U.S. patent application number 09/159131 was filed with the patent office on 2001-08-30 for method of maintaining the integrity of a seal-forming sheath, in particular a well cementing sheath.
Invention is credited to THIERCELIN, MARC J..
Application Number | 20010017209 09/159131 |
Document ID | / |
Family ID | 9511373 |
Filed Date | 2001-08-30 |
United States Patent
Application |
20010017209 |
Kind Code |
A1 |
THIERCELIN, MARC J. |
August 30, 2001 |
METHOD OF MAINTAINING THE INTEGRITY OF A SEAL-FORMING SHEATH, IN
PARTICULAR A WELL CEMENTING SHEATH
Abstract
A method of maintaining the integrity of a sheath, in particular
a cementing sheath in a well consists in calculating or estimating
variations in well pressure and/or in well temperature and/or in
the variations in in-situ stresses, which may occur during the
lifetime of the well, evaluating the stresses in the sheath as a
function of the above variations, determining the nature of the
stress likely to be first in causing deterioration of the sheath,
and the risk thereof, and evaluating the influence of the elastic
properties of the sheath, of the rock and/or of the casing on this
stress, in order to select a sheath which is capable of attenuating
this deterioration. The method is of particular application to oil,
water, gas, and geothermal wells.
Inventors: |
THIERCELIN, MARC J.;
(D'AURAY, FR) |
Correspondence
Address: |
SCHLUMBERGER TECHNOLOGY CORPORATION
IP DEPT., WELL STIMULATION
110 SCHLUMBERGER DRIVE, MD1
SUGAR LAND
TX
77478
US
|
Family ID: |
9511373 |
Appl. No.: |
09/159131 |
Filed: |
September 23, 1998 |
Current U.S.
Class: |
166/285 ;
166/250.14; 166/253.1 |
Current CPC
Class: |
E21B 33/14 20130101 |
Class at
Publication: |
166/285 ;
166/250.14; 166/253.1 |
International
Class: |
E21B 033/13; E21B
047/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 23, 1997 |
FR |
97 11821 |
Claims
Claims:
1. A method for maintaining the integrity of a sheath forming a
seal, in particular a cementing sheath, positioned around a metal
casing for an oil, gas, water, geothermal or analogous well in
rock, the sheath being subjected to mechanical and/or thermal
stresses during the lifetime of the well which can cause a risk of
cracking of the sheath by failure in tension or in shear, or by
detachment of the sheath at the casing- sheath and/or sheath-rock
interfaces, for example, the method being characterized in that it
consists in: (a) calculating or estimating pressure and/or
temperature variations in the well and/or variations in in-situ
stresses, which can occur during the lifetime of the well; (b) for
a given sheath, evaluating the various stresses which will be
applied to that sheath in particular as a function of the
variations defined above and taking into account the geometrical
characteristics of the well and of the casing, and also the
mechanical properties of the rock; (c) from the above evaluation of
the various stresses, determining the nature of the stress which is
likely to cause sheath deterioration in the first instance; (d)
evaluating the influence of the mechanical and/or physical
properties of the sheath, the rock and/or the casing on the
above-defined stress; (e) selecting a sheath with mechanical and/or
physical properties which are likely to attenuate the effects of
the above-defined stress; and (f) positioning the sheath as
selected in this way around the well casing.
2. A method according to claim 1, further comprising selecting a
sheath for which the ratio between its tensile strength and its
Young's modulus is as high as possible.
3. A method according to claim 2, further comprising taking the
elastic properties of the rock into account, and selecting a sheath
with a Young's modulus which is lower than the Young's modulus of
the rock.
4. A method according to claim 3, further comprising placing the
sheath in compression while it is being positioned around the well
casing.
5. A method according to any one of claim 1, further comprising
calculating the expansion of the sheath which is necessary to avoid
detachment of the sheath at the sheath-rock and/or sheath-casing
interfaces.
6. A method according to claim 1, further comprising increasing the
thickness of the well casing to limit its deformation when the well
pressure increases.
7. A method according to claim 1 further comprising prior to
injecting a fluid vapour into a formation traversed by the well to
stimulate production, controlling the temperature increase in the
well to attenuate the effects of temperature on the casing.
