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With the large-scale development of low-permeability reservoirs, high-temperature and high-pressure reservoirs, and unconventional reservoirs, increasing production measures have become an indispensable and important means. The application of acidizing and acidizing technology is not limited to carbonate rocks, but can also be used for the production and transformation of various reservoirs such as tight sandstone and ultra-high temperature and high pressure. The accompanying acid technology has also been widely developed, and multiple technologies such as gel acid, emulsion acid, cross-linking acid, turning acid, and environmentally friendly acid are constantly being improved. The advancement of acid systems has also driven the development of modern acidification and acid fracturing technology, achieving technical goals such as deep penetration, low damage, and uniform acid distribution to expand the scope of transformation, forming a series of efficient acidification processes technology. Below is an introduction to the recent status of the main acid systems in the deep acidification acid fracturing fluid system and the turning acidification acid fracturing fluid system.

Gelling acid is a thickening agent used to increase the viscosity of the acid solution, reduce the diffusion rate of H+, thereby reducing the rate of acid rock reaction and the filtration rate of the acid solution, increasing the effective distance of the acid solution, and achieving deep penetration. Gelling acid as an early developed acid system in China, it is relatively complete, with temperature resistance, acid rock reaction rate, and corrosion inhibition ability that can basically meet the requirements of on-site construction.

Crosslinking acid is mainly composed of acid thickening agents, acid crosslinking agents, and other supporting additives, with the main purpose of forming a network gel system. The existence of cross-linked structures can enhance the rigidity of polymers, increase the difficulty of conformational transformation, improve the salt resistance, viscosity enhancement, and temperature resistance of liquids, provide guarantees for deep penetration, low filtration loss, and high conductivity of acid liquids, and achieve the best results in deep acid pressure transformation of reservoirs. Cross linking acids are divided into surface cross-linking acids and underground cross-linking acids. Surface cross-linking acids are similar to gel fracturing fluids. During construction, they crosslink in the wellbore and are broken in the reservoir by adding a demulsifier. The cross-linking acids we usually refer to are surface cross-linking acids. Underground cross-linked acid, also known as viscous acid, reacts with rocks after injection into the formation. As the acid rock reaction proceeds, the pH value of the acid solution increases. When a specific pH value is reached, the polymer and cross-linking agent react to form a gel structure. The acid rock reaction continues, and the gel breaks and returns to its original linear fluid state.

Emulsified acid is the earliest method used to delay the rate of acid rock reaction. It is prepared by mixing acid (hydrochloric acid, hydrofluoric acid or their mixed acids) and oil (crude oil or crude oil fractions) in a certain proportion under the action of emulsifiers and their additives. It relies on the encapsulation effect of oil on acid, effectively blocking the diffusion and migration of H+to slow down the reaction between acid and rock layers and reduce the rate of acid reaction, achieving deep penetration of acid. Emulsified acid must have good stability in order to delay the acid rock reaction. Stability testing methods mainly include phase separation volume method, conductivity method, microscopic imaging method, and rheological method. The conductivity and microscopic imaging methods can provide quantitative analysis at the microscopic level; The phase separation volume method provides quantitative analysis from a macroscopic perspective by observing the amount of oil and acid precipitation; The rheological law can only be used to determine the demulsification of emulsified acids through experiments, which belongs to qualitative analysis. Due to the fact that emulsified acid is formed by the emulsification of two-phase fluids, it has high friction and limited construction displacement, which is an important factor restricting its use.

Clean self diverting acid refers to the formation of micelles of surfactants in residual acid after acid rock reaction, which increase viscosity and redirect the acid solution to relatively low-permeability or high damage reservoirs for acidification. This process is alternately repeated to achieve uniform acid distribution. When surfactants in acidic fluids encounter hydrocarbon substances, they can automatically break the gel, significantly reducing the viscosity and interfacial tension of residual acid, which is conducive to backflow. The entire acidic fluid system does not contain polymers or metal ion crosslinking agents, which will not cause damage to polymers and sulfides, ultimately achieving the goal of protecting the reservoir and improving the acidification transformation effect.