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4.6 Application (Practical) 195
The reactivity of the formation-dissolving fluid may be selected (for example, with
the use of fracture and/or acidizing simulator computer programs) on the basis of
the flow rate and formation and fluid parameters. The reactivity of the stimulation
fluid canbecontrolled byvarying therateofreaction, therateofmasstransfer, or
both. For example, the rate of reaction can be decreased by changing the type of
stimulation fluid, by changing the form of the fluid from a solution to an emulsion,
by adding appropriate salts (which change the equilibrium constant for the surface
reaction), or by increasing the pH of the stimulation fluid. The rate of reaction can
also be decreased by changing the physical processing conditions (e.g., by reducing
the pump flow rate and/or pumping pressure, or by cooling the stimulation fluid).
A matrix acidizing apparatus for conducting linear core flooding is capable of
reproducing different conditions regarding flow rate, pressure, and temperature.
The results obtained from the experiments carried out on core samples showed
that the temperature activates the reaction rate of HF–HCl acid mixtures in
sandstone acidizing. The use of higher concentrations of HF, particularly at high
temperatures, may cause deconsolidation of the matrix adversely affecting the final
stimulation results. It was also seen that the higher the flow rate the better the
permeability response, until certain optimal flow rates are reached.
In general, in creating propped fractures having wormholes in the fracture faces
far from the wellbore, simple mineral acids such as HCl, HF, or mixtures of
HCl and HF, would be too reactive, and would spend too close to the wellbore.
It would normally be necessary to use a less reactive formation-dissolving fluid
(Crowe, Masmonteil, and Thomas, 1992). Acids are not the only reactive fluids that
will dissolve formation minerals. Nonlimiting examples would be organic acids,
retarded mineral acids (such as gelled or emulsified HCl), or chelating agents. The
reactivities of organic acids such as acetic or formic acids, could be further adjusted
by including varying amounts of sodium acetate or sodium formate respectively.
The reactivities of chelating agents, such as EDTA or hydroxyethylethylenediamine-
triacetic acid (HEDTA), could be further adjusted by converting them partially or
completely into sodium, potassium, or calcium salts or by adjusting the pH with,
for example, HCl. These chelant-based materials have low reactivity, low viscosity,
but high dissolving capacity (Mella and Rose, 2006a,b).
At present, matrix acidizing treatments exhibit at least four serious limita-
tions: inadequate radial penetration; incomplete axial distribution; corrosion of the
pumping and wellbore tubing, and iron precipitation.
The first problem with acid treatment – inadequate radial penetration – is caused
by the reaction between the acid introduced into the formation and the material
in the wellbore and/or formation matrix, with which it first contacts. The material
and/or formation first contacted by the acid is usually at or near the wellbore
such that the formation near the wellbore is adequately treated and portions of
the formation more far to the wellbore (as one moves radially outward from the
wellbore) remain untouched by the acid, since all of the acid reacts before it can get
there. In fact, dissolution of the material and/or formation encountered by the acid
may be so effective that the injected acid is essentially spent by the time it reaches
a few centimeters beyond the wellbore.