Acid treatment of wells is one of the technologies used in the development of wells and their operation. Its main goal is to clean the face to intensify the influx of formation fluid. There are several modifications of this technology, depending on the mode of stimulation and geological conditions.
Purpose and principle
Acid treatment is used during drilling, operation and maintenance of oil production facilities to solve the following problems:
- treatment of the bottom-hole zone during the period of well development (for the influx of formation fluid after completion of its construction);
- intensification (increase in flow rate);
- cleaning the filter and bottom from contaminants that accumulate during operation, after water injection or well repair;
- removal of deposits in casing strings and other underground equipment.
Acids injected into the well dissolve calcium-containing rocks (limestone, dolomite and others), as well as particles of cementing compounds that remain on the bottom after cementing the annulus.
Processing types
In the practice of operation and maintenance of oil production facilities, the following types of acid treatment are distinguished:
- matrix (injection of reagent under pressure, the value of which is less than hydraulic fracturing);
- in situ acid baths (simple processing);
- under high pressure (acid fracturing, with cracking);
- interval exposure;
- thermoacid treatment.
The latter type of technology is used in situations where the pores of the reservoir in the bottomhole zone are clogged with paraffin deposits, resins and high molecular weight hydrocarbons.
Acid baths of wells are mainly carried out in the following cases:
- primary development (putting wells into operation);
- clean open filters;
- cleaning the filter blocked by casing pipes from acid-soluble materials.
Reagent Types
The basic substances used in acid treatment of wells are hydrochloric HCl and hydrofluoric acid HF, as well as their mixture (clay acid). Other acids are less commonly used:
- vinegar;
- sulfamic;
- formic;
- sulfuric;
- mixtures of organic acids.
If the geological formation is in high temperature conditions, acetic or formic acid is pumped into the formation. The use of sulfamic acid is justified in cases where the collectors consist of sulfate and iron-containing 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 industrial acid bases and transported in auto or railway tanks, painted with resistant enamel inside, with a rubber or ebony 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 ones. Work in Abyssinian wells, at shallow depths, can be carried out by a bailer for cleaning wells.
main parameters
The following factors affect the choice of 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 increase the coverage of the reservoir. For thickening, carboxyl methyl cellulose (CMC) is introduced into 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 preferred, in which the surface tension at the rock boundary is reduced. For aeration of a liquid, air, nitrogen, carbon dioxide are used.
- The mineral composition of the rocks. Formations consisting of sand, sandstones and siltstones are treated with clay.
- The temperature at the bottom. So, the use of sulfamic acid is limited by the fact that when heated to 80 ° C it decomposes by 43% with water. At temperatures above 115 ° C, concentrated hydrochloric acid is pumped into the face.
The required acid volume is calculated by the formula and depends on the following factors:
- the thickness of the acid interval of the formation;
- porosity of rocks;
- processing depth;
- well radius.
The maximum injection pressure is determined by the following criteria:
- goals and processing method;
- production string strength;
- the thickness of the bridge between the working and adjacent interval of the reservoir.
The duration of acid exposure is determined empirically - by measuring its concentration in a solution displaced at the wellhead through tubing. The average value of this parameter is in the range of 16-24 hours.
Additives
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 clogging the pores of the collector;
- potassium nitrate for the treatment of anhydrites (sulfates);
- stabilizers to preserve the dissolved products of the reaction;
- citric or acetic acid for the treatment of iron-containing 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
With acid treatment of wells using HCl, its optimal concentration is 10-16%. More saturated solutions are not used for the following reasons:
- decrease in dissolution rate;
- increase in corrosion activity;
- growth of emulsifying ability;
- increased precipitation of salts when mixed with saline formation water.
When processing sulfate-containing rocks, additives from table salt, potassium or magnesium sulfates, and calcium chloride are introduced into the working fluid. The latter substance also serves as a retardant for acid neutralization at elevated bottomhole temperatures.
Hydrofluoric acid
Hydrofluoric acid is a potent and is used to dissolve the following materials:
- silicate compounds in terrigenous formations;
- clay or cement slurry absorbed during drilling or well repair;
- cement peel on the face.
As a substitute for this reagent, ammonium fluoride bifluoride is also used, the consumption of which is 1.5 times less.
