Note: Descriptions are shown in the official language in which they were submitted.
CA 02815589 2013-05-10
APPLICATION FOR PATENT
INVENTORS: ROBERT MARTIN
GUIDO GUSTAVO NARVAEZ
QI QU
GREGORY B. ITSKOVICH
TITLE: METHOD OF REAL TIME MONITORING OF WELL
OPERATIONS USING SELF-SENSING TREATMENT FLUIDS
SPECIFICATION
Field of the Disclosure
[0001]
Downhole conditions of a self-sensing treatment fluid may be monitored in
real time by incorporating into the fluid a piezoelectric or piezoresistive
material.
Background
[0002] During
construction of a well penetrating a subterranean formation, a rotary
drill is typically used to bore through the subterranean formation to form a
wellbore. In a
typical drilling operation, a drilling mud is injected under pressure through
the drill
string. The mud returns to the surface through the drill string-borehole
annulus. Once
returned to the surface, the drilling mud contains cuttings from the drill
bit. Although
most large cuttings are removed at the surface prior to recirculating the
fluid, smaller
sized particles remain suspended within the drilling, fluid. Insufficient mud
return to the
surface may cause problems with flow and control lines within the well.
CA 02815589 2013-05-10
[0003] Once
the wellbore has been drilled, a pipe or casing is lowered into the
wellbore. Cementitious slurry is then pumped into the well, down the inside of
the pipe
or casing and back up the outside of the pipe or casing through the annular
space between
the exterior of the pipe or casing and the wellbore. The cementitious slurry
is then
allowed to set and harden as a sheath. The strength of a cement mix may be
determined
by the properties of the initial raw materials, mixing and compacting
conditions, and
specific composition such as, but not limited to, mineral binder-to-aggregate
ratio, water-
to-cement ratio, and water-to-aggregate ratio.
[0004] A
primary function of the cementing process is to restrict fluid movement
between the subterranean formation and to bond and support the casing. The
cement
sheath holds the casing in place. In addition, the cement aids in protecting
the casing
from corrosion, preventing blowouts by quickly sealing formations, protecting
the casing
from shock loads in drilling deeper wells, and selectively isolating
particular areas, such
as lost circulation or thief zones, from other areas of the wellbore, and
forming a plug in a
well to be abandoned. Cementing operations further provide zonal isolation of
the
subterranean formation and help prevent sloughing or erosion of the wellbore.
Cements
may also be used in remedial operations to repair casing and/or to achieve
formation
isolation as well as in sealing off perforations, repairing casing leaks
(including leaks
from damaged areas of the casing), plugging back or sealing off the lower
section of a
wellbore, and forming a plug in a well to be abandoned.
[0005] In
addition to their use in oil and gas wells, cementitious slurries are used
in geothermal wells, water wells, injection wells. disposal wells and storage
wells.
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[0006] After
a well is properly cemented, the well may then be subjected to a
variety of treatments. For instance, the well may be stimulated in order to
enhance the
recovery of oil or gas from the reservoir.
[0007] During
well treatment operations, including stimulation operations, cement
sheaths are subjected to axial, shear and compressional stresses induced by
vibrations and
impacts. In particular, stress conditions may be induced or aggravated by
fluctuations or
cycling in temperature or fluid pressures. In addition, variations in
temperature and
internal pressure of the wellbore pipe string may result in radial and
longitudinal pipe
expansion and/or contraction. This tends to place stress on the annular cement
sheath
existing between the outside surface of the pipe string and the inside
formation surface or
wall of the wellbore. Such stresses lead to cracking and/or disintegration of
the cement
sheath.
[0008] A
cementitious slurry should have a pumpable viscosity, demonstrate
acceptable fluid loss control, exhibit minimal settling of particles and have
the ability to
set within a practical time. In addition. the cement mix and the properties of
the
cementitious slurry must be carefully selected in order to minimize or
eliminate cracking
and/or disintegration of the cement sheath. In particular, the cement mix and
the slurry
containing the mix must be carefully tailored in order for the cement sheath
to withstand
those axial stresses, shear stresses and compressional stresses encountered
under in-situ
wellbore conditions. Further, the components of the cement mix and the
cementitious
slurry should be selected such that, when hardened, the cement sheath is not
brittle since
brittleness causes cracking of the sheath.
