U.S. patent application number 15/589562 was filed with the patent office on 2017-08-24 for compositions and methods for well completions.
The applicant listed for this patent is SCHLUMBERGER TECHNOLOGY CORPORATION. Invention is credited to Laurent Gabilly, Michel Michaux.
Application Number | 20170240795 15/589562 |
Document ID | / |
Family ID | 43902872 |
Filed Date | 2017-08-24 |
United States Patent
Application |
20170240795 |
Kind Code |
A1 |
Michaux; Michel ; et
al. |
August 24, 2017 |
COMPOSITIONS AND METHODS FOR WELL COMPLETIONS
Abstract
Well-cementing compositions for use in high-pressure,
high-temperature (HPHT) wells are often densified, and contain
weighting agents such as hematite, ilmenite, barite and
hausmannite. The weighting agents are usually finely divided to
help keep them suspended in the cement slurry. At high
temperatures, finely divided weighting agents based on metal oxides
react with the calcium-silicate-hydrate binder in set Portland,
cement, leading to cement deterioration. Finely divided weighting
agents based on metal sulfates are inert with respect to calcium
silicate hydrate; consequently, set-cement stability is
preserved.
Inventors: |
Michaux; Michel;
(Verrieres-Le-Buisson, FR) ; Gabilly; Laurent;
(Malakoff, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SCHLUMBERGER TECHNOLOGY CORPORATION |
Sugar Land |
TX |
US |
|
|
Family ID: |
43902872 |
Appl. No.: |
15/589562 |
Filed: |
May 8, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13989809 |
Jun 12, 2013 |
9644133 |
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PCT/EP2011/006364 |
Dec 7, 2011 |
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15589562 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C09K 2208/08 20130101;
E21B 33/13 20130101; C04B 14/06 20130101; C09K 8/48 20130101; C04B
28/04 20130101; E21B 33/14 20130101; C04B 22/142 20130101; C04B
20/0096 20130101; C04B 2103/408 20130101; C04B 20/008 20130101;
C04B 20/0048 20130101; C04B 2103/10 20130101; C04B 14/06 20130101;
C04B 14/36 20130101; C04B 2103/20 20130101; C04B 14/368 20130101;
C04B 2103/50 20130101; C04B 38/02 20130101; C04B 2103/0088
20130101; C04B 28/04 20130101 |
International
Class: |
C09K 8/48 20060101
C09K008/48; E21B 33/14 20060101 E21B033/14; C04B 22/14 20060101
C04B022/14; C04B 28/04 20060101 C04B028/04; C04B 14/06 20060101
C04B014/06 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 18, 2010 |
EP |
10195830.4 |
Claims
1. A well-cementing composition, comprising water and solids
comprising Portland cement, silica and an additive comprising one
or more metal sulfates in the list comprising barite, celestine and
anglesite, wherein the median particle size of the additive is
smaller than about 10 .mu.m.
2. The composition of claim 1, wherein the density of the
composition is higher than about 2035 kg/m.sup.3.
3. The composition of claim 1, wherein the additive concentration
is between about 1% and about 150% by weight of cement.
4. The composition of claim 1, further comprising one or more
additives in the list comprising: accelerators, retarders,
extenders, fluid-loss additives, dispersants, gas-generating
agents, antifoam agents, chemical-expansion agents, flexible
additives, pozzolans and fibers.
5. The composition of claim 1, wherein the solids are present in at
least two particle-size ranges.
6. The composition of claim 1, wherein the viscosity of said
composition is lower than 1000 mPa-s at a shear rate of 100
s.sup.-1.
7. The composition of claim 1, wherein the median particle size of
the additive is smaller than about 5 .mu.m.
8. A method for maintaining the compressive strength of a
well-cementing composition, comprising: (i) providing a cement
slurry comprising water, Portland cement and silica; and (ii)
incorporating into the cement slurry an additive comprising one or
more metal sulfates in the list comprising barite, celestine and
anglesite, wherein the average particle size of the additive is
smaller than about 10 .mu.m; and (iii) curing the cement slurry at
a temperature higher than or equal to about 200.degree. C.
9. The method of claim 8, wherein the density of the composition is
higher than about 2035 kg/m.sup.3.
10. The method of claim 8, wherein the additive concentration is
between about 1% and about 150% by weight of cement.
