Page 218 - Geothermal Energy Systems Exploration, Development, and Utilization

194 4 Enhancing Geothermal Reservoirs
few hours to a few days. Only rarely are they performed over several months. They
are generally carried out immediately after drilling operations are completed.

4.6.3
Chemical Stimulation

Matrix acidizing is suitable to both generate extra production capacity and to
restore original productivity in damaged wells. Matrix acidizing of sandstones
starts with the careful evaluation of the well and the accurate determination of the
nature and severity of the problem. Then, a possible treatment fluid is selected.
The first selection criterion is the nature and location of the damage. Then, the
potential compatibility problems between the rock minerals and the fluids are
examined. The composition of the fluid is further defined by performing flow tests
and checking the absence of damaging reactions. Once the treating fluids and
the sequence of fluids have been defined, treating parameters, such as volumes,
rates, and pressure are estimated or calculated and simulated. If the extension
and severity of the damage are known, economic evaluation (production prediction
vs treatment cost) can be performed and the treatment results can be optimized.
Various diverting techniques, including mechanical techniques (such as packers)
and various chemical diverting agents allow better fluid placement. To enhance the
production (or injection) capacity, most of the damage must be removed, and thus
the treating fluid must be injected in the least permeable and most damaged zones.
Finally, a comprehensive monitoring of the job effectiveness and a posttreatment
evaluation are necessary.
Various types of chemical stimulation methods have been considered. High pH
fluids seem to be a logical choice for some wellbore and/or reservoir stimulations.
The solid silica, one of the major sources for injector plugging is highly soluble in
many high pH fluids. But unfortunately, the native reservoir fluids as well as the
injected brine are often highly sensitive to a high pH value. The precipitation of
hydroxide and basic carbonate scales is a consequence of the chemical reactions
between high pH stimulation fluid and reservoir or injection brine. These scales,
particularly the hydroxides, are extremely voluminous in the pores even if their
amounts are only very small.
Fluids having a neutral pH can be successfully used in chemical stimulation
methods only in a very few and rare instances. There are some neutral pH fluids that
could be excellent solvents for certain types of damaging materials. For example,
ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) salts are
excellent chelating agents (Fredd and Fogler, 1998). Thus, scales could be removed
by solutions of these materials in neutral or near neutral pH water without causing
secondary precipitates if properly applied (Rose et al., 2007). The major problem is
cost: these materials are rather expensive and large amounts would have to be used
for most stimulation jobs. Low pH fluids, that is, acids, have by far the best chance
to be used for these chemical stimulation jobs. The standard acid treatments are
HCl mixtures to dissolve carbonate minerals and HCl–HF formulations to attack
those plugging minerals, mainly silicates (clays and feldspars).