Description
[0001] The present invention relates to a method for maintaining
the integrity of a seal-forming sheath, in particular a cementing
sheath, positioned around a metal casing for an oil, gas, water,
geothermal or analogous well.
[0002] An oil, water or gas field is usually exploited via a well
into which a metal casing has been inserted and held in place by a
cement sheath to fill the space or annulus between the casing and
the borehole. The cementing operation, i.e. putting the sheath into
position consists in injecting a cement slurry into the casing to
cause the drilling mud in particular to rise up and be evacuated
via the annulus which is then gradually filled with the slurry.
After the slurry has set and hardened, a cement sheath is obtained
which prevents any fluid communication between the various
formations through which the well passes, and which acts as a
support for the metal casing.
[0003] Well-cementing is an operation that is very difficult
because it requires several parameters to be taken into
consideration and kept under control. For example, a slurry with
too high a density can cause the rock to fracture, while a slurry
with too low a density can cause external fluids to intrude. While
slurry density is a parameter which is relatively easy to control,
this is not true of its Theological properties. Such problems,
which are inherent to any well-cementing operation, are well known
to the skilled person, and solutions generally consist in adding
various additives to the slurry, the selection of which is not
always clear and varies from one well to another.
[0004] However, even in a situation where this cementing operation
is carried out under good conditions to obtain a sheath which seals
and supports once the slurry has set and hardened, it is not long
before that sheath is subjected to mechanical and/or thermal
stresses which can cause the sheath to deteriorate, and this can
culminate in well operating conditions being put into doubt.
[0005] Such problems linked to sheath deterioration over the
lifetime of the well are not novel in themselves and are well known
to the skilled person, but up until now no practical approach has
been made to attempt to provide a solution to such problems.
[0006] A principal aim of the invention is to analyse more
precisely the mechanical and/or thermal stresses to which the
sheath may be subjected during the lifetime of the well, the
effects of these stresses and the influence of mechanical and/or
physical parameters of the cement, the casing and/or the rock on
these stresses, to obtain a solution which can clearly answer these
problems of sheath deterioration.
[0007] To this end, the invention provides a method which is
characterized in that it consists in:
[0008] calculating or estimating pressure and/or temperature
variations in the well and/or variations in in-situ stresses, which
can occur during the lifetime of the well;
[0009] for a given sheath, evaluating the various stresses which
will be applied to that sheath, in particular as a function of the
variations defined above and taking into account the geometrical
characteristics of the well and of the casing, and also the
mechanical properties of the rock;
[0010] from the above evaluation of the various stresses,
determining the nature of the stress which is likely to cause
sheath deterioration in the first instance;
[0011] evaluating the influence of the mechanical and/or physical
properties of the sheath, the rock and/or the casing on the
above-defined stress;
[0012] selecting a sheath with mechanical and/or physical
properties which are likely to attenuate the effects of the
above-defined stress; and
[0013] positioning the sheath as selected in this way around the
well casing.
[0014] In general, analysis of the data obtained by modelling and
which has served as a basis for the definition of the method of the
invention has served to identify three main types of deterioration
which can damage the sheath, namely cracking due to failure in
tension or in shear, or detachment at the interfaces with the
casing and the sheath.
[0015] An analysis of the influence of the mechanical and/or
physical properties of the sheath, of the casing and/or of the rock
on these types of deterioration has enabled the method of the
invention to be refined to attenuate the risk of these types of
deterioration occurring.
[0016] Thus, in accordance with two further characteristics of the
invention, the method includes:
[0017] taking the elastic properties of the sheath into account,
and selecting a sheath for which the ratio between its tensile
strength and its Young's modulus is as high as possible, and/or
[0018] also taking the elastic properties of the rock into account,
and selecting a sheath with a Young's modulus which is lower than
the Young's modulus of the rock.
[0019] With such provisions, the method can attenuate the risk of a
crack occurring in the sheath, in particular as a result of an
increase in well pressure and/or temperature.
[0020] If well pressure increases, the method can also include
increasing the thickness of the casing to limit its
deformation.
