Page 209 - Geothermal Energy Systems Exploration, Development, and Utilization

4.5 Definition and Description of Methods (Theoretical) 185
Table 4.1 Original McLeod’s sandstone acidizing use guidelines.

Formation Main acid Preflush

Whole rock solubility in HCl >20% Use HCl only –
High permeability (>100 mD)
High quartz (>80%); low clay (<5%) 12% HCl–3% HF 15% HCl
High feldspar (>20%) 13.5% HCl–1.5% HF 15% HCl
High clay (>10%) 6.5% HCl–1% HF Sequestered 5% HCl
High iron chlorite clay 3% HCl–0.5% HF Sequestered 5% HCl
Low permeability (≤10 mD)
Low clay (<5%) 6%HCl–1.5%HF 7.5%HCl or 10%Hac
High chlorite 3% HCl–0.5% HF 5% Hac

Hac: acetic acid.

stimulation, perforating, and production operations; therefore their natural perme-
ability can be restored. During matrix acidizing treatments the acid reacts within a
few meters form wellbore in sandstones.
The major issues that determine design of matrix acid treatments are the reservoir
characterization which comprises the understanding of formation mineralogy,
permeability,porosity,and reservoir fluid dynamics.Formationdamage assessment
in turn includes laboratory analysis, fluid compatibility, and core testing. As a
general basis, the treatments normally follow this procedure:

• Preflush (HCl) stage
• Main acid (HCl–HF) stage
• Postflush stage.
In the late 1930s Dowell (Dow Well Service) introduced the now famous
mixture of 12% HCl and 3% HF, called ‘‘Regular Strength Mud Acid,’’ whose main
objective was initially the removal of the drilling mud filter cake from the wellbore.
Hydrochloric acid (HCl), hydrofluoric acid (HF) or both have been used since
the 1980s in hydrothermal wells (Strawn, 2004) listed yet these two acids as the
most effective ones. HCl was selected to treat limestone, dolomite, and calcareous
zones whereas HF was used to dissolve clay minerals and silica. In 1984, McLeod
presented the basic guidelines for proper designing of acid treatments based
on formation mineralogy, an important issue which is often overlooked. These
guidelines are presented in Table 4.1.
These guidelines have been modified since its introduction to fill certain gaps
and must be considered as a starting point in treatment design. More recently,
some authors have called for the necessity of using non HF-based systems because
of the nature of damaging potential inherent in the reactions between sandstone
minerals and HF (Crowe, Masmonteil, and Thomas, 1992; Malate et al., 1998).
They insist on the capability of these new systems to stimulate effectively, specially
those formations with high content of HCl-soluble minerals.