Page 139 - Geothermal Energy Systems Exploration, Development, and Utilization

3.2 Drilling Equipment and Techniques 115
material), and reduction in the consequence (casing or tubing failure; or other
catastrophic incident) costs. The optimum design, however, is one that minimizes
total costs (initial costs + consequence cost). Such an approach is necessary for
geothermal wells with significantly higher temperatures and also higher pressures
than conventional HC wells. Usually, the definition of the geothermal well design
premises and performing the well design require several iterations because of
the narrow margins. For readers, interested more in operational details of HPHT
wells, the publications by Krus and Prieur (1991), French and McLean (1993), and
Seymour and MacAndrew (1994) are recommended.
One of the main tasks in reservoir drilling is to keep the formation damage
low. Formation damage is a term used throughout the industry to describe negative
interaction between the drilling operation and producing formation resulting
in an impaired near-wellbore permeability. Coincident with this permeability
impairment is a reduction in production. In a geothermal well where economic
viability is predicated on the production of prodigious amounts of heated water
and/or steam, formation damage must be understood, controlled, and minimized.

3.2
Drilling Equipment and Techniques

3.2.1
Rigs andTheirBasicConcepts

There are different types of drilling rigs in the international market; however, each
drilling rig is last, but not the least, simply a system of technical devices to lower,
pull and rotate the drillstring, circulate and clean the drill mud, and cool the mud
if necessary. Additionally, it has to offer safety equipment which allows to shut in
the well safely if necessary.

3.2.1.1 Hoisting System
The hoisting system is used to lower and pull the drillstring as well as casing
strings. It normally consists of drawworks with a wire rope, and a pulley, consisting
of crown block and traveling block which are equipped with sheaves to increase
(multiply) the load capacity of the wire rope. The hook is mounted onto the traveling
block, and the hook takes the swivel which allows the rotation of the drillstring
without turning the traveling block, too.
Other systems use hydraulic pistons or a rack instead of a pulley to move the
swivel up and down.

3.2.1.2 Top Drive or Rotary Table
These items are used to rotate the drillstring from the surface to the bit on the
bottom. The ‘‘old fashioned way’’ is the combination of a rotary table (Figure 3.1)
with a square or hexagonal kelly (Figure 3.2) and a kelly bushing fitting into the
rotary table. The kelly bushing transmits the rotation onto the kelly, which allows
traveling the string up and down in the rollers of the kelly bushing.