BACKGROUND OF THE INVENTION

Field of the Invention.

[0001] The disclosure generally relates to downhole equipment in wells. Specifically, the disclosure relates to equipment for centralizing tubing inserted into wellbores, including oil and gas wells, geothermal wells, and other wells.

Description of the Related Art.

[0002] Figure 1 is schematic cross sectional view of a typical well bore with telescoping casings disposed therein and cemented between the casings and the wellbore. A well having a well bore is formed below a surface. Deep wells are typically drilled in a telescoping manner with a large drill bit and hole size at the surface then with successively smaller bits until the desired total depth is reached. Each section has casing that is slightly smaller than the hole size installed, forming an annulus therebetween, with the goal that the last hole size as large as practical to maximize production of the hydrocarbon fluids. These casing strings have cement pumped around the outside in order to isolate or seal each zone.

[0003] To properly position the casing within the middle of the wellbore, centralizers are placed at intervals along the length of the casing. Among the various types of centralizers, one type is referred to as a "bow-spring centralizer." The bow-spring centralizer generally has a pair of collars that surround the casing diameter and a plurality of bows or ribs that bow outwardly in a resting state. The casing with the centralizer can be inserted into the well bore or a larger casing to form the annulus therebetween. The centralizer is inserted into the annulus with a starting force that compresses the bows around the casing radially inward and biases the casing toward a central portion of the well bore or larger casing. As the casing continues downward in the compressed state with a running force, the bows are biased to expand radially within the constraints of the annulus. Importantly, the bows need to be able to expand radially outward with sufficient restoring force into a larger space after passing through the annulus to maintain the centralized alignment of the casing in the larger space.

[0004] Historically, such a performance has been difficult due to a combination of factors, such as strength, available thickness of the annulus, and compressibility of the bow while balancing with sufficient restoring force, sometimes high heat temperatures, and other factors. [0005] The need for a thinner bow-spring centralizer has arisen from the operators seeking improved efficiencies in production for deeper wells and specifying casing sizes that have tighter fits, by closing the annular spacing currently down to one-quarter inch (1/4") (6.4 mm) radially, that is, down to one-half inch (1/2") (12.7 mm) on the diameter. Casing roundness and size tolerances are available to allow this. This trend has resulted in a need for a slim designed bow spring centralizer for use in casing centralization particularly in deep water applications throughout the world, including the Gulf of Mexico (GOM). The tighter annulus is likely to press the current bow-spring centralizers beyond their capabilities. Current bow-spring centralizers are typically made from ASTM 4130 steel, with the bows of the centralizer being nominally 0.167" thick. These existing centralizers are too tight for the above current standards and may get stuck. Even if the centralizers do not stick, the tighter annulus does not provide current centralizers with sufficient radial distance to compress and then expand with sufficient restoring force. The material can be made thinner to allow more radial compression and expansion, but current centralizer materials lack the desired restoring force to help maintain the centrality of the casing within the well bore or other casing to meet industry standards. Current centralizers present a risk that many operators are not willing to take.

[0006] A current option being used to comply with the requirements of a tighter annulus is a fully machined integral blade stabilizer/centralizer, or hybrid tool. The integral blade stabilizer resembles a short tube with solid lugs extending radially outward longitudinally down the tube. The hybrid tool can be described as a close tolerance machined bow-spring centralizer with a short tube that has been machined to a reduced diameter in the center and a bow- spring fabricated into the reduced diameter machined space. The reduced diameter effectively increases the thickness of the annulus between the casing and the reduced diameter, so the bow-spring can flex in a customary manner. The typical current cost of these machined parts ranges from $8,000- 20,000 each. Several units are needed per casing run.

[0007] Further, many of these wells encounter high pressure and/or high temperature("HPHT") conditions of up to 450 F, and the ASTM 4130 steel weakens rapidly with temperature.

[0008] The tighter annulus and the HTHP conditions, especially in deeper wells, has created a need for an improved centralizer that can accommodate these conditions. The need includes the ability to overcome the required thickness reduction to fit within the tighter annulus, to be able to operate at increased temperatures of deeper well that is generally greater than 200 F and averages 400 F, to provide a sufficiently strong and resilient bow at the reduced thickness that can spring back radially after passing through the tighter annulus and sufficiently centralize the casing in the wellbore.

BRIEF SUMMARY OF THE INVENTION

[0009] The present disclosure provides a bow-spring centralizer by using thinner bows of advanced alloys and thermal conditioning that can reduce the thickness of the bows compared to traditional bows, and yet provide sufficient restoring force after compression in a tight annulus with little radial thickness. The present disclosure also provides a specially shaped centralizer bow with cross sections that can reduce running force, increase stiffness and strength, and/or still maintain a sufficient restoring force.

