Causes of Decreased Discharge and Damage to a Dewatering Well’s Gravel Coat

Abstract

Drainage wells used in lignite mining have to be operated at high efficiency in order to achieve a targeted drawdown in the shortest time period possible. However, oversizing of pump units and excessive pumping can cause accelerated aging of wells and over time, decrease a well’s drainage efficiency. This article analyzes the case of a well that was operated for 10 years and then abandoned at a lignite mine. A common practice in installing dewatering wells in Poland is to protect the well screen with a pre-fabricated granular layer (called a “gravel coat”) that is attached to the screen when it is lowered into the hole. The combination of the gravel coat and well screen is surrounded by a protective gravel pack. Samples of the gravel coat were obtained from the abandoned well. The percentage by weight of precipitates in, and the porosity of the gravel coat from the abandoned well were determined and compared with the properties of new gravel coat material. Tests were conducted in the laboratory to compare hydraulic losses, apparent seepage velocity, effective velocity, and the hydraulic efficiency of the well’s gravel coat. The results of the tests explain the low hydraulic efficiency of the abandoned well and the origin of the damage to the gravel coat. The actual yield was also compared with the calculated theoretical safe yield, which was estimated based on the results of geological records from the construction of the well.