11. The method of claims 8, wherein the composition further
comprises one or more additives in the list comprising:
accelerators, retarders, extenders, fluid-loss additives,
dispersants, gas-generating agents, antifoam agents,
chemical-expansion agents, flexible additives, pozzolans and
fibers.
12. The method of claim 8, wherein the solids in the composition
are present in at least two particle-size ranges.
13. The method of claim 8, wherein the cement slurry has a
viscosity lower than 1000 mPa-s at a shear rate of 100
s.sup.-1.
14. The method of claim 8, wherein the median particle size of the
additive is smaller than about 5 .mu.m.
15. A method for cementing a subterranean well, comprising: (i)
providing a cement slurry comprising water, Portland cement and
silica; and (ii) incorporating into the cement slurry an additive
comprising one or more metal sulfates in the list comprising
barite, celestine and anglesite, wherein the average particle size
of the additive is smaller than about 10 .mu.m; and (iii) placing
the slurry into the well, wherein, the bottomhole temperature in
the well is higher than or equal to about 200.degree. C.
16. The method of claim 15, wherein the density of the composition
is higher than about 2035 kg/m.sup.3.
17. The method of claim 15, wherein the additive concentration is
between about 1% and about 150% by weight of cement.
18. The method of claim 15, wherein the composition further
comprises one or more additives in the list comprising:
accelerators, retarders, extenders, fluid-loss additives,
dispersants, gas-generating agents, antifoam agents,
chemical-expansion agents, flexible additives, pozzolans and
fibers.
19. The method of claim 15, wherein the solids in the composition
are present in at least two particle-size ranges.
20. The method of claim 15, wherein the cement slurry has a
viscosity lower than 1000 mPa-s at a shear rate of 100 s.sup.-1.
Description
BACKGROUND
[0001] The statements in this section merely provide background
information related to the present disclosure and may not
constitute prior art.
[0002] This disclosure relates to compositions and methods for
treating subterranean formations, in particular, compositions and
methods for cementing subterranean wells.
[0003] During the construction of subterranean wells, it is common,
during and after drilling, to place a tubular body in the wellbore.
The tubular body may comprise drillpipe, casing, liner, coiled
tubing or combinations thereof. The purpose of the tubular body is
to act as a conduit through which desirable fluids from the well
may travel and be collected. The tubular body is normally secured
in the well by a cement sheath. The cement sheath provides
mechanical support and hydraulic isolation between the zones or
layers that the well penetrates. The latter function is important
because it prevents hydraulic communication between zones that may
result in contamination. For example, the cement sheath blocks
fluids from oil or gas zones from entering the water table and
polluting drinking water. In addition, to optimize a well's
production efficiency, it may be desirable to isolate, for example,
a gas-producing zone from an oil-producing zone. The cement sheath
achieves hydraulic isolation because of its low permeability. In
addition, intimate bonding between the cement sheath and both the
tubular body and borehole is necessary to prevent leaks.
[0004] Portland cement is employed to cement the vast majority of
subterranean wells. Achieving optimal cement-slurry placement and
set-cement properties usually requires the incorporation of one or
more additives that modify the chemical and/or physical behavior of
the slurry. A plethora of additives exist that fall into several
categories including (but not limited to) accelerators, retarders,
dispersants, fluid-loss additives, extenders, pozzolans, weighting
agents, swellable materials, gas-generating materials, and antifoam
agents. An extensive discussion concerning additives for well
cements may be found in the following publication--Nelson EB,
Michaux M and Drochon B: "Cement Additives and Mechanisms of
Action," in Nelson EB and Guillot D. (eds.): Well Cementing
(2.sup.nd Edition), Schlumberger, Houston (2006) 49-91.
[0005] Designing cement slurries for high-pressure,
high-temperature (HPHT) wells is particularly challenging.
Generally speaking, HPHT wells begin when the bottomhole
temperature exceeds about 150.degree. C. (300.degree. F.) and the
bottomhole pressure exceeds about 69 MPa (10,000 psi). A complex
array of additives--including retarders, dispersants, fluid-loss
additives and silica stabilizers--is usually required to obtain a
slurry that operators can successfully place in the well, and a
set-cement that will provide casing support and zonal isolation
throughout the life of the well.
