CROSS-REFERENCE TO RELATEDAPPLICATION
[0001] This application claims the priority from U.S. Patent Application Serial
No. 13/542,011, filed July 5, 2012 which is incorporated herein by reference
in its entirety.
TECHNICAL FIELD
[0002] This disclosure relates to measuring mechanical properties.
BACKGROUND
[0003] Some well bores, for example some oil and gas wells, are lined with a
casing. The cemented casing stabilizes the sides of the well bore, prevents fluids
(liquids or gases) in the well bore from entering the surrounding earth formations,
and/or prevents fluids from zones other than the producing zones from entering the
well bore.
[0004] In a typical cementing operation, cement is introduced down the well
bore and into an annular space between the casing and the surrounding earth. The
cement secures the casing in the well bore, and prevents fluids from flowing vertically
in the annulus between the casing and the surrounding earth.
[0005] Different cement formulations are designed for a variety of well bore
conditions, which may be above or below ambient temperature and/or above ambient
pressure. In designing a cement formulation, a number of potential mixtures may be
evaluated to determine their mechanical properties under various conditions.
SUMMARY
[0006] Using the devices and methods described, the volumetric
shrinkage/expansion resulting from cement hydration can be directly and
continuously correlated to the Initial Stress State of curing-cement under simulated
wellbore conditions of pressure and temperature. This parameter will define the initial
distance the material is from failure or the stress state of the material prior to
additional loading. This turns out to be a key parameter when employing long term
cement-sheath modeling based on the initial state of stress added to the stress
variations that the material will be subjected to owing to the long term requirements
dictated by well operations during the economic life of the well. Even though
mechanical properties of cement are, up to some extent, known before material
placement in the well, it is important to note that there is no advantage to knowing the
total capacity of a material without prior determination of its initial stress of state as
the erroneous determination of this parameter can lead to incorrect conceptions on
whether or not a material will withstand a specific set of loadings. The test cell is
designed in such a way that both radial and axial shrinkage/expansion are
continuously monitored while cement hydrates.
[0007] In one aspect, methods for testing a sample of a fluid mixture that
hardens into a solid include: placing the sample of the fluid mixture into a test
chamber; applying a pressure to the sample in the test chamber that is different than
ambient air pressure around the test chamber; taking the samples in the test chamber
to the desire temperature setting; monitor axial dimensions and radial dimensions of
the sample over time; and identifying an initiation of gelling and hardening of the
sample by a start of changes to the radial dimensions of the sample.
[0008] In one aspect, methods for testing a sample of a fluid mixture that
hardens into a solid include: placing the sample of the fluid mixture into a test
chamber; and identifying a stress state of a sample of the cement at/after an initiation
of gelling and hardening of the sample.
[0009] In one aspect, methods for assessing a cement include: identifying a
stress state of a sample of the cement at an initiation of gelling and hardening of the
sample; using the identified stress state of the sample of the cement as an initial stress
state parameter input into a computer well model; and performing well life modeling
of the of the cement using the computer well model.
[0010] Embodiments of these methods can include one or more of the
following features.
[0011] In some embodiments, methods also include determining an initial
stress state of the sample by calculating a stress state of the sample at the identified
initiation of gelling of the sample.
[0012] In some embodiments, methods also include controlling a temperature
of the test chamber.
[0013] In some embodiments, the test chamber comprises an annular portion.
[0014] I some embodiments, methods also include developing a calibrated
stress-strain relationship for the test chamber by pressurizing the test chamber in the
absence of a sample and recording pressure and strain.
[0015] In some embodiments, methods also include applying conditions in the
test chamber after the sample cures to simulate well operation events.
[0016] In some embodiments, methods also include applying a first pressure
to bottom surfaces of the sample and a different second pressure to top surfaces of the
sample.
[001 7] In some embodiments, methods also include measuring strain at
multiple locations distributed axially along the test chamber. In some cases, methods
also include assessing heterogeneity of gelling and hardening of the sample based on
differences in the strain measured at the multiple locations distributed axially along
the test chamber.