[0021] If well temperature increases, the method can also include
controlling the increase in temperature to attenuate the effects on
the sheath.
[0022] In a further feature of the invention, the method also
includes placing the sheath in compression while it is being
positioned around the well casing.
[0023] With such an arrangement, the method can also attenuate the
risk of sheath detachment occurring, in particular following a
reduction in pressure at the sheath-rock interface.
[0024] In general, the experimental data as obtained numerically
and/or mathematically on studying the risks of the cement sheath
failing under tension or shear and the risk of sheath detachment at
the casing-sheath and sheath-rock interfaces as a result of the
mechanical and thermal stresses to which the sheath will be
subjected during the lifetime of the well have led to the discovery
that these risks can all be substantially attenuated, in particular
by adjusting the elastic properties of the cement.
[0025] Thus this data has led to the development of a method which
can be used to define the properties required for the sheath, in
particular its elastic properties, before proceeding to position it
around a well casing.
[0026] Cements for cementing sheaths which have the required
properties after setting and hardening of the cement slurry are
currently selected essentially by adjusting the rheological
properties of the slurry. This means defining numerous slurry
compositions.
[0027] Under such conditions, the method of the invention can also
be used as a tool to test slurry compositions and determine, for a
given well, their ability to withstand the strains of various
mechanical and/or thermal stress systems to which the cementing
sheath will be subjected during the lifetime of the well.
[0028] An important advantage of the invention is that carrying out
the method does not require the well to be equipped with additional
technical means to protect the cement sheath.
[0029] Further characteristics, advantages and details of the
method of the invention become apparent from the description below
which is made with reference to the accompanying drawings, given by
way of example for a cementing sheath and in which:
[0030] FIGS. 1 to 4 are graphs of the stresses to which the cement
sheath is subjected during an increase in well pressure, and the
influence of the elastic properties of the cement and rock on the
tensile strength required for the cement to avoid failure under
tension in the sheath;
[0031] FIGS. 5 to 9 are graphs of the stresses to which the cement
sheath is subjected during an increase in the temperature in the
well and the influence of the elastic properties of the cement on
the tensile strength required for the cement to avoid failure under
tension in the sheath;
[0032] FIGS. 10 to 13 are graphs of the stresses to which the
cement sheath is subjected during an increase in the pressure at
the sheath-rock interface, and the influence of the elastic
properties of the cement on these stresses; and
[0033] FIGS. 14 to 16 show graphs of the stresses to which the
cement sheath is subjected during a reduction in the well pressure,
and the influence of the elastic properties of the cement on these
stresses.
[0034] In general, the cement sheath of a well is subjected to
mechanical and/or thermal stresses over time which can be resolved
into tangential, axial and radial stresses which are in extension
or compression.
[0035] The assumption made in the study which was carried out on
these stresses was that the axial stresses are practically zero,
and essentially only the tangential and radial stresses in a plane
perpendicular to the well axis were considered.
[0036] As indicated in the preamble, an analysis of these stresses
and the data recorded during the study have enabled three principal
types of deterioration to be determined which can damage the
integrity of the cement sheath during the lifetime of the well.
[0037] I. The first type of deterioration is a risk of tension
failure of the sheath with the appearance and propagation of radial
cracks in the cement which can result in particular from an
increase in well pressure or temperature.
[0038] This type of tension failure of the sheath is essentially
caused by the action of tangential stresses which are in extension,
while the radial stresses are in compression. Since the tensile
strength of a cement is always substantially lower than its
compressive strength, the tangential stresses will be the first to
cause possible cracking of the cement.
[0039] A. An increase in well pressure can occur when drilling a
new section of the well, during leakage tests, during casing shoe
tests, when perforating the casing and when stimulating the
formation or the reservoir by hydraulic fracturing. Such a pressure
increase can be as high as 30 MPa to 40 MPa.
[0040] With reference to FIGS. 1 to 4 based on the study data, the
stress conditions in the cement are examined below for an increase
of the order of 6.90 MPa in well pressure.
