Analysis of factors affecting water level recovery data

Abstract

Water level recovery data collected in wells following controlled pumping tests are affected by both borehole and formation factors. The borehole factors comprise those effects attributed to the presence of the wellbore, such as step-increases in pumping rate, wellbore storage, well efficiency, and skin effects. The formation factors comprise those effects associated with the geologic environment in which an aquifer system occurs, such as variation of the coefficient of storage, and aquifer barrier boundaries. The recovery data should plot as a straight line on a semilogarithmic plot. Step-increases in the discharge rate during the pumping period cause the water level recovery plot to be concave downward. The curvature of the recovery data plot can be eliminated by applying a correction proposed by Harrill in 1970. However, the effect of step-increases in pumping rate on the recovery data is minimal so long as the duration of the pumping steps is less than about one-third of the total duration of pumping. The well efficiency and skin effects cause an additional component of drawdown in a pumped well, which is manifested as an initially rapid recovery rate after pumping stops. The effects of skin and well efficiency are usually dissipated within a few minutes after pumping stops. Wellbore storage effects can be critical in large diameter wells (wellbore radius greater than 0.5 feet) that penetrate aquifers with transmissivities less than about 2,700 feet squared per day. The time required to dissipate wellbore storage effects in the water level recovery data is inversely proportional to the aquifer transmissivity, and directly proportional to the borehole size. Variation of the coefficient of storage during the recovery period results in a semi-logarithmic recovery plot that is concave downward. The curvature of the recovery plot increases as the variation of the coefficient of storage increases. Variation in the coefficient of storage of one order of magnitude during the recovery period introduces an error of more than fifty percent in the transmissivity calculation at late recovery times. The recovery plot of data collected in a well influenced by a barrier boundary defines two straight line segments. The early-time straight line segment has a slope one-half that of the late-time straight line segment. Analysis of the early-time straight line yields the true aquifer transmissivity. Analysis and interpretation of water level recovery data collected in 59 wells following controlled pumping tests in aquifers of various rock types indicate that, in general, the shape of the recovery plot can be used to diagnose the presence of skin effects, low well efficiency, wellbore storage, and variation of the coefficient of storage. Analysis of data from seventeen wells in alluvial aquifers and thirteen wells in sandstone aquifers indicates that the concave downward recovery plot is the most common type of response curve. This shape of recovery curve indicates that the coefficient of storage is commonly smaller during the recovery period than during the drawdown period. Recovery data collected in twenty wells in fractured hard-rock aquifers indicate that the characteristic shape of the recovery plot predicted by Warren and Root in 1963 is generally diagnostic of flow in non-homogeneous, anisotropic, fractured aquifers. When the fracturing approaches being homogeneous and isotropic, the recovery plot can resemble data collected in non-fractured rocks. Recovery data from nine wells in composite limestone-sandstone aquifers indicate that the recovery plot is sometimes similar to the concave downward shape exhibited in sandstone and alluvial aquifers, and sometimes is similar to the shape predicted by Warren and Root for fractured rocks.