11. Applications of Nitrogen In Acidizing.

• · Atomized Acid.
• · Nitrified Acid.
• · Foamed Acid.

11.1 Atomised Acid.

Inject fluid as fine mist of droplets in gas phase:

• · Where water blocks are present or are likely to occur during acidizing.
• · Better removal due to higher mobility of gas.
• · High velocity of expanding gas aids clean-up of fines etc. from the producing formation.
• · High penetration of acid into smaller rock crevices and fractures.
• · Increased coverage due to expanded acid volume.

Gas to liquid ratios.

• · Use the correct amount of acid to remove all soluble material.
• · Near well-bore blockages: 1:1 or 2:1 should be used (Nitrogen to Liquid).
• · Larger areal coverage with given volume of acid: 4:1 to 6:1 should be used.
• · Amount of nitrogen used is dependent on:

         Bottom hole pressure.
         Bottom hole temperature.
         Calculated expansion ratio.



11.2 Nitrified Acid.

• · Effective and economic due to alleviation of need to swab.
• · Due to its low solubility, nitrogen assumes state of compressed gas bubbles. Acts as source of energy inside injected fluids.
• · Flow back, expansion of bubbles forces fluids towards well-bore and up to surface. Aids removal of formation fines etc.
• · Additional volume due to gas, increases radial penetration for given acid volume.
• · Volume of nitrogen in fluid needs to be calculated to determine correct volume of flush to be pumped.
• · Each job must be designed individually.
• · Use tables to determine ratio of gas to liquid based on well conditions.

11.3 Foamed acid

· Foam is a gas in water emulsion using ± 1.0% surfactant as emulsifier.
· Stable, low fluid loss foams at bottom hole conditions are formed using 52 - 95% gas volume with a continuous aqueous fluid phase.
· Acid recovery is a problem in low pressure reservoirs. Foam gives immediate acid clean-up and eliminates swabbing
· Foam eliminates need for extensive acid reaction rate retardation
· Foam aids removal of formation fines etc.

11.3.1 Nitrogen Retention.

• · Nitrified acid usually contains 500 to 1000 scf/bbl of nitrogen.
• · Nitrogen reduces hydrostatic head of acid, and allows natural flow.
• · Nitrogen may separate from acid in formation. In this case low pressure wells may require swabbing.
• · Use of up to 1.0% foaming agent allows retention and dispersion of nitrogen throughout acid.
• · As the acid spends, gas in fluid emulsion remains intact.
• · Gas provides energy to lift the spent acid.

11.3.2 Foamed Acid Diverting.

• · In large formations, acid will tend to enter most permeable or lowest pressure zone.
• · Foamed acid can divert the treatment from one set of perforations to another.
• · Foam viscosity can be very high (depends on shear rate).
• · Increasing the fluid viscosity by use of foam pads between acid stages will divert the acid to new intervals by increased pressure.
• · If all the acid is foamed prior to pumping it will have inherent diverting properties.

11.3.3 Foamed Acid Applications.

Any well requiring acid stimulation is a candidate for foamed acid or staged nitrified
and foamed acid treatments.

• · High pump rates are not required and can be undesirable.
• · Matrix treatments:

|             Use 60 to 70% quality foam

•  · Foamed acid fracturing:

             Use 70 to 80% quality foam

Normally foamed acid is preceded by a nitrified acid spearhead to achieve better
penetration. After initial penetration of low viscosity nitrified acid, foamed acid is
pumped at 65% to 80% quality. This procedure provides:

• · Deeper penetration due to low fluid loss.
• · Good diversion due to increased viscosity. 

In low API gravity oil wells.

• · Spearhead with solvents (xylene or toluene) to remove any heavy deposits (asphaltenes or paraffins), and allow the acid to react with the formation face.
• · Allow the solvent to soak for several hours (4 hours) before commencement of the acid treatment.

Wells with scale problems.

• · Fill tubing or casing with foamed acid over-flush with 10.0 bbl.
• · Shut in well for controlled reaction time.
• · Flow back well at highest rate possible.
• · If no corrosion test data:

          Inhibit acid for 4 hours at BHT
          Unload well after 3 hours shut-in.

11.3.4 Lower Quality Foamed Acid.

• · Foam qualities below 55% :

          Do not have stable characteristics.
          Viscosity desirable for better conductivity than with conventional acids.

• · Applications in deep wells:

           Used to lower the hydrostatic head.
           35 to 55% quality foamed acid is a more efficient fluid with respect to leak-off than gelled and
            emulsified acids, or acid containing other fluid loss control agents