[0010] The present disclosure provides a bow-spring centralizer comprising a first and second collar and a plurality of bows disposed between the collars, the bows being formed of precipitation hardening metal.The precipitation hardening metal may comprise at least one of a precipitation hardening stainless steel and a precipitation hardening nickel alloy. The plurality of bows may be sized between 0.030 inches (0.8 mm) to 0.130 inches (3.3 mm) thick. The ultimate tensile strength at room temperature of at least one of the plurality of bows may exceed 100 KSI (690 MPa). The plurality of bows may be shaped longitudinally asymmetrical or symmetrical.

[0011] At least one of the plurality of bows may be formed with a longitudinal flat portion. The flat portion may be disposed along the bow length symmetrically or asymmetrically relative to the collars. A transverse cross section of the bow may be curved along all or just part of the bow, including along a flat portion of the bow. At least a portion of at least one of the plurality of bows may be formed with longitudinal peaks and valleys, and may include both faces of the bow or just one face of the bow. The centralizer may also comprise dimples formed in at least one of the plurality of bows. The centralizer may also include a protrusion extending longitudinally from at least one end of the centralizer.

[0012] The present disclosure also provides a method of manufacturing a bow-spring centralizer, comprising forming at least a portion of the centralizer from precipitation hardening steel and heat treating at least the portion of the centralizer by precipitation hardening. The method may further comprise forming a protrusion extending longitudinally on at least one end of the centralizer. The method may further comprise forming a plurality of bows from the precipitation hardening steel. The method may comprise performing the heat treatingstep after the centralizer is formed. [0013] The method of manufacturing may further comprise cutting one or more centralizer elements in a sheet of precipitation hardening steel, forming a plurality of bows in a longitudinally curved shape from the one or more centralizer elements, andforming the centralizer with the one or more plurality of bows. The one or more centralizer elements may comprise a plurality of collars and a plurality of bows. The method may further comprise forming the one or more plurality of bows into a transversely curved structure.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0014] Figure 1 is a schematic cross sectional view of a well bore with telescoping casings disposed therein and cemented between the casings and the wellbore.

[0015] Figure 2 is a schematic cross sectional view of a well bore with casings and centralizers disposed therein.

[0016] Figure 3 A is a side schematic view of an embodiment of a centralizer according to the present disclosure.

[0017] Figure 3B is a side schematic view of another embodiment of a centralizer according to the present disclosure.

[0018] Figure 3C is a side schematic view of another embodiment of a centralizer according of the present disclosure.

[0019] Figure 4 is a schematic cross sectional partial end view of the centralizer shown in Figure 3B.

[0020] Figure 5 is a schematic cross sectional view of an alternative shaped bow of the centralizer.

[0021] Figure 6 is a schematic cross sectional view of a further alternative shaped bow of the centralizer.

[0022] Figure 7 is a schematic cross sectional view of a further alternative shaped bow of the centralizer.

[0023] Figure 8 is a schematic cross sectional partial end view of a further alternative shaped bow of the centralizer. [0024] Figure 9 is a schematic cross sectional partial end view of a bow of the centralizer with dimples.

[0025] Figure 10 is schematic partial cross sectional view of a centralizer mounted on a casing adjacent a stop collar.

[0026] Figure 11A is a schematic partial cross sectional view of an exemplary conical protrusion formed on an end of the centralizer.

[0027] Figure 11B is a schematic partial cross sectional view of another exemplary conical protrusion formed on an end of the centralizer.

[0028] Figure 11C is a schematic partial cross sectional view of another exemplary conical protrusion formed on an end of the centralizer.

