The single-well test dilemma: the skin effect and variable-rate pumping perspective

A finite-element numerical model based on COMSOL Multiphysics was developed for a variable-rate single-well pumping test considering skin effect. The variable rate was assumed to be a decreasing exponential function of time. Some important results were obtained. First, due to the variable pumping rate, the drawdown will decrease (or the water level will recover) after a certain pumping period. A larger decay constant of the pumping rate (α) causes a faster decrease of drawdown. Second, for the special case of uniform skin distribution, the drawdown inside the well with a positive skin decreases significantly as the ratio of skin hydraulic conductivity to that of the formation zone (ρ) increases, while it approaches an asymptote when ρ exceeds a certain value, where the positive (or negative) skin implies that the skin’s hydraulic conductivity is smaller (or larger) than that of the formation zone. The drawdown inside the well due to a thick positive skin is larger than that due to a thick negative skin. In the case of non-uniform skin, especially for the positive skin, the drawdown inside the wellbore varies with depth. There is more fluctuation of drawdown due to a smaller ρ for the positive skin case, and little effect can be found for the negative skin case. This finding indicates that the single-well pumping test might bring about significant errors when estimating aquifer parameters without considering the skin effect. The possible effect of a non-uniform skin should be taken into consideration when the drawdown is dependent on depth.