 19.6-lbm/gal
[2348-kg/m3] cement. There was little leeway to
adjust densities and displacement rates.
Contamination of fluids by salt-saturated mud
led to instability. Bulk shrinkage of set cement
often resulted in microannuli. In at least one
well, a microannulus was not detectable with a
cement bond log, but was discovered when pressure in the annulus rose. Finally, when compared
with conventional cements, CemCRETE slurries
set faster at the top of the liner, which reduces
the risk of fluid migration. In one well, a gas kick
occurred 14 hours after conventionally cementing
the liner and it took four days to control the well
and avoid a blowout.
Before approval for the initial use of
DensCRETE cement by PDO, numerous tests by
PDO and by Dowell in Oman and at the
Schlumberger-Riboud Product Center in France
confirmed that the advanced technology would
surpass critical performance requirements. In
addition to exceeding the performance of traditional cements in 8-hour compressive strength,
24-hour compressive strength, stability and
shrinkage, DensCRETE cement offered greater
ability to optimize slurry rheology and density
(below). A yard trial in early 1998 also demonstrated that the DensCRETE blend would not segregate during transport, remained mixable after
transport and passed relevant API tests, such as
rheology, compressive strength and fluid loss.10
The first DensCRETE operations in Oman
were performed during the second quarter of
1998 on the Sarmad-1 well, placing cement plugs
at 4100 m [13,451 ft] and 4300 m [14,108 ft]
with 21.5-lbm/gal [2576-kg/m3] slurry and a 7-in.
liner at 3850 m [12,631 ft] with 19.5-lbm/gal
[2337-kg/m3] slurry (above). Because the well
encountered a fault and fluid losses occurred just
above total depth, PDO decided to set plugs
above the fault and then cement the liner using
DensCRETE cement for both operations. The
plugged interval exceeded 200 m [656 ft] in thickness, so the plug was set in two stages.
DensCRETE Plug and Liner Cementing
Liner cement
(19.5-lbm/gal)
7-in. liner
3850 m
4100 m
4300 m
> High-pressure cementing. In the deep, highpressure Sarmad-1 well, PDO set 21.5-lbm/gal
DensCRETE cement plugs to counter faultrelated fluid losses near total depth and
then cemented the liner using a 19.5-lbm/gal
DensCRETE slurry.
To date, seven DensCRETE cement jobs have
been performed in the area for PDO, including
three liner jobs and four plugs for abandonment
of high-pressure wells. The slurry is less sensitive to salt-saturated mud contamination than
ordinary cement. As the optimized high-density
cement sets, it is less prone to forming a
microannulus because it suffers less bulk shrinkage. Even in long liners, no density gradient is
observed in the set cement column in the annulus. The column is uniform and stable, even as
the cement is setting, so the risk of a blowout is
reduced. The top of DensCRETE plugs is closer to
the theoretical top than that of conventional
plugs because the rheology of optimized highdensity slurries allows more efficient removal of
drilling fluids.
Properties
Conventional slurry
DensCRETE slurry
Compressive strength at 8 hr
0 kPa
18,616 kPa [2700 psi]
Initial set 50 psi
After 20 hours
After 4 hours
Compressive strength at 24 hr
18,275 kPa [2651 psi]
24,132 kPa [3500 psi]
Stability of set cement
(BP settling test)
0.35 kPa/m [0.297 lbm/gal]
top to bottom
0.20 kPa/m [0.169 lbm/gal]
top to bottom
Shrinkage
1.5% after 24 hours
0% after 24 hours
Separation of heavy particles
from blend during transport
High risk
Very low risk
Tolerance to density variations
Low
High
Spring 1999
Cement plug
(21.5-lbm/gal)
< Laboratory testing. In tests
conducted before the first
use of high-performance
heavyweight slurry by
Petroleum Development
Oman (PDO), DensCRETE
slurries outperformed
conventional heavyweight
slurries. This superior
performance carried over
to field applications.
27
Water-Control Applications
SqueezeCRETE technology has been used in
Alberta, Canada, for numerous squeeze jobs. In
the Halkirk field northeast of Calgary, an oil well
operated by PanCanadian Petroleum Ltd. produced 35 m3 [220 bbl] of oil nearly water-free
from the Upper Manville ”I“ Glauconitic formation upon its initial production in 1995. Within
a year, however, water production increased from
1 m3 [6 bbl] per day to more than 20 m3 [126 bbl]
per day. By late 1998, the well was completely
watered out. From knowledge of reservoir geology and performance, the water influx was
attributed to layer breakthrough.
