Page 138 - Geothermal Energy Systems Exploration, Development, and Utilization

114 3 Drilling into Geothermal Reservoirs
fluid technologies, casing and cementation procedures, well planning, drilling
operation, risk evaluation, well completion, and economic considerations. In
particular, practical borehole stability analysis and geomechanical explanations and
prognosis might be new but necessary paragraphs in a geothermal drilling chapter.
Finally, we discuss some future research and development tracks for geothermal
drilling with the aim to reduce drilling costs within the scope of well integrity and
long-term borehole stability.
3.1.1
Geothermal Environments and General Tasks

Drilling operations are directly concerned by the subsurface geological–geothermal
environment. As earlier mentioned in this book, various types of geothermal sys-
tems are defined either by temperature or by existence of high amounts of thermal
fluids. Table 3.1 summarizes the characteristics and utilization of geothermal
systems. The geological conditions of geothermal wells are generally more harsh
compared with standard shallower geothermal or oil and gas wells and might be
similar to HPHT (high pressure high temperature) wells known from HC (hydro
carbon) drilling. HPHT wells are critical due to their small design margins and
where well control is difficult to handle since HPHT wells are often characterized
by extreme pressures and temperatures coupled with small pore pressure-fracture
gradient margins. Consequently, the casing design demands maintenance of high
dimensional efficiency, and quantitative risk evaluation is necessary for more
complex casing and tubing design. In classical tubular designs, reduction in
risk results in an increase in initial costs (e.g., thicker casing wall, higher grade

Table 3.1 Summary of characteristics of geothermal systems and their utilization.

Geothermal system
High Low HDR Aquifer
Characteristics enthalpy enthalpy

◦
High temperature (>150 C) x x x (also use
of vapor)
◦
Low temperature (<150 C) x x
High reservoir pressure (>690 bar) x x
Low reservoir pressure (<690 bar) Unusual x x x
Deep reservoir (>2500 m) x x x
Shallow reservoir (<2500 m) x x x
Production + injection x x x (x)
Closed water circuit x x x x
Sedimentary rock Unusual x x
Metamorphic rock x x x Unusual
Magmatic rock x x x x