[0041] Consider a well with the following characteristics:
[0042] borehole diameter: 215.9 mm;
[0043] external diameter of casing: 117.8 mm;
[0044] internal diameter of casing: 152.5 mm;
[0045] gross casing weight: 52 kg/m;
[0046] Young's modulus of casing 200 GPa, of cement 5 GPa, and of
rock 10 GPa.
[0047] The tests were carried out using a slurry formulated with
Holnam H C4474 cement with the following composition (gal/sk=3.78
liters (1) per 94 pound (lb) (42.6 kg) sack of cement, namely 1
gal/sk=0.0881 of additive per kg of cement; 1 ppg=0.1198
g/cm.sup.3. The quantity of water is given as the percent by weight
with respect to the weight of cement.
1 Latex Stabiliser Dispersing Retarding Anti-foam D600 D135 agent
D80 agent D801 D144 Water Density (gal/sk) (gal/sk) (gal/sk)
(gal/sk) (gal/sk) (%) (ppg) Porosity 0 0 0.060 0.070 0.03 37.78
16.4 55.41 1 0.1 0.03 0.02 0.03 28.87 16.4 49.24 2 0.2 0.045 0.02
0.03 19.3 16.4 43.23 3 0.3 0.075 0.015 0.03 9.67 16.4 37.30 4 0.4
0.15 0 0.03 1.45 16.2 33.19
[0048] D600, D135, D80, D801 and D144 are additives sold by
Schlumberger Dowell.
[0049] The stress conditions in the cement were calculated assuming
the cement, the casing steel, and the rock to be thermoelastic or
poroelastic materials and the cement/rock and cement/casing
interfaces to be complete or non-existent. Further, once setting
had occurred, internal stresses in the cement were assumed to be
absent.
[0050] The risk of failure of the cement could be analysed by means
of the Mohr-Coulomb criterion which states that the stress .tau.
tending to cause failure is limited by the cohesion of the material
and by a constant which is analogous to the internal coefficient of
friction multiplied by the normal stress .sigma..sub.n exerted in a
plane perpendicular to the plane of failure.
[0051] FIGS. 1 and 2 show the radial stress conditions (FIG. 1) and
the tangential stress conditions (FIG. 2) in the sheath as a
function of the distance from the well axis, i.e., between the
casing-sheath interface and the sheath-rock interface.
[0052] Examination of these two FIGS. 1 and 2 shows:
[0053] that the radial stresses are in compression;
[0054] that the tangential stresses are in extension; and
[0055] that the tangential stress in extension is at its highest at
the casing-sheath interface.
[0056] Thus it is the tangential stress in extension as applied at
the casing-sheath interface that makes it possible to determine the
tensile strength which the cement must possess in order to avoid
the appearance and propagation of radial cracks.
[0057] The influence of the elastic properties of the sheath and of
the rock on the tensile strength required for the cement are
examined below.
[0058] FIG. 3 shows the variations in the values of this tensile
strength as a function of the Young's modulus of the cement for
various values of the Young's modulus of the rock. Curves C1 to C5
correspond to values of rock Young's modulus which are of the order
of 1 GPa, 5 GPa, 10 GPa, 20 GPa and 30 GPa respectively.
[0059] An examination of each of curves C1 -C5 shows that the
tensile strength required for the cement increases with the value
of its Young's modulus.
[0060] Now, although the study data also shows that the tensile
strength of the cement increases with the value of its Young's
modulus, it must not be concluded that a cement with a high tensile
strength will be more resistant than a more flexible cement with a
lower tensile strength.
[0061] In fact, curves C1 -C5 show that the tensile strength
required for the cement diminishes with the Young's modulus of the
rock, i.e., when the cement is more flexible than the rock, the
rock acts as the mechanical support.
[0062] As an example, a cement obtained from a slurry with the
composition given above has a Young's modulus of the order of 7800
MPa, and a tensile strength of the order of 4 MPa, shown at point A
in FIG. 3. By adding an additive such as a styrene-butadiene type
latex to this cement slurry in the following proportions: 2 gps
(point B), 3 gps (point C) and 4 gps (point D), the cement is
rendered more flexible and its Young's modulus and tensile strength
are reduced.
