# Software for Evaluating Pumping Tests on Real Wells
## 1 Introduction
### Additional Resistances (Skin Effect)  
Additional resistances cause most wells to lose their specific capacities over time. This results primarily from well aging due to mechanical, chemical, and biological processes. Mechanical degradation is most often caused by sedimentation of small particles in the well's close surroundings. Mechanical degradation may also occur when creating the well itself. All methods of well boring result in compacting and re-distributing material. Chemical degradation is most observed in areas where flowing water has higher amounts of dissolved mineral substances. This type of degradation is caused by deposition of mineral substances in individual sections of the well. The most common substances are calcium carbonate, magnesium carbonate, calcium sulfate, manganese, and iron. Biological degradation is caused by bacteria that occur naturally in most aquifer strata. These bacteria can be divided into three basic groups. The first and most widespread group consists of iron-oxidizing bacteria. These bacteria oxidize iron compounds and produce iron (III) hydroxide, an organic compound with a reddish color. This substance has a slimy character and forms deposits on well walls and pump parts and in the well's close surroundings. If the substance is exposed to wind, it hardens and can then cause even greater damage. The second group consists of sulfur-reducing bacteria. These bacteria can be found even in extremely acidic waters, with pH levels as low as 1. These bacteria produce sulfuric acid and hydrogen sulfide. The presence of these bacteria may be indicated by a foul odor. The final group consists of slime-producing bacteria, which coexist with the aforementioned types of bacteria. The slime produced most often affects parts of the pump, but may also block the well screen or pores in the aquifer stratum around the well. All forms of well degradation described above alter the hydraulic conductivity of individual well parts and the adjacent aquifer stratum. The aggregate of all these factors is called the skin effect, a term first defined by [PERSON]. The term skin effect stems from the fact that these additional resistances can, in practical applications, be replaced by an infinitesimally thin zone (see Figure 2) that influences hydraulic conductivity. There are two types of skin effects: positive and negative. A negative skin effect is caused by the elevated permeability of the aquifer stratum in the well's surroundings. Elevated permeability occurs most often in environments featuring rifts. If individual rifts reach the well, a negative skin effect may occur. In practice, however, positive skin effects are more common. Such skin effects reduce the hydraulic conductivity in the near-well region [31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41].  
Figure 2: Diagram of a real well: an aquifer stratum with saturated water level.  
Based on [9], it is possible to calculate the coefficient of additional resistances, \(W\) (skin factor), in the case of a real well, as in Equation (2).  
\[s_{w}=\frac{Q}{4\pi T}\left(ln\frac{2.246 Tt}{r_{w}^{2}S}+2W\right) \tag{2}\]  
where \(W\) is the skin factor (-).  
From Equation (2), the coefficient of additional resistances (skin factor) is  
\[W=\frac{2\pi Ts_{w}}{Q}-\frac{1}{2}ln\frac{2.246 Tt}{r_{w}^{2}S} \tag{3}\]  
and the drawdown caused by additional resistances (steady flow) is [4]  
\[s_{SKIN}=\frac{Q}{2\pi T}W \tag{4}\]  
where \(S_{SKIN}\) is the additional drawdown (caused by the skin effect) (m).