asx announcement 14 january 2009, page-20

Sidetracking 101:

The most common procedure for sidetracking a well in open hole is to set a cement plug and then go in with a directional drilling assembly. Unlike a conventional rotary assembly where the whole string and bit are turning as one, a directional assembly has a mud motor on the bottom which powers the bit. With the motor turning the bit the rest of the string can remain stationary.

Motors can be adjusted so they have a slight bend in them (usually 1-2 deg). By orienting the motor bend in the direction you want to go you can drill directionally. Directional control is provided by an MWD (Measurement While Drilling) survey tool positioned behind the motor. The MWD tool supplies hole angle and azimuth data to surface in realtime. This is achieved by a poppet or siren in the MWD tool which interrupts the mud flow through the drill string to create a pulse or sine wave which is in turn picked up at surface by a pressure sensor which sends the data to a surface computer. Plotting up of the directional data allows one to determine where the bit is in space and plan any steering.

So, once the cement kickoff plug is hard you go in with the directional assembly, point it in the direction you want to go and kick off the sidetrack. If youre drilling around junk then the sidetrack will be a simple build and drop affair - you kick off and then drop back to vertical.