[0063] Considering a rock with a Young's modulus of 10 GPa (curve
C3), the tensile strength of cements A and B will be insufficient
to avoid tension failure of the sheath on increasing the well
pressure by a value of the order of 6.90 MPa. In contrast, the
tensile strength of cements C and D will be sufficient to avoid
tension failure since points C and D are above curve C3.
[0064] FIG. 4 is analogous to FIG. 3 but for a casing of lower
weight. It can be seen that the slopes of curves C1-C5 in FIG. 4
are steeper than the corresponding curves in FIG. 3, i.e., the
tensile strengths required for the cement increase because the
casing undergoes greater deformation under the action of an
increase in well pressure.
[0065] In general, the data from the studies also shows that the
tensile strengths required for the cement vary substantially
linearly with the increase in well pressure, the value of these
tensile strengths being multiplied by two when the pressure
increase doubles.
[0066] An examination of the preceding figures also shows that the
tangential stresses become more and more compressive when the
Young's modulus of the cement is very low and the Young's modulus
of the rock is very high. Under these particular conditions, the
risk of failure of the sheath under tension is substantially
reduced.
[0067] The study data has demonstrated that the risk of failure of
the cement sheath under tension as a result of an increase in well
pressure is attenuated:
[0068] if the ratio between the tensile strength of the cement and
its Young's modulus is as high as possible; and/or
[0069] if the Young's modulus of the cement is lower than the
Young's modulus of the rock; and/or
[0070] if the thickness of the casing is increased.
[0071] B. An increase in well temperature can occur, in particular
during production of formation fluids, in which case it can reach a
value of about 100.degree. C., and during injection of steam into a
formation to stimulate production, in which case it can reach a
value of about 300.degree. C.
[0072] The study was carried out considering the following
parameters:
2 casing cement rock solid density (kg/m.sup.3) 8000 1900 2100
specific heat (5 Jkg.sup.-1K.sup.-1) 500 2100 1900 thermal
expansion coefficient (K.sup.-1) 1.3 .times. 10.sup.-5 1.3 .times.
10.sup.-5 1.3 .times. 10.sup.-5 thermal conductivity (W/mK) 15 1
1
[0073] The stress conditions in a cement with the above
characteristics are examined below using FIGS. 5 to 9 for an
increase of 55.6.degree. C. in the well temperature.
[0074] FIGS. 5 and 6 show the radial stress conditions (FIG. 5) and
the tangential stress conditions (FIG. 6) in the sheath as a
function of distance from the well axis, measurements being made
100 seconds after increasing the well temperature.
[0075] These two figures show:
[0076] that the radial stresses are in compression (FIG. 5);
[0077] that the tangential stresses are in compression towards the
casing-sheath interface and in extension towards the sheath-rock
interface (FIG. 6); and
[0078] that the tangential stress in extension is highest at the
sheath-rock interface.
[0079] Thus it is the tangential stress in extension located in the
majority of cases at the sheath- rock interface that determines the
value of the tensile strength required for the cement to avoid the
appearance and propagation of radial cracks.
[0080] The influence of the elastic properties of the sheath on the
tensile strength required for the cement are examined below.
[0081] FIGS. 7 and 8 show the variations in the value of this
tangential stress in tension at the sheath-rock interface as a
function of the time after the temperature increase. The curves in
FIGS. 7 and 8 correspond to Young's modulus values for the cement
of 10 GPa and 5 GPa respectively.
[0082] An examination of these two FIGS. 7 and 8 shows that a
cement with a low Young's modulus is more resistant than a cement
with a high Young's modulus. The tangential stress reaches a value
of the order of 8.97 MPa in FIG. 7 for a Young's modulus of the
rock of the order of 10 GPa, while this tangential stress only
reaches a value of the order of 1.3 MPa in FIG. 8 for a Young's
modulus of the order of 5 GPa.
[0083] These results are similar to those observed when studying an
increase in well pressure, namely that the rock constitutes a
mechanical support for the sheath when the Young's modulus of the
rock is higher than the Young's modulus of the cement.
