- · Soaking-Agitation.
- · Fracture Acidizing.
- · Matrix Acidizing.
4.1 Soaking-Agitation (Perforation Cleaning).
The number of soaking and agitation applications depends upon the amount of
damage that has occurred in the perforations or in the immediate area of the
wellbore. Acid solutions designed for suspension, solvent acid dispersions, or cleanup
types are normally used in soaking action. This soaking action allows the acid to
work on the acid-soluble materials and remove mud filtrate, silts and other debris
that might plug the formation. Agitation can be accomplished by one of three
methods:
1. The acid can be spotted across the perforations to allow a short
soaking period and then washed back through the annulus whilst the
work-string is moved up and down through the zone of interest.
2. Pressure is applied against the perforations without exceeding the
bottom hole fracturing pressure (BHFP) and then releasing this
pressure very quickly through the bleed off at the pumping unit. This
action is sometimes referred to as "back-surging".
3. Acid is spotted across the perforations and allowed to soak for a few
minutes, then it is "swabbed back" either through the tubing or casing.
With any of the above methods, acid may have to be applied several times before
the formation is opened for fluid entry. The use of several applications allows a
regular acid job to be performed without fear of pushing unwanted plugging material
into the natural permeability or flow channels of the formation.
Non-acid chemical treatments are used to treat for scale deposits, water blocks,
bacteria, clay damage, or water shut-off systems. These types of treatment are
applied either by injecting into the formation or by soaking for a prescribed time (up
to 24 hours).
4.2 Fracture Acidizing (Limestones and Dolomites).
In fracture acidizing, the acid is injected through natural or induced fractures at
pressures usually exceeding the formation's fracture pressure (Figure 7). This type
of stimulation enlarges or creates flow channels from the formation to the wellbore,
thus increasing the flow of oil or gas. In fracture acidizing, acid penetration depends
upon the velocity of the acid, its reaction rate with the formation, the contact area
between the fractures and the acid, and the leak-off rate of the acid.
Most experts agree that the maximum penetration of acid is achieved when the first
increment of injected acid is completely neutralised. Whilst later increments of live
acid accomplish additional etching of the fracture faces, they do not penetrate any
greater distance from the wellbore than did the first increment.
This etching creates an uneven fracture face which helps prevent the fractures from
completely closing when the pressure is released. An additional benefit to the
production of oil and gas is a zone of increased matrix permeability adjacent to the
fracture faces created by the leak-off of live acid into the formation. This increased
permeability can help improve well productivity even where almost total closure of
the fracture occurs.
Velocity of the acid in a given naturally fractured formation is determined primarily by
the injection rate. The deepest penetration can be obtained from a rate that will
produce an injection pressure just slightly below the pressure required to create
additional fractures. Any pressure greater than this optimum will widen existing
fractures and open up new ones, thus decreasing the fluid velocity.
The reaction rate of the acid probably has the greatest effect on the depth of
penetration with this method. BJ Services has developed several acid systems such
as Gelled Acid, Cross-linked acid (XL Acid) and Emulsified Acid (SRA-3) to retard
the reaction rate of hydrochloric acid with limestone and dolomite formations for
deeper penetration of live acid.
An alternative method of fracture acidizing limestone and dolomite is to pump the
treating solution at high rates and pressures to hydraulically fracture the formation,
thus achieving deep penetration of the live acid.
Since acid itself is not an efficient fracturing fluid, due to its inherently low viscosity
and high reaction rate, the use of a fluid loss additives will help to confine the acid
to the flow channels by reducing leak-off. This results in deeper penetration of the
formation with a given volume of treating solution. In addition water or brine with the
proper gelling agents and fluid loss additives may be used as a spearhead to create
the fractures. The trailing acid then enters the formation and reacts with the walls of
the induced fractures. Alternatively, Cross-linked Acids can be used as the
spearhead or main treatment for this purpose.
To obtain the maximum flow capacity with this technique, the acid must etch an
uneven pattern on the fracture faces. The fractures tend to heal after treatment, but
the creation of an uneven etching pattern can maintain communication between the
wellbore and the deep fractures.
Again, leak-off of acid to the formation adjacent to the fracture faces will create a
zone of increased permeability aiding this process. Three factors that influence the
type and amount of etching in the fracture are:
- · Rock properties.
- · Type of acid.
- · Contact Time.
In fracture acidizing the acid reacts with the faces of the fracture to produce an
irregular etch pattern. Most limestone and dolomite formations vary in acid solubility
even within the formation. Acid will attack such formations at varying rates, leaving
an unevenly etched face. Zones of more variable composition and acid solubility will
show a better, more irregular etch pattern than formations with homogeneous
composition. Another factor is naturally existing fractures. These occur at random
intervals and in random sizes contributing to the final uneven etching configuration.
4.2.2 Type of Acid.
This is an equally important factor. Chemically retarded acids are made effective by
preceding them with a hydrocarbon preflush containing an oil-wetting surfactant.
Due to the variable rock composition, the surfactant leaves a discontinuous oil film
on the fracture face. The resulting acid break-through is irregular, creating an
irregular etch pattern.
Emulsified acids are also used. Here the resulting etch patterns are influenced by
the rate at which acid penetrates the hydrocarbon outer phase of the emulsion and
reacts with formation face.
Gelled and Cross-linked Acid systems help provide leak-off control and fracture
extension during the job. The viscosity provides some retardation which helps place
live acid deeper into the fracture. These systems also have excellent insoluble fines
suspending properties. The fines are returned to the wellbore, carried by the residual
viscosity of the spent acid. A final advantage of these fluids is their ability to produce
stable, high viscosity foams for use in acid foam fracs.
