TECHNICAL FIELD

This disclosure relates generally to equipment utilized and operations performed in conjunction with a subterranean well and, in one example described below, more particularly provides for swellable packer construction.

BACKGROUND

A swellable packer may be used to seal off an annular space in a well. A swellable packer typically includes a swellable material which swells in the well, so that a seal element of the packer displaces into sealing contact with a well surface.

It will be appreciated that improvements are

continually needed in the art of constructing swellable packers. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representative partially cross-sectional view of a system and associated method which can embody principles of this disclosure.

FIG. 2 is a representative cross-sectional view of a swellable packer that may be used in the system and method of FIG. 1, and which can embody the principles of this disclosure .

FIG. 3 is a representative cross-sectional view of another example of the swellable packer.

DETAILED DESCRIPTION

Representatively illustrated in FIG. 1 is a system 10 for use with a subterranean well, and an associated method, which system and method can embody principles of this disclosure. However, it should be clearly understood that the system 10 and method are merely one example of an application of the principles of this disclosure in

practice, and a wide variety of other examples are possible. Therefore, the scope of this disclosure is not limited at all to the details of the system 10 and method described herein and/or depicted in the drawings.

In the FIG. 1 example, a packer 12 is connected in a tubular string 14 positioned in a wellbore 16. The wellbore 16 is lined with casing 18 and cement 20, but in other examples the wellbore could be uncased or open hole.

The packer 12 is used to seal off an annulus 22 formed radially between the tubular string 14 and the casing 18. For this purpose, the packer 12 includes a swellable seal element 24 which swells radially outward into sealing contact with an interior surface of the casing 18.

In examples where the wellbore 16 is not cased, the seal element 24 may sealingly contact a surface of an earth formation 26 penetrated by the wellbore. Any well surface may be sealingly contacted by the seal element 24, and so it should be understood that the scope of this disclosure is not limited to any particular well surface being contacted by the seal element.

The sealing contact and swelling of the seal element 24 results from contact between a swellable material of the seal element and an activating agent in the well.

Preferably, the swellable material swells when it is

contacted with a particular activating agent (e.g., oil, gas, other hydrocarbons, water, acid, other chemicals, etc.) in the well.

The activating agent may already be present in the well, or it may be introduced after installation of the packer 12 in the well, or it may be carried into the well with the packer, etc. The swellable material could instead swell in response to exposure to a particular temperature, or upon passage of a period of time, or in response to another stimulus, etc.

Thus, it will be appreciated that a wide variety of different ways of swelling a swellable material exist and are known to those skilled in the art. Accordingly, the scope of this disclosure is not limited to any particular manner of swelling the swellable material. Furthermore, the scope of this disclosure is also not limited to any of the details of the well system 10 and method described herein, since the principles of this disclosure can be applied to many different circumstances. The term "swell" and similar terms (such as "swellable") are used herein to indicate an increase in volume of a swellable material. Typically, this increase in volume is due to incorporation of molecular components of the activating agent into the swellable material itself, but other swelling mechanisms or techniques may be used, if desired. Note that swelling is not the same as expanding, although a seal material may expand as a result of swelling.

For example, in some conventional packers, a seal element may be extended radially outward by longitudinally compressing the seal element, or by inflating the seal element. In each of these cases, the seal element is

extended without any increase in volume of the seal material of which the seal element is made. Thus, in these

conventional packers, the seal element extends outward, but does not swell.

The activating agent which causes swelling of the swellable material is in this example preferably a

hydrocarbon fluid (such as oil or gas). In the well system 10, the swellable material swells when a fluid 28 comprises the activating agent (e.g., when the fluid enters the wellbore 16 from the formation 26 surrounding the wellbore, when the fluid is circulated to the packer 12 from the earth's surface, when the fluid is released from a chamber carried with the packer, etc.). In response, the seal element 24 seals off the annulus 22 and applies a gripping force to the inner surface of the casing 18 (or a surface of the formation 26 if uncased) .

The activating agent which causes swelling of the swellable material could be comprised in any type of fluid. The activating agent could be naturally present in the well, it could be conveyed with the packer 12, or it could be conveyed separately or flowed into contact with the

swellable material in the well when desired. Any manner of contacting the activating agent with the swellable material may be used in keeping with the principles of this

disclosure.

Various swellable materials are known to those skilled in the art, which materials swell when contacted with water and/or hydrocarbon fluid, so a comprehensive list of these materials will not be presented here. Partial lists of swellable materials may be found in U.S. Patent Nos.

