Page 229 - Geothermal Energy Systems Exploration, Development, and Utilization

4.8 Outcome 205
hydraulic fracturing. Preplanning and proper job design is essential for a successful
matrix stimulation treatment. Different techniques are being used over a variety of
reservoir and well conditions. Sophisticated laboratory equipment, expertise, and
well testing software can help to identify production or injection damage effects
and mechanisms – making it easier to select proper well candidates and optimize
job design. Except as may be helpful in adjusting the pH of the fluid system,
there are no restrictions on the order of addition of the components in the fluid
mixture.
Alternatively, any combination of the components can be premixed on site or at
a separate location (to enhance safety on location) and then another component or
components may be added later. Standard mixing equipment and methods may
be used; heating and special agitation are normally not needed but may be used.
The only acid additives necessary in a geothermal acid job are corrosion inhibitor
and inhibitor intensifier, as well as high-temperature iron-control (reducing) agent.
Corrosion inhibitors of diverse description and composition have been proposed
over the years for use with well treating acids. Treatment placement is better ensured
through the use of chemical or mechanical diversion methods and technologies,
and placement tools (coiled tubing, packers, etc.). On-site quality control is enabled
by monitors and software, enabling to determine the evolution of skin with time,
and radius of formation treated.
The application of chemical acids generally improves conditions in the reservoir,
however, with largely varying success rates. During the 1990s, the acidification
technique has been used more often, principally for the reservoir development or
to treat formation damage caused by drilling mud and scaling (mineral deposits)
in geothermal wells (Buning et al., 1995; Buning et al., 1997; Malate et al., 1997;
Yglopaz et al., 1998; Malate et al., 1999; Barrios et al., 2002; Jaimes-Maldonado and
S´ anchez-Velasco, 2003). This protocol has not really evolved since these years. In
each of the experiments proposed by the authors, the same technique is used. The
acidification occurred in three main steps:
1) A preflush, usually with hydrochloric acid (10%). The objective of the preflush
is to remove calcium carbonate materials in the formation. The preflush acid
minimizes the possibility of insoluble precipitates.
2) A main flush with hydrochloric–hydrofluoric acid mixture. A mixture of 10%
HCl – 5% HF (called mud acid) is generally prepared by dissolving ammonium
bifluoride (NH 4 HF 2 )in HCl.Amixture of 1% of HCland 56 kilosofNH 4 HF 2
will generate 1% HF solution. Regular mud acid (RMA) (12% HCl–3% HF) is
made from 15% HCl, where 3% HCl is used to hydrolyze the fluoride salts.
3) A postflush/overflush usually by either HCl, KCl, NH 4 Cl, or freshwater.

Concerning the injected amounts for the cleaning out of the geothermal wells,
the preflush volume was based on a dosing rate of 600 l m −1 of target zone. The
mainflush volume was based on a dosing rate of 900 l m −1 of target payzone (Malate
et al., 1997; Barrios et al., 2002).
In geothermal well acidizing, more acid often is better. A very successful method
of acidizing geothermal wells has been a basic, high-rate, brute-force method. High