DETAILED DESCRIPTION

[0029] The Figures described above and the written description of specific structures and functions below are not presented to limit the scope of what Applicants have invented or the scope of the appended claims. Rather, the Figures and written description are provided to teach any person skilled in the art to make and use the inventions for which patent protection is sought. Those skilled in the art will appreciate that not all features of a commercial embodiment of the inventions are described or shown for the sake of clarity and understanding. Persons of skill in this art will also appreciate that the development of an actual commercial embodiment incorporating aspects of the present disclosure will require numerous implementation- specific decisions to achieve the developer's ultimate goal for the commercial embodiment. Such implementation- specific decisions may include, and likely are not limited to, compliance with system-related, business-related, government-related, and other constraints, which may vary by specific implementation, location, and from time to time. While a developer's efforts might be complex and time-consuming in an absolute sense, such efforts would be, nevertheless, a routine undertaking for those of ordinary skill in this art having benefit of this disclosure. It must be understood that the inventions disclosed and taught herein are susceptible to numerous and various modifications and alternative forms. The use of a singular term, such as, but not limited to, "a," is not intended as limiting of the number of items. Also, the use of relational terms, such as, but not limited to, "top," "bottom," "left," "right," "upper," "lower," "down," "up," "side," and the like are used in the written description for clarity in specific reference to the Figures and are not intended to limit the scope of the invention or the appended claims. Where appropriate, one or more numbered elements may have been labeled with an letter, such as "A" or "B," (or if lettered elements, then with numbers, such as "1" or "2") to designate various members of a given class of an element. When referring generally to such elements, the number without the letter can be used. Further, such designations do not limit the number of members that can be used for that function. The various methods and embodiments of the system can be included in combination with each other to produce variations of the disclosed methods and embodiments. Discussion of singular elements can include plural elements and vice-versa. References to at least one item may include one or more items. Also, various aspects of the embodiments could be used in conjunction with each other to accomplish the understood goals of the disclosure. Unless the context requires otherwise, the word "comprise'Or variations such as "comprises'Or "comprising," should be understood to imply the inclusion of at least the stated element or step or group of elements or steps or equivalents thereof, and not the exclusion of a greater numerical quantity or any other element or step or group of elements or steps or equivalents thereof. The device or system may be used in a number of directions and orientations. The term "coupled," "coupling," "coupler," and like terms are used broadly herein and may include any method or device for securing, binding, bonding, fastening, attaching, joining, inserting therein, forming thereon or therein, communicating, or otherwise associating, for example, mechanically, magnetically, electrically, chemically, operably, directly or indirectly with intermediate elements, one or more pieces of members together and may further include without limitation integrally forming one functional member with another in a unity fashion. The coupling may occur in any direction, including rotationally.

[0030] The present disclosure provides a bow-spring centralizer by using thinner bows of advanced alloys and thermal conditioning that can reduce the thickness of the bows compared to traditional bows, and yet provide sufficient restoring force after compression in a tight annulus with little radial thickness. The present disclosure also provides a specially shaped centralizer bow with cross sections that can reduce running force, increase stiffness and strength, and/or still maintain a sufficient restoring force.

[0031] Figure 2 is schematic cross sectional view of a well bore with casings and centralizers disposed therein. A well 1 having a well bore 2 is formed below a surface 4. A first casing 6 is installed in the well bore to form an annulus 8 having a thickness Tl between the well bore and the outer periphery of the casing 6. One or more centralizers 10 can be installed with the casing 6 to bias the casing toward the center of the well bore due to the bows of the centralizer that expand outwardly. Cement or other sealing material 12 can be placed in the annulus 8 around the centralizer to seal the annulus. Similarly, a second casing 14 of smaller diameter than the first casing 6 can be inserted into the well bore to extend below the first casing as the well is bored deeper. The difference in the inner diameter of the first casing 6 to the outer diameter of the second casing 14 forms an annulus 16 therebetween, having a thickness T2. One or more centralizers 18 can be installed around the casing 14, generally before insertion into the casing 6, to bias the casing 14 toward a center of the casing 6 when inserted therein. A centralizer 18A is shown in a compressed state in the annulus 16. The annulus 16 can have a smaller thickness resulting in a tighter annulus. A centralizer 18B is shown having sprung back radially with a restoring force after having been pushed through the tight annulus 16. Importantly, the centralizer 18 needs to have sufficient restoring force after passing through the annulus 16 to be able to control and bias the casing 14toward the center of the wellbore (or further casings as successive casings are inserted into the wellbore). While the above discussion has referenced a tighter annulus 16 between the casing 6 and the casing 14, the size of the annulus 8 could also be a tight annulus, as well as further casings inserted within other casings downhole as the well is drilled deeper.

[0032] Figure 3A is a side schematic view of an embodiment of a centralizer according to the present disclosure. The centralizer 10 or 18 can include a first collar 20 and a second collar 22 coupled together with one or more bows 24A (generally referenced as "24" herein). Generally, the number of bows is two to six or more per centralizer. The bows are formed to bow outwardly in a rest state, so that compression in the annulus, described above, stresses the bows to spring back and expand radially outward when additional space is provided, such as in the well bore. The bow 24 has a thickness B. In one embodiment, the thickness of the bow is substantially the same from an upper to lower portion of the bow; in other embodiments, the thickness of the bow may change over the length of the bow. Advantageously, the material selected for the bows in the present application allow a thinner bow than typically is used for a given size, but still retains a sufficient restoring force and optimizing contact mechanics to comply with standards and/or regulations, as described herein. The centralizers 10, 18 each have an overall length Z that includes a bow length Y, the first collar length CI, and the second collar length C2. The length of the bow Y can determine the stress level on the bow for a given deflection D (shown as the maximum deflection realizing that the deflection D would be less depending on the thickness of the annulus in which the centralizer bow is compressed radially).

