Page 220 - Geothermal Energy Systems Exploration, Development, and Utilization
P. 220
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.