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[00091 The hardening process of a cement mix within the well constitutes a
series of
consecutive transitions between different states of the material, Initially,
the cement mix
is a compacted structure whose physical and mechanical properties are
determined
mainly by compressive actions of capillary pressure on "water-air" boundaries.
This state
is characterized by such chemical reactions as hydration and hydrolysis of
mineral
binders, like cement, gypsum, lime, etc., in the mix and gel formation.
Hardening and
strengthening of a cement mix is initiated immediately once compaction has
been
completed. As soon as a sufficient amount of product has accumulated per
volume unit,
the gel begins to age and a capillary-porous structure begins to form, first
as a
coagulation structure (long-range and short-range), and thereafter as
colloidal and
crystallization structures. The coagulation structure is a capillary-porous
colloidal body
having chemically active water-silicate dispersions. The colloidal and
crystalline
structures are a quasi-solid capillary porous body. The crystalline structures
then undergo
condensation and the material develops a solid capillary-porous body where
interaction
of particles and particle aggregates occur in the solid phase. At any given
state, the
material has a poly-dispersed structure of a moist capillary-porous body.
[00010] While at the beginning of the development of the hydration and
hydrolysis the
properties acquired by the material are reversible, once short-range
coagulation structure
is formed, the properties or the material become irreversible. The
reversibility at the
beginning of hydration and hydrolysis may be attributable to the long-range
coagulation
structure of the material which exhibits thixotropic properties allowing for
reverse
processes to occur under application of certain mechanical actions. In
contrast, in the
short-range coagulation structure, the formation of the initial crystalline
frame finalizes
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all mechanical transitions within the material hence preventing reverse
processes from
occurring. Thus, the crystallization strengthening process evolves from the
initial
conditions set at the time of short-range coagulation structure formation. As
these initial
conditions depend on the mechanical state of the material when its structure
still has a
long-range nature, mechanical transitions occurring at that time substantially
determine
the subsequent crystallization of the material and hence its strength.
[00011] The
liquid phase of the material is therefore an informative component
indicative of the porosity of the material and therefore of its strength.
Water (both in a
liquid and gaseous form) is always in a state of thermodynamic equilibrium
with the
porous solid phase with which it interacts. Thus, the properties of water are
changing in
strict accordance with structure formation and consequently with the strength
growth of
the hardening material.
[00012] The duration of the above hardening process is typically rather long.
For
example, in cementitious materials typical duration of hardening is of order
of one
month, at which time the cement passes through all the above states and
becomes a solid
structure of a given compressive strength. Due to the long duration of the
hardening
process, prior to reaching the final strength, the chemically active material
undergoes
many complicated physical and chemical processes, which can essentially affect
its
physical properties. It is recognized that any change, deviation and non-
observance of the
technological regulations during preparation of the chemically active
material, such as
ready-mixed or pre-cast concrete, may irreversibly reduce the properties
(e.g., strength)
of the final product.
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[00013] During hardening of the cement mix to the cement sheath, problems may
arise which weaken the structure of the cement sheath. One such problem is gas
channeling. When the cementitious slurry is first placed in the annulus of a
well,
hydraulic fluid exerts hydrostatic pressure on the sides of the well.
Initially the
hydrostatic pressure of the cement composition is great enough to ward off
naturally
occurring gases within the reservoir. As the cement mix goes through its
transition
stage from liquid to solid, it exerts less and less hydrostatic pressure on
the well. It is
in this transition stage that the hardening cement mix is susceptible to
formation gas
entering into the cement sheath. Gas entering into the cement sheath produces
pathways filled with gas. As the cement hardens, the pathways become channels
in the
hardened cement composition.
[00014] Another common problem encountered during hardening of the cement is
the loss of liquid fluid from the slurry into porous low pressure zones in the
formation
surrounding the well annulus. Fluid (liquid and/or gas) loss is undesirable
since it can
result in dehydration of the cementitious slurry. In addition, it may cause
the formation
of thick filter cakes of cement solids. Such filter cakes may plug the
wellbore. In
addition, fluid loss can damage sensitive formations. Minimal fluid loss is
desired
therefore in order to provide better zonal isolation and to minimize formation
damage
by fluid invasion.