[0033] The bows 24A can be formed longitudinally symmetrical, so that the bow curves symmetrically from a longitudinal midpoint 40 on the bow toward each of the collars 20, 22 as measured from a centerline 38 of the centralizer. An example of a symmetrical bow is illustrated as the bow on the centralizer in Figure 3A. An equal distance A toward each of the collars 20, 22 longitudinally from a midpoint 40 of the bow has the same or similar (symmetrical) radial distance El from a centerline 38 of the centralizer. In other words, at a distance A above and below midpoint 40, the symmetrical bow 24A may expand out an approximately equal distance of El.

[0034] Figure 3B is a side schematic view of another embodiment of a centralizer according to the present disclosure. The embodiment in Figure 3B has similar components as in Figure 3A, except for the shape of the bow 24B. The bow 24B can be formed longitudinally asymmetrical, so that the bow curves asymmetrically from the longitudinal midpoint 40 of the bow toward each of the collars 20, 22. An equal distance A toward each of the collars 20, 22 longitudinally from a midpoint 40 of the bow has different (asymmetrical) radial distances El and E2 from a centerline 38 of the centralizer. In other words, at a distance A above and below midpoint 40 the asymmetricalbow 24B may expand out at approximately different distances of El and E2.

[0035] Figure 3C is a side schematic view of another embodiment of a centralizer according tothe present disclosure. The centralizer 10 or 18 can include the collars 20 and 22 as described above that are coupled with one or more bows 24C. The bows are similarly formed to bow outwardly in a rest state. However, in this embodiment, the bow can be shaped with at least one substantially flat portion 36 when viewed from a side view and at least one of bow portions 38A and 38B to couple to at least one of the collars 20 and 22. Flat portion 36 may have a length F, and portions of the bow on either side of flat portion 36 may have lengths G and H. The flat portion 36 has more surface area for engagement with an inner surface of the casing or wellbore in which the centralizer is disposed. The flat portion 36 can increase a restoring force of the bow. The flat portion can be placed at various longitudinal locations along the bow 24C. For example, the flat portion can be formed at midpoint 40 from the collars 20 and 22 so that lengths G and H are approximately equal and the bow forms a symmetrical shape, such as shown in the bow shape in Figure 3A. Alternatively, the flat portion can be formed closer to one collar, so that lengths G and H are not equal and the bow forms an asymmetrical shape, such as shown in the bow shape in Figure 3B.

[0036] In these exemplary bows and others that are contemplated or feasible, the shape of the bow can be optimized for starting and running forces, and these and other shapes are contemplated or possible.

[0037] Figure 4 is a schematic cross sectional partial end view of the centralizer shown in Figure 3B. The centralizer 10, 18 has a collar 20 with the bow 24 coupled thereto. A thickness B of a cross section 26 of the bow 24 can be established. The geometry of the cross sectional shape can vary. For example in Figure 4, the bow is relatively flat in cross section. In other embodiments, such as shown in Figure 8, the bow 24 can have a radius that can correlate withthe surrounding shape of the wellbore or casing into which the centralizer is inserted.

[0038] As referenced above, a typical thickness of a prior art bow is 0.167" of ASTM 4130 that is needed for sufficient restoring force with prior designs in the field. Yet, this thickness has almost no ability to significantly compress and then expand to a significant amount when inserted in a tighter annulus, for example, of 0.25" annulus thickness. By contrast, in at least one non- limiting example, the thickness B of the present disclosure can be 1/3 to about 2/3 of the above typical thickness, that is, for example, about 0.03 inches (0.8 mm) to about 0.13 inches (3.3 mm). The thinner cross section of the material disclosed herein allows for a variety of shapes that normally would not be suitable due to the required greater thickness of traditional material in a given annulus.

[0039] Figure 5 is a schematic cross sectional view of an alternative shaped bow of the centralizer. This embodiment of the bow 24 has a cross section 26 with a series of peaks 28 and valleys 30 on one face to form "streamers" with corresponding valleys 32 and peaks 34 on the opposite face. The bow thickness B 1 can be relatively uniform in this embodiment (although in other embodiments, the thickness can vary). The peaks can form a principal contact surface while passing through the annulus, described herein. The effective bow thickness B2 is the thickness between a projected line from the peak 28 to the peak 34. The combination of the peaks and valleys can provide additional strength and stiffness to the bow 24 and can further reduce the bow thickness B l. Generally, the annular thickness Tl or T2, shown in Figure 2, will influence the effective bow thickness B2. It is noted that the peaks and valleys can also be flattened or otherwise change shape as the bow passes through the annulus described above and then spring back into a shape resembling the original shape after passing through the annulus.

[0040] Figure 6 is a schematic cross sectional view of a further alternative shaped bow of the centralizer. This embodiment of the bow 24 has a cross section 26 with a series of peaks 28 and valleys 30 on one face with relatively flat face 36 on the opposite face. The bow thickness B can vary across the width of the bow with the minimum bow thickness B3 being between a valley 30 and a face 36. The effective bow thickness B4 is the thickness between a projected line from peak 28 to the face 36. The peaks and valleys can provide additional strength and stiffness to the bow 24 and can reduce the minimum bow thickness B3 to less than the bow thickness B shown in Figure 3 A.

