12. Viscosity and Friction Pressure

12.1 Viscosity.

Since acids systems have a water base, their viscosity is about 1.0 centipoise and
they behave as Newtonian fluids with easily predicted flow properties. However,
when they are altered by adding gelling agents or polymers, or when combined with
hydrocarbons to form emulsions, they become Non-Newtonian and more complex in
behaviour. Generally, low viscosity Newtonian acids are used in matrix acidizing,
whilst viscous Non-Newtonian treating solutions are used when acidizing within
natural fractures and in fracture acidizing.
Viscosity is defined as the property of a fluid that resists the force tending to cause
the fluid to flow. The common oil field unit of measure for viscosity is the centipoise
(cp), which is expressed in centimetre-gram-seconds.

12.2 Newtonian Fluids.

A fluid undergoes continuous deformation when subjected to a shear stress, such as
when it is pumped. Some fluids follow a standard pattern of behaviour under shear
stress (i.e. when subjected to pump pressure). Specifically this standard pattern is
that shear stress is directly proportional to the rate of shear (Figure 16).
A fluid which behaves in this manner is classified as a Newtonian fluid and its flow
properties can easily be predicted from a measurement of the fluids viscosity.
Viscosity is the single rheological property needed for the flow calculations of a
Newtonian fluid.
Common examples of Newtonian fluids are water, most oils and most other liquids
that do not contain solid particles in suspension.



12.3 Non-Newtonian Fluids.

In some specialised acid treating solutions, flow properties are changed by adding
synthetic or natural polymers, or by combining the acid with a hydrocarbon such as
kerosene to form a viscous emulsion (as in BJ Services Emulsified Acid or SRA-3
acid).
These fluids are Non-Newtonian which may be defined as materials which do not
conform to direct proportionality between shear stress and shear rate (Figure 17)
Consequently, Non-Newtonian fluids do not exhibit a simple viscosity and their
consistency changes as their flow rate changes. In acid stimulation, flow rate is
related to pumping rate, pipe size, size and number of perforations and other
factors



In general, Non-Newtonian fluids become less "viscous" at higher pump rates (shear
thinning) and are often unstable at these rates (turbulence). In effect the apparent
viscosity of the fluid becomes less as it is pumped faster (Figure 18).



The effects of viscosity must be considered when designing acid treatments. In
matrix acidizing, where injection rates are low, the viscosity of the treating solution
should be kept to a minimum to prevent excessive pressures during placement and
to aid in the clean-up of the treatment when the job is complete.
On the other hand , a viscous acid has advantages when stimulating within natural
fractures or when fracture acidizing for deeper penetration. In these treatments the
higher viscosity helps to control fluid leak-off, increasing the efficiency of the acid as
a fracturing fluid.