[0006] Weighing agents are also frequently employed in cement
slurries for HPHT wells. High-density slurries are required to
exert sufficient hydrostatic pressure in the wellbore to maintain
well control. One method for increasing the cement-slurry density
is to reduce the amount of mix water. To maintain pumpability, the
addition of a dispersant is required. The principal disadvantage of
such reduced-water slurries is the difficulty of simultaneously
achieving adequate fluid-loss control, acceptable rheological
properties and slurry stability (i.e., no solids settling).
Generally, the maximum slurry density attainable by reducing the
mix-water concentration is about 2160 kg/m.sup.3 (18.0
lbm/gal).
[0007] Many HPHT wells require higher slurry densities. Under these
circumstances, materials with a high specific gravity (known as
weighting agents) are added. Such materials must meet several
criteria to be acceptable as weighting agents. The particle-size
distribution of the material must be compatible with the cement.
Large particles tend to settle out of the slurry, while small
particles tend to increase slurry viscosity. The mix-water
requirement must be low (i.e., very little water should be
necessary to wet the weighting-agent particles). The material must
be inert with respect to the cement, and must be compatible with
other additives in the cement slurry.
[0008] The most common weighting agents for Portland-cement
slurries are hematite (Fe.sub.2O.sub.3), ilmenite (FeTiO.sub.3),
hausmannite (Mn.sub.3O.sub.4) and barite (BaSO.sub.4). Their
physical properties are given in Table 1. The specific gravities
may vary from batch to batch owing to impurities that may be
present.
TABLE-US-00001 TABLE 1 Physical Properties of Weighting Agents for
Cement Slurries. Additional Absolute Water Weighting Specific
Volume Requirement Agent Gravity (L/kg) Color (L/kg) Hematite 4.45
0.201 Black 0.019 Ilmenite 4.95 0.225 Red 0.000 Hausmannite 4.84
0.209 Reddish brown 0.009 Barite 4.33 0.234 White 0.201
[0009] Large particles with a high specific gravity have a strong
tendency to settle. As shown by Stoke's law (Eq. 1), the settling
velocity of a particle is more dependent on its size than on its
specific gravity.
v = g .times. ( .rho. - .rho. L ) .times. d 2 18 .mu. L ( Eq . 1 )
##EQU00001##
[0010] where: v=settling velocity [0011] g=acceleration of gravity
[0012] .rho.=particle specific gravity [0013] .rho..sub.L=liquid
specific gravity [0014] d=particle diameter [0015]
.mu..sub.L=liquid-medium viscosity.
[0016] For example, the specific gravities of hematite and silica
sand are 4.95 and 2.65, respectively. According to Stoke's law, for
a given particle size, the hematite particle would settle about
twice as fast as the silica particle. However, for a given particle
density, if the particle size is increased to 500 .mu.m from 1
.mu.m, the settling rate increases by a factor of 250,000.
[0017] Stoke's law clearly shows that the size of the solid
materials added to a cement slurry should preferably be low in
order to minimize settling problems. Thus, the use of weighting
agents with very fine particle-size distributions would generally
enhance the stability of cement slurries.
[0018] Barite is commercially available in several particle-size
distributions, but it is not considered to be an efficient
weighting agent compared to hematite, ilmenite or manganese
tetraoxide. It has a lower specific gravity, and requires a
significant amount of additional water to wet its
particles--further diminishing its effectiveness as a weighting
agent. Therefore, although it is commonly used in drilling fluids
and spacer fluids, barite is seldom used in cement slurries.
[0019] With a specific gravity of 4.95, hematite is an efficient
weighting agent and is routinely used in the industry. It is
usually supplied with a fine particle size distribution, with a
median particle size of about 30 .mu.m. To the inventors'
knowledge, the only commercially available weighting agent with a
finer particle size is Micromax.TM., manufactured by Elkem AS,
Oslo, Norway. It is composed of hausmannite with a median particle
size of about 2 .mu.m.
[0020] Until recently, it has been assumed that barite, ilmenite,
hematite and hausmannite are inert with respect to Portland cement
hydration and the set cement. However, the inventors recently
discovered that, at high temperatures, hematite and hausmannite are
not inert. At this temperature, the calcium-silicate-hydrate
mineral xonotlite (6CaO.6SiO.sub.2.H.sub.2O) is usually the
principal binding phase in set Portland cement that has been
stabilized with silica. Hematite and hausmannite react with
xonotlite to form other minerals, including andradite
(Ca.sub.2Fe.sub.2Si.sub.3O.sub.12) and calcium manganese silicates
such as johannsenite (CaMnSi.sub.2O.sub.6). Formation of these
minerals is accompanied by a reduction of the cement compressive
strength and an increase of cement permeability. Such an effect is
potentially detrimental to the set cement's ability to provide
zonal isolation.
