Hydraulic head applications of flowmeter logs in karst aquifer studies

Fred PailletGeosciences Department

University of Arkansas

FLOW LOGGING

Flowmeter underpump indicatesflow zones and relative contribution

But estimates oftransmissivity arehighly local and notrepresentative of the flow path

AltonabedrockboreholesExample whereclosely spaced boreholes yieldcompletely different estimatesof T for thesame solutionedbedding 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 tolowest zone

Pumping flow – no drawdownon middle zone

Nuts and boltsofFlow 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 zoneRef: Molz et al, 1988, WRR

Depth Amb Ambinflow

Pump Pumpinflow

Diff ofDiffs

% ofTotal T

Above Below Diff Above Below Diff Δdiff Δ as %pump

m l/min l/min l/min l/min l/min l/min l/min %

16.8 0.00 -1.20 1.20 10.00 6.00 4.00 2.80 28

32.0 -1.20 -0.05 -1.15 6.00 1.50 4.50 5.65 57

39.8 -0.05 0.00 -0.05 1.50 0.00 1.50 1.55 15

Verify 0.00 10.00 10.00 100

Paillet (WRR, 1998)model

An alternate approach is to usea flow model to simultaneously fit models to ambient and pumped flow profiles giving directmeasurements 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 × 10-5 m2/s 5.95 m below TC32.0 4.0 × 10-5 6.8739.8 1.3 × 10-5 6.87

Flow logs in off-line drainage wellsSolution 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 m MODEL DEPTH m WL PIEZ m BGL WL MODEL m BGL

WELL 2A

OPEN HOLE OPEN HOLE 4.72 4.72

12.0-15.0 18.0-20.0 4.32 4.54

30.0-32.0 36.0-38.0 4.87 4.91

43.0-46.0 43.0-46.0 4.82 4.91

WELL 5

OPEN HOLE OPEN HOLE 2.83 2.83

4.2-6.2 8.0-12 2.81 2.71

12.3-15.2 20.0-22.0 2.81 2.84

21.2-24.2 28.0-30.0 3.39 3.45

Trolling EM FlowmeterHead values indicate an aquitard near 40 m in depth and little vertical head

gradient above 40 m

Wirelinepackersystem

Suitable for use asa standard probe run with other probes during normal wellloggingoperations

Single packer setting – Convert to readings between stationsSite 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 TESTSMonitor the propagation of drawdown outward along flow paths by measuring the evolving flow

regime in an adjacent borehole

Single fracture experiment toverify 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 m2/day• Observation well – T = 250 m2/day• Pump rate = 23 liters/min• Model response with T = 300 m2/day• Storage coefficient (S) the only variable• Test duration – 1 minute pulse

T = 300 m2/dayS CONTROLS AMPLITUDE

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0 60 120Obs flow

S = 5exp-5

S = 3exp-5

S = 1exp-4

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 DownflowHead decay time = 1/L in minutes

0 2 4 6 8 10 12 14 16 18 20-0.3

-0.2

-0.1

0

0.1

L = 2.50

L = 0.50

L = 0.25

L = 0.10

L = 0.05

L = 0.00

Time in minutes

Flo

w in

ga

llon

s p

er

min

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 T1= T2 = 50,000 ft2/day

-150.000

-100.000

-50.000

0.000

50.000

0.00 5.00 10.00 15.00 20.00

Time in minutes

Flo

w i

n l

iter

s/m

in

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