Well Acidizing

What is Well Acidizing?

Acid stimulation treatments represented many challenges for the oil industry because they are linked to uncertainty since not be defined with exact rules. However, in terms of generating immediate improvements in production rates and return potential at a reasonable price, acid stimulation processes are unmatched.

Well-acidizing refers to the stimulation of an oil reservoir with a solution containing reactive acid to improve permeability and well production. In sandstone formations, acids help to enlarge pores, while in carbonate formations, acids dissolve the entire matrix.

Acids are useful in well-stimulation for their effectiveness in the dissolution of minerals from the formation and foreign to it, such as injection sludge, clays, and carbonates that may have introduced during drilling or completion operations. The efficiency in dissolving these minerals and materials will depend on the increase in the productivity of the well.

Acidification also serves as a cushion for hydraulic fracturing, to dissolve fine and particles formed in the cannon process. Acids break emulsions in formations that are sensitive to low pH or that are stabilized by particles that the acid can dissolve. It is also used to break acid-sensitive viscous gel-type fluids, in hydraulic fracture treatments that have not ruptured after finishing treatment. In cementation operations, acid is used before the process as a preflush.

Acidification usually occurs in wells that are in the final stages of production. Among the removable acid damage we have: solids and sludge filtration during drilling, filtration invasion during the cementation process (due to the pH effect), compaction of the canyoned area, tamponed of organic deposits, migration of fluids and iron oxides during production, invasion of clay, swelling of clay during reconditioning work, the release of clay and fine, precipitation of solids formed by chemical reactions between the treatment-formation fluids and wettability change during a stimulation work.

Acid Washing

During acid washing, the main objective is to carry out a cleaning through the production pipe and in the vicinity of the well, to remove elements grade powder present in the well, or to open the holes that are covered. For this treatment, the formation is not altered since it is used to generate cleaning of fine and rubbles that may be found in the well and may cause a restriction of the flow.

Acid washing consists of using a small amount of acid, approximately 25% of the usual, in the desired position in the well and allowing it to react without external agitation. The acid circulates through the well to clean the perforated ones and the face of the formation. The most commonly used acid for this technique is HCl.

Matrix acidizing

Matrix acidification is the injection of acid into the formation at a pressure less than that of its fracture. For sandstone acidifications, matrix treatments remove or dissolve formation damage, which may exist due to plugging in the perforated area and the pore network of the formations near the well. Theoretically, the acid flows through the pore system of the rock, dissolving in its path the solids and powder trapped in the pore spaces, which prevent the flow of hydrocarbons.

Matrix acidification is applied only to formations that present damage, but there may be cases where this is unfulfilled.

The purpose of matrix acidification is to achieve radial penetration in the formation and is used when acid fracturing cannot be carried out due to risks associated with the production. It is carried out to improve capillary flow, that is, the permeability through the region of damage in the vicinity of the well.

Most matrix acidification treatments use mixtures of HCl. That is an acid that dissolves limestone and dolomites to allow the opening of conductive channels, which will give way to the formation of wormholes, thus avoiding formation damage near the well.

Fracture acidizing

The goal of this type of treatment is to inject acid into the fracture created in the formation at a pressure high enough to open natural fractures or to break the reservoir. The method is widely used in dolomites.

Initially, a higher flow rate causes an increase in pressure high enough to exceed the compressive forces of the earth and the tensional forces of the rock. At this point, the formation fails by the break and formed the fracture.

The injection flow rate has to be high enough and the formation permeability low so that the fluid loss is not excessive so that the pressure can increase enough to fracture the formation or to open natural fractures existing.

There are uses acids to record the faces of fractures promoting that they are not pairs by creating channels or "worm-holes" to prevent them from closing in the carbonic formations. The engraved fracture will act as a high conductivity channel to move fluids closer to the well.

Well Acidizing Operational Conditions - Phenomenons

The main reasons for carrying out operations of acidification in wells where desired increase is the productivity are:

• In case the production formation has an original low permeability, the treatment will allow increasing the permeability in the vicinity of the well to improve the flow of hydrocarbons from the reservoir to the producer well.
• If the reason for the low productivity is formation damage, it will create a highly permeable channel to connect the well with the formation of unchanged permeability.

Phenomena affecting well acidification operations

The most important phenomena of physical and chemical interactions of the treatment fluid with the reservoir fluids and minerals are shown below.