[0021] Therefore, in the context of HPHT wells, it would be
desirable to have weighting agents that are inert with respect to
calcium-silicate-hydrate minerals in set Portland cement, and do
not have a deleterious effect on the physical properties of set
Portland cement.
SUMMARY
[0022] Embodiments allow improvements by providing weighting agents
for Portland cement slurries that are inert in a HPHT
environment.
[0023] In an aspect, embodiments relate to well-cementing
compositions comprising water and solids comprising Portland
cement, silica and an additive comprising one or more metal
sulfates in the list comprising barite, celestine and anglesite,
wherein the median particle size of the additive is smaller than
about 10 .mu.m.
[0024] In a further aspect, embodiments relate to methods for
maintaining the compressive strength of a cement composition
comprising: providing a cement slurry comprising water, Portland
cement and silica; and incorporating into the cement slurry an
additive comprising one or more metal sulfates in the list
comprising barite, celestine and anglesite, the average particle
size of the additive being smaller than about 10 .mu.m; and curing
the cement slurry at a temperature higher than or equal to about
200.degree. C.
[0025] In yet a further aspect, embodiments relate to methods for
cementing a subterranean well comprising providing a cement slurry
comprising water, Portland cement and silica; incorporating into
the cement slurry an additive comprising one or more metal sulfates
in the list comprising barite, celestine and anglesite, the average
particle size of the additive being smaller than about 10 .mu.m;
placing the slurry into the well; the bottomhole temperature in the
well being higher than or equal to about 200.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 shows a HPHT strength-development curve for a
high-density Portland cement system containing hematite and
hausmannite.
[0027] FIG. 2 shows a HPHT strength-development curve for a
high-density Portland cement system containing hematite with two
particle-size distributions.
[0028] FIG. 3 shows a HPHT strength-development curve for a
high-density Portland cement system containing hematite and
titanium oxide (rutile).
[0029] FIG. 4 shows a HPHT strength-development curve for a
high-density Portland cement system containing barite with two
particle-size distributions.
DETAILED DESCRIPTION
[0030] At the outset, it should be noted that in the development of
any such actual embodiment, numerous implementation--specific
decisions must be made to achieve the developer's specific goals,
such as compliance with system related and business related
constraints, which will vary from one implementation to another.
Moreover, it will be appreciated that such a development effort
might be complex and time consuming but would nevertheless be a
routine undertaking for those of ordinary skill in the art having
the benefit of this disclosure. In addition, the composition
used/disclosed herein can also comprise some components other than
those cited. In the summary and this detailed description, each
numerical value should be read once as modified by the term "about"
(unless already expressly so modified), and then read again as not
so modified unless otherwise indicated in context. Also, in the
summary and this detailed description, it should be understood that
a concentration range listed or described as being useful,
suitable, or the like, is intended that any and every concentration
within the range, including the end points, is to be considered as
having been stated. For example, "a range of from 1 to 10" is to be
read as indicating each and every possible number along the
continuum between about 1 and about 10. Thus, even if specific data
points within the range, or even no data points within the range,
are explicitly identified or refer to only a few specific, it is to
be understood that inventors appreciate and understand that any and
all data points within the range are to be considered to have been
specified, and that inventors possessed knowledge of the entire
range and all points within the range. All ratios or percentages
described here after are by weight unless otherwise stated.
[0031] As stated earlier, there is a need for weighting agents that
are inert with respect to calcium-silicate-hydrate cement minerals
under HPHT conditions. The inventors have surprisingly discovered
that metal sulfates, including (but not limited to) barium sulfate
(barite), strontium sulfate (celestine) and lead sulfate
(anglesite), do not react with xonotlite, and do not cause a loss
of cement compressive strength or increased cement permeability.
Such sulfates are essentially insoluble in water.
[0032] In an aspect, embodiments relate to well-cementing
compositions that comprise water and solids comprising Portland
cement, silica, and an additive comprising one or more members of
the list comprising barite, celestine and anglesite. The
composition is preferably pumpable. Those skilled in the art will
recognize that a pumpable cement slurry usually has a viscosity
lower than 1000 mPas at a shear rate of 100 s.sup.-1. Metal
sulfates with a very fine particle-size distribution are preferred.
