1. Field of the Invention
This invention relates to the formation and lining of branch boreholes, that is to say, techniques for use in formation of a borehole which extends as a branch of a main borehole, and the lining of such a branch.
2. The Prior Art
It is well known in the exploitation of oil and gas wells to form one or more branch boreholes, usually known as "laterals", off an existing main borehole. In general, the main borehole extends vertically and the or each lateral diverges from the vertical borehole. However, in some cases the initial main borehole may itself extend at an angle to the vertical.
International patent application WO94/03698 describes various techniques for the formation of laterals. Typically, a fixed support is established in a main borehole somewhat below the level of the proposed lateral. This may conveniently be done by setting an appropriate packer in the main borehole. A whipstock is then run into the main borehole and is appropriately orientated using known techniques. The inclined surface of the whipstock is then used as a guide for one or more milling tools which mill a window in the casing of the main borehole and mill away the surrounding formation until a lateral has been started, and may be continued without milling further casing. The milling tool which has formed the opening may typically be used to continue drilling through the formation if a relatively short lateral is required. In the alternative, the milling tool may be removed and a formation drilling tool run into the well. The formation drilling tool will be guided by the whipstock through the window which has been milled in the main borehole casing and drilling of the lateral with the new tool can commence.
Several techniques have been proposed for formation of the window. For example, it has been proposed to use a starter mill for the purpose of breaking through the casing and to replace the starter mill with a main mill to mill the majority of the window. It has also been proposed to run combination mills through the window, for example an end mill followed by one or more watermelon mills. In each case, however, the intention is to produce a path which is bounded on one side by the face of the whipstock and is of the full gage diameter of the milling assembly. After the lateral has been drilled to the required depth the drilling assembly is removed. A casing may then be run into the lateral. The casing will again be guided by the sloping face of the whipstock and little difficulty should be experienced in guiding the casing through the window which has been formed in the main borehole casing, and into the lateral. The above described technique is satisfactory provided that the whipstock which is used to guide the milling tool is also used to guide the casing into the lateral. There are, however, a number of practical reasons why using the whipstock for this purpose is not wholly desirable.
Firstly, it may well be desirable to recover the whipstock for re-use or to recover the whipstock to re-open the main borehole. Obviously, if the lining installed in the lateral continues upwardly in the main borehole from the window it will not be possible to recover the whipstock after the lateral has been positioned.
Secondly, if the whipstock is used to guide the casing into the lateral there will in general be a relatively large area of sliding contact between the casing and the whipstock. The whipstock face is long and tapers only at a shallow angle. This shallow angle occurs at the point where the casing is deflected from the main borehole into the lateral. Accordingly, there may be substantially full face contact between the casing and at least part of the surface of the whipstock. Such full face contact will impose a substantial frictional drag on the casing and may render it difficult to push the casing into position.
Thirdly, if the casing to the lateral is to be cut away at the point where it enters the main borehole, the presence of the whipstock significantly complicates the cutting operation. Either the casing and the entire whipstock must be milled away by an appropriate milling tool or, if a wash-over tool or thin walled mill is used for cutting the casing, the whipstock must be designed to enter the wash-over tool or thin walled mill as the lateral casing is cut. This imposes design constraints on the whipstock which can reduce its effectiveness in performing its primary function of guiding the mill which opens up the casing window. For example, referring to the technique illustrated in FIGS. 5A-5H of WO94/03698 it will be noted that the whipstock has an external diameter substantially smaller than the internal diameter of the main borehole. The resulting clearance between the whipstock and the casing of the main borehole is used to accommodate a thin walled mill which is used to remove surplus cement and lateral casing. However, the clearance provided between the whipstock and the main borehole casing means that the whipstock is not laterally supported at the upper end thereof. In practice, a whipstock will have a significantly shallower angle relative to the axis of the main borehole than in the schematic illustrations of FIGS. 5A-5H, and accordingly the problem of inadequate support at the upper end of the whipstock will be exacerbated.