STATIC GEL STRENGTH TESTING
TECHNICAL FIELD
This disclosure relates generally to testing methods
and apparatus and, in an example described below, more
particularly provides improvements in static gel strength
testing .
BACKGROUND
For prospective cementing operations to be performed in
subterranean wells, static gel strength testing is useful to
determine how a particular cement composition will perform
in downhole conditions. In particular, a static gel strength
test can provide information as to how long it will take the
cement composition to develop sufficient gel strength to
prevent gas percolation through the cement composition.
This information is very useful because, while the
cement composition is developing gel strength, its ability
to transmit pressure is typically decreasing, thereby
decreasing hydrostatic pressure in an annulus (e.g., between
a wellbore and a casing or liner) in which the cement
composition has been placed. Unless appropriate measures are
taken, this decreased hydrostatic pressure could allow gas
in an earth formation exposed to the annulus to enter the
annulus and percolate upward through the not-yet-hardened
cement composition—a situation to be avoided.
Thus, it will be appreciated that improvements are
continually needed in the art of static gel strength
testing. Such improvements can be useful in testing the
static gel strength of cement compositions, or of other
slurries, fluids, gels, substances, etc., which develop gel
strength .
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a representative partially cross-sectional
view of a well system and associated method which can
benefit from the principles of this disclosure.
FIG. 2 is a representative partially cross-sectional
view of a static gel strength test instrument which can
embody principles of this disclosure.
FIG. 3 is a representative partially cross-sectional
view of another configuration of the instrument.
FIG. 4 is a representative side view of a blank for a
rotor which may be used in the instrument.
FIG. 5 is a representative side view of the rotor.
FIG. 6 is a representative side view of a blank and
stator blades which may be used in the instrument.
FIG. 7 is a representative cross-sectional view of a
receptacle which may be used in the instrument.
FIG. 8 is a representative side view of a consistent
gap between rotor and stator blades in the instrument.
DETAILED DESCRIPTION
For the purpose of facilitating industry-wide
collaboration and advancements regarding this very important
technology, the American Petroleum Institute (API) has
developed standards for testing static gel strength
(nominally, the gel strength developed after a cement
composition has been appropriately placed in a well).
According to an API standard well known to those skilled in
the art (e.g., API RP 10B-6), a transition time is measured
between a gel strength of 100 lb/100 ft2 and a gel strength
of 500 lb/100 ft2. Typically, it is desired for the
transition time to be less than 30 minutes, and preferably
the transition time should be less than 15 minutes.
During the static gel strength test, the cement
composition is heated, pressurized and stirred, in order to
at least approximate downhole conditions to which the cement
composition will be exposed. For example, the cement
composition may be stirred at 150 rpm while being heated and
pressurized, and then the cement composition may be stirred
at a much slower rate (e.g., 0.2 degrees /minute or other)
while a temperature of the composition is increased or
decreased to an expected downhole temperature at a location
where the composition is to be placed. The cement
composition develops gel strength while being stirred at
this much slower rate, and the transition time is measured.
The foregoing described static gel strength tests may
be performed using any suitable rotating-type static gel
strength apparatus. For example, a MACS-II(TM) test
instrument, available from Fann Instrument Company of
Houston, Texas, USA, is commonly used for such static gel
strength tests.
The present inventors have discovered that,
unfortunately, it is often the case that a rotor paddle of
the test instrument fails to adequately stir the cement
composition as it develops gel strength. Instead, a "plug"
of the gelling composition gradually forms within a
framework of the paddle, and this "plug" rotates with the
paddle relative to a stator cup of the instrument. When the
cup and paddle are disassembled following a test, a
relatively thin layer of the composition is found between
the stator cup and the rotor paddle (and its associated
"plug" ).
As a result, the gel strength measurements made during
the test when the "plug" forms are inaccurate. Although it
is generally possible to estimate what the gel strength
measurements would have been had the "plug" not formed (for
example, by extrapolating the measurements made prior to the
"plug" forming), actual accurate gel strength measurements
would be far preferable to such estimates.
Representatively illustrated in FIG. 1 is a well system
10 and associated method which can benefit from the
principles of this disclosure. In the well system 10,
various fluids 14, slurries, spacers 16, barriers, gels,
etc., can be flowed through various flowpaths in a well, and
it is beneficial to be able to accurately characterize each
of these, particularly at downhole conditions, so that well
operations can be most efficiently, safely, expeditiously
and effectively performed.
The description below focuses on static gel strength
testing for a cement composition 12 used in the well system
10. The term "cement" herein indicates a composition
typically comprising mostly Portland cement and water, with
various additives. However, it is to be clearly understood
that the scope of this disclosure is not limited to use only
with the cement composition 12.
