The present invention relates generally to operations performed and equipment utilized in conjunction with subterranean wells and, in an embodiment described herein, more particularly provides methods of casing multilateral wells and apparatus for use therein.
It is well known to those skilled in the art that it is very difficult to provide a seal between large diameter linings (casings and liners) for lateral or branch wellbores extending outwardly from an intersection between the lateral or branch wellbore and a main or parent wellbore. This due to the fact that it is generally deemed favorable to position a wellbore xe2x80x9cjunctionxe2x80x9d at the intersection of the wellbores to provide sealing for the formations intersected by the wellbores, to stabilize the wellbore intersection, to provide facilities for sealing and connecting linings extending into the wellbores, etc. In order for the junction to be installed at the wellbore intersection, it must pass through the main or parent wellbore and must, therefore, be smaller in diameter than the main or parent wellbore.
Several solutions to this problem have been proposed. One solution is to collapse the junction before it is installed in the well, and then inflate or otherwise expand the junction after it is positioned at the wellbore intersection. Unfortunately, it is difficult to ensure that the expanded junction will have a particular desired geometry after it is expanded, and the expanded junction, by its very nature, is susceptible to being again collapsed due to forces experienced in the well.
Another solution is to provide the junction with a relatively short lateral branch tube extending from a side thereof. The branch tube must be short so that the junction will fit within the diameter of the main or parent wellbore. However, this short lateral branch tube makes it difficult to reliably secure and seal casings or liners thereto. Furthermore, the fact that the branch tube extends laterally from the junction restricts its diameter (due to the necessity of transporting the junction through the main or parent wellbore), such that more than one standard casing size must be skipped in the transition to the lateral or branch wellbore as compared to a normal well casing program for normally pressurized formations. Still further, the wall thicknesses of the branch tube and the new lining of the main or parent wellbore take up space and restrict the inner diameter available through them.
A third solution is to create a junction by attaching two or more divergent tubes to the bottom of a casing. This solution utilizes an orienting cam type seal assembly that stabs into a seal bore located on top of each of these tubes, and extends back to surface to direct access to a desired tube. These tubes are limited in size by their divergence, their wall thickness, the upset created by the seal bore, and by the necessity that the inside of the casing must completely circumscribe the attached seal bores in order for an orienting cam type seal assembly to stab into a seal bore. Several standard casing sizes must be skipped in the transition from the casing to the lateral or branch wellbore as compared to a normal well casing program for normally pressurized formations.
Accordingly, it may be clearly seen that a method of casing multilateral wells is needed which resolves the problems in the art at present. Specifically, it would be highly advantageous to provide a method of casing multilateral wells in which it is not necessary to initially collapse the wellbore junction and then to expand the wellbore junction downhole, the wellbore junction has a known geometry which is, for example, well suited for connecting and sealing casings and/or liners thereto, the wellbore junction does not require the skipping of more than one standard casing size, and the wellbore junction conveniently utilizes standard wellbore and casing sizes to achieve these objectives.
In carrying out the principles of the present invention, in accordance with an embodiment thereof, a method of sealing the junction of subsea multilateral wells which have a large 20xe2x80x3 surface casing is provided in which a uniquely configured casing splitter forms a wellbore junction. The casing splitter is dimensioned so that only one standard casing size is skipped in providing the wellbore junction, and yet the casing splitter conveniently fits through standard main or parent wellbores, does not require collapse or expansion thereof, and provides ample facilities for connecting and sealing casing strings thereto.
In one aspect of the present invention, the casing splitter is dimensioned so that it fits within and through an existing 18.565xe2x80x3 wellhead bore and 20xe2x80x3 surface casing. This size of wellhead and surface casing is typical of those installed on the ocean floor for subsea production. In this manner, the casing splitter may be attached at a lower end of an otherwise standard 13xe2x85x9cxe2x80x3 intermediate casing string. The casing splitter has two 8xc2xdxe2x80x3 diameter bores formed in a lower end thereof, of a length sufficient to anchor and seal subsequent liners therein, and which share and are separated by a single wall thickness of preferably no more than 0.435xe2x80x3. The splitter is attached to the bottom of the 13xe2x85x9cxe2x80x3 casing string before it is installed.
In another aspect of the present invention, wellbores may be drilled through one or more lower connections of the casing splitter. Since only one standard casing size is skipped when utilizing the casing splitter, multiple standard liner sizes are available for use in wellbores drilled below the casing splitter. Because the lower bores of the casing splitter are parallel, they may be of any convenient length to anchor the liners therein. Multiple liner strings in the wellbores drilled through the casing splitter would typically be telescoped in progressively stepped down wellbore diameters.
In a further aspect of the present invention, the casing splitter may have a 13xe2x85x9cxe2x80x3 threaded casing connection formed on an upper end thereof, and a 9⅝xe2x80x3 flush joint threaded casing connection formed on a lower end thereof. Recesses may be machined outwardly from one or both of the 8xc2xdxe2x80x3 bores orthogonally to the other 8xc2xdxe2x80x3 bore to aid in locating and securing the subsequent liners.
In a still further aspect of the present invention, access to individual laterals may be achieved using a bent sub, or a diverter may be disposed in the casing splitter. The diverter may be positioned to permit access between the 13xe2x85x9cxe2x80x3 casing connection and one of the 8xc2xdxe2x80x3 bores while preventing access between the 13xe2x85x9cxe2x80x3 casing connection and the other bore. A latch or mule-shoe may be placed on the outside of the diverter, and a matching profile machined on the inside of the casing splitter, to assist in orientation.
These and other features, advantages, benefits and objects of the present invention will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of a representative embodiment of the invention hereinbelow and the accompanying drawings.