U.S. patent application number 12/627588 was filed with the patent office on 2011-06-02 for preparation of setting slurries.
This patent application is currently assigned to Schlumberger Technology Corporation. Invention is credited to Robert Seth Hartshorne, Carl Johnson, Nathan Lawrence, Glen Monteiro, Benoit Vidick, Jonathan Woodrow.
Application Number | 20110127034 12/627588 |
Document ID | / |
Family ID | 44066997 |
Filed Date | 2011-06-02 |
United States Patent
Application |
20110127034 |
Kind Code |
A1 |
Vidick; Benoit ; et
al. |
June 2, 2011 |
PREPARATION OF SETTING SLURRIES
Abstract
When preparing a settable slurry such as a cement slurry for
cementing an oil well, cement or other solid powder is mixed with
water and a set retarder. In order to be able to run a convenient
and rapid check on the concentration of retarder after dilution at
the site of use, a tracer material is mixed with the retarder in
known amount during manufacture. The tracer is chosen to enable its
concentration to be determined analytically after dilution at the
site of use, thereby providing a way to determine the concentration
of set retarder after such dilution. The tracer may be a
redox-active material and its concentration may be determined by
voltammetry. Tracer may likewise be mixed with additives other than
set retarder.
Inventors: |
Vidick; Benoit; (Cambridge,
GB) ; Hartshorne; Robert Seth; (Suffolk, GB) ;
Lawrence; Nathan; (Huntingdon, GB) ; Johnson;
Carl; (Blackburn, GB) ; Monteiro; Glen;
(Stavanger, NO) ; Woodrow; Jonathan;
(Leidschendam, NL) |
Assignee: |
Schlumberger Technology
Corporation
Cambridge
MA
|
Family ID: |
44066997 |
Appl. No.: |
12/627588 |
Filed: |
November 30, 2009 |
Current U.S.
Class: |
166/293 |
Current CPC
Class: |
C04B 40/0032 20130101;
E21B 33/14 20130101; C04B 40/0032 20130101; C09K 8/467 20130101;
C04B 28/02 20130101; C04B 40/0039 20130101; C04B 2103/22
20130101 |
Class at
Publication: |
166/293 |
International
Class: |
E21B 33/13 20060101
E21B033/13 |
Claims
1. A process of preparing a settable slurry by mixing a solid
powder, water and one or more additives, characterised by mixing a
tracer material with an additive before mixing that additive with
the solid powder and the water and analysing a mixture containing
at least the additive and at least part of the water to determine
the concentration of tracer therein.
2. A process according to claim 1 wherein the step of analyzing a
mixture is performed on the slurry.
3. A process according to claim 1 wherein the step of analyzing a
mixture is performed on a mixture of the additive and water before
mixing with the solid powder.
4. A process according to claim 1 wherein the solid powder is
cement, so that the slurry is a cement slurry.
5. A process according to claim 4 wherein said additive with which
the tracer is mixed is a cement set retarder.
6. A process according to claim 5 which includes a step of pumping
the cement slurry into a space between a borehole and a casing
within the borehole.
7. A process according to claim 1 wherein the tracer is mixed with
the additive at a manufacturing or storage facility and the
additive with admixed tracer is subsequently transported to another
site for preparing the slurry by mixing with the solid powder and
water.
8. A process according to claim 1 wherein analysis to determine the
concentration of tracer is carried out by electrochemistry.
9. A process according to claim 8 carried out with an array of
laminar electrodes formed on an insulating substrate.
10. A process according to claim 8 wherein the tracer undergoes
electrochemical oxidation and/or reduction and analysis is carried
out using voltammetry to determine the concentration of tracer.
11. A process according to claim 10 where analysis is carried out
using square wave voltammetry.
12. A process according to claim 8 wherein the step of analyzing a
mixture is performed on a mixture containing a mediator compound
which undergoes electrochemical oxidation and reduction the tracer
undergoes chemical reaction with the mediator compound, and
analysis is carried out using voltammetry to observe the effect of
tracer on electrochemical reaction of the mediator compound.
Description
FIELD OF THE INVENTION
[0001] This invention relates to the preparation of slurries which
set to solid form after they have been prepared. These may in
particular, but not exclusively, be cement slurries used in
wellbore cementing.
BACKGROUND OF THE INVENTION
[0002] Cement slurry is typically made by mixing cement powder,
water (sometimes referred to as the mix-water) and various
additives that may include retarders, dispersants, fluid-loss
additives and anti-foam additives.