[0084] FIG. 9 shows the variations in tensile strength required for
the cement to be able to resist a tension failure as a function of
the Young's modulus of the cement and for an increase of the order
of 111.2.degree. C. in the temperature for a given well, at a given
depth and for a given type of rock. FIG. 9 shows seven points A to
G which correspond to cements of increasing flexibility. An
examination of FIG. 9 shows that cement G which is the most
flexible is the only cement capable of avoiding tension failure of
the sheath under the conditions envisaged above.
[0085] The data demonstrates that the risk of tension failure of
the cement sheath as a result of an increase in well temperature is
attenuated:
[0086] if the ratio between the tensile strength of the cement and
its Young's modulus is as high as possible; and/or
[0087] if the Young's modulus of the cement is lower than the
Young's modulus of the rock.
[0088] Further, this risk of tension failure of the sheath can be
greatly reduced if the temperature rise can be controlled to reduce
the effects of temperature on the sheath, which is possible when
injecting steam into the formation to increase its production.
[0089] In general, the tangential stresses in extension have been
shown to be the first to deteriorate the sheath during an increase
in well pressure or temperature. However, this deterioration in the
sheath can be followed by further deterioration caused by the
action of the radial stresses which are in compression, in
particular in the case where the pressure increase in the well
persists.
[0090] II. The second type of deterioration is a risk of shear
failure of the sheath which can occur as a result of creep or
compacting of the formation, or a drop in pore pressure in the
formation which may result from overall in-situ stress conditions
becoming less compressive.
[0091] In general, all of these phenomena result in particular in
an increase in the pressure, i.e., the radial stress at the
sheath-rock interface.
[0092] The stress conditions in the cement for an increase of the
order of 6.90 MPa in the pressure at the sheath-rock interface are
examined below by considering a well with the geometrical
characteristics defined above, and referring to FIGS. 10 to 13
which are drawn up from the study data.
[0093] FIGS. 10 and 11 show the radial stress conditions (FIG. 10)
and the tangential stress conditions (FIG. 11) in the sheath as a
function of the distance from the well axis, i.e., between the
casing-sheath interface and the sheath-rock interface.
[0094] An examination of these two FIGS. 10 and 11 shows:
[0095] that the radial and tangential stresses are in compression;
and
[0096] that the maximum value for the tangential stresses and the
minimum value for the radial stresses are at the casing-sheath
interface, the sheath having its highest probability of shear
failure at this interface.
[0097] The influence of the elastic properties of the sheath on the
compressive strength required for the cement is examined below.
[0098] FIG. 12 shows the variations in the radial stresses (curve
C1) and tangential stresses (curve C2) in the sheath as a function
of the Young's modulus of the cement, at the casing-sheath
interface.
[0099] An examination of FIG. 12 shows:
[0100] that the value of the radial stresses reduces with the
Young's modulus of the cement, these stresses becoming more and
more compressive;
[0101] that the value of the tangential stresses increases with the
Young's modulus of the cement, these stresses becoming less and
less compressive; and
[0102] that as a result, the sheath acts as a mechanical support
for the casing by reducing the value of the stresses which are
applied thereto.
[0103] For a well of larger diameter, i.e., if the thickness of the
sheath is increased, the data shows that that has no notable effect
on the radial stresses which are exerted at the casing-sheath
interface.
[0104] FIG. 13 shows the variations in the compressive strength
required for the cement to avoid shear failure, as a function of
the Young's modulus of the cement and for an increase of the order
of 70 MPa in the pressure at the sheath-rock interface. The failure
criterion used was the Mohr-Coulomb type criterion, knowing that
cements have an internal angle of friction of the order of
30.degree..
[0105] As an example, a cement obtained from a slurry with the
composition defined above has a Young's modulus of the order of
7800 MPa and a compressive strength of the order of 35 MPa, which
is shown as point A in FIG. 13. By adding an additive such as a
styrene-butadiene type latex to the cement slurry in the following
proportions: 2 gps (point B), 3 gps (point C) and 4 gps (point D),
the cement was rendered more flexible and its Young's modulus and
compressive strength were reduced.
