Hydraulic head applications of flowmeter logs in karst aquifer studies Fred Paillet Geosciences Department University of Arkansas

FLOW LOGGING Flowmeter under pump indicates flow zones and relative contribution But estimates of transmissivity are highly local and not representative of the flow path

Altona bedrock boreholes Example where closely spaced boreholes yield completely different estimates of T for the same solutioned bedding planes

Using flow log data to measure hydraulic head Formulate a two variable inversion Obtain two flow profiles under different conditions Usually ambient and pumping Measure difference in open-hole water level Model flow using specified T and h values Vary T and h until match BOTH profiles simultaneously

TWO STEADY FLOW PROFILES USUALLY AMBIENT AND STRESSED Productive karst – Low Q drawdown same size as head differences so that drawdown is the same order of magnitude as the naturally occurring hydraulic head differences, biasing T measurements.

Example where ambient head differences have a major influence on the detection and characterization of flow zones in a karst aquifer. Presence of major flow zones masked by hydraulic head differences.

Ambient flow – no flow to lowest zone Pumping flow – no drawdown on middle zone

Nuts and bolts of Flow log interpretation

SUBTRACTION OF INFLOWS METHOD Remove head influence by elimination of variable but also throw out any attempt to infer hydraulic head for each flow zone Ref: Molz et al, 1988, WRR DepthAmb inflow Pump inflow Diff of Diffs % of Total T AboveBelowDiffAboveBelowDiffΔdiffΔ as % pump ml/min % Verify

Paillet (WRR, 1998) model An alternate approach is to use a flow model to simultaneously fit models to ambient and pumped flow profiles giving direct measurements of T and h

BOREHOLE FLOW MODELING Yields direct estimates for both T and h MODEL BOTH HEAD AND TRANSMISSIVITY FLOW ZONE TRANSMISSIVITY ZONE HYDRAULIC HEAD 16.8 m 2.0 × m 2 /s 5.95 m below TC × ×

Flow logs in off-line drainage wells Solution horizons in gypsum rubble aquifer

VERIFICATION OF FLOW MODEL ESTIMATES OF WL Piezometers available in the vicinity of two of the logged drainage wells PIEZ DEPTH mMODEL DEPTH mWL PIEZ m BGLWL MODEL m BGL WELL 2A OPEN HOLE WELL 5 OPEN HOLE

Trolling EM Flowmeter Head values indicate an aquitard near 40 m in depth and little vertical head gradient above 40 m

Wireline packer system Suitable for use as a standard probe run with other probes during normal well logging operations

Single packer setting – Convert to readings between stations Site directly above pumped aquifer and had assumed a strong vertical gradient in efforts to monitor heavy metal contamination. Packer data shows lateral drainage by karst bed and negligible downward gradient below 200 feet.

FLOWMETER CROSS- BOREHOLE TESTS Monitor the propagation of drawdown outward along flow paths by measuring the evolving flow regime in an adjacent borehole

Single fracture experiment to verify model predictions where there is a known analytic solution

Ambler PA cross-hole test Solution on bedding plane connects boreholes Boreholes 30 m apart Pumped well – T = 350 m 2 /day Observation well – T = 250 m 2 /day Pump rate = 23 liters/min Model response with T = 300 m 2 /day Storage coefficient (S) the only variable Test duration – 1 minute pulse

T = 300 m 2 /day S CONTROLS AMPLITUDE

A more ambitious use of the cross-hole model Leakage between fractures Two-bedding planes Pumped well has upper plane cased off Pump only from lower zone Measure flow between zones in obsv well Expect pull down flow from upper to lower

LEAKAGE BETWEEN ZONES

Effect of Leakage on Downflow Head decay time = 1/L in minutes

LEAKY AQUITARD TEST Two high T karst aquifers Upper intersects canal Pump from well completed in lower Measure borehole flow between aquifers Is aquitard between them leaky? If no leakage – pumping induces down flow that steadily increases If very high leakage flow shows short downward pulse that relaxes over time

Flow schematic

S FLORIDA KARST T 1 = T 2 = 50,000 ft 2 /day

Head decay time = 1/0.05 = 20 minutes

Conclusions T derived from flow logs is highly local h derived from logs or packers denotes large-scale flow path Two-variable interpretation can be applied to suitable pairs of flow logs to give T and h Derive estimates of head on flow paths in open boreholes Results obtained as part of the routine logging process Values not as accurate as obtained with packers – but with much less expense and effort Used as stand-alone data or to prepare for more effective straddle packer program