Pre-machined window for use in drilling lateral boreholes

摘要

A pre-machined window is disclosed for use in drilling lateral boreholes. The invention comprises a casing 10, where the casing is 'pre-machined' above ground by either, weakening an area of the casing defining the outline of a window 12 leaving the centre portion 14 intact, or by fully forming an aperture in the casing and filling the aperture with for example an easily disintegrateable material. Weakening the casing can be done for example by machining a groove 12 or by drilling a series of holes (see figure 2) in the casing. Once the casing is positioned downhole the centre portion of the window 14 can be removed by a downhole milling tool (also disclosed, see figure 14). A lug can be positioned on the inside of the centre portion of the window to help the milling tool locate the window and to aid removal of the centre portion. Various methods of sealing the junction at the window and the lateral borehole are also disclosed (see figures 4 to 11).