Due to differences in hydraulic conductivity and effects of well construction geometry, ground water flow in a wellbore typically differs from the flow field in the surrounding porous media. These differences must be well understood to use borehole flowmeters for ground water velocity and direction measurement. In this work, a lab-scale sand tank test and a small-scale field test were conducted to explore the differences between average linear velocities in the borehole and in the formation and flow directions. In the sand tank test, six variables were studied, colloidal borescope meters were used to measure flows in wellbores, and correction factors (= linear velocity in center of borehole/average linear velocity in the surrounding formation away from the well) for flow velocity were determined. In these tests, correction factors ranged between 3 and 6 for all conditions studied, without any strong correlation between correction factor and most of the variables studied. The most obvious relationship was that the correction factor appeared to increase with decreasing ground water velocity; when the flow velocity decreased to near or below 2 × 10-3 m/s, the value of the correction factors increased to equal or exceed 10. In the field test, two different types of flowmeter - the colloidal borescope meters and heat-pulse flowmeters - were used under natural gradient and forced hydraulic gradient conditions. Field test results showed no differences in correction factors when either type of borehole meters or hydraulic conditions were used. The correction factors remained between 3.9 and 6.0 consistent with other previously published results. The flow directions measured by borehole flowmeters in wellbores were consistent with the flow directions in the geological formation. The results of the sand tank test indicated the larger well screen diameter and the lower hydraulic gradient seem to have the more accurate flow directions measured by borehole flowmeters. Field and lab measurements suggested that the errors of flow direction were less than 15°, with a maximum of 21°.
All Science Journal Classification (ASJC) codes
- Civil and Structural Engineering
- Water Science and Technology