The median particle size is preferably smaller than about 10 gm,
more preferably smaller than about 5 .mu.m and most preferably
equal or smaller than about 3 .mu.m.
[0033] In a further aspect, embodiments relate to methods for
maintaining the compressive strength of a well-cementing
composition. A cement slurry is provided that comprises water,
Portland cement and silica. An additive is incorporated into the
slurry that comprises one or more members of the list comprising
barite, celestine and anglesite. The slurry containing the additive
is then cured at a temperature higher than or equal to about
200.degree. C. Metal sulfates with a very fine particle-size
distribution are preferred. The median particle size is preferably
smaller than about 10 .mu.m, more preferably smaller than about 5
.mu.m and most preferably equal or smaller than about 3 .mu.m.
[0034] In yet a further aspect, embodiments relate to methods for
cementing subterranean wells. A cement slurry is provided that
comprises water, Portland cement and silica. An additive is
incorporated into the slurry that comprises one or more members of
the list comprising barite, celestine and anglesite. The slurry
containing the additive is then placed into the well, wherein the
bottomhole temperature is higher than or equal to about 200.degree.
C. Metal sulfates with a very fine particle-size distribution are
preferred. The median particle size is preferably smaller than
about 10 .mu.m, more preferably smaller' than about 5 .mu.m and
most preferably equal or smaller than about 3 .mu.m. Those skilled
in the art will recognize that the methods may pertain to both
primary and remedial cementing operations.
[0035] For all embodiments, the slurry density is preferably higher
than about 2035 kg/m.sup.3 (17.0 lbm/gal). The additive
concentration is preferably between about 1% and about 150% by
weight of cement (BWOC). The slurry may further comprise one or
more additives in the list comprising: accelerators, retarders,
extenders, fluid-loss additives, dispersants, gas-generating
agents, antifoam agents, chemical-expansion agents, flexible
additives, pozzolans and fibers. Accelerators may be required in
slurries that are pumped in thermal-recovery wells. Such wells are
usually shallow and are cemented at a low temperature. During
production, the wells may be heated to temperatures exceeding
200.degree. C.
[0036] Furthermore, for all embodiments, the solids in the slurry
(cement+silica+metal-sulfate additive+additional solid additives)
may be present in at least two particle-size ranges. Such designs
are "engineered-particle-size" systems in which particle packing is
optimized. A thorough description of these systems may be found in
the following publication. Nelson E B, Drochon B and Michaux M:
"Special Cement Systems," in Nelson E B and Guillot D (eds.) Well
Cementing--2.sup.nd Edition, Houston, Schlumberger (2006)
233-268.
EXAMPLES
[0037] The following examples serve to further illustrate the
disclosure.
[0038] For all examples, cement-slurry preparation and strength
measurements were performed according to procedures published in
ISO Publication 10426-2. Strength measurements were performed in an
Ultrasonic Cement Analyzer (UCA).
Example 1
[0039] A solid blend was prepared with the following composition:
35% by volume of blend (BVOB) Dyckerhoff Black Label Class G cement
(median particle size .about.15 .mu.m), 40% BVOB silica sand
(median particle size .about.315 .mu.m), 10% BVOB silica flour
(median particle size .about.3 .mu.m), 5% BVOB hematite (median
particle size .about.32 .mu.m) and 10% BVOB Micromax.TM.
hausmannite (median particle size .about.2 .mu.m). To this mixture,
1.5% by weight of blend (BWOB) bentonite was added.
[0040] A fluid was prepared with the following composition: 4.17
L/tonne of blend silicone antifoam agent, 66.8 L/tonne retarder (a
blend of sodium pentaborate and pentasodium ethylenediamine
tetramethylene phosphonate [EDTMP]--weight ratio: 9.3), 0.75% BWOB
styrene sulfonate-maleic anhydride copolymer dispersant (Narlex
D72, available from Akzo Nobel), 0.8% BWOB fluid-loss additive
(UNIFLACTM, available from Schlumberger), and sufficient water to
prepare a slurry with a solid-volume fraction (SVF) of 0.61. The
slurry density was 2277 kg/m.sup.3 (19.0 lbm/gal).
[0041] The slurry was placed in a UCA instrument, and cured at a
final temperature of 302.degree. C. (575.degree. F.) and pressure
of 122 MPa (17,700 psi). The heat-up time to reach 274.degree. C.
