## 2 Materials and Methods
 \tag{9}\]  
By substituting constants in Equation (9), Equation (10) was obtained:  
\[\Delta S=-\frac{x+y}{1.6x+2\cdot y}\cdot\ln\left(1+\frac{0.0015661\cdot\left(x+ y\right)^{2}\cdot V\cdot 3\sqrt[3]{\frac{T}{\mu\cdot\pi\mu^{2}}}}{(27.08\cdot x+15.40\cdot y)\cdot(2 \cdot x+4\cdot y)\cdot\tau_{w}}\right) \tag{10}\]  
When analysing Equation (10), we can conclude that the change in the skin factor after the acidizing effect on the carbonate reservoir depends on a number of parameters given in Equation (11):  
\[\Delta S=f(x,\ y,\ V,\ T,\ \mu,\ r,\ q,\ h,\ r_{w}) \tag{11}\]  
Based on Equation (11), to predict the effectiveness of HAT, the content of calcite and dolomite in the reservoir rock, as well as its current absolute temperature, rheological properties of AC, and the radius of its diffusing particles were determined. It is also necessary to define the perforated formation thickness, the well radius, and the designed amount of AC to be injected into the formation, which can vary widely based on the BHZ condition and the expected effect. After defining these parameters and calculating the skin factor change by Equation (10), the obtained value of \(\Delta S\) was inputted into the hydrodynamic model of the production facility. The standard isothermal flow model of the three-phase (oil, water, and gas) three-component (oil, water, and oil-dissolved gas) fluid in pore space was used when building the field hydrodynamic model. Well perforation interval is characterized by the skin factor and \"well-to-reservoir connectivity\" parameters. When simulating the acid treatment, the calculated change in the skin factor was taken into account and the key technological parameters of the well operation were calculated. As a result of the calculations conducted in the hydrodynamic simulator (e.g., IRAP Tempest or Eclipse), diagrams of changes in the annual oil and water production by the basic variant and after the designed job were built. Based on the change in the annual oil and water production, a conclusion was made on the predicted effectiveness of acid treatment of the target well operating an oil-saturated reservoir with deteriorated condition of BHZ. Research Results and Obtained Factual Findings on the Predicted Effectiveness Evaluation of Hydrochloric Acid Treatment at the Fields of Perm Krai  
Hydrochloric acid treatments are used in Perm Krai to stimulate oil production from carbonate formations. The work reviews the practice of 18 acidizing jobs at the oilfields in Perm Krai. Using multiple regression analysis, the dependency of additional oil production on a number of parameters has been determined, which can be used to promptly estimate the effectiveness of hydrochloric acid treatment (Equation (12)):  
\[220.0315\cdot x+1065.9062\cdot y+195.8524\cdot S_{1}-116.9743\cdot Q_{1}-17878.5183=\Delta Q_{cum} \tag{12}\]  
As shown in Figure 4, the relationship can be characterised as tight, however, this equation is of little practical use (\(R^{2}\) = 0.70). To determine the possibility of practical application of obtained Equation (10) for predicting the effectiveness of the designed acid treatment, statistical analysis has been conducted. For each of reviewed procedure, the expected change in the skin factor is determined, calculated based on the presented technique of prediction of change in the skin factor after the HAT job. Table 1 provides a comparison of the actual change in the skin factor for the wells under study and the predicted value obtained using the given technique.