Objective In order to solve the problem of poor production effect and short acidification validity period of low-permeability reservoir with calcium-rich cement by using soil acid acidification, the research of the mechanism and improvement measures of strong acid acidification has been carried out.

Method The mineral composition, pore throat structure, ionic composition of residual acid and flowback fluid, and solid-phase composition of this reservoir core before and after the strong acid acid rock reaction were tested and analyzed by using NMR T₂ spectroscopy, electron microscopy, ICP, and other testing methods, to clarify the reasons for the poor effect of the strong acid put into production and the improvement measures.

Result Strong acid is mainly used to fully dissolve carbonate rock cement, and locally dissolve potassium feldspar, plagioclase feldspar and clay minerals, but the core dissolution is not uniform, and there exists obvious interface between dissolved and non-dissolved areas. The frequency distribution of medium-coarse pore throats in the range of 1.14 to 18.38 μm in the core increased by more than one times after strong acid acidification, and the fine pore throats smaller than 1.14 μm slightly increased or decreased. Acidification produces high concentrations of Ca²⁺, Fe³⁺, Si²⁺, Al³⁺ and so on, which are easy to form secondary precipitates such as Fe(OH)₃, CaF₂, Na₂SiF₆. Meanwhile, the incompletely dissolved formation particles are easy to strip off and clog. In the return stage, the acidified secondary precipitates and incompletely dissolved particles cause the medium-coarse pore throat after acid etching to be divided and blocked, resulting in the reduction of permeability of the core after acid etching, and the reduction is more than 50%.

Conclusion Improvement measures such as strengthening the inhibition ability of Fe³⁺, controlling the rate of fluid production in wells after acidizing and unplugging, and adopting a multi-hydrogen acid system are proposed, and the effect of acidizing of this type of reservoir has been greatly improved after the application in field, and the validity period of the first implemented well has been more than 8 months.