4.2.3 Contact Time.
The pumping rate and the total volume of acid pumped determine the contact time
of live acid with the fracture faces. Contact time has a direct bearing on the amount
of etching obtained. Depth of penetration is not increased appreciably by increasing
the volume of the treatment, as there seems to be an optimum volume above
which, large amounts of acid may smooth out any irregularities in the etch pattern.
Any additional benefits seen from a treatment having a contact time greater than the
spending time of the acid, can be attributed to the additional flow conductivity that
results from acid etching and increased permeability adjacent to the fracture faces
caused by leak-off of the live acid.
4.2.4 Spearhead Acid Control Technique.
When designing acid fracturing treatments, determination of the volume required for
effective acid penetration to a specific depth is difficult to achieve. Acid reaction
rates in the fracture cannot be accurately predicted, and the true acid leak-off rate to
the formation from the fracture face is difficult to determine.
By using Spearhead Acid Control (SAC) techniques these problems can be
minimised. With this technique a non-acidic, high viscosity, low fluid loss, aqueous
spearhead, is pumped ahead of the acid. This fluid creates the fracture and places a
temporary protective film over the fracture faces. This film restricts fluid leak-off and
under certain conditions, delays the reaction of the acid with the formation during
placement into the fracture system.
Shortly after placement, the acid disperses the protective film, allowing leak-off and
acid reaction with the formation to occur. When this reaction is complete, the
"pillaring" effect resulting from the uneven solubility of the formation leaves long
fractures of high conductivity.
Since the leak-off and reaction time of the acid are effectively controlled, the acid
can be considered as a normal fracturing fluid for the purposes of calculations when
designing the job.
4.3 Matrix Acidizing.
In matrix acidizing, acid flow is confined to the formations natural pores and flow
channels at a bottom hole pressure less than the fracturing pressure (Figure 8). The
purpose is to increase the permeability and porosity of the producing formation. This
method is used primarily in sandstone formations.
During a matrix acidizing job, the contact area between the acid and the formation is
very large; therefore, friction pressure increases rapidly with increased pumping
rates. Due to the high friction pressures, matrix acidizing must be conducted at low
injection rates, and is therefore, usually limited to removing shallow formation
damage (wash jobs).
After the flow channels are enlarged, the materials creating the damage can be
removed from the formation with the acid as it is produced back. In treating
formation damage such as mud filter cake and scale, care must be taken to treat at
less than the fracture pressure of the formation to avoid fracturing past the damaged
area.
For maximum penetration when matrix acidizing, the acid should have a low
viscosity and low surface tension. Gelled and emulsified acids should not be used
for matrix acidizing because their viscosity and interfacial tension greatly increase
the injection pressures and impede the flow back of the spent acid.
In both fracture and matrix acidizing, effective stimulation depends upon the
permeation of the producing formation with an extensive network of channels that
will serve as a gathering system for the transport of oil and gas from the low
permeability rock to the wellbore.
4.3.1 Matrix Acidizing Horizontal Wells.
Horizontal wells are normally targeted for thin formations with good vertical
permeability and reservoirs that suffer from coning problems. Matrix stimulation of
long horizontal sections have been shown to be successful compared to fracture
treatments in reservoirs with relatively low permeabilities (0.5 to 1.5 md) and small
vertical height (less than 50 ft). Outside of this narrow range, reservoirs where
vertical wells are normally considered candidates for fracture treatments, will also
require hydraulic fracturing when drilled horizontally.
When matrix acidizing horizontal wells, all aspects of job design that apply to vertical
wells should be considered. Placement of fluids along the length of the horizontal
section is critical to the success of any stimulation. “Bull-heading” acid into horizontal
wells has generally proven to be unsuccessful. In most cases the acid has a
tendency to enter the formation in a zone close to the vertical section thus, obtaining
only partial stimulation.
To overcome this problem, various methods have been applied to improve the
distribution of stimulation fluids along the well. These methods include chemical
diverters, mechanical isolation devices, partial perforation techniques (leaving blank
portions in the casing, and coiled tubing
.
In general coiled tubing has proven to be the most effective method for acid washes
and matrix stimulation and can be used to obtain adequate coverage. With this
technique, the coiled tubing is placed at the end of the horizontal section and
withdrawn towards the vertical section whilst pumping acid and diverter stages. At
the same time an inert fluid is pumped down the to aid placement and reaction of
the acid at the point of injection. However, the stimulation fluid is likely to follow the
path of least resistance making diversion and zonal isolation essential.
This technique is better discussed in other literature (Matrix Stimulation Methods for
horizontal Wells. Economedes, Naceur and Klem. JPT July 1991). However, as the
section to be treated in a horizontal well can be several thousand feet long,
treatment volumes will be large and particular attention should be paid to the
economic aspects of such a treatment.
For example, in a vertical well a small treatment may require that 100 gallons of acid
per foot of pay be pumped to remove the damage. If the zone of interest was 50 ft in
length, the total volume of acid required would be 5000 gallons. If this same
treatment were required for damage in a horizontal section with 1000 ft of length in
pay, a "small" treatment of 100,000 gallons of acid would be required. This type of
volume and greater is commonly pumped in matrix stimulation of horizontal wells
with excellent results.
Drawbacks to this method of stimulation include long acid to tubing exposure times
(due to limitations on pump rate through coiled tubing), and difficulty in achieving
successful diversion, particularly in limestone formations as fracturing pressures
would not be exceeded.
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