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196  4 Enhancing Geothermal Reservoirs
                                 The second problem that limits the effectiveness of matrix acidizing technology
                               is incomplete axial distribution. This problem relates to the proper placement
                               of the acid- containing fluid, that is,, ensuring that the fluid is delivered to
                               the desired zone or zones (i.e., the zone that needs stimulation) rather than
                               another zone or zones. This behavior is exacerbated by intrinsic permeability
                               heterogeneity (common in many formations), especially the presence of natural
                               fractures and high permeability streaks in the formation. Again, these regions of
                               heterogeneity attract large amounts of the injected acid, hence keeping the acid
                               from reaching other parts of the formation along the wellbore where it is actually
                               desired most. In response to this problem, numerous techniques have evolved
                               to achieve more controlled placement of the fluid, diverting the acid away from
                               naturally high permeability zones, and zones already treated, to the regions of
                               interest.
                                 Techniques to control acid placement (i.e., to ensure effective zonal coverage)
                               can be roughly divided into either mechanical or chemical techniques. Mechanical
                               techniques include packers and coiled tubing (flexible tubing through which the
                               acid can be delivered with more precise location within the wellbore). Chemical
                               techniques include foaming agents, emulsifying agents, and gelling agents to
                               modify the acid-containing fluid itself. Coiled tubing plays a major role in ma-
                               trix stimulation and is largely viewed as a tool to aid placement and diversion
                               of acids (Pasikki and Gilmore, 2006). However, the challenge of zonal coverage
                               becomes increasingly difficult with larger intervals and/or when there are large
                               permeability contrasts within the formation to be stimulated. Conventional acid
                               placement techniques are less effective for the long, open-hole, or liner-completed
                               intervals typically encountered in geothermal wells. Hightemperature foam sys-
                               tems may improve zone coverage. Gelling agents for thickening acid have been
                               shown to be ineffective in geothermal liner completions. The best way to max-
                               imize acid coverage in geothermal wells is by pumping at maximum injection
                               rates.
                                 The third problem with acid treatments is their susceptibility to the temperature
                               of geothermal reservoir. The effects of high formation temperatures, for instance,
                               vary widely according to the details of the particular fluid treatment. In some acid
                               treatments, the high temperature has a tendency to accelerate corrosion of metal
                               in the wellbore. High temperature reduces the efficiency of corrosion inhibitors
                               and increases their cost. Protecting the tubulars against corrosion requires careful
                               selection of acid fluids and inhibitors (Buijse et al., 2000), while cooling the well
                               by injecting a large volume of water preflush may reduce the severity of the
                               problem.
                                 Another limitation of known acid treatments is iron precipitation. The dissolved
                               iron tends to precipitate, in the form of ferric hydroxide or ferric sulfide, as the
                               acid in the treatment fluid becomes spent and the pH of the fluid increases.
                               Precipitation of iron is highly undesirable because of damage to the permeability of
                               the formation. Therefore, acid treatment fluids often contain additives to minimize
                               iron precipitation, for example, by sequestering the iron ions in solution using
                               chelating agents such as EDTA.
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