[00015] It should be understood that the above-described discussion is
provided for
illustrative purposes only and the scope of the claims should not be limited
by the
preferred embodiments set forth in the examples, but should be given the
broadest
interpretation consistent with the description as a whole.
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_
[00016] With the expansion in drilling of offset wells, it is becoming
increasingly
important to ensure minimal disturbance to surrounding facilities during
drilling,
during cementing and after formation of the cement sheath. For instance, there
is an
increasing need to minimize fluid losses, minimize gas channeling and to
maintain the
integrity of the cement sheath. In addition, there is an increasing need to
prevent
compromises to the well which may be attributed to the lack of mud and cement
returns which cause problems with flow and control lines. Maximizing the
integrity of
the well to varying downhole conditions requires reliable monitoring.
Summary of the Disclosure
[00017] In an embodiment of the disclosure, a method of monitoring one or more
downhole conditions within a wellbore or the returns of a drilling mud is
provided
which comprises pumping into the wellbore a self-sensing treatment fluid
containing a
piezoelectric or piezoresistive material and assessing the electrical
resistivity of the
self-sensing treatment fluid containing the piezoelectric or piezoresistive
material
within the wellbore.
[00018] In another embodiment of the disclosure, a method of evaluating the
stability of a set cement within a wellbore in real time at downhole
conditions is
provided which comprises comprising monitoring the electrical resistivity of a
cement
hardened from a cementitious slurry containing a cement mix and at least one
piezoelectric or piezoresistive material or mixtures thereof.
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[00019] In another embodiment of the disclosure, a method of monitoring in
real time
the strengthening process of a cementitious slurry is set forth which
comprises pumping
into the wellbore penetrating the reservoir a self-sensing treatment fluid
containing a
piezoelectric or piezoresistive material and assessing the electrical
resistivity of the self-
sensing treatment fluid containing the piezoelectric or piezoresistive
material within the
wellbore.
[00020] In another embodiment of the disclosure, a method of monitoring in
real time
one or more downhole conditions within a wellbore is disclosed which comprises
pumping into the wellbore a cementitious slurry containing a cement mix and a
piezoelectric or piezoresistive material wherein the cementitious slurry is
hardened within
the wellbore to form a cement sheath, the method further comprising assessing
the
electrical resistivity of the cement mix or the hardened cement containing the
piezoelectric or piezoresistive material within the wellbore.
[00021] In still another embodiment of the disclosure, a method of
monitoring one or
more downhole conditions in a wellbore is disclosed which comprises pumping
into the
wellbore a cementitious slurry containing a piezoelectric or piezoresistive
material in an
amount sufficient for a monitoring device to measure the electrical
resistivity of a cement
set from the cementitious slurry: hardening the cement; and then assessing one
or more
downhole conditions using a monitor receptive to the piezoelectric or
piezoresistive
material within the set cement.
[00022] In still another embodiment of the disclosure, a method of
cementing a pipe or
casing in a wellbore is disclosed which comprises introducing into the
wellbore a
cementitious slurry comprising a piezoelectric or piezoresistive material,
wherein the
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piezoelectric or piezoresistive material is present in the cementitious slurry
in an amount
sufficient to be monitored, when the cementitious slurry has been hardened, by
a monitor
receptive to the piezoelectric or piezoresistive material; and allowing the
slurry to harden
to a solid mass.
[00023] In yet another embodiment of the disclosure, a method of monitoring
the
returns of a drilling mud or a cementitious slurry in a wellbore is provided
which
comprises monitoring the electrical resistivity of the returns .from a
drilling mud or a
cementitious slurry containing a piezoelectric or piezoresistive material.
Brief Description of the Drawings
[00024] In order to more fully understand the drawings referred to in the
detailed
description of the present disclosure, a brief description of each drawing is
presented, in
which:
[00025] FIG. 1 is a top view showing the arrangement of electrical and
thermocouple
leads on an insulator ring within a wellbore.