[0003] When the cement slurry is going to be used to fill the space
between a drilled bore hole and a casing inserted into that
borehole, the slurry must flow for a considerable distance before
it reaches its final position where it is required to set. It is
therefore normal to include a retarder to delay setting. Typically
the retarder and other additives are supplied to the rig-site as
manufactured products, which may be stock solutions, and these are
added to the mix-water before adding the cement powder. So, when
making up the mix water a stock solution of retarder might be
diluted 100-fold or more.
[0004] It is very important that the correct amount of retarder is
added. Using too much retarder will delay set of the cement
unnecessarily and hence increase the "waiting on cement" delay
during which the well site stands idle because further work cannot
be done until the cement has set. On the other hand premature
setting of the cement can be hugely expensive to rectify.
[0005] Yet, there is currently no satisfactory technology for
checking that the concentration of retarder as diluted in the
mix-water used to make the cement slurry is correct. The only
available test is to take a sample of the mix water (with the
additives in it, but before the cement powder is added) and run a
thickening test either in a fixed laboratory which is not at the
rig site or in a portable laboratory if available at the site of
intended use. This process is time-consuming, potentially taking up
to 8 or 9 hours (depending on the thickening time) and so results
in job delays and (depending on the outside temperature) in the
possible degradation of the quality of the mix water.
SUMMARY OF THE INVENTION
[0006] The invention provides a way to check the concentration of
an additive in a cement slurry and/or in the mix-water before
cement powder is added. The invention can be embodied as a facile,
inexpensive, and accurate measurement technique which can be
carried out rapidly at the site of use without requiring a fully
equipped laboratory. Although the invention has been conceived in
the specific context of checking retarder concentration when
cementing a wellbore, it could also be applied to other additives
and/or in other contexts.
[0007] According to a first aspect of this invention a process of
preparing a settable slurry comprises mixing a solid powder, water
and one or more additives, and the process is characterised by
[0008] mixing a tracer material with an additive before mixing that
additive with the solid powder and the water and by [0009]
analysing a mixture containing at least the additive and some or
all of the water to determine the concentration of tracer
therein.
[0010] The solid powder is likely to be a cement powder although it
might possibly be some other material such as gypsum plaster which
forms a settable slurry with water. The additive may be a set
retarder for cement. An advantage of using a tracer substance which
is not itself acting as retarder or functional additive is that
there is then freedom to select the tracer as a substance which can
be detected analytically.
[0011] It is envisaged that the tracer could be added to retarder
or other additive at the manufacturer's factory, or could be added
by a commercial intermediary or even by the end user while the
additive is in storage prior to use. Addition of tracer while the
additive is stockpiled as a concentrate could be checked by
laboratory analysis if so desired.
[0012] When mixing cement at the site of the well to be cemented,
the concentration of the tracer could be determined after mixing
the retarder or other additive with the mix-water, e.g. after
mixing all the retarder with the calculated total quantity of water
but before mixing with the solid cement powder. Alternatively or
additionally the concentration of tracer could be determined in a
sample taken from the mixed cement slurry which is ready to be
placed in the position where it will set. It is also possible that
the retarder or other additive might be mixed with some of the
total quantity of water, but not all of it, and the concentration
of tracer determined at that stage. Depending on the result, a
further quantity of water might then be added to adjust the
dilution.
[0013] It is possible that the tracer could be detected by a
spectroscopic method, for example using a tracer with a distinctive
fluorescence. For instance fluorescein has been used as a tracer in
applications where the objective is to identify a path of flow. It
exhibits strong fluorescence under visible or ultraviolet
illumination and is thereby detectable at concentrations of 10 ppm
and even less, as for instance mentioned in Society of Petroleum
Engineers paper 50768. A number of other fluorescent materials have
been employed as tracers, including compounds of the lanthanide
series of elements, as mentioned in U.S. Pat. No. 5,979,245 and
WO2007/102023.
[0014] In some preferred forms of this invention the concentration
of tracer is determined by an electrochemical measurement. The
tracer may then be a compound which is capable of undergoing
electrochemical reduction and oxidation and the analytical
procedure may use the current flow during this electrochemical
oxidation and reduction to determine the concentration of
tracer.
[0015] More specifically, the tracer may be determined by one of
the available forms of voltammetry which applies potential to the
electrodes and measures the current flow.
