## 2 Factors Impacting the Intensity of Hydroabrasive Wear of the Pumping Stages of the ESP  
Mechanical impurities contained in the pumped fluid can be divided into one-component and multi-component fluids according to their composition [18]. The classification of abrasive particles by their origin is as follows:  
* rock particles carried into the well during development and operation of the well;
* corrosion products from borehole equipment;
* particles contained in well-killing fluid;
* unsecured propping agent after hydraulic fracturing;
* particles of solid sludge formed by interaction of chemically incompatible fluids [19; 10]. The concentration of mechanical impurities in the well fluid is generally not a constant value. A one-time increase in the concentration of abrasive particles (up to two orders of magnitude) is observed when the ESPs are started and wells are operated at stable production after workover or current repairs, short-term shutdowns of ESPs, and changes in operation parameters of the reservoir pressure maintenance systems. The intensity of abrasive wear in the pumping stages of an ESP depends on the number of influencing factors and can generally be described by Equation (1):  
\[\frac{dK_{i}}{dt}=f(R,F,\rho,H_{c},C,K,H_{d},F_{d},\omega) \tag{1}\]  
where \(K_{i}\)--relative mass change of the wearing part of the pumping stage of ESP; \(t\)--time of the exposure of the abrasive particles to the wearing part; \(dK_{i}/\)dt--hydro abrasive wear rate; \(R\), \(F\), \(\rho\), \(H_{c}\), \(C\)--size, shape, density, hardness, and concentration of abrasive particles of mechanical impurities, respectively; \(K\)--corrosiveness of the medium; \(H_{d}\)--hardness of the surface of the wearing part; \(F_{d}\)--shape of the surface of the wearing part; \(\omega\)--rotational velocity of driving shaft of the pump [20; 21]. The intensity of the wear of parts of the pumping stages is not uniform along the length of the pump section [22]. Figure 1 shows the results of the accelerated bench tests, published in [9], from an evaluation of the wear of thrust washers of the pump section assembled from 50 stages of VNN5A-100. The tests were carried out on a bench with a closed circulation of fluid through the test pumping section for 6 h. The pumped fluid was water with silica sand having a concentration \(C\) = 10 g/L. Coarse silica sand having a grain size \(R\) = 800... 1000 \(\upmu\)m was used in the first stage of the testing, and fine sand having a grain size \(R\) = 10... 20 \(\upmu\)m was used in the second stage. The analysis of the obtained results showed that coarse particles of silica sand are more abrasive to the thrust washers of the first stages of the pumping section. Subsequently the abrasion ability of coarse particles of mechanical impurities decreases. Fine silica sand particles accumulate and settle in cavities of the pumping stages. The concentration of such particles in cavities of local accumulation increases in the direction of the flow from the inlet to the outlet of the pump section. Therefore, the thrust washers of the successive pumping stages (in the direction of the flow) are worn by the fine sand more intensively than the thrust washers of the stages closest to the flow inlet of the pump section (Figure 1, position 2). Six basic zones of intensive development of hydroabrasive wear of pumping stages of an ESP were determined in previous papers [11; 23] (Figure 2). These are: the practically closed cavity 1 between the lower impeller disk and the upper disk of the guide apparatus; area 2 of fluid flow rotation in the guide apparatus; area 3 of fluid flow rotation in the stage impeller; area 4 of fluid flow rotation at the fluid inlet to the channels of the guide apparatus of the next stage; area 5 of the upper impeller disk; and area 6 of the concave side of the guide apparatus vane.