# Patent

## Title
Methods for completing subterranean wells

## Abstract

Well cement compositions comprise water, an inorganic cement and one or more fillers having a thermal expansion coefficient (CTE) that is lower than about 3×10−6/° C. The presence of low-CTE fillers may reduce cement-sheath shrinkage when the well temperature falls during stimulation treatments, cyclic steam injection or when the well temperature reaches equilibrium at the top of a long casing string.

## Background
BACKGROUND
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This disclosure relates to equipment and methods for completing subterranean wells; in particular, wells in which large temperature fluctuations may occur.
During completion of a subterranean well, drilling and cementing operations are performed to provide a conduit through which desirable fluids originating within the formation may flow. The cementing operation involves placing a competent cement sheath inside the annular region between the external surface of a tubular body such as well casing, and the borehole wall. The cement sheath supports the casing and provides a hydraulic seal between producing formations. The presence of a hydraulic seal is commonly called zonal isolation.
Well cementing is a difficult operation because it requires several parameters to be considered and controlled. Such parameters include density, setting time, rheological properties, fluid-loss rate, set-cement strength and permeability. Control of these parameters is inherent to any well-cementing operation, and is well known to the skilled person. Solutions generally involve incorporating various additives into the cement slurry. Detailed information about well cementing may be found in the following publication: Nelson E B and Guillot D (eds.): Well Cementing, 2nd Edition, Schlumberger (2006).
When well cementing is successful, and a cement sheath has been formed that provides casing support and zonal isolation once the slurry has set and hardened, it may not be long before the sheath is subjected to mechanical and/or thermal stresses that can lead to deterioration. Cement systems employed in thermal-recovery wells are particularly prone to problems that lead to loss of zonal isolation. One type of thermal-recovery well involves the injection of steam into the wellbore, commonly known as steamflooding. Steamflooding may consist of introducing steam into an injection well and sending the steam through the formation to one or more production wells. Another technique involves cyclic steam injection, during which steam is injected into a single well for a limited period. After the steam-injection period, the well is placed into production. Heating the reservoir reduces the viscosity of oil in the formation, making production more efficient. Steamflood wells are usually less than 915 m (3000 ft) deep, and are frequently deviated (30° to horizontal). The circulating temperatures during primary-cementing operations are often less than 40° C. (104° F.). During injection, the steam temperature usually approaches about 315° C. (600° F.).
When heat is initially supplied, the temperature rise is normally controlled to prevent undue thermal shock to the casing and cement. Nevertheless, because of thermal expansion, high levels of stress are built up in the pipe and the cement sheath.
A substantial amount of work has been performed for many years to devise cementing techniques that minimize the effects of thermal stresses. Such methods include the placement of thermal packers and the inclusion of a sliding sleeve in the casing string that can move freely in response to thermal stress. Another procedure involves holding the casing in tension during primary cementing to minimize axial casing deformation when thermal stress is eventually applied.
More recently, a method was developed that involves applying internal casing pressure after the primary cementing has been performed, and while the cement slurry is setting and hardening. The internal pressure may vary from about 15.9 MPa to 138 MPa (2300 psi to 20,000 psi). This process prestresses the casing, and gives the cement sheath an improved ability to withstand the application of heat during the steam-injection process. Modeling software is used to analyze the anticipated well conditions during steam production, and determine the optimal amount of casing pressurization. All of the above techniques are aimed at maintaining zonal isolation.
There are also situations during which the casing and cement may experience a temperature decrease. For example, cold fluids may be injected into subterranean formations during hydraulic fracturing operations, or to maintain reservoir pressure. In other cases involving long casing strings or long liners, the cement at the top may initially be hotter as it sets than the surrounding formation. In addition to the heat of cement hydration, the slurry would have been further heated during its journey along the bottom of the string. As the casing-cement-formation system returns to equilibrium, the temperature decreases. In addition, casing and cement cooling may occur in shallow sections of deepwater wells. Such cooling may also induce stresses and deformation in the casing and cement sheath that could lead to the loss of zonal isolation.
SUMMARY
Improved cementing techniques in the realm of steam-injection wells and other wells in which large temperature fluctuations occur would be advantageous.
The present disclosure provides improved compositions and methods for cementing thermal-recovery wells and maintaining zonal isolation during production. The improved compositions comprise an inorganic cement and one or more fillers that have very low or negative coefficients of thermal expansion.
In an aspect, embodiments relate to cement compositions. The compositions comprise water, an inorganic cement and one or more fillers having a thermal expansion coefficient lower than about 3×10−6/° C.
In a further aspect, embodiments relate to methods for cementing a subterranean well having a borehole. A casing string is installed into the borehole, and a cement sheath is placed between the exterior surface of the casing string and the formation wall or between the exterior surface of the casing string and another casing string, the cement sheath comprising water, an inorganic cement and one or more fillers having a thermal expansion coefficient lower than about 3×10−6/° C.
In yet a further aspect, embodiments relate to well-construction methods. A borehole is drilled into the earth, thereby forming a subterranean well. A casing string is installed into the borehole, and a cement sheath is placed between the exterior surface of the casing string and the formation wall or between the exterior surface of the casing string and another casing string, the cement sheath comprising water, an inorganic cement and one or more fillers having a thermal expansion coefficient lower than about 3×10−6/° C.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B illustrate the mechanism by which fillers with low thermal expansion coefficients may behave in a cement matrix.
FIG. 2 shows the results of simulations showing the effect of cement thermal expansion coefficient on the size of a microannulus resulting from cooling the well.