[0018] In some embodiments, methods also include performing shear and/or
hydraulic bond testing on the sample in the test chamber.
[0019] In some embodiments, methods also include identifying the initiation
of gelling and hardening of the sample by a start of changes to the radial dimensions
of the sample.
[0020] In some embodiments, methods also include applying a pressure to the
sample in the test chamber that is different than ambient air pressure around the test
chamber.
[0021] In some embodiments, methods also include monitoring axial
dimensions and radial dimensions of the sample over time.
[0022] In some embodiments, methods also include applying conditions in the
test chamber after the sample cures to simulate well operation events.
[0023] In some embodiments, performing well life modeling comprises
simulating at least one of cementing, pressure testing, swabbing, hydraulic fracturing,
and production.
[0024] In some embodiments, methods also include simulating application of
stresses to a virtual cement sheath in the computer well model estimate a distance to
failure for the cement under different conditions.
[0025] The described methods and systems can provide one or more of the
following advantages.
[0026] Both chemical shrinkage and bulk shrinkage of cements are influenced
by temperature and pressure conditions. These methods and systems can be used to
determine stress changes experienced by cement due to hydration
shrinkage/expansion while cement cures under the downhole conditions (e.g., below
or above atmospheric temperatures and above atmospheric pressures). In particular,
these methods and systems can provide a calculation of the initial stress state a cement
sheath will experience in specific downhole applications. This parameter is critical
when modeling long term events that occur during the life of a well.
[0027] This technique measures a stress change that a cement will experience
and can directly address the concerns about determining the initial stress state of
cement. The initial stress state of set cement calculated using these methods and
systems is anticipated to be more accurate than prior methods that investigate
chemical shrinkage or bulk shrinkage but not both. In particular, these methods and
systems avoid the complicated and controversial analysis necessary to derive an initial
stress state of the set cement that can be used for stress analysis from other
approaches.
[0028] The development of analytical models to simulate material behavior
under certain conditions applies knowledge of various parameters such as geometry,
failure criteria, loading history, constitutive law (relation between physical parameters
that allow for material characterization), and the initial stress of state. Besides
determining the initial state of stress of hydrating cement, the device and method
described herein can also be employed to determine various physical parameters that
comprise constitutive law that emulate the behavior of cement. Other parameters
employed to characterize cement behavior such as shear and hydraulic bond under
different wellbore conditions can also be correlated to the shrinkage/expansion
measurements.
The method apparatus described herein also has the potential to measure
hydrostatic pressure loss experienced by the cement during hydration, which is
directly related to static gel strength; as well as widely considered a reason for early
gas migration.
[0029] The details of one or more embodiments are set forth in the
accompanying drawings and the description below.
DESCRIPTION OF DRAWINGS
[0030] FIG. 1A is a schematic of a testing apparatus. FIGS. IB-IE are,
respectively, an exploded perspective view, a exploded cross-sectional view, an
exploded side view, and an assembled side view of t e testing apparatus of FIG. 1A.
[0031] FIG. 2 is a schematic illustrating axial compression and radial
expansion of a sample.
[0032] FIG. 3 shows a plot of the relationship between strain and pressure
used in calibration of a testing apparatus. This will allow for correlation of hydrating
shrinkage/expansion to the state of stress of cement.
[0033] FIG. 4 shows axial extension and radial strain over 48 hours as a
sample cures.
[0034] FIG. 5 shows volume change due to axial extension, volume change
due to radial strain, and total volume change over 48 hours as a sample cures.
[0035] FIG. 6 shows volume change due to radial strain over 48 hours as a
sample cures.
[0036] FIG. 7 shows axial pressure and radial pressure over 48 hours as a
sample cures.
[0037] FIG. 8 compares the change in volume of samples as a function of time
for different curing pressures.
[0038] FIGS. 9A and 9B, respectively, show the initial stress state and the
pressure-drop (owing to hydration) of cement samples subjected to different curingpressures.