[00026] FIG. 2 depicts an illustrative method for measuring in real time
physical
properties affecting the integrity and stresses and/or strains placed on the
cement sheath
within a wellbore.
[00027] FIG. 3 illustrates a resistivity curve over time during hardening
of a
cementitious slurry containing a cement mix and a piezoelectric or
piezoresistive material
and illustrates the transition from the reversible stage to the irreversible
stage of the
hardening process.
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Detailed Description of the Disclosure
[00028] Characteristics and advantages of the present disclosure and
additional
features and benefits will be readily apparent to those skilled in the art
upon
consideration of the following detailed description of exemplary embodiments
of the
present disclosure and referring to the accompanying figures. It should be
understood
that the description herein and appended drawings, being of example
embodiments,
and the scope of the claims should not be limited by the preferred embodiments
set
forth in the examples, but should be given the broadest interpretation
consistent with
the description as a whole.
[00029] In showing and describing embodiments in the appended figures, common
or similar elements may be referenced with like or identical reference
numerals or are
apparent from the figures and/or the description herein. The figures are not
necessarily
to scale and certain features and certain views of the figures may be shown
exaggerated in scale or in schematic in the interest of clarity and
conciseness.
[00030] As used herein and throughout various portions (and headings) of this
patent application, the terms "disclosure", "present disclosure" and
variations thereof
are not intended to mean every possible embodiment encompassed by this
disclosure
or any particular claim(s). Thus, the subject matter of each such reference
should not
be considered as necessary for, or part of, every embodiment hereof or of any
particular claim(s) merely because of such reference.
CA 02815589 2013-05-10
[00031]
Certain terms are used herein and in the appended claims to refer to
particular
elements and materials. As one skilled in the art will appreciate, different
persons may
refer to an element and material by different names. This document does not
intend to
distinguish between elements or materials that differ in name. Also,
the terms
"including" and "comprising" are used herein and in the appended claims in an
open-
ended fashion, and thus should be interpreted to mean "including, but not
limited to. . . ."
Further, reference herein and in the appended claims to elements and
components and
aspects in a singular tense does not necessarily limit the present disclosure
or appended
claims to only one such component, materials or aspect. but should be
interpreted
generally to mean one or more, as may be suitable and desirable in each
particular
instance.
[00032]
Downhole conditions affecting the integrity of a wellbore and the returns of
drilling mud may be monitored in real time by pumping into the wellbore a self-
sensing
treatment fluid containing one or more piezoelectric or piezoresistive
materials and
measuring changes in electrical resistivity.
[00033] The self-sensing treatment fluid may be a drilling mud or a cement
mix. The
cement mix may be pumped into the wellbore as a cementitious slurry. The
piezoelectric
materials render significant piezoresistivity to the cementitious slurry or
drilling mud and
allows for the measurement of electrical resistance.
[00034] In an embodiment. downhole conditions. hardening of the cement, and
the
integrity' of a cement sheath may be monitored in real time by pumping into
the well a
treatment fluid which includes a piezoelectric or piezoresistive material and
measuring
changes in electrical resistivity. Monitoring in real time of resistivity
changes provides,
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for an accurate assessment of the stability of hardened cement within a
wellbore during
the life of the well. Thus, the integrity of the wellbore may be enhanced
during the
setting of the slurry as well as during the lifetime of the well.
[00035] The self-sensing treatment fluid may be a drilling mud or a cement
mix. The
cement mix may be pumped into the wellbore as a cementitious slurry. Inclusion
of
piezoelectric or piezoresistive material in the fluid render significant
piezoresistivity to
the cementitious slurry or drilling mud and allows for the measurement of
electrical
resistance. Thus, enhanced capabilities arc provided for the changing
conditions within
the wellbore which may affect drilling mud returns, fluid losses, gas
channeling and the
integrity of the hardened cement sheath.