DETAILED DESCRIPTION
At the outset, it should be noted that in the development of any such actual embodiment, numerous implementations or specific decisions must be made to achieve the developer's specific goals, such as compliance with system related and business related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. In addition, the compositions used/disclosed herein can also comprise some components other than those cited. In the summary of the disclosure and this detailed description, each numerical value should be read once as modified by the term “about” (unless already expressly so modified), and then read again as not so modified unless otherwise indicated in context. Also, in the summary of the disclosure and this detailed description, it should be understood that a concentration range listed or described as being useful, suitable, or the like, is intended that any and every concentration within the range, including the end points, is to be considered as having been stated. For example, “a range of from 1 to 10” is to be read as indicating each and every possible number along the continuum between about 1 and about 10. Thus, even if specific data points within the range, or even no data points within the range, are explicitly identified or refer to only a few specific points, it is to be understood that inventors appreciate and understand that any and all data points within the range are to be considered to have been specified, and that Applicant possessed knowledge of the entire range and all points within the range.
As discussed earlier, it would be advantageous to improve cement-design, primary-cementing and completion methods for wells in which large temperature fluctuations occur.
It was disclosed that, when particles with a thermal expansion coefficient (CTE) higher than set cement are incorporated into the cement, the resulting CTE of the composite increases. A higher CTE is beneficial in situations during which the well temperature is artificially increased (e.g., steam-injection wells). The composite cement system with the higher CTE expands in a similar fashion to the casing, thereby reducing the stresses induced in the cement sheath.
However, there are situations during which the casing and cement may experience a temperature decrease. For example, cold fluids may be injected into subterranean formations during hydraulic fracturing operations, or to maintain reservoir pressure. In other cases involving long casing strings or long liners, the cement at the top may initially be hotter as it sets than the surrounding formation. In addition to the heat of cement hydration, the slurry would have been further heated during its journey along the bottom of the string. As the casing-cement-formation system returns to equilibrium, the temperature decreases. In addition, casing and cement cooling may occur in shallow sections of deepwater wells.
As a well cools, the casing may shrink, whereas the diameter of the wellbore may remain effectively constant. Therefore, the annular space between the casing and the formation wall may increase. In addition, the cement-sheath thickness may decrease because of thermal contraction. The CTE of standard set Portland cement is approximately 9×10−6/° C. In a typical configuration (7-in. casing (17.8 cm) in an 8.5-in. (21.6 cm) open hole), a 50° C. temperature reduction will reduce the cement thickness by about 9 μm. A 9 μm gap may be sufficient to allow interzonal communication.
The Applicant has determined that it is possible to reduce the thermal contraction of the cement sheath by adding fillers with very low or negative CTEs to the cement slurry. The composite system may then have a CTE between that of the cement matrix and that of the filler inclusions.
A non-limiting example of a filler with a low thermal expansion coefficient is INVARTM (available from Imphy Alloys, France)—a nickel/iron alloy containing 36 wt % Ni and 64 wt % Fe. Depending on the purity of the alloy, the CTE may be as low as 0.6×10−6/° C.
A non-limiting example of a filler with a negative CTE is zirconium tungstate—Zr(WO4)2. This material has a CTE of approximately −7×10−6/° C. The negative value indicates that the material expands when cooled and contracts when heated.
Both INVARTM and Zr(WO4)2 maintain these CTEs over a temperature range that is useful for well cementing applications: 20° C. to 100° C. for INVARTM and up to 700° C. for Zr(WO4)2. Table 1 presents calculated CTEs for composite cement systems containing either INVARTM or Zr(WO4)2. The CTEs are volume weighted averages of the filler and matrix.







