Borehole flowmeter applications in irregular and large-diameter boreholes

Abstract

High-resolution flowmeter measurements such as those obtained with heat-pulse and electromagnetic flowmeters are often analyzed to produce in situ permeability profiles of heterogeneous aquifers. However, the borehole environment exerts a strong influence on the accuracy of flow log data and its interpretation. As many as five different types of corrections need to be applied to many flow-log data sets: (1) Adjustments to differentiate between very low-flow and no-flow environments; (2) normalization to account for changes in the magnitude of the flow regime attributed to changes in pumping rate or relaxation of drawdown when measurements are made during water-level recovery; (3) multiplication by a constant factor to account for leakage around the flowmeter measurement section related to ineffective sealing of the annulus by packers or flexible-disk diverters; (4) correction of continuous flow logs collected while trolling by adjusting the zero-point and scale of the log to match a few stationary flow data points; and (5) suppression of the effects of diameter variations on trolled flow logs by collecting data with an under-fit diverter and developing calibration curves representing bypass factor as a function of local borehole diameter. Specific examples of these corrections applied to heat-pulse and electromagnetic flowmeter data sets are given for logs obtained in open boreholes in igneous, metamorphic and sedimentary bedrock, and in screened boreholes in unconsolidated sediments. Scatter in flow measurements related to the efficiency of diverter operation in the field act to effectively limit the permeability detection capability of both heat-pulse and electromagnetic flowmeters to about two orders of magnitude regardless of the dynamic range and accuracy of either flowmeter as demonstrated in smooth-walled calibration tubes.