This document provides information about acidizing oil and gas wells. It discusses that acidizing involves pumping acid into wells or formations to improve productivity or injectivity. There are three main types of acid treatments: acid washing, matrix acidizing, and fracture acidizing. The document outlines key factors in acid selection like formation type and permeability. It also discusses common acid systems and additives used. Methodology sections provides details on calculating skin factor and models for predicting acidization. The conclusion states that acidizing is widely used for well stimulation and hydrochloric acid and hydrofluoric acid are most commonly used, sometimes in blends.

2. contents
1.ABSTRACT
2.INTRODUCTION
3.METHADOLOGY
4.CONCLUSION
5.REFERENCE

3. ABSTRACT
 Oil and gas operator shave used acid treatment to improve well productivity for almost 120 years .
 Acidizing is one of the most widely used and effective means available to oil and gas operators for improving
productivity of wells .
 Acidizing is commonly performed on new wells to maximize their initial productivity and on aging wells to
restore productivity and maximize the recovery of the energy resources.

4.  With the development of effective corrosion inhibitors, the use and further development of acid
treatment (acidizing) of oil and gas wells proliferated, leading to the establishment of the well
stimulation services industry.
 Oil and gas operator shave used acid treatment (acidizing) to improve well productivitity for almost 120
years Acidizing predates all other well stimulation techniques,including hydraulic fracturing.
INTRODUCTION
 Acidizing involves pumping acid into the well bore or geological formation that is capable of
producing oil and/or gas. The purpose of any acidizing is to improve a well's productivity or
injectivity.
There are three general categories of acid treatments:
a. Acid washing
b. Matrix acidizing
c. Fracture acidizing.

5. KEY FACTORS OF ACID SELECTION
 There are majorly two key factors that dominate the treatment selection and design
process when planning an acid job. They are
1.Formation type
2. Formation permeability
Formation type:
Knowing the type of formation being acidized and details of its composition (mineralogy) is critical
to achieving positive results. In carbonate formations, the acid job design is typically based on the
use of hydrochloric acid (HCL).
.
Formation permeability:
Formation permeability determines the pumping pressure required to place the acid into the formation. In
general, the lower the permeability, the higher the pumping pressure. In high permeability formations the acid
can be pumped into the matrix of the formation at relatively low pumping pressures

6. METHADOLOGY
 In the removal of near-wellbore formation damage, acidizing finds its primary application. In a well producing
in a radial-flow regime, most of the pressure drop to the well bore occurs within a short penetration distance into
the reservoir
 Skin is a mathematical representation of the degree of damage present. It can be represented, qualitatively, by
the Hawkins’ equation, as follows:
where k is the formation permeability
𝑘𝑠 is the permeability of the altered (damaged) zone extending to radius
𝑟𝑠 is the radius of the altered zone
𝑟𝑤 is the well bore radius
 Acid systems in current use can be classified as mineral acids, dilute organic acids, powdered organic acids,
hybrid (or mixed) acids, or retarded acids. The most common numbers of each category are given in below
table.

7.  The permeability was observed during injection of HF-HCI mixtures to monitor changes that
occurred for various acid concentrations and applied pressure gradients (flow rates) indifferent and
stones. A plot of permeability change as a function of the amount of acid injected was
subsequently called an Acid Response Curve

8. Graph showing effect of HF concentration on core response to HF-HCL mixtures

9. Graph showing effect of acid flow rate on Berea core response %HF- 13 wt % HCL.

10. Graph showing solubility of𝐂𝐎𝟐 in spent acidizing solution.
Distance –time graph of a model – predicted acidization using 4wt% HF for a sandstone formation containing 9wt%
kaolinite and 91wt% inertquartz at 𝟐𝟓°C.

11. Distance-time graph of a model –predicted acidization using 4wt%HF/11wt% HCL for a sandstone formation
containing 9 wt% kaolinite and 91 wt% inert quartz at 𝟐𝟓°C.

12. Distance-time graph of a model-predicted acidization using 4 wt%/11wt% HCL for a sandstone formation containing
9wt% kaolinite, 12wt% calcite, and 79wt% inert quartz at 𝟐𝟓°C.

13. The HF—HCI spent acid front was allowed to advance only half way into the core. Following a 2-hr shut-in period,
flow was reversed and the spent acid was flushed out with 3 wt% NH-CI brine. The core was cut into sections and the
permeability of each was measured. Three distinct permeability regions were found.
 The general approach to sandstone acidizing is as follows:
1. Select an appropriate candidate well for acid stimulation.
2. Design an effective treatment
3. Monitor the treatment for subsequent improvement.