(525.degree. F.) was 100 min, and the total heat-up time to reach
302.degree. C. was 240 min. The time to reach 122 MPa was 100 min.
The UCA chart is shown in FIG. 1.
[0042] The strength reached a maximum value after about 100 hr.
Then the strength began to decrease, and reached a plateau after
about 400 hr. The UCA test was terminated after 500 hr. At that
time the strength had stabilized.
[0043] The cement sample was removed from the UCA and cored for
measurement of actual compressive strength and water permeability.
The compressive-strength result was 20.6 MPa (2990 psi). The water
permeability was 0.77 mD, which those skilled in the art would
recognize as being too high. For proper zonal isolation, the
maximum allowable permeability value is generally considered to be
0.1 mD.
[0044] Next, the sample was ground to a fine powder and dried first
with acetone and then with ethyl ether. The crystalline composition
of the powder was analyzed by x-ray diffraction. The cement matrix
was mainly composed of johannsenite. A small amount of xonotlite
(the expected cement mineral at this temperature) was detected. The
presence of hausmannite (Mn.sub.3O.sub.4) was not noted.
Example 2
[0045] A solid blend was prepared with the following composition:
35% by volume of blend (BVOB) Dyckerhoff Black Label Class G cement
(median particle size 15 .mu.m), 40% BVOB silica sand (median
particle size .about.315 .mu.m), 10% BVOB silica flour (median
particle size .about.3 .mu.m), 5% BVOB hematite (median particle
size .about.32 .mu.m) and 10% BVOB hematite (median particle size
.about.3 .mu.m). To this mixture, 1.5% by weight of blend (BWOB)
bentonite was added. The difference between this blend and the one
of Example 1 is the replacement of 10% BVOB Micromax.TM. with the
same volume of very fine hematite.
[0046] A fluid was prepared with the following composition: 4.17
L/tonne of blend silicone antifoam agent, 66.8 L/tonne retarder (a
blend of sodium pentaborate and pentasodium EDTMP--weight ratio:
9.3), 0.75% BWOB styrene sulfonate-maleic anhydride copolymer
dispersant (Narlex D72, available from Akzo Nobel), 0.8% BWOB
fluid-loss additive (UNIFLAC.TM.) and sufficient water to prepare a
slurry with a solid-volume fraction (SVF) of 0.61. The slurry
density was 2280 kg/m.sup.3 (19.03 lbm/gal).
[0047] The slurry was placed in a UCA instrument, and cured at a
final temperature of 302.degree. C. (575.degree. F.) and pressure
of 122 MPa (17,700 psi). The heat-up time to reach 274.degree. C.
(525.degree. F.) was 100 min, and the total heat-up time to reach
302.degree. C. was 240 min. The time to reach 122 MPa was 100 min.
The UCA chart is shown in FIG. 2.
[0048] The strength reached a maximum value after about 150 hr.
Then the strength began to decrease, and was still decreasing after
1260 hr when the test was terminated.
[0049] The cement sample was removed from the UCA and cored for
measurement of actual compressive strength and water permeability.
The compressive-strength result was 12.2 MPa (1770 psi). The water
permeability was 0.15 mD, which those skilled in the art would
recognize as being too high. For proper zonal isolation, the
maximum allowable permeability value is generally considered to be
0.1 mD.
[0050] Next, the sample was ground to a fine powder and dried first
with acetone and then with ethyl ether. The crystalline composition
of the powder was analyzed by x-ray diffraction. The cement matrix
was mainly composed of andradite and quartz. Small amounts of
xonotlite and hematite were detected.
[0051] Another UCA test was performed with this cement formulation.
In this case, the test was terminated after only 216 hr. The
compressive strength of the cement core was 27.4 MPa (3975 psi),
and the water permeability was below 0.007 mD (the detection limit
of the equipment). The cement matrix was mostly composed of
xonotlite, quartz and hematite. This result shows that xonotlite
was initially the principal binding phase but, with time, was
consumed by reacting with hematite.