[00036] Further, measuring resistivity changes attributable to the presence
of the
piezoelectric or piezoresistive material in hardened cement enables
identification within
the well of those locations where cemented material has been or is being
subjected to
high stress conditions. Thus, the disclosure provides a method of evaluating
the stability
of hardened cement as it is being exposed to downhole conditions by the use of
self-
sensing materials in cement mixes.
[00037] Monitoring of the electrical resistivity of the hardened cement. as
well as the
slurry as it is being pumped into the wellbore, provides the operator or
service provider
the ability to ascertain changes in stress and/or strain within the well
and/or temperaturc
variations which cause a change in the electrical resistivity. Such changes
reflect
conditions to which the cementitious slurry or set cement (as well as drilling
mud) in the
borehole are exposed. The method described may be practiced onshore as well as
offshore.
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[00038] Factors which may be monitored and/or interpreted in real time
include stress,
strain, temperature and/or chemical reactions within the wellbore. For
instance, a change
in strain, temperature or pressure within the wellbore or a change within the
wellbore
caused by a chemical reaction may change the proximity between the
piezoelectric or
piezoresistive materials, thus affecting the electrical resistivity.
[00039] The use of piezoelectric or piezoresistive materials in a cement
mix may
further be used to monitor the strengthening process of a slurry containing
the cement
mix during hardening within the wellbore. As such, real time changes may be
monitored
within the wellbore in the cementitious slurry prior to or during- hardening
of the slurry.
Thus, tailoring of the slurry may be maximized during pumping of tile slurry
at downhole
conditions within the wellbore.
[00040] Detection of the electrical resistivity of the piezoelectric or
piezoresistive
material may be assessed by monitoring the voltage produced by one or more
thermocouples placed at pre-determined locations on the casing within the
wellbore.
[00041] In an embodiment, electrical resistivity may be detected with
specially
designed casing placed within the well. For instance, a well casing may be
modified with
outside rings at set spacing for monitoring of changes in the electrical
resistivity and
temperature of cement materials.
[00042] Direct real-time observation of the location of the advancing
cementitious
slurry front in the borehole may also be determined as the slurry is being
pumped into the
wellbore and advances around the casing during the construction phase of the
well. By
monitoring the advancement of the fluid front, loss of fluids into the
formation may be
mitigated since changes may be made to the slurry.
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[00043] In still another embodiment, a method is disclosed for forecasting
the strength
of a cement mix pumped into the wellbore as a slurry during the hardening
stages of the
slurry. In addition, the hardening process of the cementitious slurry may be
monitored by
use of a self sensing treatment fluid, preferably at early hardening stages.
[00044] Thus, one can identify functional transitions between different
transitional
stages and using said functional transitions to determine a strengthening
state of the
chemically active material.
[00045] By assessing the hardness of a cement slurry using resistivity
measurements,
cement manufacturers and operators of cementing operations may optimize their
products, for example, by monitoring of the strengthening process and
adjusting the
various manufacturing steps in real-time, while the properties of the final
products are
still adjustable. Cement manufacturers and operators are thus provided
sufficient time to
adjust the preparation process according to needs before the cement mix enters
thc
irreversible hardening stages.
[00046] In addition, the presence of piezoelectric or piezoresistive
materials in a
cement mix may be used to predict thc stability and performance of the cement
mix at in-
situ downhole conditions prior to pumping a cement slurry into the well
containing the
cement mix.
[00047] As such, the disclosure provides a method for designing a cement
slurry
which will be suitable at downhole conditions by use of resistivity data of
cement mixes.
This resistivity data may be obtained from a learning set of cement mixes. As
such, the
stability and/or strength of the cement mix prior to being hardened within the
well and
prior to pumping a cement slurry containing the cement mix may be forecasted.
As such,
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the operator may forecast the strength of a cement mix at the well site prior
to pumping a
slurry containing the cement mix into the well.
[00048] In addition, the presence of a piezoresistive or piezoelectric
material in the
cementitious slurry enables monitoring of the slurry prior to as well as
during hardening
of the cement within the well. Real time monitoring of compressive strength
during the
hardening of the cementitious slurry provides the ability to ascertain the
transition time
between the slurry and the set cement.