14.  The six-step process to successful sandstone acidizing is as follows:
1.As certain whether acid- removable kind am age is present.
2. Determine appropriate fluids, acid types, concentrations, and treatment volumes.
3. Establish the proper treatment additive program.
4. Determine the treatment placement method.
5. Ensure proper treatment execution and quality control.
6. Evaluate the treatment.
 The additives used in acid areas follows:
• Corrosion inhibit or and inhibit or intensifier
• Iron control agent
• Water-wetting surfactant
• Mutual solvent
• Alcohols
• Non emulsifier/ demulsifier
• Anti-sludging agent
• Clay stabilizer
• Fines – stabilizing agent
• Foaming agent
• Calcium carbonate / calcium sulphate scale inhibitor
• Friction reducer
• Acetic acid (additive to HCL-HF)

15.  CORROSION IN HIBITOR:
A corrosion inhibitor is always necessary. It must be added to all acid stages
(pickling treatment, acid pre flush, main acid, and acid over flushes). Corrosion inhibitors are often
cationic polymers that oil-wet sandstones.
 In general, a corrosion inhibitor concentration greater than 1% may be questionable, unless down
hole temperature is greater than 250°F. At higher temperatures, inhibitor concentration should
generally not exceed 2%, unless corrosion protection requirements are very stringent.
Corrosion inhibition—acceptable24-hourlimits
 IRON CONTROL AGENT:
Iron control is required in any acid treatment. Therefore, an iron control agent is almost always needed.
There are many service company iron control products. However, products fall into two general categories:
(1) iron complexing or iron-sequestering agent sand
(2) iron-reducing agents.

16.  Iron control is critical in the acidizing of injection wells, because iron rust and scale are deposited
in the wellbore and at the formation face.
Properties of common iron control agents:
 A water-wetting surfactant should not be added beyond a concentration of 1%. High concentrations
of surfactant can cause emulsion and foaming problems in production processing equipment.

17.  Non emulsifiers are surfactant additives that can be added to acid to prevent acidoil emulsification. There
commended non emulsifier concentration range is0.1%–2.0%( depending on the results of laboratory
testing).The preferred rangeis0.1%–0.5%
 A clay stabilizer is often recommended for the purpose of preventing migration and /or swelling of clays
following an acid treatment. Common clay stabilizers are either poly quaternary amines or polyamines
A clay stabilizer is most effective when added to the over flush only
 The effective clay stabilizer concentration range is 0.1%–2.0%
 Fines-stabilizing agents (FSAs) are additives that can effectively prevent clay migration / swelling and
the migration of non clay siliceousfines.11 One such additive is
 This FSA may be used in acid treat mentor in non-acid treatments in a brine carrier fluid. There
commended FSA concentration is 0.5%–1.0%.
 The use of nitrogen and a foaming agent can assist return production of spent acid in gas wells by
reducing fluid gravity and surface tension of the fluids injected. There commended foaming – agent
concentration range is 0.3%–0.8%.

18.  Pipe friction can increase acid treatment injection pressures, thereby lowering injection rates, which may
be undesirable. This is especially true in smaller-diameter injection strings, such as coiled tubing.
Friction pressure represents increased pumping energy resulting from fluid drag on pipe.
 There commended friction reducer concentration is 0.1%–0.3%.
 Addition of acetic acid to the HCL-HF stage may reduce the precipitation of certain alumino silicates,
which occurs as the pH of the HCL –HF mixturerises with acid spending.
 There commended concentration is 3%.

19. CONCLUSION
Today, acidizing is one of the most widely used processes for stimulating oil and gas wells. Two types of acids
are most commonly used; hydro chloric acid in all formation types and hydro fluoric acid in sand stone sand
certain shales. Other types of acids, such as organic acids, may also be used in specialized situations. Since
geologic formations are near homogeneous, blends (particularly for sandstone formations) of HCL and HF are
usually pumped with the blend ratios based on formation mineralogy
 The success of matrix acidization depends on choosing the correct acid formulation based on the reservoir
mineralogy and petro physical properties.

20. REFERENCE
1. Ohia, Nnaemeka; Igwilo, K. &Duru, U “WELL PRODUCTIVIY ENHANCEMENT USING
MATRIX ACIDIZING – ANIGERDELTA CASE STUDY” Federal University of Technology,
Owerri– Nigeria ISSN: 2395-3470.
2. Reservoir Simulation 2nd Edition Michael. J. Economides Kenneth G.Nolte.
3. Successful Matrix Acidization Using Organic Acid System for Sand stone Reservoir of Western On
shore Fields of India – Sohil Shah, S.K.Singh,V.K.Bahuguna, and K. W.Rajendra, Oil & Natural Gas
Corp.
4. “Acidization of sand stone with buffered Hcl Acid system” by M.J. Smithand G.J.Clark, Amerigo
R&D Ltd; A.R. Hendrick son Intl, SPE 1482615.
5. “Effect of oil saturation on rock propagation during Matrix Acidization of carbonate rocks“, by
R.Kumar ,SPE 169330

21. THANK YOU