3385367, 7059415 and 7143832, the entire disclosures of which are incorporated herein by this reference.

As another alternative, the swellable material may have a substantial portion of cavities therein which are

compressed or collapsed at surface conditions. Then, after being placed in the well at a higher pressure, the material is expanded by the cavities filling with fluid.

This type of apparatus and method might be used where it is desired to expand the swellable material in the presence of gas rather than oil or water. A suitable

swellable material is described in U.S. Published

Application No. 2007-0257405, the entire disclosure of which is incorporated herein by this reference.

Preferably, the swellable material used in the seal element 24 swells by diffusion of hydrocarbons into the swellable material, or in the case of a water swellable material, by the water being absorbed by a super-absorbent material (such as cellulose, clay, etc.) and/or through osmotic activity with a salt-like material. Hydrocarbon-, water- and gas-swellable materials may be combined, if desired. It should, thus, be clearly understood that any

swellable material which swells when contacted by a

predetermined activating agent may be used in keeping with the principles of this disclosure. The swellable material could also swell in response to contact with any of multiple activating agents. For example, the swellable material could swell when contacted by hydrocarbon fluid, or when contacted by water.

One disadvantage of conventional swellable packers is that a large difference in modulus of elasticity is present at an interface between the swellable material and a base pipe of the packer. Modulus of elasticity (also known as elastic modulus) is a tendency of a substance to be deformed elastically when a force is applied to it. The elastic modulus of a substance is a slope of its stress—strain curve in an elastic deformation region. A stiffer material will have a higher elastic modulus.

The large difference in elastic modulus between the base pipe and the seal element in conventional swellable packers can be detrimental to their differential pressure holding capabilities. For example, shear failures can occur at or near the interface between the swellable material and the base pipe.

However, swellable materials with relatively high elastic modulus typically do not have favorable swelling capabilities (such as, rate or total amount of swelling upon exposure to an activating agent, etc.). Thus, swellable materials with relatively low elastic modulus are generally used in conventional swellable packers. These low elastic modulus materials contribute to the large elastic modulus difference mentioned above, which results in shear failures. Referring additionally now to FIG. 2 , an example of the packer 12 is representatively illustrated in an enlarged scale cross-sectional view. The packer 12 may be used in the FIG. 1 system 10 and method, or it may be used in other systems and methods .

In the FIG. 2 example, the packer 12 includes a

generally tubular base pipe 30 . The seal element 24

surrounds the base pipe 30 , and is restrained longitudinally by two end rings 32 .

The end rings 32 may be attached to the base pipe 30 in any manner. For example, the end rings 32 could be clamped onto, welded to, integrally formed with, molded onto, bonded to, threaded onto, or otherwise secured to the base pipe 30 . In other examples, the end rings 32 may not be used.

The seal element 24 may be retained on the base pipe 30 in any manner. For example, the seal element 24 may be retained longitudinally between the end rings 32 , molded onto the base pipe 30 , bonded to the base pipe, slipped onto the base pipe, wrapped onto the base pipe, etc. The scope of this disclosure is not limited to any particular technique for positioning or retaining the seal element 24 on the base pipe 30 .

The seal element 24 in this example includes layers 34 , 36 , 38 of respective swellable materials 40 , 42 , 44 .

Although three layers 34 , 36 , 38 are depicted in FIG. 2 , any number of layers may be used.

A modulus of elasticity of each of the swellable materials 40 , 42 , 44 is different. The inner layer 40 has a greater modulus as compared to the middle layer 42 , and the middle layer 42 has a greater modulus as compared to the outer layer 44 . Thus, the modulus of the seal element 24 decreases in a direction radially outward from the base pipe 30 .

In this manner, the swellable material 40 having the greatest modulus is positioned adjacent the base pipe 30 (e.g., forming an interior inner surface 46 of the seal element 24 in contact with the base pipe), so that the difference in modulus between the base pipe and the

swellable material is reduced. The swellable material 44 having the least modulus is positioned on an exterior of the seal element 24 (e.g., forming an outer surface 48 of the seal element which contacts a well surface), so that the better swelling characteristics of the material can be used for sealingly contacting the well surface.

By increasing the modulus of the swellable material positioned adjacent the base pipe 30 , shear failures due at least in part to a large difference in modulus may be avoided, or at least reduced. However, desirable swelling characteristics can still be obtained by using relatively low modulus swellable material away from the base pipe 30 .