[0039] FIG. 10 shows a plot of the relationship between axial curing-pressure
and pressure-drop/axial pressure for different curing-pressures.
[0040] FIG. 11 compares the rate of volume change over time for different
applied pressures.
[0041] FIG. 12A is a schematic of a testing apparatus. FIGS. 12B-12E are,
respectively, an exploded perspective view, an exploded cross-sectional view, an
exploded side view, and an assembled side view of the testing apparatus of FIG. 12A.
[0042] FIG. 13A is a schematic of a testing apparatus. FIGS. 13B-13E are,
respectively, an exploded perspective view, an exploded cross-sectional view, an
exploded side view, and an assembled side view of the testing apparatus of FIG. 13A.
[0043] Like reference symbols in the various drawings indicate like elements.
DETAILED DESCRIPTION
[0044] Cements can be used, for example, to seal an annular space in wellbore
between a well casing and the surrounding formation. Understanding the
shrinkage/expansion properties of cements under different conditions can be
important in designing/choosing an appropriate cement for a specific application. The
described devices and methods incorporate a test chamber capable of directly and
continuously measuring sample shrinkage/expansion at different pressure and
temperature conditions. Both axial shrinkage/expansion and radial
shrinkage/expansion of the sample are continuously measured and correlated to the
initial stress state of cement sheath under simulated wellbore conditions.
[0045] As used herein, "cement" and "cement composition" encompass a fluid
mixture that hardens into a solid, and may be any agent suitable to bond casing or
other elements (e. g. tubulars) to well bore walls or to other tubing used for downhole
applications. Some examples of cement include hydraulic cement (e. g. , Portland
cement formulations), non-hydraulic cement (e. g. , polymer resin formulations), and
mixtures thereof having, for instance, silica, Pozzolans, cross-linked polymers,
ceramics, among other components. As used herein, "curing" refers to the reactions
through which cement hardens from a fluid mixture into a solid. In some instances,
the devices and methods discussed herein can be used to measure mechanical
properties at temperatures and pressures that simulate downhole conditions.
[0046] FIG. 1A illustrates a testing system 100 that includes a test cell 110 and
a data acquisition system 112. FIGS. IB-IE further illustrate features of the test cell
110 which includes a test chamber 114, a top piston 116, and a bottom piston 118. In
this description, terms of relative orientation such as upper, lower, above, and below
are used relative to the orientation of embodiments shown in the figures being
discussed. Although such terms do not require that the illustrated devices be oriented
as shown in the figures, the test cell 110 will typically be oriented as shown in FIG.
1A during use.
[0047] As used herein, "piston" encompasses driving mechanisms including,
for example, hydraulic pistons, power screws, and linear actuators. Thus, the piston
does not necessarily seal against the pressure vessels described below.
[0048] In testing system 100, the test chamber 114 is a hollow metallic
cylinder. The test chamber 114 is formed of material which is structurally stable
enough to contain a sample at pressures and temperatures simulating downhole
conditions (e.g., up to 10,000 psi and 600 F) are applied to the sample and which
measurably deforms as the sample shrinks/expands during curing. In one prototype,
the test chamber 114 was machined from brass and, in another prototype, the test
chamber 114 was machined from bronze alloy. Alternatively, the test chamber 114 can
be formed using casting, laminating, or molding techniques from materials including,
for example, steel, alloys, or composite fibers with a resin structure. Ports 120 (see
Figures IB-IE) extend through walls of the test chamber and provide access for
sensors (not shown) used to measure sample conditions. For example, test chamber
114 defines a first port for a thermocouple used to measure sample temperature and a
second port for a pressure sensor.
[0049] The top piston 116 is operable to apply a load to a sample in the test
cell 114. Testing system 100 includes a load frame 122 operable to generate loads
transmitted to the sample in the test cell 114. A prototype testing system was
implemented with an Instron series 5884 load frame. Some testing systems include
other mechanisms (e.g., power screws, linear actuators, and pressure pumps) can be
used to generate loads transmitted to the sample in the test cell 114. The top piston
116 defines a port 132 extending through the top piston. A side bleeding channel 133
extends at an angle from the port 132. A screw (not shown) is employed to block the
bleeding channel 133 once the cell 114 is filled with the slurry and all the air is
removed.
