School of Engineering
Department of Petroleum Engineering
Dr. Muhammad Amin
Rebaz Abdulqadr Hamad
Aras Bahri Salim
Dlvin Taher Abdullah
Some, petroleum exists in a formation but is
unable to flow readily into the well because
the formation has very low permeability.
• Natural low permeability formation.
• Formation damage around the wellbore
caused by invasion of perforation
fluid and charge debris.
the reduction of permeability in a reservoir
rock caused by the invasion of drilling fluid
and treating fluids to the section adjacent to
die wellbore. It is often called skin damage.
any of several operations used to increase
the production of a well or a treatment
performed to restore or enhance the
productivity of a well such as:
• The pumping of acid into the wellbore to
remove near-well formation
damage and other damaging substances.
• This procedure commonly enhances production
by increasing the
effective well radius.
The two basic types of acidizing are
characterized through injection rates and
Injection rates below fracture pressure are
termed Matrix acidizing.
Injection rates above fracture pressure are
termed Fracture acidizing.
Matrix acidizing is applied primarily to remove skin
damage caused by drilling , completion, workover ,
well-killing, or injection fluids, and by precipitation
of scale deposits from produced or injected water.
During matrix acidizing the acids dissolve the
sediments and mud solids within the pores that are
inhibiting the permeability of the rock.
mostly used in sandstone formations.
Due to the extremely large surface area contacted
by acid in a matrix treatment, spending time is very
short. Therefore, it is difficult to affect formation
more than a few feet from the wellbore.
Fracture acidizing is an alternative to hydraulic
fracturing and propping in carbonate reservoirs.
In fracture acidizing, the reservoir is
hydraulically fractured an then the fracture
faces are etched with acid to provide linear flow
channels to wellbore.
As such, the application of acid fracturing is
confined to carbonate reservoirs and should
never be used to stimulate sandstone, shale, or
Long etched fractures are difficult to obtain,
because of high leak off and rapid acid reaction
with the formation
WELL STIMULATION ACIDS
The basic types of acid used are:
Also, various combinations of these acids are
employed in specific applications.
Acidizing can cause a number of well
problems. Acid may :
(1) release fines
(2) create precipitants
(3) form emulsions
(4) create sludge
(5) corrode steel
Additives are available to correct these and
a number of other problems
• Surfactants should be used on all acid jobs to
reduce surface and interfacial tension, to prevent
emulsions, to water-wet the formation, and to
safeguard against other associated problems.
Suspending Agents: Most carbonate formations
contain insolubles which can block formation pores
or fractures if fines released by acid are allowed to
settle and bridge.
Suspension should be differentiated
from dispersion. Dispersed particles
usually settle in a short time.
A suspending surfactant, such as
Halliburton's HC-2, in concentrations of
about five gallons per 1,000 gallons of
acid may suspend fines for more than 24
hours, and possibly as long as seven
days. Suspending agents are usually
polymers or surfactants
Some crudes, particularly heavy asphaltic crudes,
form an insoluble sludge when contacted with acid,
with greater problems experiences with high strength
acid. Dissolved Fe(III) in acid appreciably increases
the possibility of sludge.
The primary ingredients of a sludge are usually
Sludges may also contain resins and paraffin
waxes, high-molecular weight hydrocarbons,
formation fines, clays, and other materials.
The addition of certain surfactants can prevent the
formation of sludge by keeping colloidal material
dispersed. These sludge-preventing surfactants
usually prevent an emulsion
Corrosion inhibitors for acid are chemical
additives that reduce the rate of corrosion
of steel by acid.
There are two primary reasons for using
(1) to protect the acid pumping and handling
(2) to protect well equipment.
Factors that govern the degree of
acid attack on steel are:
1) type of steel including hardness
3) type of acid
4) acid concentration
5) acid contact time
The objective of acidizing limestone and dolomite
wells is to remove
damage near the wellbore or to create linear flow
channels by fracturing and
Acid may also be used in sandstone wells to
dissolve carbonates in the form
of sand grain cementing materials, discrete particles,
and carbonate scale.
The time required for a specified volume
and concentration of HCl acid to
spend to about 3.2% in a selected
formation under given conditions is
defined as Acid Reaction Time.
A major problem in fracture acidizing of
carbonate formations is that acids tend
react too fast with carbonates and spend
near the wellbore.
Factors controlling the reaction rate of
area of contact per unit volume of acid
physical and chemical properties of
flow velocity of acid
Retardation of Acid:
To achieve deeper penetration in
fracture acidizing, it is often desirable to
acid reaction rate. This can be done by
chemically retarding the acid
The use of gelled acid for fracture
acidizing has increased to the point that
it is now the most used technique.
The introduction of more temperature-
stable gelling agents with ready
application up to temperatures of about
400°F has been a major factor in
selecting gelled acid for acid fracturing.
Two types of gelling systems, polymers
and surfactants, are in common use.
For many years the primary retarded acid for
fracture acidizing was an acid-in-oil emulsion. This
type retarded acid is very functional but is no
longer the primary fracture acid method used.
It has limited temperature range and stability, with
high viscosity and high friction loss.
It does, however, have the ability to restrict contact
between the acid and formation, to reduce fluid
loss, and to retain large quantities of the treating
fluid in the fracture.
Acid-Retardation of HCl is obtained by
the addition of unique surfactants to the
acid which form protective films on the
surface of limestone or dolomite.
These films retard reaction rate in much
the same way that an acid corrosion
inhibitor protects metal.
In addition to retarding acid reaction
rate, chemical retarders tend to promote
nonuniform etching of fracture faces,
thus increasing fracture conductivity.
The objective of hydraulic fracturing for well
stimulation is to increase well productivity
by creating a highly conductive path
(compared to reservoir permeability) some
distance away from the wellbore into the
Usually the conductivity is maintained by
propping with sand to hold the fracture
Acid fracturing involves most of the
same considerations as hydraulic
fracturing except that conductivity is
generated by removing portions of the
fracture face with acid, leaving etched
channels after the fracture closes
During a fracture job or hydraulic fracturing, a
service company injects large volumes of fracture
fluids under high pressure into the well to fracture
the reservoir rock . Fracture jobs are done either in
an open-hole or a cased well with perforations.
Common fracture fluid is a gel formed by water
and polymers, long organic molecules that form a
thick liquid when mixed with water.
Oil-based fracture fluid and foam-based fracture
fluids using bubbles of nitrogen, or carbon dioxide
can also be used to minimize formation damage.
The fracture fluid is transported out to the fracture
job in large trailers
A fracture job is done in three steps:
First, a pad of fracture fluid is injected into the well
by several, large, pumping units on trucks to initiate
fracturing the reservoir.
Next, a slurry of fracture fluid and propping agents
are pumped down the well to extend the fractures
and fill them with propping agents. Propping agents
or proppants are small spheres that hold open the
fractures after pumping has stopped.
The propping agents are usually well sorted quartz
sand grains, ceramic spheres, or aluminum oxide
pellets. The well is then back flushed in the third
stage to remove the fracture fluid.