Wells – Guide for device or conduit – Surrounding existing device or tubing
Patent
1996-08-14
1998-07-14
Suchfield, George A.
Wells
Guide for device or conduit
Surrounding existing device or tubing
1662417, 1662421, 1753255, E21B 1710
Patent
active
057789762
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to the construction of oil, gas, geothermal or other wells having a casing inserted into the well bore, and cemented into place. More particularly, but not exclusively, the present invention relates to improvements in casing installation equipment which may find application in the construction of centralizers, float shoes and float collars.
BACKGROUND OF THE INVENTION
The improvements in casing installation equipment described herein may find application in the construction of float collars, float shoes and such related components as are used in casing installation. The details of such improvements are discussed below with particular reference to casing centralizers, although it is understood that such techniques may be applied to the abovementioned related components.
When the drilling stage of a well is completed a casing string is lowered into the bore of the well. The casing serves to prevent the collapse of unstable portions of the formation through which the well is being drilled, provide a smooth bore through which the production fluids and/or gas may flow and prevent pressure loss and/or fluid and gas migration between zones.
The casing is secured within the well bore by cementing. In this process, a cement slurry is pumped downward into the casing and up within the annular volume created between the casing outer wall and the bore surface. It is essential that the cement provides a uniform shell of substantially constant thickness surrounding the casing. To this end, adequate stand-off must be maintained between the bore wall and the outside surface of the casing.
In practice, it is virtually impossible to produce a well bore which is perfectly straight. A consequence of this being that the casing frequently rests against the bore wall over portions of the well length. This problem is further exacerbated when drilling volcanic formations in which large hard rock intrusions ("ghoulies") are encountered. In this latter case the drill string departs from the vertical, thereby forming a deviated bore path through which the casing string must pass.
If insufficient stand-off is maintained, the upward flow of the cement slurry is impeded thus increasing the likelihood of forming cavities in the cement. Such voids can lead to the undesireable migration of gas or fluid from one zone to another. In some instances catastrophic failure of the well can result from migration of high pressure gas or fluid up the outside of the casing due to inadequate cement placement.
To provide the required degree of standoff, casing centralizers spaced apart at regular intervals along the casing string, are used to hold the casing in the center of the well bore.
Casing centralizers are generally constructed in the form of a metal cage incorporating two end collars with an internal diameter such that the casing fits closely within the bore of the centralizer collars. The two collars are connected longitudinally by bow springs thereby forming a cylindrical cage which holds the casing off from the formation via the resilient action of the bow springs.
Bow spring centralizers can fail in situations where pronounced well deviations produce lateral forces which compress the bow springs sufficiently to allow the casing to lie against the well bore. In this situation, inadequate standoff may produce cement voids leading to failures as described above. In addition, the relatively flimsy construction of such centralizers can result in mechanical failures and/or jamming under conditions often encountered downhole, such as passing through key seats. A further disadvantage of bow spring centralizers is that they exhibit high axial drag or "starting force" due to the sustained tension of each bow spring against the wall of the well bore.
An alternative type of centralizer commonly used incorporates rigid metal strips tapering at each end which replace the resilient bow springs discussed above. Centralizers of this type are rigid in construction and lend themselves to cast manufacturin
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Austoil Technology Limited
Suchfield George A.
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