[0050] The bottom piston 118 is fixed in place relative to the test chamber 114
acting only as a bottom cap. However, some test cells include bottom pistons that are
moveable relative to the test chamber 114.
[0051] During use, the temperature of fluid in the test can range from below
ambient condition temperatures to the high temperatures associated with downhole
conditions (e. g. , up to 1000 degrees Fahrenheit). The pressure of the fluid in the
pressure vessel can range from atmospheric pressure to the high pressures associated
with downhole conditions (e. g., up to 60,000 psi). The components of the pressure
vessel can be made from materials which are strong (e. g. , able to maintain structural
stability when subjected to high pressures), are durable (e. g. , resistant to corrosion
by the anticipated pressurizing fluids in the anticipated temperature and pressure
ranges), and can be formed with the precision necessary to maintain substantially
pressure-tight engagement between the components under testing conditions. For
example, the test chamber 114, the top piston 116, and the bottom piston 11 can be
machined from stainless steel. Alternatively, the test chamber 114, the top piston 116,
and the bottom piston 11 can be formed using casting, laminating, or molding
techniques from materials including, for example, steel, alloys, or composite fibers
with a resin structure.
[0052] Seals between inner walls of the test cell 114 and outer surfaces of the
pistons 116, 118 limit (e.g., substantially prevent or prevent) fluid flow out of the test
cell 114 between the inner walls of the test chamber 114 and outer surfaces of the
pistons 116, 118. Test cell 110 includes O-ring seals 124 attached to both the bottom
and top pistons in order to avoid fluid losses. In some embodiments, testing systems
use other sealing mechanisms including, for example, matching threads, gaskets, or
metal-to-metal seals.
[0053] Some testing systems 100 include temperature-control mechanisms to
simulate downhole temperatures during testing. External or internal heating elements
may be employed to keep the desired temperature on the cement slurry; or the testing
apparatus could be placed in an oven for heating purposes. Examples of external
heating elements include heating coils or stainless steel heating bands and, internal
heating coils include, for example, internal electrical resistances inside the hydraulic
fluid. There are applications where the temperature below ambient conditions are
present in the wellbore, Cooling coils can be employed to take the cement specimen
to the desire conditions and allow for its controlled curing. A double purpose
heating/cooling system may be employed, where a hot fluid is employed when
temperatures higher that ambient conditions are required; or a refrigerant is employed
when temperature below ambient conditions are required.
[0054] In some embodiments, pressure and temperature controllers are used in
such way that (a), downhole conditions are simulated during cement transferring,
curing and testing; and (b) these conditions are accurately maintained or shift
according to the downhole conditions. For instance, cement slurry and testing
apparatus can be preheated during mixing. The testing apparatus can be heated in a
sequence that simulates the temperature conditions that a cement system would
encountered from mixing, placement, and curing during the cementing a wellbore
casing string. In addition, the test apparatus can simulate other well operation events
that the cement system may be exposed to including, for example, pressure testing,
steam injection, fracturing, and hydrocarbon production. As anticipated, tests
performed using a prototype testing apparatus have confirmed that changes in the
curing temperature and pressure change the properties or mechanical response of the
cement sample.