[0041] Figure 7 is a schematic cross sectional view of a further alternative shaped bow of the centralizer. The bow 24 can have a non-uniform cross section 26 that varies between a cross sectional portion 42 not having peaks and valleys and a portion 44 with peaks and valleys (or other variations). The number of portions and location of portions across the width of the bow 24 can vary.

[0042] Further variations can include forming the peaks and valleys along a longitudinal portion of the bow. For example, the longitudinal portion having the peaks and valleys could be the portion that initially enters the annulus, which can assist with reducing a starting force and/or running force for the bow to be compressed as it enters into and/or travels through the annulus.

[0043] Figure 8 is a schematic cross sectional partial end view of a further alternative shaped bow of the centralizer. The transverse cross section of the bow can be curved with a radius R that can generally correlate with an inner surface of a wellbore or casing into which the centralizer is inserted. The radius R can more fully engage the inner surface with additional area of contact surface compared to a shape that does not correlate with the inner surface (such as that shown in FIG. 4) to adjust forces exerted by the centralizer on the inner surface. In some embodiments, the radius can substantially match the shape of the wellbore or casing, and in other embodiments the radius can be slightly larger or smaller to vary the force exerted by the centralizer. The overall curved surface shape can be used in conjunction with any of the other cross sectional shapes described herein including Figures 5 - 7 or other variations in shapes.

[0044] Figure 9 is a schematic cross sectional partial end view of a bow of the centralizer with dimples. A further variation is to form small indentions or protrusions46 and 48 (herein "dimples") that either extend outward from the centralizer (see dimples 46), inward toward interior surfaces of the centralizer (see dimples 48), or both. The dimples may affect the starting and/or running force for the bows. Dimples, corrugated shapes with the streamers, and other deformations described herein can be formed prior to heat treating the precipitation hardening of the heat treating process.

[0045] Figure 10 is a schematic partial cross sectional view of a centralizer mounted on a casing adjacent a stop collar. Generally, a centralizer 10, 18 can be positioned at various locations along the casings 6, 14. To hold the centralizer at that location, a pair of stop collars 50 adjacent each centralizer collar 20, 22 are generally coupled to the casings to restrict the centralizer movement along the casing to a space between the stop collars 50. The stop collars 50 can be thermally sprayed onto the casing, mechanically coupled to the casing such as with a clamp, or otherwise coupled in position on the casing. [0046] In the present disclosure, the centralizer collars 10, 18 can be substantially thinner than a typical centralizer as described herein and meet the requirements of a centralizer within the limited annular clearances between the wellbore and the casing. Further, the stop collars are required to be correspondingly thinner than a typical stop collar as well due to the same limited annular clearances. To assist the thinner centralizer abutting a corresponding thinner stop collar from becoming dislodged from the intended position by radially expanding over the stop collar, an end 56 of the centralizer collar (see Figures 11A-11C) adjacent the stop collar 50 can be formed to engage the stop collar and restrict radial expansion. In at least one embodiment, a thinned portion of the end 56 of the collars 20, 22 can be made to engage the stop collar 50. The engagement shape of the end 56 can take several forms, including one or more of the nonlimiting examples illustrated in Figures 11A, 11B, and 11C, described below, among others. Generally, the outside diameter 58 of a centralizer collar is effectively reduced at the end 56 of the collar, resulting in a generally circular protrusion 54 at the end with a transition surface 52 between the outside diameter 58 of the collar 20, 22 and the protrusion. The protrusion 54 therefore has a smaller outside diameter at a point of contact with the stop collar 50 than the remaining diameter of the collar. The smaller outside diameter protrusion can engage a corresponding smaller diameter surface on the stop collar 50. Generally, a greater longitudinal force on the centralizer causes a greater force of the centralizer end against the stop collar, which resists radial expansion of the centralizer collars 20, 22.

[0047] Figure 11A is a schematic partial cross sectional view of an exemplary conical protrusion formed on an end of the centralizer. The centralizer 20, 22 has at least one end 56 formed with a smaller outside diameter than the centralizer collar outside diameter 58, thus forming a protrusion 54at the end of the collar with a transition surface 52 between the outside diameter 58 and the protrusion. As shown in FIG. 11 A, the protrusion can be relatively sharp and can engage the end of the stop collar 50. With sufficient force on the centralizer, the protrusion can engage the stop collar and cause a deformation in the stop collar that assists in restricting the centralizer collar from radially expanding over the stop collar.

