DILUTION LOGGING METHODS FOR CHARACTERIZING HETEROGENEOUS AQUIFERS

The borehole dilution method, where the fluid column is replaced with water of a known solute concentration before repeated fluid-column logging, provides a means to characterize inflow intervals in heterogeneous aquifers. The method has had limited use because it typically requires proprietary numerical codes and specialized logging equipment. In this study, we explore the potential to perform dilution logging with a simple code and readily available fluid-column logging equipment.

A simple mass-balance code produces a close fit to results from dilution experiments with initial conditions established by either dispersing brine in the fluid column or replacing the fluid column with distilled water. The time required to establish steady flow after the start of pumping, the time to log the column, and the mixing induced by the probe have minimal effects on the ability to model solute variations at inflow zones.

The high-resistivity limit inherent in many fluid-column probes affects the ability to characterize early times in experiments where distilled water is used for the initial column condition. Dilution logging has the important advantage over flowmeter logging that variation in borehole diameter and the effectiveness of flow diverters do not introduce uncertainty in the measurement. However, the distortion of inflow effects at minor producing zones in the presence of much greater flow from other zones limits the dynamic range of the application in approximately the same way as for flowmeter measurements; thus requiring repeat measurements at two or more pumping rates to increase production from some zones while moving the more productive zones out of the effective measurement range. The two major advantages of dilution methods are the ability to provide an unambiguous estimate of flow in a rough-walled borehole, and the ability to characterize weak inflows in boreholes without major permeable zones. The disadvantage is the more cumbersome establishment of initial conditions for repeat dilution experiments at different pumping rates.

Results are illustrated by model data sets for a fractured bedrock aquifer, a karst aquifer with major and minor producing zones, and a sedimentary aquifer with many low and very low permeability inflow zones.