[0016] One possible form of voltammetry varies the potential
applied to a working electrode over a sufficient range to bring
about the oxidation or reduction reaction of the tracer while
recording the current flow as the potential is varied. This form of
voltammetry may be a linear scan over a range of applied potential
or may be a cyclic scan over a range of potential and back again,
so as to bring about both the oxidation and reduction of the
tracer. A discussion of cyclic voltammetry can for instance be
found in "Electrochemistry, Principles, methods and applications"
by C M A Brett and A M O Brett (OUP 1993) pages 174-199. The
recorded current shows peaks at the potentials associated with the
electrochemical reduction and oxidation. The concentration of the
compound undergoing redox reaction, which for the present invention
will be the compound acting as tracer, may be calculated from the
observed current, may be found using a previously constructed
calibration curve or look-up table, or may be determined by means
of additional experiments with deliberately added tracer as
illustrated in one of the Examples below.
[0017] Another possible form of voltammetry which may be used is
square wave voltammetry in which the potential applied to a working
electrode is provided by a square wave superimposed on a staircase
baseline, so that the potentials applied at the peaks and troughs
of the square wave increase progressively. Current flow is measured
close to the end of each peak and trough of the square wave. A
description of this technique can be found at pages 219-221 of the
same textbook.
[0018] Linear or cyclic voltammetry may be entirely adequate when
used in this invention, but the present inventors have appreciated,
as a further feature of some forms of this invention, that square
wave voltammetry may be advantageous for this invention because it
can observe the redox reaction of a tracer while excluding
inteference by other chemical species which may be present. It also
provides good sensitivity and can be carried out quickly.
[0019] Examples of materials which undergo electrochemical
reduction and oxidation reactions, and which may be used as
tracers, include ferrocyanide ions, quinones and anthraquinones,
phenylene diamine and its derivatives and ferrocene and its
derivatives such as ferrocene sulfonates. Voltammetry with such
compounds has been described in WO2005/066618, WO2007/034131 and
references cited therein. Another category of materials which may
be used as redox active tracers come from the family of purines
(organic compounds with fused pyrimidine and imidazole rings)
especially purines which incorporate keto groups such as xanthine.
Various other organic chemicals are redox active, including
oxidisable hydroxy acids such as ascorbic acid. Bromides and
iodides may also be used as tracers: voltammetry with these has
been described by Wu et al in J. Anal. Chem Vol 60, pp1062-1068
(2005).
[0020] Another possibility is to choose a metallic ion as tracer
and estimate its concentration by means of adsorptive stripping
voltammetry (ASV). In this case the metallic species is first
accumulated at the electrode surface by the imposition of a
reducing potential (more negative than that of the redox species of
interest). The reduction process results in electrochemical-induced
deposition of the analyte onto the electrode surface. The deposited
analyte is then subsequently `stripped` from the electrode surface
via application of an oxidative potential (relative to the analyte
species) yielding an oxidative peak wave and thus the analytical
signal used to detect the species.
[0021] A further possibility for electrochemical determination of a
tracer is to select as a tracer a substance which does not itself
undergo electrochemical redox reaction at an electrode but instead
is a substance which reacts with another compound which itself
undergoes electrochemical reaction. Such coupling between a species
to be determined and the electrochemistry of a mediator compound
has been described in the context of the electrochemical
determination of hydrogen sulfide in WO2001/063094 and
WO2004/011929. Ferrocene carboxylate and sulphonate have been
suggested as possible mediator compounds in Electroanalysis Vol 18
pp1658-63 (2006) and in Electrochimica Acta Vol 52 pp499-50 (2006).
A number of ferrocene sulphonates for possible use in this way have
been described in Journal of Organometallic Chemistry Vol 692
pp5173-82 (2007). What is contemplated for the present invention is
to apply this approach to the determination of a compound
deliberately added as a tracer rather than analytical determination
of a compound which may happen to be present. Another species which
can couple to electrochemical redox reactions of a mediator
compound is nitrate, as for instance disclosed by Kim et al in
Biotechnology and Bioprocess Engineering Vol 10 pages 47-51
(2005).
[0022] For carrying out electrochemical determination of the tracer
an electrochemical cell may be set up with a sample of the cement
slurry or the mix-water being used as the electrolyte. The
electrodes for such a cell may be provided by conventional
electrodes but preferably they are provided as an array of three
electrodes in laminar form deposited on an insulating laminar
support. Such laminar electrodes may be formed on the support by
screen printing of conductive pastes, as described in
WO2004/011929. For example the working and counter electrodes could
be strips containing conductive carbon while the reference
electrode could be a strip containing both silver and silver
chloride
[0023] An advantage of voltammetry as an analytical technique is
the small size and portability of the apparatus required.
