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4.6 Application (Practical) 187
strongly related to dissolution pattern of the matrix. Recent studies have shown
that one of the most important factors that determine the etching pattern is the
heterogeneity of the rock. In sandstones, the variations in permeability, porosity,
and mineralogy may drive the acid to follow certain paths, the highly permeable
channels called wormholes. There is experimental evidence that wormhole disso-
lution patterns can be achieved particularly using high HF concentrations and
elevated temperatures; however, the risk of precipitation and rock deconsolidation
might be significantly increased. Reaction rates are affected for kinetics; among
the factors that strongly influence the mineral reactions are acid concentration and
temperature. Dissolution reaction rates are proportional to the HF concentration
for most sandstone minerals. The dissolution of minerals is a thermally activated
phenomenon; thus, the rates increase greatly as a function of temperature, and the
penetration depths of live acid diminish accordingly.
Sandstone matrix acidizing cannot be considered as an exact and predictable set
of rules; thus, the appropriate design of treatments almost never has only one right
answer. That is an inherent problem of the complex and heterogeneous nature of
most sandstone matrices (Davies et al., 1992). The interactions between different
minerals and the injected acid depend on the chemistry, as well as temperature,
pressure, pore-size distribution, surface morphology, and pore-fluid composition.

4.6
Application (Practical)

4.6.1
Hydraulic Stimulation

The equipment and materials as well as the modifications of existing equipment
necessary to perform a hydraulic stimulation treatment are summarized in this
chapter (Figure 4.6).
In most cases the well head (Figure 4.7) has to be modified to withstand increased
wellhead pressures during the stimulation treatments. Furthermore, it is necessary

Figure 4.6 Main equipment necessary to perform a hy-
draulic stimulation treatment (left pump aggregats, middle
tanks, left container for proppants) (from treatments in
Groß Sch¨onebeck 2007). (Please find a color version of this
figure on the color plates.)