[0055] Testing systems can include sensors to measure parameters used to
calculate properties of samples being tested. For example, testing system 100 includes
sensors to measure the axial and radial deformation of samples being tested. The
sensors are in communication with the data acquisition system 112. In testing system
100, a strain gauge 1 6 monitors the radial deformation of the sample due to the
cement slurry volume change. Sensors associated with the mechanism applying a load
to the top piston 116 (e.g., load frame 122) monitor axial deformation of the sample
and applied load. Some testing systems include other monitoring mechanism
including, for example, linear variable displacement transducers (LYDTs),
extensometers, lasers, DVRTs, or fiber optic strain gauges, can be used in addition to
or in place of the strain gauges to measure relevant parameters. Pressure and
temperature sensors can be included to measure pressures and temperatures present
during testing. Pressure, temperature, and strain sensors can be used as feedback to
control the test process. For example, pressure sensors can control the pump to
pressure up or down dependent upon a controlled set point. Likewise, the piston
loading the test specimen can be actuated in a direction depending on the deflection or
strain measurements experienced by the sample.
[0056] In the prototype, a 1 0 ohms strain gauge connected with a high speed
-USB- 1 2 data acquisition card 128 monitored radial deformation of the sample
due to the cement slurry volume change. Bluehill® software for Instron load frame
continuously recorded the axial displacement of samples and the applied load on the
top piston during testing. The strain gauge and the Instron load frame communicated
data to a desktop computer 130 with Bluehill® and LabVIEW software installed.
[0057] For a test chamber 114 that is a thick walled cylinder, the relationship
between hoop strain of the outer surface and the strain in the inner surface is given by
the Lame's solution for thick-wall cylinder as
where eq, a is strain in the inner surface of the test chamber 114, eq, b is strain in the
outer surface of the test chamber 114, a is the inner radius of the test chamber 114
(see Figure 2A), b is the outer radius of the test chamber 114 (see Figure 2A), and v is
the Poisson ratio for the material used to form the test chamber 114. Given the known
dimensions of the test chamber a, b and the Poisson ratio, it is possible to calculate
the strain in the inner surface of the test chamber 114 ( eq, a ) based on the strain in the
outer surface of the test chamber 114 (eq, b ) as measured using the strain gauge 126.
[0058] Before use, the test system 100 is calibrated to develop the correlation
between pressure applied to a sample and strain in the inner surfaces of the test
chamber 114. The test chamber 114 is filled with water and pressure is applied to the
water using the load frame 122. The applied pressure and strain in the outer surfaces
of the test chamber 114 are measured and strain in the inner surfaces of the test
chamber 114 is calculated based on the measured strain in the outer surface of the test
chamber 114 using the equation above. Figure 3 presents the data obtained when the
prototype test cell 110 was calibrated using this approach. For the prototype test cell
110, the regression analysis performed using an Excel spreadsheet indicated that
P = 44,725,46.7 eq, a + 16.1
where P is pressure (psi) and represents the stress on the sample. This equation can be
used to convert strain measured on outer surfaces of the test chamber 114 to the stress
state of the sample due to hydrating shrinkage/expansion. The relationship, which is
specific to each individual test cell 110, can be used to determine downhole stresses
on a cement sample.
[0059] Prior to testing a sample, a user assembles the test chamber 114 and the
bottom piston 118, the chamber is filled to a desired level with a slurry of cement
being tested. Once the chamber is filled, the top piston 116 is placed in the test
chamber 114 while port 133 is used as a bleeding port for air removal. This is
followed by positioning a special design screw (not shown) in port 132 to seal the test
cell 110. After the test cell 110 is placed in the loading device (e.g., load frame 122),
the user operates the testing system 100 to bring the slurry to conditions that simulate
downhole temperature and pressure. For example, the load frame 122 can be used to
apply pressure to the slurry via the top piston 116. As shown in Figure 2B, the sample
changes shape both radially and axially during testing. The test system 100 records
the displacement of top piston 116 and the radial dimensions of the vessel while
controlling the pressure applied to the top piston 116. The relationship developed
during the cell calibration of pressure versus strain can be used to determine the stress
change in the cement during testing. It has been observed that shrinkage/expansion
occurring while the cement is still a liquid only results in the movement of the piston
because the cell is still under constant pressure. In contrast, shrinkage/expansion that
occurs once the cement starts to gel and/or harden results in both movement of the
piston 116 and radial changes in the cell dimensions. The radial change in the cell
dimensions result due to cement hydration which is reflected in a change of the
pressure the cement applies to the cell. This is directly related to the stress that the
cement experiences. The stress measured by testing cell 100 can then be used as input
in well modeling. These models essentially create a virtual wellbore and simulate
several events that occur during the life of the well. For example, a virtual wellbore is
constructed by the software and then progresses through the life of the well simulating
events such as cementing, pressure testing, swabbing fluids out, hydraulic fracturing
and production. Stresses are applied to the cement sheath and analysis estimates the
cement's remaining capacity for failure. In order to determine this distance to failure
due to the different events, the initial state of stress of the cement sheath is a key
model input.