Analytical determination of tracer in accordance with this
invention may be carried out with a beaker or similar container to
hold a sample of slurry or mix-water, a disposable three-electrode
array, a potentiostat to supply the electrical potential and
observe current and a computer to control operation and store and
display results.
[0024] A potentiostat can be fairly small and requires electrical
power as the only supply to it. The computer could be a
conventional lap-top PC. It would also be possible for the
potentiostat and computer to be provided as a small single-purpose
battery powered handheld unit.
[0025] Cement retarder may be a material which is already used for
that purpose. Some conventional cement retarders include sodium
pentaborate, calcium glucoheptonate, lignosulphonate and derivaties
thereof, and a combination of phosphoric acid and the pentasodium
salt of ethylenediamine tetra(methylene phosphonic acid). These
materials may be shipped to a rig-site as aqueous stock solutions
containing between 2 and 25 wt % of the retarder. The amount of
extent of dilution will depend on the requirements of the
individual cementing job but it is likely that the concentration of
retarder in the mix water will lie between 0.001% and 0.1% by
weight. The concentration of any other additive may also lie within
such a range.
[0026] It will be desirable that the amount of tracer is less than
the amount of retarder, possibly no more than 10% by weight of the
retarder and possibly even less than this. Consequently, the
concentration of tracer in the water or cement slurry at the time
of testing is likely to lie in a range from 1 micromolar up to 10
millimolar. The amount may be at least 5 or at least 10 micromolar.
It may be no more than 1 millimolar or no more than 500
micromolar.
[0027] Although it is a significant objective to check the
concentration of retarder, it would be possible to check one or
more other concentrations (in addition or as an alternative
application of this invention) For instance the retarder could
incorporate a ferrocene derivative as tracer and a dispersant could
incorporate xanthine as a tracer. These give voltammetric peaks at
different values of applied potential and so the concentrations of
both could be determined concurrently by voltammetry.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIGS. 1 and 2 are schematic diagrams illustrating an
embodiment of the invention;
[0029] FIG. 3 schematically illustrates apparatus for testing;
[0030] FIG. 4 shows the square wave voltammetry response of
solutions of retarder with and without xanthine tracer;
[0031] FIG. 5 shows the square wave voltammetry response of
solutions of retarder and other additives with and without
t-butylferrocene sulphonate tracer;
[0032] FIG. 6 shows the linear voltammetry response of solutions
containing ascorbic acid as tracer, and
[0033] FIG. 7 is a a graph constructed with data from FIG. 6.
DETAILED DESCRIPTION
[0034] As shown schematically in FIG. 1, retarder and other
additives are made by their manufacturers at factories 10, 11.
These are manufactured as stock solutions containing a
predetermined concentration of the retarder or other additive. They
are shipped to a well service company which holds stocks in a
warehouse 12. A tracer is added to the retarder in predetermined
amount by the manufacturer at its factory 10. The weight ratio of
retarder to tracer is thus fixed at that point. In the warehouse 12
the service company takes a sample from each batch of retarder and
analyses it in laboratory 14 as a part of routine quality control
procedure. This analysis confirms the presence and concentration of
the tracer relative to the concentration of retarder. When
required, the retarder and other additives are shipped to a
rig-site 16 where a well has been drilled.
[0035] As shown schematically in FIG. 2 the well 17 has been
drilled and steel casing 18 has been placed in it. Retarder
indicated by arrow R, plus other additives A1, A2 etc (eg antifoam,
dispersant, and fluid-loss control additives) are mixed with water
W by a mixer 20 and the thus-prepared mix-water is stored in a
holding tank 21 before it is mixed with cement powder C in a second
mixer 22 to form a cement slurry S which is delivered to the well
17 where the slurry is pumped down the well inside the steel casing
18 and forced back up into the annular space 24 around the casing
18 where it sets.
[0036] Samples of the mix water are taken from the tank 21 and are
tested using apparatus shown in FIG. 3. This consists of a hand
held computer 26 connected to a hand-held potentiostat 28 which is
connected to an electrode array 30 consisting of three electrodes
screenprinted onto an insulating substrate as described in
WO2004/011929.
[0037] This electrode array is immersed into the sample 32 of
mix-water and voltammetry is carried out using the computer 26 to
control operation of the potentiostat and to receive, store,
process and display the results.