[00049] Further, real time monitoring of changes in resistivity of the
cement sheath
during the life of a well can be used to determine if the cement shcath cracks
or has
cracked under temperature, pressure or other downhole conditions. Voids within
the
casing and cracks and other failures of the cement sheath may further be
located by
monitoring- the resistivity of the cement sheath. By accurately locating the
location of
cracks in the cement sheath, squeeze cement treatments may be more effectively
and
expeditiously performed.
[00050] Further, installation of the casing as well as cementing of the
casing to the
wellbore can be more closely' controlled since conditions within the wellbore
in real time
may be monitored.
[00051] In addition, rig costs may be decreased since real-time monitoring
of wellbore
conditions may more accurately assess waiting-on cement times which are used
to
determine the requisite time to resume operations. Such improvement may most
notably
substantially reduce the cost of operation in deepwater installation as well
as in the
maintenance of the well during deepwater operations.
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[00052] Monitoring of the temperature of the cementitious slurry in real
time during
placement of the slurry and hardening of the slurry further provides the
ability to
correlate actual temperature to pump rate and pressure during the cementing
operation
and monitoring of the heat of hydration of the cement and its effect on the
properties of
set cement.
[00053] Real time properties and parameters of a set cement and conditions
downhole
may be determined by monitoring the electrical resistivity of the
piezoelectric or
piezoresistive materials within the set cement. These piezoelectric or
piezoresistive
materials are present in the slurry which hardened into the set cement. Real
time
monitoring of hardened cement may occur at any point during the lifetime of
the well.
Thus, casing performance over the service life of the well may be performed.
Since the
properties of the hardened cement may be monitored during the service life of
the
wellbore, maintenance of the well may be vastly improved.
[00054] Real time monitoring further prevents fluid movement between zones by
enhancing selective perforation of a zone of interest. For example, real time
monitoring
as described herein may be used to prevent fluid movement between oil
producing zone
and a water producing zone and to prevent any communication between one zone
and the
other.
[00055] Further, the drilling of a well may be monitored in real time by
measuring the
electrical resistivity of the mud having an incorporated piezoelectric or
piezoresistive
material. The monitoring of mud returns minimizes or eliminates problems which
may
be encountered in flow lines and control lines within the well.
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[00056] Real
time variations may be monitored downhole during drilling of the well.
Such real time variations may be seen from changes in the drilling mud as well
as in
returns. With the sensing capabilities within the drilling mud, it is possible
to monitor the
front of the drilling mud as it advances during drilling. By monitoring the
advancement
of the fluid front, loss of fluids may be mitigated by changing the
constituency of the
mud.
[00057] Thus,
monitoring the advancing front of the drilling mud and/or cementitious
slurry provides better control of fluid losses in various rock formations and
minimizes
fluid loss.
[00058] The
piezoelectric or piezoresistive material for inclusion in the drilling mud
and/or cementitious slurry arc sensitive to the changes in the composition of
the mud or
slurry, sensitive to the changes chemical reactions which may occur within the
mud or
slurry, sensitive to changes in the downhole environment including temperature
conditions and pressure conditions and/or sensitive to stress conditions
downhole.
[00059] In a
preferred embodiment, the drilling mud or cementitious slurry contains a
piezoelectric or piezoresistive material which may be influenced by downhole
strain
and/or stress. Strain and/or stress typically occur within the wellbore during
setting of'
the slurry as well after the cement has been set. Strain within the wellbore
changes the
proximity between piezoelectric members and thus affects the electrical
resistivity.
[00060]
Resistivity in the stress and transverse directions increases upon tension and
decreases upon compression. With increased tension, distance between the
piezoelectric
or piezoresistive materials increases as resistivity increases. With
decreased
compression, the distance between the piezoelectric or piezoresistive
materials decrease
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as resistivity decreases. The cement containing the piezoelectric material is
thus capable
of sensing its own strain due to piezoresistivity, i.e., the effect of strain
on the electrical
resistivity.