[0060] A prototype testing system 100 was implemented using a brass test
chamber 114 that provided an adequate ratio of resistance and flexibility to generate
anticipated radial deformations. The top 116 and bottom pistons 118 were made of
stainless steel-3 16. The top piston 114 had bleeding port 133 for air removal and a
port 132 for positioning of special design screw (not shown) for sealing the test cell
100.
[0061] The prototype testing system was used to test properties of cements
under various conditions. Initial experiments were conducted on a 16.4 lb/gal slurry
over 48 hours. The load frame 112 applied pressure at a load rate of 2,000 lb-f/min
until a constant load of 25,450 lbf (4,962 psi) was achieved.
[0062] Figure 4 shows the extension (axial displacement) and the strain (radial
displacement) curves with respect to time. During the first 12 to 15 minutes of the
test, the strain and axial displacement rapidly increased during the pressurization
stage. The increase in strain during 3-8 hrs might have been a result of heat of
hydration considering that the strain gages were not temperature compensated. From
8.5 to about 18 hours, there was a rapid decrease for both radial strain and axial
displacement which is in agreement with the period where heat of hydration kicks in.
The rate of change of radial strain and axial displacement slowed down after 18 hrs,
however, these properties kept decreasing owing to hydration.
[0063] Figures 5 and 6 illustrate the total shrinkage volume and the shrinkage
volume due to the axial and radial displacement. Figure 7 shows the change in stress
over time. A total shrinkage volume of 3.38% was observed after 48 hours. Most
(97%) of the total shrinkage was due to the axial displacement, which corresponds to
be 3.28%. Conversely, the radial displacement accounted for 0.1% of volume
shrinkage, what would seem to be an insignificant amount of the total volume
shrinkage. It is important to note that the radial shrinkage, a minute percentage of the
total volumetric shrinkage, resulted in a pressure drop equivalent to ~2000 psi (see,
e.g., Figure 7).
[0064] The prototype testing system 100 demonstrated the capability to
measure stress changes experienced by the cement due to hydration
shrinkage/expansion. In addition to providing accurate volume shrinkage results, this
method of testing also provides the capability of identifying the precise time at which
shrinkage measurement should commence considering its effect on the stress state of
cement.
[006S] FIG. 8 compares the change in volume of samples as a function of time
for different curing pressures. As expected, the increase in curing pressure resulted in
an increment of volumetric shrinkage, FIG. 8. The trends are generally similar to
those discussed above with respect to Figure 5. FIGS. 9A and 9B, respectively, show
the influence of curing pressure on the stress state of cement. Figure 9B reveals that
increasing the curing pressure results in greater pressure drop owing to cement
hydration. Moreover, FIG. 10 illustrates a linear correlation among pressure drop due
to hydration and curing pressure, which is indicative of potential prediction stress
state of cement at certain ages.
[0066] FIG. 11 compares the rate of volume change over time for different
applied pressures. Owing to the acceleration effect of the increased pressure causing
early static gel strength (SGS) and strength development, it was expected that higher
curing pressures would reduce the time at which the maximum shrinkage rate takes
place. It is important to notice that the shape of this plot resembles that of heat of
hydration, which further confirms the evolution of volumetric shrinkage is hydration
dependent, as well as the stress state.
[0067] The prototype cell demonstrated the ability to provide accurate results
in determining the initial stress state of cement due to cement shrinkage/expansion.