In the FIG. 1 example, the composition 12 is flowed
into an annulus between a tubular string (such as, a casing
or liner string) and a drilled wellbore. When hardened, the
composition 12 will seal off the annulus and prevent fluid
migration between formations penetrated by the wellbore,
protect the tubular string, and serve various other
purposes. In other examples, the composition 12 could be
used to plug an interior of the tubular string. Thus, the
scope of this disclosure is not limited to any particular
purpose for which the composition 12 is used.
One example of a static gel strength test instrument 20
which can embody the principles of this disclosure is
representatively and schematically illustrated in FIG. 2 .
For clarity of illustration and explanation, FIG. 2 does not
depict pumps or heaters used to pressurize and heat the
composition 12, but preferably the instrument 20 does
include such pump(s) and heater (s).
A suitable static gel strength test instrument which
may be modified for use as described herein is the MACSII
(TM) instrument mentioned above. However, the scope of
this disclosure is not limited to use or modification of any
particular type of static gel strength test instrument.
In this example, the instrument 20 includes a motor 22,
a torque sensor 24, a rotor 26 and a stator 28. The motor 22
rotates the rotor 26 relative to the stator 28, and the
torque sensor 24 measures torque due to shearing of the
composition 12 in the instrument 20.
However, in other examples, other configurations of
instruments may be used, static gel strength of other
compositions may be investigated, etc. Therefore, it should
be understood that the principles of this disclosure are not
limited to the instrument 20 described herein and depicted
in the drawings .
One example of another configuration of the instrument
20 is representatively illustrated in FIG. 3 . In this
example, the rotor 26 serves as a receptacle for the
composition 12, and is rotated by the motor 22 positioned
beneath the rotor.
In contrast, the FIG. 2 configuration has the stator 28
serving as a receptacle for the composition 12, with the
rotor 26 being rotated by the motor 22 positioned above the
rotor. This demonstrates that a variety of differently
configured instruments can incorporate the principles of
this disclosure, and those principles are not limited to the
details of any specific examples described herein.
One feature of the instrument 20 as depicted in FIGS. 2
& 3 is that helical blades 30 are provided on the rotor 26,
and on the stator 28. The helical blades 30 on the rotor 26
effectively homogenize (or at least maintain homogenization
of) the composition 12, in part by ensuring that a volume of
the composition at a bottom of the receptacle is urged
upward toward a top of the receptacle.
Referring to FIG. 3 , the helical blades 30 on the
stator 28 are configured so that they intermesh with the
blades on the rotor 26, and the composition 12 is sheared in
a space or gap between the blades. Preferably, the gap
between the blades 30 is constant along the length of the
gap, to thereby provide for consistent shearing of the
composition 12 between the blades. Minimal variation in the
gap between the blades 30 could be present, but preferably
not to an extent which unacceptably degrades the resulting
measurements .
In a method of performing static gel strength tests on
the composition 12, the composition is first dispensed into
the receptacle, and the rotor 26 is rotated relative to the
stator 28 by the motor 22. The rotation of the rotor 26, in
conjunction with the helical shapes of the blades 30
effectively homogenizes the composition 12 (or at least
maintains homogenization of the composition) .
Note that, in the FIG. 2 configuration, the blades 30
on the rotor 26 are axially spaced apart into separate
flights, with the flights being separated by the blades on
the stator 28. The blades 30 on the stator 28 are helically
spaced apart on an inner generally cylindrical surface 32 of
the stator. Of course, other configurations of elements in
the instrument 20 may be used, in keeping with the scope of
this disclosure.
Preferably, the blades 30 on the stator 28 have the
same shape and curvature as the blades on the rotor 26, so
that the gap between the blades is uniformly consistent as
one blade displaces past another, thereby preventing
interference between the blades, but providing for uniform
intermeshing . One method of producing such complementarily
shaped blades 30 is representatively illustrated in FIGS. 4-
6 , for the instrument 20 of FIG. 2 , but it should be
understood that this is merely one example of how the blades
could be produced, and other methods may be used in keeping
with the scope of this disclosure.
For clarity in the description below, the blades on the
rotor 26 are indicated with reference number 30a, and the
blades on the stator 28 are indicated with reference number
30b, it being understood that in other examples the specific
blades could be on different ones of the rotor and stator,
the blades could be differently configured, etc.
In the example of FIGS. 4-6, the rotor 26 begins as a
cast or molded blank 34 having double helix blades 30a
formed thereon. As depicted in FIG. 4 , the blades 30a extend
outwardly from a generally cylindrical surface 36 on the
rotor 26. In other examples, the blades 30a could extend
inwardly, the blades could extend from a non-cylindrical
origin, different numbers of blades may be used, etc.
