4.2.6. Well efficiency
Well efficiency (Ew) is considered as the ratio of aquifer head loss BQ (just outside the casing of the theoretical drawdown) whereas the total drawdown has been measured as Sw inside the well of pumping. However, it is the ratio of head loss of the laminar flow to the total of the laminar and turbulent flow of head loss. On the basis of Rorabaugh (1953), it has calculated on as follows:
EW=\(\left[100\times\left(\frac{\frac{Q}{S_{W}}}{\frac{Q}{\text{BQ}}}\right)\right]\)(6)
Rewritten the equation (6), it can be formulated in following ways:
EW=\(\left[\left(\frac{\text{BQ}}{S_{W}}\right)\times 100\right]\)(7)
Values of EW has been considered acceptable as 70% acceptable by Kresic (2006) and it has indicated in a designed properly and well developed. When EW=100%, the well loss term is identified the values as CQ2 is zero. Several factors have affected the efficiency of well, the most significant being the drilling process, screen design of the screen, size of the gravel pack and development system. When screen length is insufficient, openings of the screen are too short, incomplete penetration of well in the aquifer, insufficient development of well are the main cause of low well efficiency. The aquifer pore spaces have plugged by the drilling fluid and silty and fine materials infiltration. The factor of well development has been indicated by Bierschenk (1963), the well loss to formation loss ratio is 100. On the basis of Bierschenk (1963) classification, when the development factors are <0.1, it has represented the reflect “very effective” development, when the values are indicated the ranges 0.1 to 0.5 which means the development is “effective”, when the values range is 0.5 to 1.0, it has indicated the development “fairly effective” and when the values range is >1.0, it is indicated the development “poorly effective”. The equation of the development factors on total ranges have calculated as follow:
\(\left[\left(\frac{C}{B}\right)\times 100\right]\) (8)