However, some embodiments of testing system 100 include modifications that can
provide even more precise results and/or more realistically simulate the environment
for cement at downhole conditions.
[0068] FIG. 12A is a schematic of a test cell 210. FIGS. 12B-12E are,
respectively, an exploded perspective view, an exploded cross-sectional view, an
exploded side view, and an assembled side view of the test cell 210 of FIG. 12A. The
test cell 210 is substantially similar to the test cell 110 discussed above but includes
additional sensors, a temperature control system, a modified bottom piston with a
pressure port for application of fluid pressure (for pore pressure simulation), and
different size-screens that also enable the application of pore pressure to the sample.
[0069] The test cell 210 includes 3 strain gauges distributed along the test
chamber 214. The hardening of a cement slurry is not homogenous but rather starts at
the bottom of the sample and proceeds upward. Use of multiple strain gauges is
anticipated to provide more accurate radial displacement measurements as well as
provide insight into the heterogeneity of the hardening process.
[0070] The test cell 210 also includes temperature control system with three
thermocouples 227 placed to measure temperature at the outer surface of test chamber
214, at the outer surface of the cement sample, and within the cement sample. This
allows for heat of hydration monitoring. The temperature control system operates by a
Eurotherm controller to achieve downhole temperature conditions based on data from
the thermocouples 227. The thermocouples can also be used to monitor the
temperature changes of the sample as the cement slurry cures.
[0071] The test cell 210 includes an end cap 218 rather than a bottom piston.
Various mesh size screens 219 are disposed adjacent the end cap 218. A 320-mesh
size screen allows for pore pressure simulation by allowing water to flow through the
porosity of the samples and avoid the sample flowing towards the pore pressure fluid
source. Additionally, a 60-mesh size screen is employed to provide stability to the
320-mesh screen. Fluid communication with a port 221 is defined extending through
the end cap 218. This configuration enables the application of fluid pressure to the
cement sample. Either water or oil can be employed for this purpose. Furthermore, an
additional pressure transducer can be included to determine the cement pore pressure.
A hydraulic pump can be employed as the pore pressure source.
[0072] FIG. 13A is a schematic of a testing apparatus. FIGS. 13B-13E are,
respectively, an exploded perspective view, a exploded cross-sectional view, an
exploded side view, and an assembled side view of the testing apparatus of FIG. 13A.
As discussed above, downhole applications for cements include filling the annular
space between a well casing and the surrounding formation. Testing system 300 is
substantially similar to testing system 200 but has a two-cylinder test cell 310 to
simulate the annular systems such as pipe-in-open hole and pipe-in-pipe downhole
environments. The test cell 310 includes a hollow top piston 316 sized to fit in the
annulus between the test chamber 214 and an inner pipe 314. Three strain gauges 126
are disposed on the inner surface of inner pipe 314. This modified
shrinkage/expansion test cell enables simulation of the downhole cement environment
including the formation (represented by the external pipe or the top piston), downhole
temperature and hydrostatic pressure (controlled by the heat-temperature control
system and the top piston, respectively), exposure to external fluids (simulated by the
various size mesh screens and application of pore pressure); and the casing
(represented by the internal pipe). Furthermore, the weight of fluids above the cement
sheath can also be simulated by the load applied to the top piston. The data recorded
by inner and outer strain gages can be employed to determine the absolute radial
deformation of the cement sample.
[0073] Shear and hydraulic bond testing can also be performed with this piece
of equipment. Correlating these two parameters to cement hydration will result in a
key piece of information to evaluate gas migration considering its direct relationship
to hydraulic bond. Furthermore, hydrostatic pressure loss correlation to cement
hydration and to the transition period are also achievable with this device and directly
related to SGS and therefore to gas migration.
[0074] A number of embodiments have been described. Nevertheless, it will
be understood that various modifications may be made.