10
5200
176
4160
137
3640
Temperature, °C
156
117
3120
98
2600
Temperature
78
2080
58
1560
39
1040
19
520
0
0:00
0
1:45
3:30
5:15
7:00
8:45 10:30 12:15 14:00 15:45 17:30 19:15
Elapsed time
> Rapid development of compressive strength. High-density optimized slurries develop
compressive strength sooner than their conventional counterparts. In this example
from the Al Shomou-4 well, the 22-lbm/gal DensCRETE slurry achieved a strength of
5000 psi in only 17 hr.
The attractive economics for remedial work
prompted action. On the basis of known wellbore
integrity, a bridge plug was set above existing
perforations at 1266.5 m [4154 ft] and the zone
above it was reperforated, but water production
continued. After reviewing geological, reservoir
and completion data, the water influx was
ascribed to poor cement behind the bridge plug.
Because of a large drawdown and close water
1000
WOR
WOR'
proximity, the Dowell DESC Design and
Evaluation Services for Clients engineer was
asked to verify that water coning was occurring.
Water-control diagnostic plots, which display
raw historical production data versus time on a
log-log scale, help identify water sources, such
as differentiating bottomwater coning from
multilayer channeling (below). Systematic flow
model numerical simulations were performed to
WOR
WOR'
100
WOR or WOR'
WOR or WOR'
4680
Compressive strength
1
0.1
0.01
0.001
10
1
0.1
0.01
0.0001
0.001
1
10
100
1000
1
10
100
Time, days
1000
10,000
10
100
Time, days
1000
10,000
Time, days
10
10,000
WOR
WOR'
WOR
WOR'
1000
1
100
WOR or WOR'
WOR or WOR'
> Water-control diagnostic
plots. Log-log plots of the
actual water-oil ratio (WOR)
and its derivative (WOR’)
versus time help differentiate
between water-control
problems, including water
coning and channeling, during production. Systematic
flow model numerical
simulations produced
characteristic curves. These
curves are used to diagnose
problems and then decide
the appropriate remedy. The
theoretical representation of
bottomwater coning (upper
left) is similar to the actual
field example below it.
Simulated multilayer
channeling (upper right)
also mimics actual multilayer
channeling observed in the
field (lower right).
196
Compressive strength, psi
The WOC time for conventional cements to
develop adequate compressive strength under
the conditions in the southern Oman fields is at
least 28 hours. DensCRETE cement achieves high
compressive strength in as few as 15 hours (the
worst case to date has been 26 hours) and ultimately develops higher compressive strength
than standard high-density cement (right). The
decrease in WOC time has proven especially
important in drilling exploration wells, and there
has been a decreased need to repeat plugs or
remediate liner cements. Thus, PDO plans to
continue to use DensCRETE cement for highpressure cementing operations.11
0.1
0.01
10
1
0.1
0.01
0.001
0.001
10
0.0001
100
1000
Time, days
28
10,000
1
Oilfield Review
11. For more on the use of high-density slurries in well
construction: Adamson K, Birch G, Gao E, Hand S,
Macdonald C, Mack D and Quadri A: “High-Pressure,
High-Temperature Well Construction,” Oilfield Review 10,
no. 2 (Summer 1998): 36-49.
12. In a hesitation squeeze, a portion of the slurry is pumped,
then pumping stops to expose the slurry to differential
pressure against the zone of interest in stages over
a period from several minutes to several hours. This
pressure, higher than necessary for fluid movement, is
applied to force filtrate from the cement slurry, leaving
only solid material in the area requiring repair. This procedure is repeated until all the slurry has been pumped.
The dehydrated cement remaining in the zone forms a
seal with a higher compressive strength and lower permeability than the original slurry design.
Spring 1999
10,000
1,000
100
WOR or WOR'
produce characteristic curves for different types
of water production. On the basis of the watercontrol analysis for the Halkirk well, the diagnosis was a high-permeability layer with water
breakthrough (right). This problem was complicated by a microannulus that allowed water flow
behind the casing.
