Definition - What does Borehole Enlargement mean?
Borehole enlargement occurs when the borehole becomes larger than originally intended. It may result from mechanical damage, hydraulic erosion or the nature of the material through which the bore passes. Mechanical damage can occur when drill string pipe joints batter and abrade the sides of the bore. Hydraulic erosion happens when the sides of the bore are weathered mechanically (washed away) by drilling mud or by water occurring naturally in the bore. Bores through shale – a low-density rock that sloughs away when it encounters pressurized mud – can also lead to undesirably oversized boreholes.
Borehole enlargement may also refer to the process of borehole expansion. Borehole expansion is the intentional enlargement of the circumference of a borehole, for example, when a new pipe is inserted into an old bore.
Trenchlesspedia explains Borehole Enlargement
Borehole enlargement can cause a number of expensive problems. The increase in the interior space of the borehole can mean additional costs for more difficult mud logging; the enlarged borehole may deviate from the project's design path. Cementing the hole can only be done with more than a modicum of difficulty because the borehole walls may go into full collapse, or they may simply require more materials, and the resulting profit declines. Hole cleaning – the ability of the drilling mud to entrap and remove cuttings and other debris from the hole – may be compromised by rock or soil that "gets lost" in the enlarged area.
Borehole expansion is often undertaken when a new pipe or conduit is being placed inside an older bore of a smaller size. Borehole expansion is performed using a special two-way bit that is moved back and forth in the existing bore until the bore meets the project conduit or pipe diameter.