Wells – Guide for device or conduit – Surrounding existing device or tubing
Patent
1992-08-06
1993-11-16
Britts, Ramon S.
Wells
Guide for device or conduit
Surrounding existing device or tubing
E21B 1710
Patent
active
052614883
DESCRIPTION:
BRIEF SUMMARY
The invention relates to centralizers for oil well casings.
As an oil well is drilled, the casings are lowered from a derrick in a string into the borehole so formed and drilling mud is circulated down the casing string, through the end of the final casing, back up through the annular space between the casing string and the borehole. At intervals, the casing string is cemented in position by pumping a suitable cement into the casing string which passes out of the final casing and into the annulus between the casing string and the borehole, displacing the mud, and which, when set, holds the casing string in position.
It is important that all of the annular space between the casing string and the surrounding borehole is filled with cement and that the cement is firmly bonded to both the casing string and the borehole. In order to achieve this, it is important that all of the drilling mud is displaced from this space by the cement.
The mud is unlikely to be placed successfully if the casing string is not located centrally in the borehole.
If the casing string is close to, or touching, the borehole then pockets may be formed where mud collects and is not displaced by the cement.
Such centralization of the casing string is important where the borehole is vertical, but is even more important where the borehole is at an angle to the vertical, particularly where it is horizontal or nearly horizontal. This is because in such angled or horizontal boreholes gravity tends to drop the casing string on to the surrounding borehole.
It has been usual to centralize the casing string by the use of centralizers. These have customarily been of two forms. A first form has a number of bowed springs arranged around a casing and connected to sleeves which are mounted on the casing. The portions of the springs remote from the casing surface engage the surrounding borehole to centralize the casing but do not impede substantially the flow of cement and mud. A second form comprises a one-piece sleeve which fits over the casing and is provided with angularly-spaced, longitudinally-extending ribs which engage the surrounding borehole with the passages formed between the ribs allowing flow of cement and mud.
Though such centralizers have been used successfully for many years in vertical or near-vertical boreholes, they are less satisfactory when used in horizontal or near-horizontal boreholes. This is because to pass casing into such horizontal or near-horizontal boreholes requires the casing string to pass from a vertical section of the borehole to the horizontal or near-horizontal section around a corner and the projecting springs or ribs on the presently used centralizers can catch on such a corner and prevent or impede the passage of the casing.
For this reason, it has become customary to use few, if any, centralizers in horizontal or near-horizontal boreholes. This can produce the cementing problems described above and can also produce problems should explosive perforation of the casing be required in such a borehole.
U.S. Pat. No. 2,490,350 discloses a centralizer for oil well casing which centralizer comprises an annular mounting by which the centralizer may be mounted on an outer surface of a casing and a plurality of members carried by the mounting at spaced positions therearound, wherein the members are held by a control device in a collapsed disposition in which the members extend along said casing closely adjacent said outer surface of the casing, said control device being remotely operable so that the members move from said collapsed disposition to a deployed disposition in which the members extend away from said mounting for engagement with an associated borehole.
Thus, by holding the members in a collapsed disposition until the centralizer and the associated casing string has been deployed, the members do not impede the passage of the casing string into and through the borehole.
The disadvantage of this arrangement is that the control device passes through the wall of the casing thus creating a localised area of low burst
REFERENCES:
patent: 2490350 (1949-12-01), Grable
patent: 2656890 (1953-10-01), Brandon
patent: 3343608 (1967-09-01), Solum
patent: 4523640 (1985-06-01), Wilson et al.
patent: 4543998 (1985-10-01), Thomerson
patent: 4651823 (1987-03-01), Spikes
patent: 4688636 (1987-08-01), Hennessey
Gullet Paul D.
Jansch Manfred
Britts Ramon S.
McClung Guy
Tsay Frank S.
Weatherford U.K. Limited
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