[0048] Figure 11B is a schematic partial cross sectional view of another exemplary conical protrusion formed on an end of the centralizer. The centralizer 20, 22 has at least one end 56 formed with a smaller outside diameter than the centralizer collar outside diameter 58, thus forming a protrusion 54 with a transition surface 52 between the outside diameter 58 and the protrusion. The protrusion 54 at the end can be more rounded than shown in Figure 11 A, which may be beneficial in particular configurations of the collar / stop engagement. The protrusion contacts the end of the stop collar and helps maintain the radial position of the centralization collar from expansion.

[0049] Figure 11C is a schematic partial cross sectional view of another exemplary conical protrusion formed on an end of the centralizer. The centralizer 20, 22 has at least one end 56 formed with a smaller outside diameter than the centralizer collar outside diameter 58, thus forming a protrusion 54 with a transition surface 52 between the outside diameter 58 and the protrusion. Rather than having a substantially straight angled transition surface 52 as in FIGs. 11A and 1 IB, the protrusion 54 can be a stepped portion having a shoulder as a transition portion 52. The protrusion contacts the end of the stop collar and helps maintain the radial position of the centralization collar from expansion.

[0050] While various methods exists for manufacturing the bow-spring centralizers, an exemplary non-limiting method could include cutting the material through waterjetor laser cutting, forming the centralizer shape, welding portions together such as the cylindrical ends, inert atmosphere heat treating including a vacuum atmosphere, and low temperature thermal treatments as a type of further heat treating. In other embodiments, the collars can be formed separately from the bows and the components welded together, and heat treated. The shapes and variations described herein and others that are in keeping with the underlying principles disclosed herein can be optimized using modeling and simulations using computational fluid dynamics, finite element analysis, thermal analysis and applying computer aided engineering integration of different meshers, solvers, and other codes as appropriate for design capability, performance, and reliability. Exemplary factors could include optimizing contact pressure between two curved surfaces that depends on that type and radius of curvature, magnitude of contact force, elastic modulus, and Poisson's ratio of contact surfaces, among others.

[0051] From the hundreds of possible metals, the inventors have realized that the material for the centralizer can advantageously be a precipitation-hardening, corrosion-resistant steel. This type of material differs from customary materials used in the oil field service industry that typically rely on ASTM 4130 (or other similar materials, such as ASTM 4140, 4340). Expectations in the oil field industry would teach away from precipitation-hardening steels due to the perception that many such steels are subject to stress corrosion cracking under oil field conditions. Contrary to such expectations, the inventors utilize a thermal conditioning procedure that allows such steels to harden into a high strength steel to allow a relatively thin centralizer bow yet reduce the stress corrosion cracking tendency. These steels have ultimate tensile strengths at room temperature that exceed 100 KSI (690 MPa), many exceed 140 KSI (965 MPa), and various levels therebetween, and some are higher. Further, these steels have higher temperature resistance to loss of strength compared to the customary ASTM 4130 steel and similar steels.

[0052] The selected materials can withstand the higher stress and the higher temperatures that current designs are not in general able to meet performance criteria. Precipitation hardening stainless steels and their desired upper operating temperatures include, but are not limited to, 15- 5PH (600 F), 17-4PH (600 F), 17-7PH (500 F), 15-7MoPH (900 F). Precipitation hardening nickel alloys and their desired operating temperatures include, but are not limited to, alloy 718 (1300 F+) andalloy750 (1300 F+), and otherprecipationhardening nickel alloys.

[0053] Further, the embodiments of Figures 5, 6, and 7 will generally have less radial surface contact with the casing or well bore into which the centralizer is inserted. Less surface contact is believed to reduce the amount of running force needed to push the centralizer downhole during installation and may also reduce the amount of starting force to compress the centralizer bows into the casing or well bore. A reduction in the amount of running force and/or starting force can have significant impacts on a string of casing miles in length with potentially hundreds of centralizers installed thereon that are inserted into a well bore.

[0054] One example of a suitable precipitation-hardening metal is 17-7PH stainless steel. Exemplary specifications for this precipitation hardening steel shows a composition of the following with all numerical values given in weight percent with the balance being iron with trace percentages of other elements:

Chromium 16.00-18.00

Nickel 6.50-7.75

Aluminum 0.75-1.50

Carbon 0.09 max.

Manganese 1.00 max.

Phosphorus 0.040 max.

Sulfur 0.030 max.

Silicon 1.00 max.

[0055] Another example of a suitable precipitation-hardening steel is alloy 718. Exemplary specifications for this precipitation hardening steel shows composition of the following with all numerical values given in weight percent with the balance being iron with trace percentages of other elements:

Chromium 17.00-21.00 Nickel (plus Cobalt) 50.00-55.00

Niobium (plus Tantalum) 4.75-5.50

Molybdenum 2.80-3.30

Titanium 0.65-1.15

Aluminum 0.20-0.80

Cobalt 1.00 max.