TABLE 1







Predicted thermal expansion coefficients for cement systems


containing INVAR TM or Zr(WO4)2 as fillers.









Composite thermal expansion coefficients (×10−6/° C.)









Filler (vol %)
INVAR TM
Zr(WO4)2












0
9.0
9.0


10
8.2
7.4


20
7.3
5.8


30
6.5
4.2


40
5.6
2.6


50
4.8
1.0


60
4.0
−0.6


70
3.1
−2.2









When low- or negative-CTE materials are present as fillers, the useful effect may only be present when the temperature falls. This effect is illustrated in FIGS. 1A and 1B. FIG. 1A illustrates what may occur when the temperature decreases. The cement matrix 10 shrinks onto the filler particles 20, and the filler may prevent further shrinkage. For example, if a 50° C. temperature reduction occurs, and the composite system contains 50 vol % Zr(WO4)2, the CTE may be 1.0×10−6/° C. and the cement thickness may decrease by only 1 μm (assuming the same well geometry described earlier). In a more extreme case in which 70 vol % Zr(WO4)2 is present, the cement composite may expand upon 50° C. cooling by 2 μm.
When the temperature increases (FIG. 1B), the matrix may expand to a greater extent than the filler particles (in the case of Zr(WO4)2 the filler may shrink), leaving void spaces 30 between the filler particles and the matrix. In such cases, because the filler particles are no longer in contact with the matrix, the CTE may become that of the matrix.
Another technique by which low- or negative-CTE materials may be employed to improve zonal isolation may be to incorporate low-CTE metals in one or more casing strings. In such applications, the CTE of the metal may be lower than about 9×10−6/° C., or lower than about 6×10−6/° C., or lower than about 3×10−6/° C. In a non-limiting example, INVARTM alloy may be used to fabricate lengths of casing string. Such casing strings may be present in the well along with conventional casing, or the entire casing string may be made from INVARTM alloy. Alternatively, conventional casing may include an outer layer composed of INVARTM alloy. When steam is injected into the well, the casing comprising INVARTM alloy may not expand significantly as the temperature increases, and there may be significantly less stress induced in the cement sheath. The CTEs of INVARTM alloy at various temperatures are given in Table 2.







TABLE 2







Coefficients of thermal expansion for INVAR TM alloy


at various temperatures.












Temperature

CTE













° F.
° C.
(×10−6/° F.)
(×10−6/° C.)
















