Page 234 - Geothermal Energy Systems Exploration, Development, and Utilization

210 4 Enhancing Geothermal Reservoirs
4.9.3
Chemical Stimulation

Geothermal injection wells are prone to having a naturally low injectivity and/or
exhibiting serious injectivity losses at various stages of their life. This is especially
true in the case of high temperature geothermal operations. The main reasons for
sometime rapid injectivity losses are the need to inject very large amounts of brine
per well and the plugging of originally good injectors due to specific conditions of
a geothermal operation. The new drilled or completed geothermal injection well
differs drastically from the same well after large amounts of heat-depleted (cooled)
brine are injected. The damage caused by drilling and completions operations is
quite different from the damage caused by a prolonged injection of heat-depleted
brine and also require special considerations (O’Sullivan and McKibbin, 1993).
Some field studies involving suspended particles measurement and monitoring
in a geothermal operation showed that the majority of the suspended particles
entering the injection well consist of silica and iron compounds. These suspended
particles can eventually result in the plugging of the pore spaces of the reservoir,
thereby decreasing the well injectivity. In addition, various chemical inhibitors
are added during the production of the brine to combat scale (and/or corrosion)
problems in the producing of brine. Excess amounts of these chemicals can remain
in the brine and enter the injection wells. This can damage the well by blocking
the pores. The heat-depleted brine has a composition somewhat different from
the formation brine and can create incompatibility problems. The stimulation of
injection wells generally consists of repairing the near-wellbore damage described
above. Chemical methods (acidizing or use of chemicals other than acids) are
commonly used for such stimulation.

4.10
Case Studies

4.10.1
Groß Sch¨ onebeck

4.10.1.1 Introduction
The aim of stimulation treatments in the geothermal research wells in Groß
Sch¨ onebeck is the enhancement of productivity of the reservoir targets as a pre-
requisite for geothermal power generation of the Rotliegend Formation as an
EGS (G´ erard et al., 2006; Huenges et al., 2007). Optimum economic utilization of
reservoirs is mainly due to adequate planning including reservoir modeling and un-
derstanding of the processes and interaction of the system ‘‘borehole – reservoir.’’
Hydraulic stimulation treatments were carried out in the two wells at the Groß
Sch¨ onebeck drill site. In the first well GrSk3/90, several treatments were performed
(in 2002 and 2003). The second well GrSk4/05 was drilled in 2006 and stimulation
treatments were performed in August 2007.