Phenomena	Description
Mass transfer between acid molecules and mineral surface	The reaction of acids with minerals takes place at the boundary between solid and liquid. There not occur within one of the phases. For the reaction to occur, the acid is transported to the surface of the material by convection. The reaction rate is influenced by the characteristics of the mineral, the properties of the acid (type and concentration), the temperature of the reservoir, and depends on the velocities of mass transfer and reaction on the surface of the mineral.
Change in the poral structure	The physical change of the porous structure caused by the dissolution of some minerals is the mechanism of increased permeability. The poral structure change is different for sandstones and carbonates, so different models are used to approximate each of the processes.
Precipitation of reaction products	Secondary chemical reactions occur when acidification occurs, particularly in sandstones, involving precipitation of some of the reaction products. These solid precipitates can block the porous space and avoid achieving the objectives of stimulation.
Emulsions	Formation of emulsions or changes in the wetness of the reservoir, in the vicinity of the well.

Parameters to be considered in well acidification operations

The main parameters to be considered in acidification operations include the volume of the acid, injection rate and injection pressure, as detailed below.

Volume of the Acid:

The volume of acid should be large enough to treat formation damage near the well and small enough to reduce the cost of treatment. The acid volume required depends heavily on the depth of the damaged area.

The total volume of fluids to be pumped during the operation consists of an initial volume for conditioning the formation (pre-wash) containing dissolved acid in a fluid base that is usually water, the corresponding volume of solution for treatment, which is a volume of base fluid more acidic at a higher concentration than the pre-wash fluid and finally an adequate displacement volume.

The pre-wash volume aims to create a physical barrier between acid and water formation, preventing the precipitation of substances. The selected stimulation fluid will depend on the type of lithology to be stimulated. Finally, the displacement fluid is intended to displace the harmful precipitates from the vicinity of the well, ensure the total reaction of the acid, and facilitate the removal of reaction products.

Injection Rate:

The injection rate is selected based on the mineral’s dissolution and the depth of the damaged area. The rate to inject the acid is usually a little less than the maximum rate at which the reservoir can be injected without breaking it. The process of determining the injection rate is complicated because the radius of the damaged area, the dissolution effects of the minerals, and the possible precipitation of the reaction products are rarely accurately known.

Injection Pressure:

In most acid treatment operations, only is monitored the surface pressure of the production pipe. It is necessary to predict the injection pressure at the design stage for the selection of the pumping equipment.

Additives to be considered in well acidification operations

To carry out the operations of well-acidification are used additives to guarantee the effectiveness of the treatment and to protect the formation of collateral effects. Among these additives, it can find:

• Acid Corrosion Inhibitors. Its chemical substances that, added in small concentrations to the corrosive medium, act on the metal-solution interface, significantly reducing the speed of corrosion of the metal material exposed to it. For the application in acidification treatments, inhibitors based on organic compounds that have functional groups with at least one heteroatom of nitrogen, sulfur, oxygen, phosphorus, and multiple, double or triple bonds are very effective.
• Acid Emulsifiers. Its formulations usually contain the acid in the internal phase and 10 to 30% kerosene or diesel in the external phase. The increase in viscosity created by emulsification retards the rate of reaction of the acid with the rock, increasing the depth of penetration of the acid into the formation.
• Non-Emulsifiers. Its substances used to prevent the formation of emulsions in situ before is put stimulated well into production.
• Surfactants. It is used to reduce interfacial tension, to alter the wetness of the formation, and to accelerate cleaning. When surfactants are added its compatibility with the corrosion inhibitor and other additives should be ensured.
• Anti-Sludge Agents. Its surfactants used to prevent the formation of asphalt sludge after acidification treatments.
• Iron Control/Sequestering Agents. It is employed to prevent precipitation of the iron dissolved by the acid, which reduces the permeability of the formation. The most common sequestering agents are organic acids, such as citric acid, lactic acid, and acetic acid.
• Acid Gelling Agents. It is used to increase the viscosity of the fluid and reduce the rate of acid transfer with the fracture. Its use has been limited to low-temperature formations because most available gelling agents (usually water-soluble polymers) degrade rapidly in acid solutions at temperatures greater than 130° F.
• Mutual Solvents. These materials have appreciable solubility in water and oil, such as alcohols. Methanol or propanol, in concentrations of 5 to 20% by volume of acid, is used to reduce interfacial tension. The use of these alcohols accelerates and improves the cleaning of the formation, particularly in gas-producing wells. One such very effective additive is Ethylene-Glycol Monobutyl Ether (EGMBE).
• H2S Scavengers. These chemicals reduce the content of H2S, a toxic gas that is produced during production operations. Scavengers can be regenerable as amines or non-regenerable as solid that reacts selectively with H2S to form other compounds such as iron sulfide.
• Friction reducers. Its additives that when dissolved in the fluid reduces the pressure drop by friction through the pipes. It is usually based on organic polymers.
• Loss reducers for filtering. It consists of two agents or products: an inert solid particle that enters the formation by bridging near the fracture surface and a gelatinous material that blocks the pores formed by the granular solid material.

