Page 189 - Geothermal Energy Systems Exploration, Development, and Utilization

3.11 Recent Developments, Perspectives in R&D 165
3.10.1.2 Vertical Wells versus Deviated Wells
Particularly in cases where special targets like fault systems have to be reached
by the wells different approaches are possible. As an example a ‘‘hydrothermal
doublet,’’ consisting of a production and a reinjection well, will show the different
possible alternatives.
When a doublet is planned it is necessary to keep a certain minimum distance
between both wells at the pay zone if same formation is used for both wells
(otherwise the cold water–front from the reinjection well will very quickly affect the
temperature in the surrounding of the production well). Anticipating the minimum
distance to be 2 km the following three alternatives are possible:
• Two vertical wells with 2 km distance between the slots and the targets of
production and reinjection well. This gives in tendency the following advantages
(+) and disadvantages (−):
– two drill pads necessary (−);
– long surface pipeline between the wellheads necessary (−);
– short overall drill length (+);
– no directional drilling costs (+).
• One vertical and one deviated well with 2 km departure
– one drill pad (+);
– short surface pipeline between the wellheads (+);
–long overall drill length (−);
–1 × high directional drilling costs (−).
• Two deviated wells with 1 km departure
– one drill pad (+);
– short surface pipeline between the wellheads (+);
– medium overall drill length (−);
–2 × moderate directional drilling costs (−).
Because of the complexity it is recommended to carry out a complete calculation
for all alternatives which are applicable to a specific project in order to find out the
optimum solution.

3.11
Recent Developments, Perspectives in R&D

3.11.1
Technical Trends

In most of the cases drilling-related technological developments start in offshore
oil and gas applications due to the extremely high cost of such drilling operations
which can reach up to a half million US dollars per day! Taking into account these
costs a new technology which accelerates drilling operation can save money even if
the additional costs for the new technology are high. If the new technology enters
the drilling market successfully it normally lasts some years until it is common