DESCRIPTION OF PREFERRED EMBODIMENTS
Referring first to FIGS. 1 to 3, a casing centralizer 10 is a unitary annulus comprising a generally cylinder body 12, and an array of five equiangularly-spaced blades 14 integrally formed with the body 12. A cylindrical bore 16 extends longitudinally and coaxially through the body 12, the bore 16 having a substantially uniform diameter dimensioned to be a clearance fit around the well bore casing (not shown in FIGS. 1 to 8). Each of the blades 14 (see also FIGS. 4 and 5) not only extends between longitudinally opposite ends of the body 12, but also extends circumferentially part-way around the periphery of the centralizer. The skewing of the blade 14 ensures that their respective radially outer edges 18 collectively provide a circumferentially substantially uniform well bore-contacting surface for the centralizer 10, as most particularly shown in FIGS. 2 and 3.
Each of the blades 14 has a respective radially inner root 20 integral with the body 12. In each of the blades 14, the root 20 has a greater circumferential width than the outer edge 13, ie the cross-section of each blade 14 tapers towards the well bore-contacting periphery of the centralizer 10. The individual and collective shapes of the blades 14, and of the longitudinal fluid flow passages defined between adjacent pairs of the blades 14, gives the centralizer 10 improved flow characteristics and minimises the build-up of trapped solids during use of thecentralizer 10.
Longitudinally opposite ends of the blades 14, and of the body 12, are chamfered to assist in movement of the centralizer 10 up/down a well bore.
Although the blades 14 are shown separately from the body 12 in FIGS. 4 and 5 (and while the blades 4 could be separately formed and subsequently attached to the body 12 by any suitable means) it is preferred that the entire centralizer 10 is fabricated as a one-piece article.
The centralizer 10 may be manufactured entirely from a plastics, elastomeric and/or rubber material. Alternatively, the centralizer may comprise a metal body coated, or partially coated, with a plastic elastomeric and/or rubber material.
Examples of possible plastic, elastomeric and/or rubber materials are polytetrafluoroethylene (PTFE), polyetheretherketone, carbon reinforced polyetheretherketone, polyphthalamide, polyvinylidene fluoride, polyphenylylene sulphide, polyetherimide, polyethylene, polysulphone, polyethersulphone, polybutyleneterephthalate, polyetherketoneketone, polyamides, rubber & rubber compounds, phenolic resins or compounds, thermosetting plastics, thermoplastic elastomers, thermoplastic compounds or thermoplastic polyester resins.
The plastics elastomeric and/or rubber material may contain a filler. Examples of possible fillers are glass, carbon, PTFE, silicon, molybdenum disulphide, graphite, oil or wax, or any combination of these materials.
Use of a plastic, elastomeric and/or rubber material gives a number of advantages, including:—chemical resistance, such as resistance to acid; non-sparking (ie sparks are not generated if the centraliser in 10 collides with steel); and, materials such as PTFE give superior bearing properties.
Since the bore 16 is clearance fit around the casing and since the bore 16 lacks any means of tightly gripping a normally dimensioned casing, the centraliser 10 can not only rotate freely around the casing but also move freely along the casing (unless and until the centraliser collides with an obstruction, for example a protruding casing joint). Thus to provide longitudinal restraint for the centralizer 10 to retain the centraliser substantially at its preferred location along the casing but without impairing the relative rotatability of centralizer and casing, use is made of a stop collar 50, as illustrated in FIG. 6.
FIG. 6 shows a modified form of casing centralizer 100, fitted around hollow tubular casing 102 which is located within a well bore 104. The modified centralizer 100 is essentially the same as the centralizer 10 described above, and differs principally in the dimensions and proportions of its blades 106. In particular, the blades 106 are circumferentially wider at the lower end of the centralizer 100 than they are at the upper end. FIG. 6 also illustrates the manner in which the centralizer will hold casing out of direct contact with the well bore and centrally within the well bore, in preparation for subsequent cementing.
FIG. 7 shows a modified plastic centralizer 110 located around a length of casing 112. The centralizer 110 has blades R1, R2, R3 and R4 spaced around its outer surface to contact the inner surface of the wellbore and to centralize the casing 112 therein. The blades R extend axially along the centralizer but can alternatively extend around the outer circumference of the centralizer like the blades 106.
The centralizer 110 is axially divided along the midline of opposing blades R2 and R4 to form two half shells 110a and 110b, so that the blades R2 and R4 are formed only when the opposing faces F of the half shells 110a and 110b are joined together. Half shell 110a has two threaded sockets S in each of the faces F of R2 and R4 to receive bolts B protruding through the faces F of the other half shell 110b. The bolts B engage in the sockets S and pull the faces F together when the centralizer 110 is made up around the casing 112 and the bolts tightened.
The centralizer 110 can be formed from polytetrafluoroethylene (PTFE), polyetheretherketone, carbon reinforced polyetheretherketone, polyphthalamide, polyvinylidene fluoride, polyphenylylene sulphide, polyetherimide, polyethylene, polysulphone, polyethersulphone, polybutyleneterephthalate, polyetherketoneketone, polyamides, rubber & rubber compounds, phenolic resins or compounds, thermosetting plastics, thermoplastic elastomers, thermoplastic compounds of thermoplastic polyester resins.
The centralizer 110 is useful with coil tubing applications, but may also be used for casing and screens to afford protection from acids and other harmful chemicals downhole.
In the case of casing located within larger diameter casing, centralizers can be employed on the inner casing to hold it out of direct contact with the outer casing.
Advantages of the invention are that the use of a plastic, elastomeric and/or rubber material for the centraliser helps to provide chemical resistance, such as resistance to corrosion from acid. Other advantages are that the materials are generally non sparking and that certain materials for example PTFE, have superior bearing properties.