Page 210 - Geothermal Energy Systems Exploration, Development, and Utilization

186 4 Enhancing Geothermal Reservoirs
HF is the only common acid that dissolves clay, feldspar, and quartz fines.
For years mixtures of HF and HCl have been the standard acidizing treatment
to dissolve the minerals that cause damage. These treatments are preceded by a
preflush of either HCl varying between 7.5 and 15% or weaker acids such acetic
acid to dissolve carbonates and avoid precipitation of calcium fluoride. The readily
HCl-soluble minerals are mainly calcite, dolomite, and siderite which additionally
do not generate precipitates. The reactions are as follows:

Calcite 2HCl + CaCO 3 −→ CaCl 2 + H 2 O + CO 2
Dolomite 4HCl + CaMg (CO 3 ) 2 −→ CaCl 2 + MgCl + 2H 2 O + 2CO 2
2
Siderite 2HCl + FeCO 3 −→ FeCl 2 + H 2 O + CO 2

Siliceous minerals are dissolved by hydrofluoric acid and its chemistry is
much more complex than HCl when reacting with carbonates (Walsh, Lake, and
Schechter, 1982; Pournik, 2004). Quartz, clay, and feldspars are the main siliceous
particles involved in damage of sandstones. The primary chemical reactions in
sandstone acidizing are as follows:

Quartz SiO 2 + 4HF −→ SiF 4 silicon tetraflourude + 2H 2 O

SiF 4 + 2HF −→ H 2 SiF 6 fluo silicicacid
Clays (kaolinite) Al 4 Si 4 O 10 (OH) 8 + 24HF + 4H −→ 4AlF 2 + 4SiF 4 + 18H 2 O
+
+
(montmorillonite) Al 4 Si 8 O 20 (OH) 4 + 40HF + 4H −→ 4AlF 2 + 8SiF 4 + 24H 2 O
Feldspars (Mg, Na or K) KAlSi 3 O 8 + 14HF + 2H −→ K + AlF 2 + 3SiF 4 + 8H 2 O
+
+
The main acid stage requires the greatest emphasis because of the damage
mechanisms, directly associated to precipitation of products from the HF reactions
(Pournik, 2004). Secondary reactions may occur between fluosilicic acid H 2 SiF 6 ,a
byproduct of primary reaction, and aluminum-silicates, clays, and feldspars. These
reactions are considered to have adverse effects since silicon can be precipitated as
hydrated silica, which contributes to damage if mobile inspite of the presence of
HCl to reduce the pH to prevent silica and fluosilicate precipitation. Conversely,
some authors believe these reactions are beneficial because they retard HF reactions
allowing deeper penetration. Additionally, HF dissolves native clays and feldspars,
and when reacting with quartz may also cause formation deconsolidation by
weakening the matrix. Precipitation will always take place, associated with HF
concentration among other things; however, it tends to be more severe if HF acid
treatments are not properly displaced. Acid stimulation techniques have to account
for both chemistry and treatment execution to accurately predict the effectiveness
since the effect of these precipitates could be minimized if they are deposited far
from the wellbore (Entingh et al., 1999). Careful selection of mixtures, additives,
acids formulations, and treatment volumes must be accounted to minimize these
secondary adverse effects.
Reservoir geology and mineralogy are the relevant issues for successfully re-
moving the acid-soluble particles present in reservoirs; removal mechanisms are