most cases the above three types of configurations usually apply to straight/vertical wellbores at most low to medium angle wellbores. For high angle and horizontal wellbore careful weight control of the BHA is a must. In this instance the weight may be applied by running the drill pipe in compression in the high angle section. The high angle may help to stabilise the drill pipe allowing it to carry some compression. BHA tools Stabiliser A stabiliser is used within a column of drill collars. They help guide the bit in the hole. They play a major part in directional drilling as it helps determine the well-bore path and angle. It is used to equalize the load on the bit; prevent whirl of the lower assembly; minimize bit walk; minimize bending and vibrations that cause tool joint wear; prevent collar contact with the sidewall of the bore-hole; minimize key-seating with differential pressure; restrict lateral movement of the lower BHA as such they lower the strain on the drill collar and BHA connections. Solid stabilisers have no moving or replaceable parts. The blades and the mandrel can be one piece (integral) or welded to the mandrel (weld-on/welded blade). The blades may be either straight or spiral. The working surface can be impregnated with tungsten carbide or diamonds inserts. Replaceable blade stabilisers can maintain full gauge stabilization, but their blades can be changed with tools no machining or welding required Sleeve type stabilisers have replaceable sleeves that can be changed in the field. The sleeves are either rotating or non-rotating. Reamers are stabilisers that have cutting elements embedded on their fins, and are used to maintain a gauged well-bore. They can be used to drill out doglegs and key-seats in hard formations. Due to the cutting ability of the reamer the bit does less work in maintaining well-bore gauge and more work drilling. Underreamer An underreamer is used to enlarge the well-bore, usually beginning at some point under the surface. It does this by utilizing expandable cutters that only deploy at the designated time or depth. It is not to be confused with hole opening which occurs from the surface and in most cases the hole opener tool is of a fixed diameter. The underreamer utilises an increase in mud pressure or flow rate to deploy the expandable cutters. A corresponding pressure drop across the tool would indicate that the tool has fully