Simulation and Interpretation of Borehole Flowmeter Results Under Laminar and Turbulent Flow Conditions

摘要

The hydraulic conductivity of an aquifer is frequently assumed to be directly proportional to the change in measured velocity in the analysis of flowmeter profiles. In screened wells, the measured velocity can be affected as much by the well screen, gravel pack, and flowmeter itself as by the hydraulic conductivity profile of the adjacent aquifer where the overall transmissivity of the aquifer is high. An existing ground-water flow model was modified to allow for simulation of the presence of a flowmeter in a screened or open borehole when there is turbulent flow throughout the well-aquifer system. Pipe flow through the flowmeter, annular flow around the flowmeter, and slot flow across the well screen are simulated with an equivalent hydraulic conductivity that varies as a function of Reynolds number. Solving the problem sequentially with two models of the well-aquifer system minimizes numerical instabilities. A transient model with time-varying specified heads in the wellbore is simulated first to define the far-field flow profile in the aquifer. The lateral, specified-flow boundary for the second model is defined by the far-field flow profile simulated in the first model. The velocity profile is simulated by sequentially changing the conductances in the wellbore between stress periods to approximate moving the flowmeter up the borehole. Interpretations of hydraulic conductivity profiles from flow profiles in screened wells completed in a highly transmissive aquifer can be problematic. Bypass flow through the annular space can occur if the annular space between the screen and aquifer were left open with most the measured flow being near the top of the screen. Annular fill that has a much lower hydraulic conductivity than the hydraulic conductivity of the aquifer reduces the contrast in hydraulic conductivity inferred from a flow profile. An aquifer with beds of differing hydraulic conductivity could be incorrectly interpreted as homogeneous from the flow profile if the hydraulic conductivity of the annular fill is much less than the hydraulic conductivity of the aquifer. Thus, flow profiles in a screened well may provide misleading estimates of the hydraulic conductivity profile in the aquifer.