In FIG. 5 , the rotor 26 is representatively illustrated
after material has been removed from the blades 30a to
accommodate the blades 30b on the stator 28. Note that, in
this example, the chosen peripheral shape of the blades 30b
is trapezoidal (in lateral projection), to provide a desired
length of a desired gap between the blades 30a, b for
shearing the fluids. In other examples, different shapes
(e.g., rectangular, circular, polygonal, curved,
combinations of shapes, etc.) of the blades 30b may be used.
As depicted in FIG. 5 , the blades 30a are axially
spaced apart along the rotor in four sets of flights. In
other examples, more or fewer sets of flights may be used,
as desired.
In FIG. 6 , it may be seen that the blades 30b for the
stator 28 are cut from another blank 38 having a double
helix formed thereon, similar to the double helix blades 30a
on the blank 34 of FIG. 4 . In this technique, the
trapezoidal shape is cut from the helixes 50 on the blank
38, thereby yielding multiple blades 30b which have
substantially the same helical pitch (slope) and curvature
as the blades 30a on the rotor 26.
Note that it is not necessary for the blank 38 to have
a double helix formed thereon. Any number of helixes may be
used in keeping with the scope of this disclosure. Indeed,
the blades 30b could be formed by casting, molding, etc.,
and without cutting them from a helix, if desired.
Referring additionally now to FIG. 7 , a receptacle 40
of the stator 28 is representatively illustrated. The
receptacle 40 is provided with a series of opposing recesses
42 which are helically spaced apart along the inner
cylindrical surface 32 of the receptacle.
The recesses 42 are used in this example to position
the blades 30b on the stator 28. In other examples, the
blades 30b could be otherwise positioned, configured or
arranged.
Referring additionally now to FIG. 8 , an enlarged scale
representative view of the blades 30a, b in the instrument 20
is illustrated in lateral projection. The blades 30a, b are
depicted in FIG. 8 as if "flattened" laterally, so that the
blade 30b has its trapezoidal perimeter, and the blades 30a
are axially separated by trapezoidal cutouts, as in the
example of FIGS. 5 & 6 . However, it will be appreciated
that, in this example, the blades 30a, b are actually helical
in shape.
Preferably, a gap 44 between the blades 30a, b is
constant, or at least substantially consistent, so that the
composition 12 is sheared between the blades consistently.
However, some variation in the gap 44 may be permitted, if
desired.
In one example, an instrument 20 can have a consistent
gap 44 of .150 in. (-3.8 mm) between the blades 30a, b and a
base angle 46 of 70 degrees, with a tip width 48 on the
blade 30b of .090 in. (-2.3 mm). Using this example, the
cement composition 12 can be subjected to a static gel
strength test, without a "plug" forming in a framework of
the rotor 26. Indeed, in situ homogenization can be
efficiently carried out, if desired, thereby enabling
accurate "mix while measure" techniques in which the
composition 12 is mixed and homogenized in the instrument 20
prior to performing the static gel strength test per se.
It may now be fully appreciated that the above
disclosure provides significant advancements to the art of
static gel strength testing. In examples described above, a
composition can be tested for static gel strength, without a
"plug" forming within a framework of a paddle/rotor of a
static gel strength test instrument, but instead maintaining
homogenization of the composition during the test, so that
accurate static gel strength measurements can be obtained.
A method of performing a static gel strength test on a
composition 12 is provided to the art by the above
disclosure. In one example, the method can include: placing
the composition 12 into a static gel strength test
instrument 20; stirring the composition 12 with at least one
helical blade 30 of the instrument 20; and measuring
resistance to rotation between a stator 28 and a rotor 26 of
the instrument 20.
The resistance to rotation measurement may be made by
sensing torque applied to the rotor 26, measuring deflection
of a biasing device which resists rotation of the stator 28,
etc. The scope of this disclosure is not limited to any
particular technique for measuring resistance to rotation of
the rotor 26 relative to the stator 28.
The stator 28 may comprise the helical blade 30.
Alternatively, or in addition, the rotor 26 may comprise the
helical blade 30.
In one example described above, the at least one
helical blade 30 comprises at least one first helical blade
30a on the stator 28 and at least one second helical blade
30b on the rotor 26. The first helical blade 30a may be
spaced apart from the second helical blade 30b by a
substantially consistent gap 44.
If multiple helical blades 30 are used, the blades 30
can be helically spaced apart on a cylindrical surface 32 of
the stator 28.
If multiple helical blades 30 are used, the blades 30
can be axially spaced apart on the rotor 26.
A static gel strength test instrument 20 is also
described above. In one example, the instrument 20 can
include a rotor 26, and a stator 28 having at least one
first helical blade 30a. The static gel strength test
instrument 20 characterizes gelation of a composition 12.