Example 3
[0052] Next, titanium oxide (TiO.sub.2, also known as rutile) was
used. Its specific gravity is 4.15
[0053] A solid blend was prepared with the following composition:
35% by volume of blend (BVOB) Dyckerhoff Black Label Class G cement
(median particle size .about.15 .mu.m), 40% BVOB silica sand
(median particle size .about.315 .mu.m), 10% BVOB silica flour
(median particle size .about.3 .mu.m), 5% BVOB hematite (median
particle size .about.32 .mu.m) and 10% BVOB rutile (Ti-Pure R-902,
available from DuPont Titanium Technologies--median particle size
.about.0.6 .mu.m). To this mixture, 1.5% by weight of blend (BWOB)
bentonite was added. The difference between this blend and the one
of Example 1 is the replacement of 10% BVOB Micromax.TM. with the
same volume of titanium oxide.
[0054] A fluid was prepared with the following composition: 4.17
L/tonne of blend silicone antifoam agent, 66.8 L/tonne retarder (a
blend of sodium pentaborate and pentasodium EDTMP--weight ratio:
9.3), 0.75% BWOB styrene sulfonate-maleic anhydride copolymer
(Narlex D72, available from Akzo Nobel), 0.8% BWOB fluid-loss
additive (UNIFLAC.TM.) and sufficient water to prepare a slurry
with a solid-volume fraction (SVF) of 0.61. The slurry density was
2235 kg/m.sup.3 (18.65 lbm/gal).
[0055] The slurry was placed in a UCA instrument, and cured at a
final temperature of 302.degree. C. (575.degree. F.) and pressure
of 122 MPa (17,700 psi). The heat-up time to reach 274.degree. C.
(525.degree. F.) was 100 min, and the total heat-up time to reach
302.degree. C. was 240 min. The time to reach 122 MPa was 100 min.
The UCA chart is shown in FIG. 3.
[0056] The strength reached a maximum value after about 200 hr.
Then the strength began to decrease and reached a plateau after
about 900 hr. XRD analysis revealed that the cement matrix was
mainly composed of titanite (CaTiSiO.sub.5) and schorlomite
[Ca.sub.3(Fe,Ti).sub.2((Si,Ti)O.sub.4).sub.3]. Very small amounts
of xonotlite and rutile were detected.
Example 4
[0057] A solid blend was prepared with the following composition:
35% by volume of blend (BVOB) Dyckerhoff Black Label Class G cement
(median particle size .about.15 .mu.m), 40% BVOB silica sand
(median particle size .about.315 .mu.m), 10% BVOB silica flour
(median particle size .about.3 5% BVOB barite (median particle size
.about.17 .mu.m) and 10% BVOB barite (median particle size
.about.1.5 .mu.m). To this mixture, 1.5% by weight of blend (BWOB)
bentonite was added. The difference between this blend and the one
of Example 1 is the replacement of 10% BVOB MicromaxTM with the
same volume of very fine barite, and the replacement of 5% BVOB
hematite with the same volume of barite with a larger median
particle size.
[0058] A fluid was prepared with the following composition: 4.17
L/tonne of blend silicone antifoam agent, 66.8 L/tonne retarder (a
blend of sodium pentaborate and pentasodium EDTMP--weight ratio:
9.3), 0.75% BWOB styrene sulfonate-maleic anhydride copolymer
(Narlex D72, available from Akzo Nobel), 0.8% BWOB fluid-loss
additive (UNIFLACTM) and sufficient water to prepare a slurry with
a solid-volume fraction (SVF) of 0.6. The slurry density was 2222
kg/m.sup.3 (18.54 lbm/gal).
[0059] The slurry was placed in a UCA instrument, and cured at a
final temperature of 302.degree. C. (575.degree. F.) and pressure
of 122 MPa (17,700 psi). The heat-up time to reach 274.degree. C.
(525.degree. F.) was 100 min, and the total heat-up time to reach
302.degree. C. was 240 min. The time to reach 122 MPa was 100 min.
The UCA chart is shown in FIG. 4.
[0060] The strength reached a maximum value after about 150 hr.
Then the strength began to slowly decrease, and reached a plateau
after about 500 hr. The UCA test was terminated after 600 hr.
[0061] The cement sample was removed from the UCA and cored for
measurement of actual compressive strength and water permeability.
The compressive-strength result was 26 MPa (3770 psi). The water
permeability was 0.008 mD. Unlike the previous tests, these results
were acceptable.
[0062] Next, the sample was ground to a fine powder and dried first
with acetone and then with ethyl ether. The crystalline composition
of the powder was analyzed by x-ray diffraction. The cement matrix
was mainly composed of xonotlite, quartz and barite, indicating the
barite behaves as a chemically inert filler under HPHT
conditions.
* * * * *