1 Matrix acidizing is a well stimulation technique in which an acid is injected into the formation in order to dissolve some of the minerals present and hence increase permeability in the near-wellbore vicinity.
What is the difference between matrix acidizing and acid fracturing?
In matrix acidizing ,the acid is injected at pressure below the fracture pressure of the formation while in acid fracturing the acid injected at pressure above the formation fracture pressure.
Treating techniques:
1-Uncontrolled Acidizing:
A-Remove fluid standing in the hole by swabbing
B-Pump the acid into the casing
C-Follow the acid with sufficient displacement fluid to force all the acid out into the formation
Advantages of uncontrolled acidizing:
1-It is cheaper
2-Less time consuming
3-The reaction products are readily removed from the formation
Its disadvantages:
There is no control over where the acid will go and it is possible for the fluid to be wasted on an unproductive zone.
2-Selective acidizing: the acid is directed into a desired section of the formation
A-Conventional Method:
1-The well is filled with oil followed by the acid to displace the oil in the tubing, plus the annulus.
2-Then casing outlet is closed then.
3-Further pumping of acid results in the acid’s being forced out into the formation.
B-The packer method:
1-The well is filled with oil, after which acid is pumped down the tubing and spotted over the pay zone
2-The packer then prevent further travel of the acid up the annulus of the well and is forced out into formation.
C-The selective electrode technique
1-Tubing is lowered so that the seating nipple is opposite the zone and acidizing electrode is run on wire line and seated in the nipple.
2-The acid-oil interface is kept at any desired position by the use of two pumps, one pumping acid down the tubing, the second pumping oil down the annulus.
3-The level of the interface is determined electrically at the surface.
D-Radioactive-tracer Technique
In this case, a small amount of radioactive isotope is dissolved in the acid, and the level of the acid-oil interface in the annulus is detected by Geiger counter hung on a wire line in the tubing.
E-Ball sealers
It is possible to acidize the less permeable section by dropping ball sealers.
These ball sealers enter the more permeable section, wedge these balls into the perforations and divert the acid into the less permeable zone.
F-Temporary Plugging Agents
Selective acidizing either in perforated casing or open hole may be accomplished by the use of temporary chemical plugs which block off the most permeable section.
The materials used are self liquefying, either by interaction of two or constituents or by dissolution in the well fluids.
Selective Acidizing
Advantage
1-Keeping the acid out of unproductive zones
2-Stimulate tight section
3-Preventing the acid from entering water zones
Disadvantages
1-More expensive 2-The treatment is difficult to conduct
3-Longer time is required for the well to clean up afterwards.
Stage Acidizing
The well is treated with two or more separate stages of acid, rather than one stage.
At the first stage, we clean up the contaminated zone near the wellbore so that later stages can penetrate further distances.
Chemicals
1-Acids
HCL is used usually for acidizing in oil fields
In case of sulfuric acid, the reaction product calcium sulfate being insoluble and remaining in the formation, plugging flow passages.
Other acids such as acetic acid and citric acid have been used, but their high cost limits their use.
2-Inhibitors
Inhibitors are chemical materials which greatly retard the reaction rate of the acid with metals.
Their use is to avoid damage to casing, tubing, pumps and valves
Their use only slow the reaction down, eliminating 95 to 98 percent of the metal loss
3-Intensifiers
Intensified acid is admixtures of inhibited hydrochloric and hydrofluoric acids.
The fluoride present speeds up the reaction rate of the acid and enable the acid to dissolve insoluble minerals found in dolomite in small amounts.
4-Surfactants
Surfactants lower the surface tension of acid solution.
The presence of surfactant facilitates the penetration of the acid solution into the microscopic formation.
Another advantage is the demulsifying action of it which inhibits the occurrence of emulsions.
5-Demulsifiers
When the crude oil is mixed or agitated with acid or spent acid, emulsions may be formed which could block the formation.
Demulsifies are chemical agents which counteract the natural emulsifiers in the crude oil.
6-Silicate Control
These silicates will swell in spent acid and hence swollen silicate particles may block.
The silicate control agents either:
1-Keep the acid pH at a definite value at which silicate particles occupies the smallest volume
2-Causing shrinkage of the silicate particles by replacing the adsorbed water molecules with a water repellant organic film
7-Hot Acid
By heating the acid, the reaction rate is increased and amore effective treatment result.
It is better to heat the acid at the bottom of the wellbore.
This is accomplished by allowing the acid to react with magnesium at bottom.
8-Fluid Loss Control
Special additives are available which are designed to prevent “bleeding off "of acid into small formation pores.
The additives consist of ground vegetable material which blocks the finer pores but is later destroyed by hydrolysis.
9-Iron Retention
The iron compounds which are dissolved will re-precipitate as a bulky, gelatinous hydroxide when the acid becomes spent.
Various complexing agents are used to tie up the dissolved iron in complex ions, retaining it in solution even after the acid has become spent.
Precipitation of acid reaction products:
The most common damaging precipitates in sandstone acidizing are:
1-Calcium fluoride:
It results from reaction between HF and Calcite.
Inclusion of an adequate HCL preflush ahead of the HF/HCL stage prevents its formation.
2-Colloidal silica:
Its precipitation occurs at fairly slow rate.
To minimize its damage, we inject the acid at high rates, so that the precipitation zone is rapidly displaced away from the wellbore.
Also, spent acid should be produced back.
3. Ferric hydroxide:
The ferric ions precipitate from spent acid solution when the pH is greater than 2.
The ferric ions result from the dissolution of rust in tubing by the acid solution.
4-Asphaltene sludge:
The contact of crude oil by acid causes their formation.
Surface active additives have been used to prevent their precipitation.