[0106] Thus cements A, B, C and D have compressive strength which
is largely sufficient to avoid shear failure of the sheath under
the conditions defined above.
[0107] In general, a rigid cement will resist a compressive stress
better, but a cement with a ratio between its compressive strength
and its Young's modulus which is as high as possible will also be
satisfactory.
[0108] III. The third type of deterioration is a risk of detachment
of the sheath at its interface with the casing and/or the rock.
[0109] Such detachment can result from:
[0110] a reduction in the pressure inside the well when the density
of the drilling mud used to drill a new section of the well is
reduced or when the pore pressure in the reservoir increases;
or
[0111] a reduction in the temperature in the well or in the
pressure at the sheath-rock interface during injection of a cold
fluid into the formation during hydraulic fracturing, for
example.
[0112] In general, the tangential stresses become compressive,
while the radial stresses are more and more in extension and can
cause the sheath to become detached.
[0113] A reduction in well pressure can be treated as the
application of a radial stress in extension at the casing-sheath
interface. Under these conditions, the radial and tangential stress
conditions are generally similar to those shown in FIGS. 1 and 2
for an increase in well pressure, but with the opposite sign.
[0114] In other words:
[0115] the radial stresses are in extension with a maximum value at
the casing-sheath interface, which can cause the sheath to become
detached at this location; and
[0116] the radial stresses are also in extension at the sheath-rock
interface, which can also cause the sheath to become detached at
this location.
[0117] Detachment of the sheath can occur at one and/or the other
interface depending on the degree of adhesion of the cement to
these interfaces.
[0118] The influence of the elastic properties of the sheath and
the rock on the tensile strength required to avoid detachment of
the sheath when the well pressure is reduced are examined
below.
[0119] FIG. 14 shows the variations in tensile strength required
for the cement at the casing-sheath interface to prevent detachment
of the sheath, as a function of the Young's modulus of the cement
and for various values of the Young's modulus of the rock. Curves
C1 to C5 were produced which correspond respectively to values of 1
GPa, 5 GPa, 10 GPa, 20 GPa and 30 GPa for the Young's modulus of
the rock, and for a reduction of the order of 6.9 MPa in the well
pressure.
[0120] Examination of FIG. 14 shows that, in contrast to FIGS. 3
and 4 regarding an increase in well pressure:
[0121] that the cement tensile strength required to avoid
detachment of the sheath increases with the Young's modulus of the
rock, since the presence of hard rock prevents the sheath from
deforming; and
[0122] that the cement tensile strength required to prevent the
sheath from becoming detached also increases with the Young's
modulus of the cement, but this increase is smaller for high values
for the Young's modulus of the cement.
[0123] It could be concluded that it is desirable to have a sheath
the cement of which has a high Young's modulus but in practice the
stresses in extension are difficult to evaluate at the two
interfaces of the sheath. In effect, the adhesion of the cement can
vary depending on the presence or absence of a cake between the
cement and the rock. This cake can be a film of drilling mud which
forms during the well cementing operation when the drilling mud is
evacuated via the annulus.
[0124] The study has demonstrated that to avoid detachment of the
sheath at the interfaces, i.e., the appearance of a micro-annulus,
the best solution is to place the cement under compression while it
is being positioned around the casing.
[0125] Thus the cement will store a certain amount of elastic
energy which it can then release on expanding during contraction of
the casing caused by a reduction in well pressure. However, a
micro-annulus may be created at one of the interfaces if the
precaution of controlling the degree of contraction of the casing
and the degree of expansion of the cement is not taken.
[0126] A cement under compression can be produced by using either a
cement foam, i.e., a cement into which a gas such as nitrogen has
been injected, or a cement which expands during setting to stress
it.
[0127] FIG. 15 shows the radial stress conditions in cement as a
function of the distance from the well axis, once the cement has
expanded by an amount of the order of 0.5% for a Young's modulus of
the order of 1 GPa and a rock Young's modulus of the order of 10
GPa.
[0128] An examination of FIG. 15 shows that the radial stresses are
in compression from the casing-sheath to the sheath-rock interface,
indicating that the cement is properly in compression. The study
has also shown that an increase in the Young's modulus of the
cement increases the radial stresses at the casing-sheath interface
without substantially modifying the stresses at the sheath-rock
interface.