[00061] Real time monitoring using piezoelectric or piezoresistive
materials further
enables identification of the location of a sheath of hardened cement. In
addition, the
length of the cement sheath supporting the casing within the wellbore may be
determined
by measuring the electrical resistivity of piezoelectric or piezoresistive
materials within
the cement. The integrity of the cement sheath and the location of possible
bonding or
cement sheath failure may be determined by measuring the piezoresistive
response along
the length of the wellbore. Placement of the monitoring device along pre-
determined
locations makes it possible to identify the type or location of potential
cross flow between
zones of the formation through the cement matrix or channels. Determination of
the
length of the set cement supporting the casing is often important prior to
further drilling.
[00062] The use of piezoelectric or piezoresistive materials in real time
monitoring is
especially advantageous in those instances where cement returns are not
returned to the
surface such that the uppermost portion of the cement sheath is difficult to
accurately
locate. The location or the set cement may be determined by monitoring the
changes in
resistivity as the slurry sets downhole and water within the slurry decreases.
As such,
lost circulation of fluids during the return of cement up into the annulus may
be readily
identified and the height of the cement column may be elevated.
[00063] In addition, changes in resistivity may be used to determine if the
cement
sheath cracks under temperature, pressure or other downhole conditions.
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[00064] Locating voids within the casing and cracks and other failures of
the cement
sheath may further be addressed by monitoring the well in real time. By
accurately
locating the location of cracks in the cement sheath, squeeze cement
treatments may be
more effectively and expeditiously performed.
[00065] Suitable piezoelectric or piezoresistive materials include quartz,
tourmaline.
ceramics, conductive fibers and polymeric materials.
[00066] Exemplary ceramics include zirconate titanate, barium titanate,
lead niobate
and silicon carbide.
[00067] Suitable polymers include those of asymmetric structure, such as
polyvinylidene fluoride as well as anti-static plastics.
[00068] Exemplary conductive fibers include carbon fibers, carbon black and
metal
particles, such as steel. The length of the fibers is typically between from
about 5-60 !_tm
and are typically are no greater than 15 1..tm. With conductive fibers,
resistivity in the
stress and transverse directions increase upon tension as the fibers are
pulled out from the
less conductive cement matrix.
[00069] The amount of piezoelectric or piezoresistive materials in the
cementitious
slurry or drilling mud must be an amount sufficient to impart to the drilling
mud, slurry
or set cement a measurable change upon exposure to variations in physical
parameters as
well as to downhole variations in stress. strain and/or compression.
Typically, the
amount of piezoelectric or piezoresistive material is present in the
cementitious slurry
between from about 0.1% to about 10% BWOC. preferably from about 0.1% to about
8%
BWOC. In a drilling mud, the amount of piezoelectric or piezoresistive
material may be
between from about 0.1% to about 5%.
19
CA 02815589 2013-05-10
[00070] During construction of the well, the casing may be modified with
outside
rings at predetermined locations to monitor the changes in the electrical
resistivity and
temperature of the drilling mud and cementing material stabilizing the casing
and
borehole. FIG. 1 illustrates a top view of a wellbore wherein electrical and
thermocouple
leads A, B. C, and D are held within receptors on insulator ring 1 placed
within the
wellbore. Each of the leads is used to measure a physical property, such as
stress (A),
strain (B), temperature (C) and chemical reaction (D). The monitoring ring
with
electrical lead and thermocouples may be used to monitor the changes in the
resistance
and temperature in the slurries.
[00071] Instruments for monitoring detection in changes in resistivity may
be located.
on a surface platform. Such instruments may continuously monitor such changes
or be
used intermittently. External transducers on electronic indicators with cables
running to
the surface platform to capture the data. The cables. thermocouples and
electrical
external casing devices may be installed onto the casing assembly at the
surface platform
while the casing is running into the wellbore (before the slurry is pumped
downhole).
The change in electrical resistance may be measured by the gage factor,
defined as the
fractional change in resistance per unit strain.
[00072] FIG. 2 is a schematic of a deepwater producing well wherein
multiple
insulator rings 1, 2, and 3 contain electrical and thermocouple leads A, B. C
and D placed
at predetermined locations within the well. Each of the leads is extended to a
monitoring
device. (FIG. 2 shows, for illustration purposes, thermocouple leads A and C
on insulator
ring 1 being fed into monitoring device 5 and monitoring device 6 though each
of the
leads may be fed into the same monitoring device and multiple monitoring
devices may
CA 02815589 2013-05-10
be used for reading other conditions within the wellbore. FIG. 2 shows
monitoring
device 5 for reading stress and monitoring device 6 for reading temperature
from
thermocouples on insulator rings placed at predetermined locations within the
well.)
