Process considerations for trolling borehole flow logs

Abstract

Horizontal hydraulic conductivity with depth is often understood only as a depth-integrated property based on pumping tests or estimated from geophysical logs and the lithology. A more explicit method exists for determining hydraulic conductivity over small vertical intervals by collecting borehole flow measurements while the well is being pumped. Borehole flow rates were collected from 15 deep monitoring wells on the Nevada Test Site and the Nevada Test and Training Range while continuously raising and lowering a high-precision impeller borehole flowmeter. Repeated logging passes at different logging speeds and pumping rates typically provided nine unique flow logs for each well. Over 60 km of borehole flow logs were collected at a 6.1-cm vertical resolution. Processing these data necessitated developing a methodology to delete anomalous values, smooth small-scale flow variations, combine multiple borehole flow logs, characterize measurement uncertainty, and determine the interval-specific lower limit to flow rate quantification. There are decision points in the data processing where judgment and ancillary analyses are needed to extract subtle hydrogeologic information. The analysis methodology indicates that processed measurements from a high-precision trolling impeller flowmeter in a screened well can confidently detect changes in borehole flow rate of ∼0.7% of the combined trolling and borehole flow rate. An advantage of trolling the flowmeter is that the impeller is nearly always spinning as it is raised and lowered in the well and borehole flow rates can be measured at lower values than if measurements were taken while the flowmeter was held at a fixed depth. Copyright © 2006 The Author(s).