CHARACTERIZING LARGE-SCALE AQUIFER PROPERTIES WITH LOCAL-SCALE BOREHOLE FLOWMETER MEASUREMENTS

Borehole flow logs, routinely used to characterize the heterogeneous hydraulic properties of formations, apply only to the immediate vicinity of the borehole. Thus, flow-log interpretation yields a statistically limited subset of the hydraulic properties of the regional aquifer. A prohibitively large set of such measurements would be needed to provide enough data to estimate average large-scale aquifer properties. An alternate approach is based on an annular boundary-layer model used to relate the local borehole flow regime to the regional flow in the surrounding aquifer or aquifers. In this model, the borehole is surrounded by an annular region where an arbitrary number of fractures, bedding planes, or permeable lenses provide hydraulic connections between the borehole and the far-field aquifers. Conduits connecting the borehole to the far field are simulated as ideal confined aquifers characterized by transmissivity (T) and storage coefficient (S). Each of these conduits is identified as an inflow or outflow point in the borehole using the flowmeter data. Borehole flow is driven by differences between the water level in the borehole and the far-field hydraulic boundary conditions (H) for each conduit. The method can be used in the steady state mode to solve for T and H, or in the transient mode to estimate T, S, and H. In the transient mode, the outer boundary conditions are allowed to vary over time in the form of H(t). In either mode, hydraulic-head conditions at the outer edge of the annular boundary region are used to constrain the properties of the regional aquifer, because the far-field conditions can be related to regional-flow conditions. For example, transient -flowmeter experiments can be used to characterize large-scale flow between aquifer units when regional drawdown is induced at the borehole by pumping from nearby water-supply wells. Application of this technique is illustrated for heterogeneous fractured-rock or karst aquifers in New York, Massachusetts, and Minnesota.