Formation Evaluation for Acidizing - Selection of the Acidification Treatment

To select appropriate acid stimulation it is taken into account that this:

1. must have the ability to react giving soluble products,
2. it must be possible to inhibit it before reactions with ferric materials,
3. have ground transportation facilities minimizing danger,
4. be low cost, and
5. have a simple manufacturing process.

The selection of fluids for matrix acidification of carbonates is simpler than for sandstones. In a carbonate reservoir, the mineralogical composition of the rock is not so complex, and the reactions between the acids and calcite or dolomite do not tend to form precipitates.

The selection of the acid system depends on:

• The mineralogical composition of the rock
• The solubility of rock in acid
• The reaction rate of stimulation acid with rock
• The temperature of the formation

The acids used in well-acidification are classified as mineral, dilute organic, organic powder, hybrid, or mixed and delayed. The most used are mineral organic acids such as hydrochloric acid, hydrofluoric acid, and mixtures of hydrochloric acids - hydrofluoric, as shown in the table below.

Acid	Use / Application	Considerations
Hydrochloric acid	Any permeability sandstones and HCl solubility >20% Carbonate formations	High corrosion power in the well and the pipes. High toxicity in humans.
Hydrofluoric acid	Any permeability sandstones, presence of iron and chlorite > 5%	It can corrode glass, steel, and rock. HF is created at the project site by mixing hydrochloric acid and ammonium fluoride and then immediately injected into a well.
Mixtures of hydrochloric acids - hydrofluoric	High permeability sandstones k > 100 md High quartz content (80%), low clay content (<5%) - 10% HCl - 3% HF High feldspar content (>20%)- 13.5% HCl - 1.5% HF High clay content (>10%) - 6.5% HCl - 1% HF High chlorite clay/iron content -3% HCl - 0.5% HF Low permeability sandstones k < 10 md Low clay content (< 5%)- 6% HCl - 1.5% HF High chlorite content - 3% HCl - 0.5% HF	Corrosion characteristics in HF-HCl mixtures are comparable to those produced by HCl, which is why similar corrosion inhibitors are used.

Design of the Acidification Treatment

The general steps for the design of well-acidification are the following:

• Select the most suitable candidates for wells, assessing the severity of the damage, its location, the radius of penetration of the damage, and whether an acid can remove said 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.
• Quality control.
• Monitoring of treatment.
• Evaluation of results.

The operational procedure in an acidification work is as follows:

1. Pickling. It is a procedure for washed the injection pipe before starting with the pumping of the acid treatment fluids. It consists of injecting iron control agents into the production pipe, casing, and coiled tubing to avoid possible contamination of the main treatment.
2. Crude oil displacement (solvent) 10-75 gal/ft
3. Displacement of formation water 12-25 gal/ft
4. Acetic acid pre-flow for cleaning 25-100 gal/ft
5. HCl pre-flow to avoid secondary reactions 25-200 gal/ft
6. Main treatment (HCl-HF, organic acid, HF according to the type of formation) 25-200gal/ft
7. Overflush (which must be the same acid to maintain the balance of the system
8. Displacement

Environmental Management Considerations

In oil operations up to 500 substances are pumped underground, among which 17 are toxic to aquatic organisms, 38 are acutely toxic, 8 are proven carcinogenic and 7 are mutagenic elements. Among others, the presence of acids, antioxidants, biocides, benzenes, xylenes, carbon disulfides, and pyridine compounds stands out.

Factors to be prevented or mitigated related to environmental impact include:

• Damage to existing infrastructure
• The occurrence of damage to vegetation, soil, bodies of water produced by the activity
• Degradation of soil conditions
• Fractionation of the ecosystem involvement of bodies of water
• Deterioration of the quality of surface water and groundwater, caused by the dumping of liquid waste and spills or leaks of acids