The ENGINE work-package 4 "Drilling, stimulation and reservoir assessment" is
investigating the current stage of development of these three key technologies
relating to Enhanced Geothermal Systems (EGS) and Unconventional Geothermal Resources
(UGR). Several research projects have been carried out over the past 30 years at
widely different geological conditions and others are being planned. Economic
development of such geothermal resources depends very much on improving the output of
wells by stimulation and drilling techniques and also by lowering the cost of
drilling. The results obtained are incorporated into the reservoir assessment
together with the available geoscientific information. Recent technological advances
in these areas will be examined and ways to integrate them into “Best Practices”. Two
ENGINE workshops are planned to this end and it is hoped that the partners to ENGINE
will contribute their experience and ideas and thereby further the development of
these resources and geothermal utilization in general.
Out of several worldwide EGS (including HDR) projects only a small number of projects
are still under development. One of the main reasons in the long development time is
the learning curve that can be tracked by the evolution of different expressions for
the geothermal subsurface heat exchanger. Originally, the HDR (hot dry rock) concept
started with the Los Alamos project, some 30 years ago with the idea of targeting the
heat exchanger in unfractured rock and improving the low permeability only by
mechanical stimulation of a penny-shaped crack. It was anticipated that an artificial
fracture with sufficient large surface area (roughly 2 km²) would be created and
expected that the stress components (orientation and magnitude) as well as the rock
mechanical parameters describing fracture propagation were most significant for the
successful development of a reservoir. However, after having effectively created the
fracture system, the final hydraulic link between the injection and production holes
did not reveal the necessary flow impedance of < 0.2GPa/(m3/s).
Especially under the light of the Japanese projects, the significance of the natural
subsurface flow field was discovered, leading to the acronym HWR (Hot Wet Rock).
Existing fracture systems around the boreholes attributed a positive effect to the
reservoir, however tended to create unwanted consequences: Large fluid losses were
encountered.
In contrast, the European project in Soultz-sous-Forêts targeted on a well-pronounced
heat anomaly in the Upper-Rhine Graben was successfully conducting a long-term flow
circulation in 1997 without any fluid losses. The natural flow field was providing
the necessary fluid from large neighboring fault systems. In parallel, conventional
geothermal systems mainly in the U.S. started re-injecting the produced steam or
balancing the water budget. With this development the initial difference between HDR
and conventional systems started diminishing since of the latter one were already
operating in highly convective subsurface. As such, the acronym EGS (Enhanced
Geothermal Systems) now stands for geothermal projects in a hydrothermal environment
that requires stimulation procedures to circulate fluid across the deep heat exchanger.
It is now generally agreed that an EGS resource assessment has to focus on those
locations that have optimum hydraulic and stress settings. Clearly, can the site
evaluation under central European conditions be optimized, however, experience has
shown that the thermal target parameter (i.e. ~200°C) is only exceptionally
encountered at depths below 5km. Rarely the hydraulic permeability in the undisturbed
target rock will be sufficient for hydraulic circulation at economic operation
conditions.
At EGS, the host rock generally needs to be stimulated by appropriate measures.
Today, chemical and mechanical stimulations are commonly used. Depending on their
chemical reaction behavior, individual minerals clogging fracture walls can be
dissolved by chemical treatment. The injection of acids is performed with modest flow
rate creating pressure levels below values needed for mechanical stimulation.
Acidization is used for the removal of skin damage associated with drilling
operations and to increase formation permeability in undamaged wells. The operation
of matrix acidizing has not changed in the last 30 years and is composed of three
main steps (André & Vuataz, 2005): 1) preflush, usually with hydrochloric acid, 2)
mainflush usually with a hydrochloric – hydrofluoric acid mixture, and 3)
postflush/overflush usually with soft HCl acid solutions or with KCl, NH4Cl solutions
and freshwater. An improvement of the well conditions is generally
observed, however with largely varying success rates.
Also for mechanical stimulation the success of improvement cannot be anticipated.
There are two main mechanisms that act during injection of massive flow injection.
Mechanical fracturing of intact rock or induced shearing events. Depending on the
stress regime, the downhole overpressure may not always reach the magnitude of the
minimum stress (P ~ Smin) in case of induced shearing. Recently, numerical
simulations of complex fracture geometries have been performed that relate the
shearing slippage to located microseismic events. This leads us to a more integrated
understanding of the complex interplay between the mechanical and the hydraulic
processes. Kohl & Mégel (2005) have demonstrated that slippage events are likely to
occur as function of pressure development in adjacent fractures.
With increasing numerical capabilities, the stimulation behavior may become more
predictable. In the near future there is the prospect that designing stimulation
measures beforehand, without having created irreparable damage to the host rock.

References
André L., Vuataz F.-D. (2005) - Technical note on the potential of acid treatments in
geothermal reservoirs: A bibliographic review and numerical simulations on the Soultz
EGS reservoir, CREGE internal report, Sept. 15, 2005.
Kohl T., Mégel T. (2005) - Coupled Hydro-mechanical modelling of the GPK3 reservoir
stimulation at the European EGS site Soultz-sous-Forêts. Proc., 30th Workshop on
Geothermal Reservoir Engineering, Stanford University, Stanford, California, January
31- February 2, 2005.

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