15. Acid Systems and Additives for Fracturing.

Variables influence acid penetration before becoming spent include the volume acid
used, fluid-loss control, acid concentration, injection rate, formation temperature,
fracture width, and the composition of the formation.

15.1 Materials and Techniques for Acid Fluid-Loss Control

Controlling of fluid loss during acid fracturing of carbonate formations presents
problems unique to reactive fluids. Fluid-loss additives and gelling agents normally
used in non-reactive fracturing fluids are seldom stable in acid and are therefore
ineffective due to rapid degradation. This has led to the development of special acidstable
additives required for acid fracturing treatments.
In addition to the problem of degradation, as acid flows across the faces of carbonate
fracture, it constantly erodes the fracture surfaces, making it difficult for wall-building
fluids to form an effective filter cake.
Acid tends to selectively enlarge certain large pores and hairline fractures, which
results in "worm-holes" and channels perpendicular to the fracture faces. This further
complicates the problem of leakoff and causes the rate of acid fluid loss to increase
with time. Consequently, excessive fluid loss is generally considered to be the
controlling factor that limits fracture growth and fracture extension when fracturing low
to moderate-temperature carbonate formations.
Laboratory studies have shown that most acid fluid loss occurs from the worm-holes
rather than uniformly into the face of the core. Nierode and Kruk (1973) suggested
that acid fracturing fluids require much higher concentrations of fluid-loss additive for
effective fluid-loss control than do non-reactive fluids. They also concluded that the
only effective additive is a product composed of a mixture of oil-soluble resins. Oilsoluble
resins eliminate the possibility of conductivity impairment (in oil wells or gascondensate
wells) when compared to 100-mesh sand in the fracture. However, acid
fluid-loss additives have not been used extensively because of performance limitations
and high cost.

15.1.1 Viscous Pads.

One of the technique most commonly used for fluid-loss control involves the use of a
viscous pad preceding the acid. The pad is used to initiate the wide fracture and to
deposit a filter cake which will act as a barrier to fluid leakoff.
Low-viscosity linear gel pre-pads and high-viscosity cross-linked gel pads will also
increase fracture width, which improves acid penetration and fracture conductivity.
Multiple stages of a viscous pad, alternating with acid stages, will further improve acid
fluid-loss (to the worm-holes) control, and therefore, the efficiency of the treatment is
improved. This technique is widely used in acid fracturing treatments.

15.1.2 Foamed Fluids.

The use of foamed acid is one of the most effective methods for controlling acid fluid
loss. Fluid-loss control is further enhanced by the use of a viscous pad preceding the
foamed acid. However, foaming the acid does reduce the effective amount of acid
available for etching since there is less acid present per unit volume of fluid injected.
Therefore, 28% HCl should be used in preparing the foamed acid to maximise the
amount of acid available for fracture etching. The primary advantages of foamed acid
are its low fluid-loss and improved cleanup characteristics.
15.2 Materials and Techniques for Acid Spending Control.
Another major factor limiting penetration of live acid along fractures in carbonate
formations is the spending of the acid. The acid reacts constantly with fracture
surfaces and decreases in strength during its travel down the fracture. Once acid
strength falls below about 10% of the original concentration, it is no longer capable of
providing sufficient etching for acceptable fracture conductivity.
Higher acid concentrations increase penetration distance due to the greater amount of
available acid. The more concentrated acid has a higher viscosity and generates
more reaction products during spending, and both factors act to reduce the reaction
rate.

15.2.1 Viscous Fluids.

Fracture width also has a significant influence on penetration distance. An increase in
width results in an increase in acid penetration distance in both limestone and
dolomite. This demonstrates the importance of using a viscous pad fluid preceding
acid injection or the use of viscous acid, such as gelled acid or crosslinked acid. Highviscosity
cross-linked gels are more widely preferred as pad fluids than low-viscosity
linear gels since they have the advantage of creating wider fractures.
Temperature accelerates the reaction of acid on carbonate, an increase in
temperature decreases acid penetration. Acid penetration distance in limestone is
relatively less sensitive to temperature compared to that in dolomite. Pre-pads and/or
pad fluids that precede an acid injection treatment will cool the tubular goods, which
reduces corrosion, and cool the fracture, which reduces acid reaction rate and
enhances live acid penetration.
At temperatures above 200 °F (93 °C), certain acrylamide-base copolymers, of a type
commonly used to thicken acid, can be used in preparing pad fluids since they have
good acid and temperature stability. The presence of a high-viscosity pad in the
fracture promotes viscous fingering of the acid, which decreases the reactive surface
area to which the acid is exposed. This fingering also tends to increase the effective
conductivity of the etched fracture.

15.2.2 Chemical Retarders.

Retarders such as alkyl sulfonates, alkyl phosphonates, or alkyl amines reduce acid
reaction rates by forming a hydrophobic film on the carbonate surfaces. This films act
as a barrier which inhibits acid contact with the formation face and thus slows the acid
reaction with the formation. Some retarders slow the reaction rate by blanketing
carbonate surfaces with a thin layer of carbon dioxide foam which can be a stabilised
by the presence of foaming agents.

15.2.3 Organic Acids.

Acetic and formic acids are sometimes used as retarded acids since they react at a
much slower rate than hydrochloric acid at high temperatures. Their cost per unit
dissolving power is higher than HCl, however, they are less corrosive, and therefore,
can be inhibited at high temperatures for long periods of time. Inhibited acetic acid
does not attack chrome plating, and small amount of formic acid in HCl may serve as
inhibitor aid and reduce HCl acid corrosion.
15.3 Materials and Techniques for Improved Fracture Conductivity.
For an acid fracturing treatment to be effective, the wall of an acidized fracture must
be etched sufficiently that conductive channels remain after the treatment. If the
fracture faces are etched uniformly, the conductivity after the fracture closure is very
low. Fortunately, several factors promote uneven etching of the fracture faces, such as
mineral composition. Acid reacts with different minerals at different rates resulting in
non-uniform etching.
The rate of acid reaction is also greatly affected by the acid flow velocity. Faster
reaction rate at high flow rate results in the erosion of the fracture faces in areas of
more rapid acid flow and creates erosion patterns. Once these channels develop, the
acid tends to flow selectively along a few of the larger channels and most of the
fracture faces remain relatively un-etched. This not only promotes increased fracture
conductivity, but also increases live acid penetration.
Rock strength and closure stress are important factors affecting ultimate fracture
conductivity. Crushing of fracture faces can result in loss of conductivity if the rock is
too soft or closure stress is too high. Soft chalk formations are very prone to this
problem.
The injection of a viscous pad fluid ahead of the acid is the most commonly used
technique to maximise fracture conductivity. The presence of higher viscosity pad fluid
promotes viscous fingering of the thinner acid which follows. This selective acid flow
increases penetration distance and tends to create deep channels with good