Carbon 0.08 max.

Manganese 0.35 max.

Phosphorus 0.015 max.

Sulfur 0.015 max.

Silicon 0.35 max.

Boron 0.006 max.

Copper 0.30 max.

[0056] A further example of a suitable precipitation hardening material is 15-7MoPH stainless steelwith molybdenum. Specifications for the material list a composition of the following with all numerical values given in weight percent with the balance being iron with trace percentages of other elements:

Chromium 14.00-16.00

Nickel 6.50-7.75

Molybdenum 2.00-3.00

Aluminum 0.75-1.50

Carbon 0.09 max.

Manganese 1.00 max.

Phosphorus 0.040 max.

Sulfur 0.040 max.

Silicon 1.00 max.

[0057] Some precipitation hardening metals, but not all, are subject to hydrogen embrittlement with extended exposure to environmental conditions, including salt water. The general operational requirement of a centralizer is to be sufficiently strong for one pass downhole before the annulus is cemented for permanency. Thus, in such instances where the particular precipitation hardening metal might be susceptible to embrittlement, a simple coating can be applied, including paint, or tape that can be easily removed if desired, and other protective coatings for a temporary period before use. [0058] The above materials can be heat treated for precipitation hardening, which can vary depending on the particular participation hardening steel used and the performance results desired. Below are representative heat treatment processes that are believed to be suitable for the desired use described herein and other heat treatments can be found in the relevant art.

[0059] For the 17-7PH stainless steel, the metal is generally purchased in an annealed state (or can be annealed subsequent to the purchase). An exemplary annealing process includes heating the metal to a temperature of 1950 F +/- 25 F (1066 C +/- 14 C). Optionally, fabrication can be performed in this condition. The subsequent heat treatment to develop the strength and hardness includes three overall steps: austenite conditioning, cooling to transform the austenite to martensite, and precipitation hardening. For austenite conditioning, the steps can include heating to 1750 F +/- 15 F (954 C +/- 8 C), holding for 10 minutes, and air cooling to room temperature. For cooling to transform the austenite to martensite, the steps can include within an hour of the air cooling to room temperature starting to cool to -100 F +/- 10 F (-73 C +/- 5.5 C), holding for eight hours, and then air warming to room temperature. For precipitation hardening, the steps include heating to 950 F +/- 10 F (510 C +/- 5.5 C), holding for one hour, then air cooling to room temperature. This process develops a high strength participation-hardened metal of about 230 KSI ultimate tensile strength (UTS), 201 KSI yield strength (YS), with elongation of approximately 7% at room temperature. Each of the above steps can be varied to produce different results with the metal.

[0060] For alloy718, the metal is generally heat treated by solution annealing and precipitation hardening (age hardening). An exemplary heat treatment process includes solution annealing at a temperature between 1700-1950 F followed by rapid cooling, usually in water, plus precipitation hardening between 1325-1450 F for six to ten hours, and sometimes followed by a lower aging temperature of 1200 F for several more hours. Within these general parameters, three exemplary variations are as follows. One variation is solution annealing at 1700-1850 F, followed by rapid cooling, usually in water, plus precipitation hardening at 1325 F for eight hours, furnace cooling to 1150 F, holding at 1150 F for a total aging time of 18 hours, followed by air cooling. The UTS is about 180 KSI, the YS is about 150 KSI, and elongation is about 12% at room temperature. A second variation is solution annealing at 1900-1950 F, followed by rapid cooling, plus precipitation hardening at 1400 F for ten hours, furnace cooling to 1200 F, holding at 1200 F for total aging time of 20 hours, followed by air cooling. The UTS is about 180 KSI, the YS is about 150 KSI, and elongation is about 15% at room temperature. A third variation is solution annealing at 1850-1900 F, rapid cooling, and aging at 1450 F for six to eight hours, and then air cooling. The UTS is about 150 KSI, the YS is about 140 KSI, and elongation is about 20% at room temperature. Each of the above steps can be varied to produce different results with the material.

[0061] For the 15-7PH stainless steelwith molybdenum, the metal is generally purchased in an annealed state or can be annealed subsequent to the purchase. An exemplary annealing process includes heating the metal to a temperature of 1950 F +/- 25 F (1066 C +/- 14 C). Optionally, fabrication can be performed in this condition. The subsequent heat treatment to develop the strength and hardness includes three overall steps: austenite conditioning, cooling to transform the austenite to martensite, and precipitation hardening. For austenite conditioning, the steps can include heating to 1750 F +/- 15 F (954 C +/- 8 C), holding for 10 minutes, and air cooling to room temperature. For cooling to transform the austenite to martensite, the steps can include within an hour of the air cooling to room temperature starting to cool to -100 F +/- 10 F (-73 C +/- 5.5 C), holding for eight hours, and then air warming to room temperature. For precipitation hardening, the steps include heating to 950 F +/- 10 F (510 C +/- 5.5 C), holding for one hour, then air cooling to room temperature. This process develops a high strength participation- hardened metal of about 240 KSI ultimate tensile strength (UTS), 225 KSI yield strength (YS), with an elongation of approximately 6% at room temperature. Each of the above steps can be varied to produce different results with the material.

