## 1 Introduction  
The bottom-hole zone (BHZ) condition deteriorates during the entire well operation due to damage of flow channels with different particles. Since the contaminated area near the well reduces the reservoir-to-well hydrodynamic connectivity, the well productivity does not reach the maximum possible value, while carbonate rocks are characterized by low flow properties per se. The necessity of detailed study of properties of carbonate reservoir and methods of their development is propelled by the presence in them of one-third to half of hydrocarbon reserves worldwide ([PERSON] et al., 2015 [1]). Hydrochloric acid treatment (HAT) is the most effective and cost-efficient method of oil production stimulation at the deteriorated state of near-wellbore zone (NWZ) as of today ([PERSON] et al., 2019 [2]; [PERSON] et al., 2020 [3]; [PERSON] et al., 2019 [4]; [PERSON] et al., 2019 [5]; and [PERSON] et al., 2019 [6]). However, acidizing does not always produce the planned results due to insufficient consideration of key factors such as flow properties of reservoir, bottom-hole zone structure, and its parameters before well stimulation, as well as their behaviour in the process of treatment with acid composition (AC). The success of chemical treatment in the bottom-hole zone is demonstrated by a negative change in the skin factor, which is calculated from the [PERSON] formula (Equation (1)) ([PERSON], 1956 [7]):  
\[S=\left(\frac{k}{k_{d}}-1\right)ln\frac{r_{d}}{r_{w}} \tag{1}\]  
Equation (1) takes into account the difference in permeability of the near-wellbore and far field zones (Figure 1), based on which, the degree of the bottom-hole zone contamination is evaluated. The acid treatment process is considered to be quite complex and inexplicable in the details of the process of dissolution of minerals from carbonate reservoir rocks by hydrochloric acid. Therefore, when forecasting the technological effect of the considered method of stimulation of hydrocarbons production, there are many issues ([PERSON] et al., 2019 [8]) arising associated with the inability to forecast the nature of changes in the void space of the productive formation near the well. Conducting flow studies on core samples under reservoir conditions is one of the ways to address these issues ([PERSON] et al., 2019 [9]; [PERSON] et al., 2020 [10]). Computer tomography allows to determine the change of pore space, namely, the size and position of the channel that has broken through the sample, which is called a \"wormhole.\" However, in the article ([PERSON] et al., 2013 [11]), the authors point to the unreliability of the results obtained during the studies on standard samples of rock formation using a flow unit in relation to the real acid treatments at the oil field, which is referred to high anisotropy of reservoir properties of carbonate oil- and gas-saturated rocks ([PERSON] et al., 2019 [12]). Most of scientific research works is aimed at studying the process of \"wormholing\" forming in rocks under the effect of acid compositions. The authors of the work ([PERSON] and [PERSON], 2018 [13]) have analysed the existing models describing this process in detail. The application of the studied models is considered in the publication ([PERSON] et al., 2019 [14]), where semiempirical models of [PERSON], [PERSON], and [PERSON] have been used, which allow to predict the direction of AC spreading in the pore space of BHZ and to estimate the increase in permeability coefficient in this area. The obtained flow characteristics allow to calculate additional fluid inflow using available formulas of plane radial flow of fluids to hydrodynamically imperfect wells due to reduction of additional flow resistance provided by change in the skin factor. An example of initial evaluation of the hydrochloric acid treatment effectiveness on carbonate rocks is also given in the work ([PERSON] et al., 2019 [15]); its authors have developed an acidizing effect simulator that allows to calculate changes in the structure of the rock near the well.