Simple hydrochloric acid treatment
Simple machining is done with a single pumping unit. Before acid injection, the well is flushed with water to preliminarily remove particles of sludge and other contaminants. If there are deposits of paraffin or resins at the bottom and in tubing (tubing), then organic solvents — kerosene, liquefied propane-butane fraction, and others — are used as the washing liquid. Processing in depleted fields can be done using a bailer for cleaning the well.
Preliminary activities also include the following operations:
- installation of an underground repair unit at the wellhead;
- extraction of downhole equipment (for production wells);
- tubing descent to the lower perforations of the processed interval:
- wellhead equipment with fittings for piping and check valve;
- piping of the pumping unit with tubing, acid carrier, tankers with displacement fluid;
- hydraulic testing of pressure pipelines under pressure 1.5 times higher than the working one.
Next, 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 squeezing liquid are injected. As the last in production wells, crude degassed oil is used. What the acid treatment process looks like can be found in the figure below.
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 for dissolution, after which the chemical reaction products are removed by the pump by backwashing.
Interval Technology
When opening an oil and gas bearing formation having layers with different permeability, simple acid treatment 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 flow of an acid solution through the annulus is prevented by cementing it. After processing the selected area of the reservoir, proceed to the next.
Acid fracturing and thermoacid exposure
Acid treatment of wells under high pressure is carried out during the operation and development of formations with heterogeneous permeability. Simple acid baths in such cases are ineffective, because the acid "leaves" in well-permeable layers, while other areas remain unreached.
Before injection of the reagent, isolation of interlayer with high permeability is done using packers (similar to previous technology). Preparatory activities are carried out according to the scheme of simple acid treatment of wells. The casing is protected by installing a packer with an anchor on the tubing.
An emulsion prepared from a solution of hydrochloric acid and oil is used as a working reagent. What the layout of the Azinmash-30A unit looks like for injecting acid into the formation is shown in the figure below.
This unit is equipped with three-plunger horizontal high pressure pumps. Sometimes 2 pumping units are used for processing. Other units are also being produced in the oil industry - UNC-125x35K, ANTs-32/50, SIN-32, manufactured on the KrAZ or URAL chassis. A typical arrangement of installations includes an off-road wheeled chassis, an installation platform on which the main technological equipment is installed, high-pressure pumps, a tank for transporting and supplying reagent, an acid-resistant manifold consisting of pressure and intake pipelines.
During thermoacid exposure, reaction tips are lowered into the well. Their inner cavity is filled with magnesium in the form of chips or granules, and the outer surface has perforated holes. When exposed to magnesium, a large amount of thermal energy is released.
Corrosion Protection
The reagents used in acid treatment of wells are corrosive environments in relation to metals. The corrosion rate of parts made of St3 steel at a temperature of 20 ° C and a HCl concentration of 10% is 7 g / (m 2 ∙ h), and for a mixture of 10% HCl and 5% HF - 43 g / (m 2 ∙ h) . Therefore, to protect the metal of the equipment, inhibitors are used:
- formalin;
- catapine;
- urotropin;
- I-1-A inhibitor;
- unicol and others.
Safety Acid Treatment
When conducting acid treatment on the formation, toxic and flammable substances are used. If they leak or spill, environmental damage can be caused.
A preliminary acid treatment plan is being developed, which is approved by the chief engineer of NGDU. Work is carried out according to the tolerance and technological regulations. The following measures are used as security measures:
- Residues of chemicals and flushing liquids are collected in special containers for subsequent disposal.
- Monitoring the concentration of acid vapor is carried out using a gas analyzer.
- Pumping equipment and tanks are installed at a distance of not less than 10 m from the wellhead; car cabs are placed in the opposite direction.
- During the injection of acids, only those workers who are directly involved in the maintenance of equipment remain near the units; all other persons are removed to a safe distance.
- It is forbidden to carry out work in strong winds, fog and in the dark.
- Before depressurization, the system does not carry out repair and installation work on pipelines and technological equipment.
- To protect against the influence of acids, personal protective equipment is used - special clothing (rubber aprons, boots), rubber gloves, glasses, masks, gas masks.
The field should also have an emergency supply of workwear and chemicals to neutralize acids (lime, chalk, chloramine and others). All working and engineering personnel are required to undergo periodic training and certification for knowledge of safety regulations in accordance with the schedule approved by the head of the enterprise.