Examples

[0062] Specimens of the typical ASTM 4130 material used for centralizers were cut from a commercially manufactured bow spring centralizer. The exemplary centralizer bows are about 21 inches long when flat (that is, not bowed in an arc), about 1.5 inches wide, and about 0.167 inches thick. Two other sets of bow springs were prepared of 17-7PH stainless steel and alloy 718 of about the same dimensions but only about 0.092 inches thick so that these bow springs could fit within a tighter annulus with less radial thickness that is desired for current drilling needs.

[0063] For the 17-7PH stainless steel, the heat treat of an annealed bar after being formed in an equivalent shaped bow-spring as the ASTM 4130 bow was austenite conditioning at 1735-1765 F for 10 minutes in a vacuum atmosphere, then cooling to -100 F +/- 10 F for eight hours an air atmosphere, then precipitation hardening at 950 F +/- 10 F for one hour in a vacuum atmosphere.

[0064] For the alloy 718, the heat treat of an annealed bar after being formed in an equivalent shaped bow-spring as the ASTM 4130 bow was solution annealing at a temperature between 1725-1825 F in a vacuum atmosphere, followed by rapid cooling plus precipitation hardening at 1325 F +/- 15 F for 495 minutes +/- 15 minutes in a vacuum atmosphere, and then holding at 1150 F +/- 15 F for another 480 minutes for a total of about 16 to 16.5 hours.

[0065] The bows were each measured to determine the maximum height of the arc in the bow in an uncompressed state when its ends were resting on a horizontal surface. Each bow was then subjected to a sufficient load on the arc to push the bow substantially flat on the horizontal surface, held momentarily, and then the load was released to simulate the bow passing through a tight annulus between a casing and pipe and then springing back to reform an arc after passing through the annulus. The resulting height after the flattening was then measured to determine the resulting permanent deformation by the difference in heights and percentage of change as a spring back percentage.

[0066] The results are shown in the below table:

Delta free

Prior to Set After Set length Spring Back

Specimen Solid— Free Solid— Free =Column D %

Temperature Mat'l # Length Length - E (E/D* 100)

Ambient 4130 5 2.244 1.903 0.341 85

Ambient 17-7PH 1 2.308 2.225 0.083 96

Ambient 17-7PH 2 2.36 2.3 0.06 97

Ambient 718 1 2.395 2.242 0.153 94

Ambient 718 2 2.415 2.295 0.12 95

150 DF 4130 2 2.32 1.961 0.359 85

150 DF 17-7PH 3 2.378 2.298 0.08 97

150 DF 17-7PH 4 2.425 2.345 0.08 97

150 DF 718 3 2.418 2.328 0.09 96

150 DF 718 4 2.38 2.291 0.089 96

300 DF 4130 3 2.35 1.698 0.652 72

300 DF 17-7PH 5 2.462 2.372 0.09 96

300 DF 17-7PH 6 2.447 2.357 0.09 96

300 DF 718 5 2.438 2.198 0.24 90

300 DF 718 6 2.42 2.167 0.253 90 450 DF 4130 4 2.338 1.703 0.635 73

450 DF 17-7PH 7 2.46 2.379 0.081 97

450 DF 17-7PH 8 2.43 2.351 0.079 97

450 DF 718 7 2.38 2.281 0.099 96

450 DF 718 8 2.387 2.208 0.179 93

[0067] The results show that even with thinner material in the precipitation hardening steels of the two exemplary materials, the spring back percentage was advantageously far greater than the typical ASTM 4130 material. The thickness can be reduced by at least 30% and more, thereby allowing the fabrication of the thinner bow springs that can meet the application requirements. The spring back percentages were advantageously even more noticeable at the higher tested temperatures.

[0068] The order of steps can occur in a variety of sequences unless otherwise specifically limited. The various steps described herein can be combined with other steps, interlineated with the stated steps, and/or split into multiple steps. Similarly, elements have been described functionally and can be embodied as separate components or can be combined into components having multiple functions.

[0069] The invention has been described in the context of preferred and other embodiments and not every embodiment of the invention has been described. For example, other sizes could be similarly designed with the resulting differences in flow volumes described above. Obvious modifications and alterations to the described embodiments are available to those with ordinary skill in the art given the teachings disclosed herein. In conformity with the patent laws, the claims determine the scope or range of equivalents, rather than the disclosed exemplary embodiments, with the understanding that other embodiments within the scope of such claims exist.