Although the layers 40 , 42 , 44 are depicted in FIG. 2 as being cylindrical and having substantially equal

thickness, in other examples the layers could be otherwise configured. Thus, the scope of this disclosure is not limited to the details of the packer 12 as depicted in the drawings or described herein.

Referring additionally now to FIG. 3 , another example of the packer 12 is representatively illustrated. In this example, the seal element 24 comprises a swellable material 50 which has a modulus that varies across its thickness.

Preferably, the modulus of the swellable material 50 decreases in a direction radially outward from the base pipe 30 . Thus, a relatively high modulus portion of the swellable material 50 is positioned adjacent the base pipe 30 (in order to reduce the difference in modulus between the material and the base pipe), and a relatively low modulus portion of the swellable material is positioned toward the outer surface 48 of the seal element 24 (which sealingly contacts a well surface).

It may now be fully appreciated that the above

disclosure provides significant advancements to the art of constructing swellable packers. In examples described above, differences in modulus between base pipes and swellable materials can be reduced, while still maintaining acceptable swelling characteristics for swellable seal elements.

A swellable packer 12 for use in a subterranean well is provided to the art by the above disclosure. In one example, the packer 12 can include a base pipe 30 and a swellable seal element 24. The seal element 24 can have a modulus of elasticity which decreases in a direction outward from the base pipe 30.

A maximum modulus of elasticity of the swellable seal element 24 may be adjacent the base pipe 30.

The swellable seal element 24 may include multiple layers 34, 36, 38 of swellable materials 40, 42, 44. The modulus of elasticity can be greater in an inner one of the layers 34, as compared to an outer one of the layers 38. The modulus of elasticity of the layers 34, 36, 38 preferably decreases in succession in the direction outward from the base pipe 30.

The swellable seal element 24 may comprise a swellable material 50, and the modulus of elasticity may decrease in the swellable material 50 in the direction outward from the base pipe 30. A method of constructing a packer 12 for use in a subterranean well is also described above. In one example, the method can comprise: providing a swellable seal element 24 for the packer 12, the swellable seal element 24 having a modulus of elasticity which decreases from an interior to an exterior of the seal element 24.

The method can also include securing the seal element 24 to a base pipe 30, with the modulus of elasticity of the seal element 24 decreasing in a direction outward from the base pipe 30.

A system 10 for use with a subterranean well can include a swellable packer 12 comprising a swellable seal element 24 which swells and sealingly contacts a well surface (e.g., an inner surface of the casing 18, a surface of the formation 26, etc.). The seal element 24 may have a modulus of elasticity which decreases in a direction toward the well surface.

Although various examples have been described above, with each example having certain features, it should be understood that it is not necessary for a particular feature of one example to be used exclusively with that example. Instead, any of the features described above and/or depicted in the drawings can be combined with any of the examples, in addition to or in substitution for any of the other features of those examples. One example's features are not mutually exclusive to another example's features. Instead, the scope of this disclosure encompasses any combination of any of the features. For example, the swellable materials 40, 42, 44 of the FIG. 2 example could have varying moduli, as with the swellable material 50 of the FIG. 3 example.

Although each example described above includes a certain combination of features, it should be understood that it is not necessary for all features of an example to be used. Instead, any of the features described above can be used, without any other particular feature or features also being used.

It should be understood that the various embodiments described herein may be utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., and in various configurations, without departing from the principles of this disclosure. The embodiments are described merely as examples of useful applications of the principles of the disclosure, which is not limited to any specific details of these embodiments.

In the above description of the representative

examples, directional terms (such as "above," "below,"

"upper," "lower," etc.) are used for convenience in

referring to the accompanying drawings. However, it should be clearly understood that the scope of this disclosure is not limited to any particular directions described herein.

The terms "including," "includes," "comprising,"

"comprises," and similar terms are used in a non-limiting sense in this specification. For example, if a system, method, apparatus, device, etc., is described as "including" a certain feature or element, the system, method, apparatus, device, etc., can include that feature or element, and can also include other features or elements. Similarly, the term "comprises" is considered to mean "comprises, but is not limited to."

Of course, a person skilled in the art would, upon a careful consideration of the above description of

representative embodiments of the disclosure, readily appreciate that many modifications, additions,

substitutions, deletions, and other changes may be made to the specific embodiments, and such changes are contemplated by the principles of this disclosure. For example,

structures disclosed as being separately formed can, in other examples, be integrally formed and vice versa.

Accordingly, the foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the invention being limited solely by the appended claims and their equivalents.