Because of low oil prices and the fact that the
mature Halkirk field is undergoing waterflooding,
workover costs must be minimized to achieve
acceptable economic results. Considerable effort
is made to mitigate the risk and impact of unsuccessful treatments. Therefore, procedures with a
high probability of success are favored. In this
well, a conventional cement squeeze was
deemed too risky. The SqueezeCRETE treatment
was predicted to have a much higher probability
of success, so the economics for that treatment
were acceptable.
SqueezeCRETE slurry was placed across the
perforations from 1263 to 1265.25 m [4144 to
4151 ft] as a balanced plug, and a hesitation
squeeze was performed.12 After 24 hours, the
cement was drilled out and successfully pressure- and swab-tested. Following reperforation,
the zone is producing 28 m3 [176 bbl] of oil,
3100 m3 [110 Mcf] of gas and 0.32 m3 [2 bbl]
of water per day, reversing the water cut
from 99.5% before to only 1.1% after the
squeeze operation.
In another well in southern Alberta,
PanCanadian wanted to shut off old perforations
and complete a deeper interval. Because the
slurry feed rate into the old perforations was less
than 20 L/min [5.3 gal/min], ordinary slurries
would not be effective. After acid was spotted
across the perforations to increase the injection
rate, only minor improvement occurred.
SqueezeCRETE slurry was then batch mixed and
1.2 m3 [8 bbl] placed across the perforations, followed by a hesitation squeeze. After 48 hours,
the cement was drilled out and the perforations
were successfully pressure tested and swab
tested. The lower interval was subsequently perforated and completed. Without a highly
injectable remedial system like SqueezeCRETE
WOR
WOR'
Wellbore water
holdup
High-permeability layer
water breakthrough
10
1
Initial wellbore
fluid cleanup
0.1
Post-treatment WOR
0.01
0.001
10
100
1000
10,000
Cumulative production time, days
> Halkirk water-control diagnostic plot. The PanCanadian Halkirk well produces oil from several
layers. A log-log plot of the actual water-oil ratio (WOR) and its derivative (WOR’) versus cumulative
production time illustrates water breakthrough from a high-permeability layer. Increasing hydrostatic pressure from wellbore water holdup significantly reduced oil production, and the water
cut reached 99.5%. Successful shutoff of the water layer restored the previous oil production and
reduced the water cut to 1.1%.
slurry, the operator might have risked impairing
the additional completion by using a casing patch
to shut off the abandoned perforations.
SqueezeCRETE cement has the potential to
address stringent well plugging requirements as
some of the many shallow gas wells in western
Canada are abandoned. Its high injectability and
low permeability can repair gas leaks better than
traditional cementing materials.
Successful water-shutoff jobs have been
performed using engineered squeeze cements
elsewhere. In one case in the North Sea, oil production increased from 2000 to 4000 bbl per day
[317 to 635 m3/d] while water production
decreased from 7000 to 1500 bbl per day
[1112 to 238 m3/d]. This sharp reduction in
water production made gas lift unnecessary
after production resumed.
Also in the North Sea, BP Amoco plc successfully abandoned a reservoir section in a well from
its Bruce platform using a single optimized
cement plug. After remedial completion efforts
and other attempts to isolate and abandon the
reservoir failed, SqueezeCRETE slurry was
pumped through coiled tubing across the perforations and then squeezed. BP Amoco plc was then
able to sidetrack an adjacent wellbore to reach
the reservoir.
Merely pumping a superior slurry does not
always effect the desired repair. Sound completion engineering concepts, proper design and
execution are critical ingredients for successful
well remediation.
Present and Future Value
of Optimized Cements
CemCRETE technology has proven its value on
several fronts. Its early development of compressive strength saves rig time because drilling
operations can resume sooner. The reliability of
the technology decreases the need for remedial
block squeezes or repetition of plugs. Repairs of
faulty cement and casing are more effective than
ever before. The risk and expense of stagecementing operations are avoided with a singlestage operation using CemCRETE technology.
Well designs can be optimized to avoid costly
casing strings.
The lower porosity and permeability of
set cements using CemCRETE technology will
allow safer abandonment of wells and isolation
of aquifers from hydrocarbon zones. Lowpermeability cements are more resistant to
corrosive brines and there is less bulk shrinkage
as the cement sets, resulting in better zonal isolation over time. Studies are nearing conclusion
on the enhanced durability of the new systems
over conventional cements when perforating.
The successful application of CemCRETE
technology in 257 wells during 1998 provides a
foundation for expansion of this versatile technology from specialized initial applications to
mainstream cementing operations.
—GMG
29