Impedance spectroscopy 8 may be used to provide real time monitoring of
physical
properties.
[00073] The piezoresistive response may be measured by other methods than
those
illustrated in FIG. 2 and FIG. 3. For instance piezoresistive response may be
measured
by using specially modified centralizers which are able to measure and
transmit signals.
[00074] Assessment of cement hardness may further make use of the Nernst-
Einstein
equation for resistivity measurements. Since the liquid phase is in
thermodynamic
balance with the solid phase, which adsorbs or absorbs it, at an early stage,
the physically
bound water assures the physical mechanical properties of the mix. Since the
liquid
phase is formed during the initial stage of hardening, the hardness assessment
can be
done significantly faster compared to the standard technique used in the
industrial
applications.
[00075] The Nemst-Einstein equation establishes a linear correlation between
structural electrical resistivity. R*sTR, and viscosity, n, and represents the
grounding for
the application of resistivity measurements to the express cement hardness
assessment:
wherein k is some substance-specific coefficient, depending on physicochemical
characteristics of a substance.
[00076] In an
embodiment, a teaming set (or etalon) of resistivity measurements for a
range of cement mixes may be obtained at pressure and temperature ranges which
may be
21
CA 02815589 2013-05-10
expected under downhole conditions. Experimental values may be determined by
use of
a specialized cell. In a preferred embodiment, the measurement of the physical
parameter is preferably substantially in a constant volume. A constant volume
of the
slurry during the measurement ensures that any functional dependence of the
measured
parameter can be attributed to the strengthening of the material, rather than
to its volume.
Each sample within the learning set is further characterized by some hardness.
[00077] FIG. 3 illustrates a resistivity curve over time during hardening
of the cement
slurry and illustrates the transition from the reversible stage to the
irreversible stage when
a short-range coagulation structure was formed. As illustrated, the cement was
seen to set
at 3000 ohmm after 35 hours. Transitional points may be identified by
calculating a
derivative of the time-dependence and finding zeros on the curve. Generally.
such
transitional points may be characterized by a sign inversion of an nth
derivative of time-
dependence, where n is a positive integer. Inversion of the measurements of
the learning
set may then be used to ascertain the hardness of the set cement downhole at a
desired time
or during a desired time interval, typically during the first hours (such as
10 hours).
[00078] The data obtained from the teaming set may be used to calculate a
residual
norm A, in each node of the three-dimensional (pressure, temperature and time)
parameters space and parameters corresponding to the node may then be
selected,
providing the minimal value of
1' ¨ '
A ¨ _____________________________ k )2
where S,':,S1 are experimental and reference resistivity data corresponding to
the test
cement and learning set correspondingly. NT is the number of time discretes
used in the
CA 02815589 2015-03-16
analysis. The hardness of the test cement may then be estimated as to be the
same as the
hardness of the etalon sample from the learning set, providing the best fit of
experimental
data.
[00079] The disclosed method provides a method to monitor the strengthening
process
of the cementitious slurry during the stage in which the properties of the
slurry may be
adjusted prior to hardening of the slurry, according to the needs of the
specific
application for which the cementitious slurry is being used. Once the cement
slurry
passes the reversible stage it enters the irreversible stages of hardening, in
which control
on the quality of the set cement is often very limited. During the
irreversible hardening
stages, the compressive strength, R, as a function of time T, has a
logarithmic shape:
R log(r)
= õ
log(n)
wherein Rõ denotes the compressive strength of the material at time r = n.
[00080] By
measuring resistivity change of the slurry while the slurry is still within
the
liquid phase, porosity and strength of the material may be assessed.
[00081] The scope of the claims should not be limited by the preferred
embodiments set forth in the examples, but should be given the broadest
interpretation
consistent with the description as a whole.
23
