Page 144 - Geothermal Energy Systems Exploration, Development, and Utilization

120 3 Drilling into Geothermal Reservoirs
are similar, but have reaming edges with rough hard facing which enables to
ream tight spots in the open hole.
1 1
• Drill collars: DCs are thick-walled pipes (e.g., 9 in. × 3 in. = 241 mm ×
2 2
1
89 mm; 76 mm wall) which give high weight (e.g., 9 in. about 310 kg m −1 ;
2
about 3t per joint!) and a high bending stiffness, so that these joints are well suited
for high axial loads. Each joint is normally approximately 9.5 m long (Figure 3.6).
They are used to generate the ‘‘weight on bit’’ (WOB) to assure penetration of the
bit cutters into the formation to be drilled. The ‘‘neutral point’’(transition from
compression to tension) should always be in the DCs in order to prevent buckling
in parts with low bending stiffness. Both ends of each joint are equipped with
threads to allow quick connection and disconnection (male thread on lower end
and female thread on upper end).
• Jar: A jar is a device which allows producing high kinetic (shock) energy onto
a stuck drillstring section below the jar. There are different types (mechanically
or hydraulically operated) in the market. The general working principle is the
same for all types: The free part of the drillstring is pulled into tension to
store energy, and the jar releases this stored energy after a time delay by an
immediate elongation in a travel joint, which allows a certain upward travel until
the movement is restricted again by hitting of the hammer part of the travel joint
onto an ‘‘anvil’’ in the housing of the tool. Thus, the kinetic energy of the fast
moving DCs below the jar is transmitted as a ‘‘blow’’ to the stuck point.
• Shock sub: A shock sub is a device to dampen axial vibrations of the drillstring,
which are induced by the drill bit. Most of the tools use belleville springs and/or
rubber elements as dampeners but some tools use hydraulic fluid dampeners.
• Heavy wall drillpipe: Heavy weight drillpipe (HWDP) is an ‘‘intermediate’’
between the extremely heavy DCs and the normal drillpipe. It is capable of
withstanding some axial load due to a thicker wall than the normal drillpipe.
Each joint is equipped with up-set ends on which the ‘‘tool joints’’ (i.e., the
thicker threaded ends) are friction-welded. Additionally, it is equipped with an

Figure 3.6 Drill collars (right) and drillpipe (left). (from
Schlumberger Oilfield Glossary 2009.)