Wells – Guide for device or conduit
Reexamination Certificate
1999-04-30
2001-04-03
Tsay, Frank (Department: 3672)
Wells
Guide for device or conduit
C166S244100
Reexamination Certificate
active
06209638
ABSTRACT:
BACKGROUND—FIELD OF THE INVENTION
The present invention relates generally to accessory equipment for oilfield tubular strings, especially casing strings used in casing earth boreholes drilled for oil and natural gas production. More specifically, the present invention relates to casing accessory equipment, including casing string centralizers and “float” equipment used in cementing the casing strings in place, which have bow spring centralizers mounted thereon such that the centralizers may collapse and permit the accessory equipment to readily pass through tight annular clearances (such as through wellheads and the like) no larger than the largest body diameter of the accessory equipment, then permit the centralizers to spring back out to properly centralize the casing string in open hole or a larger casing string.
BACKGROUND—RELATED ART
Earth boreholes drilled for oil and natural gas wells typically have one or more “casing strings” run and cemented in place during the course of the drilling program. A typical drilling sequence is to drill a length of open hole in the earth (perhaps several thousand feet in length), then lower a casing string having an outer diameter somewhat smaller than the diameter of the drilled hole to a position usually near the bottom of the open hole section. The casing string (which also may be several thousands of feet long) is usually comprised of a number of joints, each being on the order of forty feet long, connected to one another by threaded connections or other connection means. Cement is then pumped down through the inner bore of the casing, exits the bottom of the casing, and is displaced upwardly to a desired “cement top” depth in the annulus between the casing and the open hole. The cement supports and anchors the casing in place, and (ideally) forms a hydraulic bond between the casing string and the wall of the borehole. It is important, then, for the cement to be displaced to the required depth, and for the casing to be substantially centered in the borehole, so that a uniform cement sheath may be formed around the casing string (that is, if the casing were touching the borehole wall on one side, a cement bond could not be formed at that location).
Several types of casing accessory equipment assist in properly placing the cement so as to ensure a good hydraulic bond between the casing string and the wall of the borehole. “Float equipment” (which includes float shoes and float collars) includes usually relatively short (perhaps three to four feet long) sections of tubular member, with the bore of the tubular member having a casting therein, in which a check or one-way valve is seated. Usually, a “float shoe” is affixed to the very bottom end of the casing string; one or more joints of casing, referred to as “shoe joints” are then made up above the float shoe; then a “float collar” is made up, with the remainder of the casing string made up to complete the casing string. The one way valves in the float shoe and float collar permit cement (and other fluids) to be pumped down through the casing string, out the casing shoe and up around the casing string, but prevent any flow back into the casing string. The outer diameter of the tubular member used in float equipment is typically somewhat larger than the nominal diameter of the casing string to which it is attached, typically roughly equal to the diameter of the casing “collars” which comprise part of the threaded connections between casing joints.
“Centralizers” are often mounted on casing strings to center the casing string in the borehole and obtain a uniform thickness cement sheath around the casing string. The centralizers provide blades (of different possible configurations, as discussed herein) extending out from the casing wall and contacting the borehole, thereby holding the casing string off of direct contact with the borehole, and substantially centralizing the casing therein. To accomplish that goal, the centralizer blades typically form a total centralizer diameter roughly the diameter of the borehole in which the casing string is run.
Different types of centralizers have been used. One type comprises a solid central tubular body having a plurality of solid blades integral with the central body, the blades extending out to the desired diameter. Yet another type in use for many years are “bow spring” centralizers, which typically comprise a pair of spaced-apart bands which can be opened to encircle a casing string, then locked in place on the casing; and a number of outwardly bowed, resilient bow spring blades connecting the two bands, spaced around the circumference of the bands. The configuration of bow spring centralizers permits the bow spring blades to at least partially collapse as the casing string is run into the borehole and passes through any restricted diameter location, such as a piece of equipment having an inner diameter smaller than the at-rest bow spring diameter, then spring back out after passage through the reduced diameter equipment. One type of casing accessory equipment which comprises a bow spring centralizer is a “casing string centralizer”, which comprises a relatively short tubular member (on the order of three to four feet), which is made up into the casing string at selected locations over its length. However, even when collapsed, a conventional bow spring centralizer mounted on a casing string presents a diameter necessarily larger than the nominal casing diameter.
Centralization of a casing string near its bottom end, in particular around the float equipment, is usually considered especially important to securing a uniform cement sheath and consequently a hydraulic seal around the bottom end of the casing string. Toward that end, placement of centralizers very near or even on the float equipment is desirable.
The advent of drilling offshore wells in very deep water depths has given rise to arrangements of subsea wellheads, casing strings and the like with very tight clearances for passage of casing strings therethrough. Often, drilling devices such as underreamers and bi-center (or eccentric) drill bits are used to drill a borehole below a given casing string, in which the borehole has a diameter greater than the inner diameter of the casing string above the open hole section being drilled. Although it is desirable to centralize the succeeding casing string, including the casing float equipment, when the succeeding casing string is run, mounting a conventional bow spring centralizer on the casing and on conventional float equipment (which as described above, typically has an outer diameter somewhat larger than the nominal casing diameter) usually results in a diameter (even with the centralizer collapsed) too large to permit passage of such float equipment through restricted diameter passageways. In addition, such conventionally mounted bow spring centralizers do not shield the bands of the centralizers from scraping and catching on obstructions, ledges and the like, which pose further serious problems.
It is an object, then, of the present invention to provide improved casing accessory equipment especially adapted for use in situations where casing strings must be run through very close clearance passages. It is a further object of the present invention to provide casing accessory equipment, in particular casing string centralizers and float equipment, having a reduced diameter central section with a bow spring centralizer mounted in the reduced diameter central section, wherein the bow spring centralizer may be collapsed to a diameter no greater than that of the float equipment, and wherein the bow spring centralizers are “dragged” through tight clearance locations rather than being “pushed” therethrough. It is a further object of the present invention to provide casing accessory equipment in which the bands of the bow spring centralizers are shielded from catching on obstructions. Further objects of the present invention will become apparent through a reading of the ensuing description and the drawings.
SUMMERY OF THE PRESENT INVENTION
The casi
Lambert Jesse D.
Tsay Frank
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