EXAMPLE 1
[0038] To demonstrate suitable electrochemistry, square wave
voltammetry was carried out on a solution simulating mix-water
containing a conventional retarder sodium pentaborate.
[0039] In this example, a glassy carbon working electrode was used
with a standard calomel reference electrode. Voltammetry was
carried out using a potentiostat from Eco Chemie BV, Utrecht,
Netherlands.
[0040] The results obtained are shown in FIG. 4 where curve 40 is
the response when no xanthine was added. Curve 42 is the response
when xanthine was added to the solution at a concentration of 40
.mu.M. In the absence of xanthine no distinct redox active waves
were observed in the electrochemical potential range scanned
across. The lack of electrochemical activity in the pentaborate
solution provided a background response and is important when using
redox active tracers to detect the cement additive. Upon the
addition of xanthine to the solution, a redox peak 44 emerges at
ca. +0.75 V (vs. saturated calomel electrode, SCE), consistent with
the theoretical redox potential for the oxidation of xanthine.
[0041] Experiments were also carried out using lower concentrations
of xanthine. These showed that the peak at ca. +0.75 V increases in
intensity as the concentration of added xanthine is increased. It
was apparent that the limit of detection was about 10 .mu.M
xanthine, a sensitive limit of detection which demonstrates that
this is a useful method for detecting the tracer species in
retarders. It is envisaged that the concentration in the mix-water
would exceed this value.
EXAMPLE 2
[0042] Square wave voltammetry was also carried out on solutions
containing a mixture of calcium glucoheptonate (retarder), and
polynapthalene sulphonate (dispersant). Voltammetry was carried out
using a glassy carbon electrode. The tracer was t-butylferrocene
sulfonate. The results are shown in FIG. 5.
[0043] Curve 50, obtained in the absence of t-butylferrocene
sulfonate, shows no redox waves. By contrast, in curve 52,
following addition of t-butylferrocene sulfonate to the solution a
redox peak 54 emerges at +0.35 V (vs. SCE), consistent with a 1
electron oxidation of the ferrocene species to the ferricenium ion.
The well defined nature of the peak enables such a species to be
used as a tracer species.
EXAMPLE 3
[0044] An experiment was carried out to demonstrate the
quantitative detection of ascorbic acid as a tracer in a cement
retarder stock solution. Voltammetry in this experiment was carried
out using a glassy carbon working electrode with a standard calomel
reference electrode.
[0045] A sample of a simulated mix-water fluid was prepared that
comprised a typical concentration of retarder (and associated
chemical tracer) after dilution from a stock solution at the
rig-site. The concentration of ascorbic acid in this simulated
mix-water was 0.29 mM. A voltammetric linear scan was performed at
a scan rate of 0.1 volt per second and the resulting plot of
current against applied potential is shown as curve 56 in FIG. 6.
It is the characteristic Faradaic signal associated with the
oxidation of ascorbic acid. Curve 58 is a base line curve obtained
with a similar solution omitting the ascorbic acid.
[0046] A 100 mM standard solution of ascorbic acid was prepared in
deionised water., 80 uL aliquots of this ascorbic acid standard
solution were added consecutively to the retarder solution. After
each addition a voltammetric scan was carried out. The resulting
curves are shown in FIG. 6 and, as indicated by the vertical arrow,
the presence of increasing amounts of ascorbic acid led to
increasing current at +0.8 volt relative to the reference
electrode.
[0047] Following each of eight such separate additions, the peak
height at +0.8 volt was plotted versus the added ascorbic acid
concentration. The plot is illustrated in FIG. 7. The negative
intercept on the abscissa of the graph gives the ascorbic acid
concentration in the simulated sample. As illustrated in FIG. 7,
the Y=0 value on the X-axis was at a value of -0.27 mM and
therefore the concentration of ascorbic acid in the simulated
sample had been determined by this simple experimental procedure to
be 0.27 mM which corresponds well to the actual value of 0.29
mM.
[0048] This example has thus shown that a redox-active species
(such as ascorbic acid) can usefully be used as a chemical tracer
to `tag` a cement retarder solution at a known ratio of tracer to
retarder. Following dilution of the cement retarder solution, prior
to mixing with cement, the concentration of the chemical tracer can
be determined via a voltammetric standard addition experiment. From
this, and the known ratio of tracer to retarder, the actual
concentration of the retarder after dilution can be determined
easily.
* * * * *