#Well drawdown test skin#

The linear well-loss coefficient, B 2, is assumed independent of time.įor a well experiencing no well loss, Jacob's drawdown equation reduces to aquifer loss: Δ h w t = B 1 r w, t Q Where r w is the nominal radius of the well. The linear head-loss coefficient, B, consists of two components, aquifer loss and linear well loss, which Kruseman and de Ridder (1994) define as B 1 and B 2, respectively, as follows: B r w *, t = B 1 r w, t + B 2 The effective well radius, r w *, is defined as the radial distance from the center of the pumped well at which the theoretical drawdown in the aquifer (aquifer loss) is equal to the total linear head loss in the well (i.e., total drawdown in the well neglecting turbulent loss). Īre linear well loss and nonlinear well loss, respectively. Is drawdown in the pumped well, t is time since pumping began, B is a time-dependent linear (laminar) head-loss coefficient, r w * is effective radius of the pumped well, Q is pumping rate, and C is a nonlinear (turbulent) well-loss coefficient. Jacob (1947) proposed the following drawdown equation to account for linear and nonlinear head losses in the pumping well at time t: Δ h w t = B r w *, t Q + C Q 2 Typically, aquifer properties and well-loss coefficients are estimated from a step-drawdown test by fitting mathematical models (type curves) to drawdown data through a procedure known as curve matching (Figure 1). Estimation of aquifer properties and well loss by matching Theis (1935) type-curve solution with well loss to time-drawdown data from a step-drawdown test assuming a nonleaky confined aquifer (data from Clark 1977).

#Well drawdown test skin#

In addition to estimating hydraulic properties of an aquifer system such as transmissivity and hydraulic conductivity, the goal of a step-drawdown test is to evaluate well performance criteria such as well loss, well efficiency, wellbore skin factor and effective well radius. Each step should be of sufficient duration to allow dissipation of wellbore storage effects. Each step is typically of equal duration, lasting from approximately 30 minutes to 2 hours ( Kruseman and de Ridder 1994).

In a step-drawdown test, the discharge rate in the pumping well is increased from an initially low constant rate through a sequence of pumping intervals (steps) of progressively higher constant rates. A step-drawdown test (or step test) is a single-well pumping test designed to investigate the performance of a pumping well under controlled variable discharge conditions.