200
93
0.72
1.30



300
149
1.17
2.11



500
260
2.32
4.18



700
371
4.22
7.60










Another low-CTE metal that may be used in the present application is KOVARTM alloy—a Fe/Ni/Co alloy available from Carpenter Technology Corporation, Reading, Pa., USA. The alloy comprises about 46% Fe, 29% Ni and 17 wt % Co. The CTE of KOVARTM alloy may vary from about 4.9×10−6/° C. and 6.2×10−6/° C.
Yet another metal that may be used in the present application is OSPREYTM alloy—a silicon/aluminum alloy available from Sandvik Materials Technology, Sandviken, Sweden. The alloy may comprise between 27% and 70% Si. The CTE of OSPREYTM alloy may vary from about 7.4×10−6/° C. and 16.8×10−6/° C.
Alternatively, the low-CTE metal may be present as a coating around conventional casings. Conventional casings may comprise carbon steel, stainless steel, titanium, INCONELTM and MONELTM. INCONELTM is an alloy composed primarily of nickel and chromium, and is available from Special Metals Corporation. MONELTM is an alloy composed primarily of nickel (up to 67%) and copper, and is available from Special Metals Corporation.
In an aspect, the disclosure relates to compositions comprising an inorganic cement and one or more fillers having a thermal expansion coefficient lower than about 3×10−6/° C. The filler may comprise zirconium tungstate, or an alloy comprising between about 36 wt % and 38 wt % Ni and about 62 wt % and 64 wt % Fe, or a combination thereof. The filler concentration may be between about 1 wt % and 75 wt % by weight of cement, or between about 5 wt % and 50 wt %. The average particle size of the filler may be between about 10 μm and 500 μm, or between about 50 μm and 250 μm. When used in subterranean applications, the compositions may be pumpable. Those skilled in the art will recognize that a pumpable fluid in the context of well cementing has a viscosity lower than about 1000 mPa-s at a shear rate of 100 s−1 at the temperatures to which the fluid is exposed during a cementing operation, and during the time necessary to place the composition in the well.
In a further aspect, the disclosure relates to methods for cementing a subterranean well having a borehole. A casing string is installed in the borehole, and a cement sheath is placed between the exterior surface of the casing string and the formation wall, or between the exterior surface of the casing string and another casing string. The cement sheath may comprise an inorganic cement and one or more fillers having a thermal expansion coefficient lower than about 3×10−6/° C. The filler may comprise zirconium tungstate, or an alloy comprising between about 36 wt % and 38 wt % Ni and about 62 wt % and 64 wt % Fe, or a combination thereof. The filler concentration may be between about 1 wt % and 75 wt % by weight of cement, or between about 5 wt % and 50 wt %. The average particle size of the filler may be between about 10 μm and 500 μm, or between about 50 μm and 250 μm. The well may be a thermal recovery well (e.g., a steamflood well) or a well that experiences cooling after the cement sheath sets (e.g., a well in which fluids are injected that cause wellbore cooling).
In yet a further aspect, the disclosure relates to well-construction methods. A borehole is drilled into the earth, thereby forming a subterranean well. A casing string is installed in the borehole, and a cement sheath is placed between the exterior surface of the casing string and the formation wall, or between the exterior surface of the casing string and another casing string. The cement sheath may comprise an inorganic cement and one or more fillers having a thermal expansion coefficient lower than about 3×10−6/° C. The filler may comprise zirconium tungstate, or an alloy comprising between about 36 wt % and 38 wt % Ni and about 62 wt % and 64 wt % Fe, or a combination thereof. The filler concentration may be between about 1 wt % and 75 wt % by weight of cement, or between about 5 wt % and 50 wt %. The average particle size of the filler may be between about 10 μm and 500 μm, or between about 50 μm and 250 μm.
For all aspects, the inorganic cement may comprise Portland cement, calcium aluminate cement, lime-silica blends, zeolites, geopolymers, Sorel cements, chemically bonded phosphate ceramics, cement-kiln dust and combinations thereof.
The compositions disclosed may further comprise dispersing agents, fluid-loss-control agents, set retarders, set accelerators, foaming agents, gas generating agents, antifoaming agents, extenders, swellables particles, weighting agents, lost-circulation control agents and combinations thereof.
EXAMPLE
The following example serves to further illustrate the disclosure.
Several simulations were performed to calculate the effects of cooling a well on the size of a casing/cement microannulus. The CTE of the cement sheath was varied. The casing had a low CTE—1.3×10−6/° C. Two simulators were employed in the present study. One is described in the following publication. Thiercelin M J et al.: “Cement Design Based on Cement Mechanical Response,” SPE Drilling & Completion 13 (4) December 1998, 266-273. The other is a finite element analysis model (ABAQUSTM, available from ABAQUS Inc.). The simulations correspond to a cooling of the well (e.g., due to water injection). Three cases were simulated.
1. Conventional Cement: CTE=9×10−6/° C.
2. Low CTE Cement: CTE=2×10−6/° C.
3. Negative CTE Cement: CTE=−2×10−6/° C.
The rest of the simulator input parameters are given in Table 3.







TABLE 3. 





Input parameters for simulations involving


different cement-sheath CTEs.