Another static gel strength test instrument 20 can
include a stator 28 having at least one first helical blade
30a, and a rotor 26 having at least one second helical blade
30b. However, it is not necessary in keeping with the scope
of this disclosure for both of the rotor 26 and stator 28 to
comprise helical blades 30.
Although various examples have been described above,
with each example having certain features, it should be
understood that it is not necessary for a particular feature
of one example to be used exclusively with that example.
Instead, any of the features described above and/or depicted
in the drawings can be combined with any of the examples, in
addition to or in substitution for any of the other features
of those examples. One example's features are not mutually
exclusive to another example's features. Instead, the scope
of this disclosure encompasses any combination of any of the
features.
Although each example described above includes a
certain combination of features, it should be understood
that it is not necessary for all features of an example to
be used. Instead, any of the features described above can be
used, without any other particular feature or features also
being used.
It should be understood that the various embodiments
described herein may be utilized in various orientations,
such as inclined, inverted, horizontal, vertical, etc., and
in various configurations, without departing from the
principles of this disclosure. The embodiments are described
merely as examples of useful applications of the principles
of the disclosure, which is not limited to any specific
details of these embodiments.
The terms "including," "includes," "comprising,"
"comprises," and similar terms are used in a non-limiting
sense in this specification. For example, if a system,
method, apparatus, device, etc., is described as "including"
a certain feature or element, the system, method, apparatus,
device, etc., can include that feature or element, and can
also include other features or elements. Similarly, the term
"comprises" is considered to mean "comprises, but is not
limited to."
Of course, a person skilled in the art would, upon a
careful consideration of the above description of
representative embodiments of the disclosure, readily
appreciate that many modifications, additions,
substitutions, deletions, and other changes may be made to
the specific embodiments, and such changes are contemplated
by the principles of this disclosure. For example,
structures disclosed as being separately formed can, in
other examples, be integrally formed and vice versa.
Accordingly, the foregoing detailed description is to be
clearly understood as being given by way of illustration and
example only, the spirit and scope of the invention being
limited solely by the appended claims and their equivalents.
WHAT IS CLAIMED IS:
1 . A method of performing a static gel strength test
on a composition, the method comprising:
placing the composition into a static gel strength test
instrument ;
stirring the composition with at least one helical
blade of the instrument; and
measuring resistance to rotation between a stator and a
rotor of the instrument.
2 . The method of claim 1 , wherein the stator
comprises the helical blade.
3 . The method of claim 1 , wherein the rotor compr
the helical blade.
4 . The method of claim 1 , wherein the at least one
helical blade comprises at least one first helical blade on
the stator and at least one second helical blade on the
rotor .
5 . The method of claim 4 , wherein the first helical
blade is spaced apart from the second helical blade by a
substantially consistent gap.
6 . The method of claim 1 , wherein the at least one
helical blade comprises multiple helical blades, and further
comprising helically spacing apart the helical blades on a
cylindrical surface of the stator.
7 . The method of claim 1 , wherein the at least one
helical blade comprises multiple helical blades, and further
comprising axially spacing apart the helical blades on the
rotor .
8 . A static gel strength test instrument, comprising
a rotor; and
a stator having at least one first helical blade,
wherein the static gel strength test instrument
characterizes gelation of a composition.
9 . The static gel strength test instrument of claim
8 , wherein the at least one helical blade comprises multipl
first helical blades, and wherein the first helical blades
are helically spaced apart on the stator.
10. The static gel strength test instrument of claim
8 , wherein the rotor includes at least one second helical
blade .
11. The static gel strength test instrument of claim
10, wherein the first and second helical blades are spaced
apart from each other by a substantially consistent gap.
12. The static gel strength test instrument of claim
10, wherein the first and second helical blades have a
substantially same pitch.
13. The static gel strength test instrument of claim
10, wherein the first and second helical blades have a
substantially same curvature.
14. A static gel strength test instrument, comprising:
a stator having at least one first helical blade; and
a rotor having at least one second helical blade.
15. The static gel strength test instrument of claim
14, wherein the first and second helical blades have a
substantially same pitch.
16. The static gel strength test instrument of claim
14, wherein the first and second helical blades have a
substantially same curvature.
17. The static gel strength test instrument of claim
14, wherein the first helical blade comprises a portion of a
helix.
18. The static gel strength test instrument of claim
14, wherein the first helical blade is spaced apart from the
second helical blade by a substantially consistent gap.
19. The static gel strength test instrument of claim
14, wherein the at least one first helical blade comprises
multiple first helical blades, and wherein the first helical
blades are helically spaced apart on a cylindrical surface
of the stator.
20. The static gel strength test instrument of claim
14, wherein the at least one second helical blade comprises
multiple second helical blades, and wherein the second
helical blades are axially spaced apart on the rotor.