Matrix acidizing

Matrix acidizing is one of the technologies used in the development of wells and their operation. Its main goal is to clean the bottom hole to stimulate the flow of formation fluid. There are several modifications of this technology, depending on the regime of impact on the reservoir and geological conditions.
Purpose and principle
Acid treatment is used in the drilling, operation and maintenance of oil production facilities to solve the following problems:

• treatment of the bottomhole zone during well development (for inflow of formation fluid after the completion of its construction);
• intensification (increase in production rate);
• cleaning the filter and bottomhole from contamination accumulated during operation, after water injection or well workover;
• elimination of deposits in casing strings and other underground equipment.

Acids injected into the well dissolve calcium-containing rocks (limestone, dolomite, etc.), as well as particles of cementitious compounds that remain at the bottom after cementing the annulus.
Types of acidizing
In the practice of operation and maintenance of oil production facilities, the following types of acid treatment are distinguished:

• matrix (injection of a reagent under pressure, the value of which is less than hydraulic fracturing);
• in-situ acid baths (simple treatment);
• under high pressure (acid fracturing, while fracturing occurs);
• interval impact;
• thermal acid treatment.

The latter type of technology is used in situations where the pores of the reservoir in the bottomhole zone are clogged with wax deposits, resins and high molecular weight hydrocarbons.
Acid wells are mainly carried out in the following cases:

• primary development (putting wells into operation);
• cleaning of uncased filters;
• cleaning the filter, covered with casing pipes, from acid-soluble materials.

Reagent types
The basic substances used in acidizing wells are hydrochloric HCl and hydrofluoric HF acids, as well as their mixture (clay acid). Other acids are less commonly used: acetic; sulfamic; formic; sulfuric; mixtures of organic acids.
If the geological formation is in high temperature conditions, then acetic or formic acid is injected into the formation. The use of sulfamic acid is justified in cases where the reservoirs consist of sulfate and iron-bearing carbonate rocks, since their reaction with hydrochloric acid leads to the precipitation of gypsum or anhydrous calcium sulfate. The working solution of the reagent is prepared at commercial acid bases and transported in road or railroad tanks, painted inside with resistant enamel, with a rubber or ebonite coating. Acid treatment is carried out not only in oil wells, but also in water injection (to maintain reservoir pressure), as well as in artesian wells. Work in Abyssinian wells, at shallow depths, can be carried out with a well cleaning thief.
Pure acids are rarely used. The following substances are used as additives to them in the oil industry: corrosion inhibitors - to prevent the destruction of casing, tubing and other equipment; complexing compounds that prevent the formation of a gel or iron hydroxide that clogs the pores of the reservoir; potassium nitrate for the treatment of anhydrites (sulfates); stabilizers for keeping the reaction products in a dissolved state; citric or acetic acid for the treatment of iron-bearing carbonate rocks; surfactants, or intensifiers (OP-10, OP-7 and others) to improve the wettability of rocks and facilitate the removal of reaction products from the bottom.
Hydrochloric acid
When acidizing wells using HCl, its optimal concentration is 10-16%. More saturated solutions are not used for the following reasons: a decrease in the dissolution rate; increased corrosivity; increase in emulsifying ability; increased precipitation of salts when mixed with saline formation water. When processing sulfate-containing rocks, additives from sodium chloride, potassium or magnesium sulfates, and calcium chloride are added to the working fluid. The latter also serves as an acid neutralization inhibitor at elevated downhole temperatures.
Hydrofluoric acid
Hydrofluoric acid is a potent one and is used to dissolve the following materials: silicate compounds in terrigenous formations; clay or cement slurry absorbed during drilling or well workover; cement crust at the bottom. Ammonium fluoride bifluoride is also used as a substitute for this reagent, the consumption of which is 1.5 times less.
Simple hydrochloric acid treatment Simple treatments are done with one pumping unit. Before acid injection, the well is flushed with water to preliminarily remove cuttings and other contaminants.
If there are deposits of paraffin or resins at the bottomhole and in the tubing (tubing), then organic solvents are used as the flushing fluid - kerosene, liquefied propane-butane fraction, and others. Depleted fields can be treated with a clean-out thief. Preliminary measures also include the following operations: installation of a workover unit at the wellhead; extraction of downhole equipment (for operating wells); running tubing to the lower perforations of the treated interval: equipping the wellhead with fittings for piping and a check valve; piping of a pumping unit with tubing, acid carrier, tank trucks with displacement fluid; hydrotesting of discharge pipelines under pressure 1.5 times higher than the working one. Then, acid is pumped into the well in a volume equal to the tubing cavity, after which the annular valve is closed. Then the remainder of the reagent and the displacement fluid are injected. Crude degassed oil is used as the latter in production wells.
After pumping the full volume, close the buffer valve, disconnect the pump and other equipment. The acid remains in the well for the required time to dissolve, after which the products of the chemical reaction are extracted by a pump by backwashing.
Interval technology
When opening an oil and gas reservoir with layers with different permeabilities, simple acidizing of wells leads to the fact that it affects only the most permeable layer. In such cases, it is advisable to use interval technology. To isolate each layer, 2 packers are installed in the well. The overflow of acid solution through the annulus is prevented by cementing it. After processing the selected area of the formation, they move on to the next one.
Acid fracturing and thermal acid treatment
High pressure acid treatment of wells is carried out during the operation and development of reservoirs with heterogeneous permeability. Simple acid baths are ineffective in such cases, because the acid "leaves" in well-permeable layers, and other areas remain uncovered. Before injection of the reagent, the layers with high permeability are isolated using packers (similar to the previous technology). Preparatory measures are carried out according to the scheme of simple acidizing of wells. The casing is protected by installing an anchored packer on the tubing. An emulsion prepared from a solution of hydrochloric acid and oil is used as a working reagent.
The following factors influence the choice of the reagent composition:
Fracturing of the rock. With a high value of this indicator, it is advisable to use thickened acids and foams. This helps to improve the sweep. For thickening, carboxymethylcellulose (CMC) is added to the acid.
Contamination of the bottom of the well with mineral suspensions and low permeability of the porous reservoir. In this case, to improve the penetration of the reagent, carbonated acids are preferable, for which the surface tension at the boundary with the rock is reduced. For aeration of the liquid, air, nitrogen, carbon dioxide are used.
Mineral composition of rocks. Seams consisting of sand, sandstones and siltstones are treated with clay acid.
Downhole temperature. So, the use of sulfamic acid is limited by the fact that when heated to 80 ° C, it decomposes by water by 43%. At temperatures above 115 ° C, concentrated hydrochloric acid is pumped downhole.
The required volume of acid is calculated by the formula and depends on the following factors: the thickness of the reservoir interval exposed to acid attack; porosity of rocks; processing depth; borehole radius.
The maximum injection pressure is determined by the following criteria: purpose and treatment method; production casing strength; the thickness of the dam between the working and adjacent interval of the formation.
The duration of the acid exposure is determined empirically - by measuring its concentration in the solution displaced at the wellhead through the tubing. The average value of this parameter is in the range of 16-24 hours.

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