[0129] FIG. 16 shows the radial stresses of FIG. 15 after a
reduction in well pressure of the order of 6.90 MPa. An examination
of FIG. 16 shows that these radial stresses are always in
compression, i.e., cement adhesion is maintained at both
interfaces. In other words, with a cement under compression, a
comparative examination of FIGS. 14 and 16 shows that the radial
stresses are in compression and not in extension.
[0130] However, the study has also shown that for a circularly
shaped well, expansion of the cement can lead to a risk of
detachment of the casing at the casing-sheath interface, in
particular if the cement is more rigid than the rock. In order to
reduce the risk of detachment and encourage expansion of the cement
towards the casing, it is desirable to select a value for the
Young's modulus of the cement which is lower than the Young's
modulus of the rock. It is also desirable to calculate the amount
of expansion of the cement sheath as a function of the variation in
load. Too little expansion would not be sufficient to avoid
detachment of the sheath, while too much expansion would damage the
sheath.
[0131] Thus the study has led to the conclusion that a risk of
detachment of the sheath can be avoided:
[0132] if the ratio between the tensile strength of the sheath and
its Young's modulus is as high as possible; and/or
[0133] if the Young's modulus of the cement is lower than the
Young's modulus of the rock; and/or
[0134] if the cement expands during setting to place it in
compression.
[0135] The same overall conclusion can be drawn as that drawn by
the study regarding avoiding the risk of sheath cracking.
[0136] In general, the study has also demonstrated that the
conditions for reducing the risk of detachment of the sheath as a
result of a reduction in well pressure is overall the same as in
the case of an increase in well pressure with the additional
condition of keeping the cement in compression with this pressure
drop.
[0137] The risk of sheath detachment can occur as a result of a
variation in the in-situ stresses, in particular when the pore
pressure in the reservoir increases. These stresses can increase by
an amount of the order of 30 MPa. In other words, the in-situ
stresses become more compressive, but the effective stresses in the
cement become less compressive. The effective stress is the total
stress minus a function of the pore pressure. This effective stress
is the stress which controls deformation of the solid material.
[0138] In general, the data shows that the radial and tangential
stresses are in extension but the radial stresses are in extension
to a greater extent than the tangential stresses and the highest
value of these radial stresses is at the casing-sheath
interface.
[0139] Overall, the conditions are thus similar to those
corresponding to a reduction in well pressure, i.e., with a risk of
sheath detachment which is a function of the adhesion of the cement
to the casing and to the rock.
[0140] Finally, the data shows that the influence of the pore
pressure in the formation on the stresses in the sheath is globally
similar to an increase in pressure, i.e., in the radial stress at
the cement-rock interface, if the pore pressure falls, and is
globally similar to a reduction in the cement-rock pressure if the
pore pressure increases.
[0141] The above study of the principal types of deterioration of
the cementing sheath which can occur during the lifetime of the
well has enabled a method to be developed which can be used to
prepare a cement slurry which can avoid these types of
deterioration in the sheath for a given well and, conversely, it
has enabled a determination to be made as to whether a given cement
slurry is capable of avoiding sheath deterioration for a given
well.
[0142] This method uses computer programs which use the data
concerning the characteristics of the borehole and the well casing,
and also data on the elastic properties of the rock traversed by
the well, this data being obtained by taking samples, for example.
The software then estimates the variations in pressure and/or
temperature in the well and/or variations in the in-situ stresses,
which can occur during the lifetime of the well.
[0143] In general, variations in well pressure and/or in
temperature can be calculated quite accurately, while this is not
the case for variations in in-situ stresses which must be estimated
on the basis of mathematical models.
[0144] The software then determines the stress conditions in the
sheath resulting from the above variations which have been
calculated or estimated, the type of deterioration which is likely
to occur first and its risk, and the influence of the elastic
properties of the sheath, of the casing and/or of the rock, in
order to eliminate this risk of deterioration and as a result to
select the elastic properties required for the sheath and for a
given well.
* * * * *