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Well Tests to Characterize Idealized Lateral Heterogeneities

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## PowerPoint Slideshow about ' Well Tests to Characterize Idealized Lateral Heterogeneities' - cruz-raymond

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Steeply Dipping Beds

Unconfined Aquifer

Neighboring

Matrix

Matrix

Strip

T1S1

Tm Sm

T2 S2

Tm Sm

Ts

Ss=Sm

L

w

L

L

L

Conceptual Models2-Domain Model

3-Domain Model

Method – Analytical

- Transient analytical solution using Method of Images (Fenske, 1984)

Methods – Numerical

- Transient numerical model using MODFLOW
- 2-Domain – Tr and Sr were varied
- 3-Domain - Tr and w of the strip were varied.
- Grid optimized for small mass balance errors
- The properties of the model were selected so that the drawdown and time from the numerical model were dimensionless

Dimensionless Time

- Drawdowns were evaluated at three dimensionless times to illustrate effects during development of drawdown fields.
- Dimensionless time used for type curves
- Dimensionless time used in drawdown fields

Ds = 4.1 T = 0.55

to = 0.029 S = 0.017

Ds = 2.3 T = 1

Graphical Evaluation – 2-DomainEstimate Aquifer PropertiesSL=0.029

TL=0.55

SL=0.021

L

TE=1

SE=0.0179

TL=0.55

SL=0.25

TE=1

SE=0.0179

TL=0.55

SL=0.136

TL=0.55

SL=0.27

TL=0.55

SL=0.068

TL=0.55

SL=0.06

TL=0.55

SL=0.021

L

L

Critical Region

- An early semi-log straight line can be determined by
- The second derivative was compared to plots with a variety of curves. An early SLSL could be identified by a second derivative of 0.2 or less from 0.3<tdL<2.5.

Critical Region

- Observation points confined to a region that is within 0.3 to 0.5 of the distance between the pumping well and the linear discontinuity

Strip Transmissivness & Conductance

- Hydraulic properties of the strip depend on strip conductivity and width
- Strip K greater than matrix
- Strip K less than matrix

Ds = 2.3 T = 1

to = 0.028 S = 0.017

Ds = 2.3 T = 1

Graphical Evaluation – 3-DomainEstimate Aquifer PropertiesDetermine Properties of Strip

- SLSL analysis on the first line will give T and S of the area near the well.
- Take the derivative of time and determine the maximum or minimum slope.
- Using equations from curve fitting determine Tssd or Cd of the layer.
- Solve for Tssor C

Non-Uniqueness

Overlying Leaky Layer without storage

Dual Porosity

s

Streltsova, 1988

Streltsova, 1984

Overlying Leaky Layer with storage

Unconfined Aquifer w/delay yield from storage

s

Neuman, 1975

Streltsova, 1984

Log (t)

Log (t)

Determining Hydraulic Properties

Tm = 0.05 ft2/min

Sm = 2x10-4 ???

- Using Semi-Log Straight-Line Analysis :
- Minimum slope using the derivative curve is 0.5
- Tssd=34=Ksw/KaL
- Tss = 24 ft2/min

w = 10 to 20 ft

Ts = 26 to 52 ft2/min

Ts/Tm = 500 to 1000

L = 280 ft Distance to fault

b = 21.5 ft screened thickness

Conclusions 2-Domain Model

Using the Jacob method to analyze well tests:

- Piezometers r < 0.25L gives T, S of local region.
- Piezometers r > 0.25L gives average T of both regions.
- Piezometers r > 0.25L unable to predict S

Conclusions – 2-Domain

- Piezometers in neighboring region also give average T of both regions.
- L can be determined from intersecting SLSLs using a piezometer within the critical region

Conclusions 3-Domain Model

- Drawdown for low conductivity vertical layer controlled by conductance.

C=Ks/w

- Drawdown for high conductivity vertical layer controlled by strip transmissivness.

Tss=Ks*w

- Feasible to determine properties of a vertical layer from drawdown curves.

Conclusions

- Analyzing piezometers individually is a poor approach to characterizing heterogeneities.
- Drawdown curves non-unique. Require geological assessment.

Acknowledgments

- Funding
- Geological Society of America
- Brown Foundation
- National Science Foundation
- Others…

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