Acidification Design

Generalities

For the vast majority of applications, the acidification design is used in sandstone reservoirs with the objective of sensitizing the removal of formation damage, prior to field work.

In formations where the quartz content is approximately 95%, it is possible to stimulate the formation by quartz solution.

However, it can be stimulated in carbonate deposits, reaching a damage value of -2.

Depending on the treatment you want to apply, there are three types of acidification: acid washing, which has the purpose of removing deposits from the walls of the well or to open blocked perforated intervals, generally plugged with scales.

Another type of acidification is matrix stimulation, which is nothing more than the injection of an acid into the formation at a pressure lower than the fracture pressure in a radial manner.

Finally, acid fracturing, which involves injecting acid at a pressure high enough to produce a hydraulic fracture within the formation.

With this type of acidification, highly conductive flow channels are obtained that, with a good proppant agent, can remain for a long period of time after the treatment has been applied.

Other uses of Acidification

Acidification often serves as a hydraulic fracturing cushion, to dissolve fines and particles formed in the perforating process.

Breaks emulsions into formations that are sensitive to low pH or that are stabilized by particles that acid can dissolve.

It is also used to break acid-sensitive viscous gel type fluids in hydraulic fracture treatments, which have not broken after completing a treatment.

In cementing operations it is used before the process as a preflush.

General Steps for Acidification Design

• Select the most suitable well candidates, evaluating the severity of the damage, its location, radius of penetration of the damage and whether an acid can remove the aforementioned damage.
• Design the most appropriate treatment according to: the type of acid that can remove the damage from the formation and its compatibility with the formation and the fluids contained in it, maximum operating flow rate.
• Quality control.
• Treatment monitoring.
• Result assessment.

Acid concentrations to use according to the type of Formation

The main treatment will depend on the solubility it has with the minerals present. Generally the concentrations to use (this should not be used as a cooking recipe) are the following:

Formations with Permeabilities > 100 mD

• Quartz > 80%, Clays 12% HCl – 3% HF
• Clay < 8%, Feldspar 7.5% HCl – 1.5% HF
• Feldspar > 15% ==> 13.5% HCl – 1.5% HF
• Clays > 10% ==> 6.5% HCl – 1% HF
• Feldspar > 15%, Clays > 10% ==> 9% HCl – 1% HF
• Presence of iron, Chlorite > 8% ==> 10% CH3COOH – 0.5% HF

Formations with Permeabilities 20 – 100 mD

• Clay > 5% ==> 6% HCl – 1.5% HF
• Clay 9% HCl – 1% HF
• Feldspar > 10% ==> 12% HCl – 1.5% HF
• Feldspar > 10%, Clay > 10% ==> 9% HCl – 1% HF
• Presence of iron, Chlorite > 8% ==> 10% CH3COOH – 0.5% HF

Permeabilities < 20 mD

• Sol. HCl < 10%, Clays 5% HCl – 1.5% HF
• Clays > 8% ==> 3% HCl – 0.5% HF
• Chlorite > 5% ==> 10% CH3COOH – 0.5% HF
• Feldspar > 10% ==> 9% HCl – 1% HF

Removable damage in an Acification

• Solids and mud filtrate during drilling. 
• Filtrate invasion during the cementation process (due to the pH effect). 
• Compaction of the perforating and formation of debris.
• Plugging of organic deposits. 
• Calcium carbonate, fluid migration and iron oxides during production. 
• Invasion of solids.
• Clay swelling during reconditioning work.
• Release of clays and fines.
• Precipitation of solids formed by chemical reactions between treatment-formation fluids. 
• Change of wettability during stimulation work.

Operational procedure in an Acidification job

• Pickling: consists of injecting iron control agents into both the production piping, casing and coiled tubing, to avoid possible contamination of the main treatment.
• Crude oil (solvent) displacement 10-75 gal/ft.
• Formation water displacement 12-25 gal/ft.
• Acetic acid preflow for cleaning 25-100 gal/ft.
• HCl preflush in order to avoid secondary reactions 25-200 gal/ft.
• Main treatment (HCl-HF, organic acid, HF according to the type of formation) 25-200 gal/ft.
• Overflush (which must be the same acid to maintain the balance of the system.
• Displacement.