[0075] For example, in some embodiments, some cement test cells 110
includes a modified piston 116 that defines a channel that can be used to controUably
vent gases as interior spaces of the cement test cell 110 during with a pressurized
cement slurry. U.S. Patent Number 5,869,750 and U.S. Patent Publication Number
201 1/0094295 discuss methods and equipment that can be used in preparing and
testing a slurry of a cement sample without exposure to ambient pressure conditions.
The entire contents of these references are incorporated herein by reference.
[0076] In another example, some testing systems 100 incorporate a control
pressure mechanism. For example, the control pressure mechanism can be provided
using the Instron load frame employed in the prototype system. The control pressure
mechanism can also be provided using an added syringe injection pump. The syringe
pump approach requires modifying the top piston and adding a top end cap. The
modified testing system can be used to develop a relationship between the axial
pressure, the pressure applied to the cement by the cylindrical cell, and the strain
changes resulted from the volumetric shrinkage.
[0077] Accordingly, other embodiments are within the scope of the following
claims.

WE CLAIMS:-
1. A method for testing a sample of a fluid mixture that hardens into a solid,
the method comprising:
placing the sample of the fluid mixture into a test chamber;
applying a pressure to the sample in the test chamber that is different than
ambient air pressure around the test chamber;
monitor axial dimensions and radial dimensions of the sample over time; and
identifying an initiation of gelling and hardening of the sample by a start of
changes to the radial dimensions of the sample.
2. The method of claim 1, further comprising determining an initial stress
state of the sample by calculating a stress state of the sample at or after the identified
initiation of gelling of the sample.
3. The method of claim 1, further comprising controlling a temperature of the
test chamber.
4. The method of claim 1, wherein the test chamber comprises an annular
portion.
5. The method of claim 1, further comprising developing a calibrated stressstrain
relationship for the test chamber by pressurizing the test chamber in the absence
of a sample and recording pressure and strain.
6. The method of claim 1, further comprising applying conditions in the test
chamber after the sample cures to simulate well operation events.
7. The method of claim 1, further comprising applying a first pressure to
bottom surfaces of the sample and a different second pressure to top surfaces of the
sample.
8. The method of claim 1, further comprising measuring strain at multiple
locations distributed axially along the test chamber.
9. The method of claim 8, further comprising assessing heterogeneity of
gelling and hardening of the sample based on differences in the strain measured at the
multiple locations distributed axially along the test chamber.
10. The method of claim 1, further comprising performing shear and/or
hydraulic bond testing on the sample in the test chamber.
11. A method for testing a sample of a fluid mixture that hardens into a solid,
the method comprising:
placing the sample of the fluid mixture into a test chamber; and
identifying a stress state of a sample of the cement at or after an initiation of
gelling and hardening of the sample.
12. The method of claim 11, further comprising identifying the initiation of
gelling and hardening of the sample by a start of changes to the radial dimensions of
the sample.
13. The method of claim 11, further comprising applying a pressure to the
sample in the test chamber that is different than ambient air pressure around the test
chamber.
14. The method of claim 11, further comprising monitoring axial dimensions
and radial dimensions of the sample over time.
15. The method of claim 11, further comprising developing a calibrated
stress-strain relationship for the test chamber by pressurizing the test chamber in the
absence of a sample and recording pressure and strain.
16. The method of claim 11, further comprising applying a first pressure to
bottom surfaces of the sample and a different second pressure to top surfaces of the
sample.
17. The method of claim 11, further comprising applying conditions in the test
chamber after the sample cures to simulate well operation events.
18. A method for assessing a cement, the method comprising:
identifying a stress state of a sample of the cement at an initiation of gelling
and hardening of the sample;
using the identified stress state of the sample of the cement as an initial stress
state parameter input into a computer well model; and
performing well life modeling of the of the cement using the computer well
model.
1 . The method of claim 18, wherein performing well life modeling
comprises simulating at least one of cementing, pressure testing, swabbing, hydraulic
fracturing, and production.
20. The method of claim 18, further comprising simulating application of
stresses to a virtual cement sheath in the computer well model estimate a distance to
failure for the cement under different conditions.