Rock







Open hole
8.5
inch
Young's modulus
24000
MPa










Casing
7 inch, 38.7
Poisson's ratio
0.25



kg/m


Standoff
100%
CTE
13 × 10−6/° C.















Casing


Cement







Young's modulus
200000
MPa
Unconfined
48
MPa





Compressive





Strength











Poisson's ratio
0.27
Tensile strength
4.8
MPa


CTE
1.3 × 10−6/° C.
Young's modulus
8500
MPa










Temperature ramp
100 to 50° C.
Poisson's ratio
0.15



in 50 minutes









The results of the simulations are shown in FIG. 2, which shows the microannulus-gap sizes that may be observed upon well cooling. The total microannulus gap is the sum of the widths of both the inner and the outer microannuli. As the CTE of the cement decreases and becomes negative, the cumulative size of the microannulus falls below 10 μm.
Although various embodiments have been described with respect to enabling disclosures, it is to be understood that the preceding information is not limited to the disclosed embodiments. Variations and modifications that would occur to one of skill in the art upon reading the specification are also within the scope of the disclosure, which is defined in the appended claims.

## Claims

1. A composition, comprising water, an inorganic cement and a filler comprising one or more metal alloys, zirconium tungstate or combinations thereof, wherein the thermal expansion coefficient of the composition is lower than 3×10−6/° C.

2. The composition of claim 1, wherein the filler comprises an alloy comprising between 36 wt % and 38 wt % Ni and 62 wt % and 64 wt % Fe.

3. The composition of claim 1, wherein the filler concentration is between 1% and 75% by weight of cement.

4. The composition of claim 1, wherein the average particle size of the filler is between 10 μm and 500 μm.

5. The composition of claim 1, wherein the inorganic cement comprises Portland cement, calcium aluminate cement, lime-silica blends, zeolites, geopolymers, Sorel cements, chemically bonded phosphate ceramics, cement-kiln dust and combinations thereof.

6. A method for cementing a subterranean well having a borehole, comprising:
(i) installing a casing string into the borehole; and
(ii) placing a cement sheath between the exterior surface of the casing string and the formation wall or between the exterior surface of the casing string and another casing string, the cement sheath comprising water, an inorganic cement and a filler comprising one or more metal alloys, zirconium tungstate or combinations thereof, wherein the thermal expansion coefficient of the cement sheath is lower than 3×10−6/° C.

7. The method of claim 6, wherein the filler comprises an alloy comprising between 36 wt % and 38 wt % Ni and 62 wt % and 64 wt % Fe.

8. The method of claim 6, wherein the filler concentration is between 1% and 75% by weight of cement.

9. The method of claim 6, wherein the average particle size of the filler is between 10 μm and 500 μm.

10. The method of claim 6, wherein the inorganic cement comprises Portland cement, calcium aluminate cement, lime-silica blends, zeolites, geopolymers, Sorel cements, chemically bonded phosphate ceramics, cement-kiln dust and combinations thereof.

11. The method of claim 6, wherein the well is a thermal recovery well.

12. The method of claim 6, wherein the well experiences cooling after the cement sheath sets.

13. The method of claim 6, wherein the casing string comprises a metal having a coefficient of thermal expansion lower than 9×10−6/° C.

14. The method of claim 13, wherein the metal consists of an alloy containing between 32 wt % and 42 wt % Ni and between 58 wt % and about 68 wt % Fe.

15. A well-construction method, comprising
(i) drilling a borehole into the earth, thereby forming a subterranean well;
(ii) installing a casing string into the borehole; and
(iii) placing a cement sheath between the exterior surface of the casing string and the formation wall or between the exterior surface of the casing string and another casing string, the cement sheath comprising water, an inorganic cement and a filler comprising one or more metal alloys, zirconium tungstate or combinations thereof, wherein the thermal expansion coefficient of the cement sheath is lower than 3×10−6/° C.

16. The method of claim 15, wherein the filler comprises an alloy comprising between 36 wt % and 38 wt % Ni and 62 wt % and 64 wt % Fe.

17. The method of claim 15, wherein the casing string comprises a metal having a coefficient of thermal expansion lower than 9×10−6/° C.