ANALYSIS OF WEZLL INTERFERENCE EFFECTSFRACTURED BEDROCK AQUIFERS

PRESEmATION NOTE!3

For=

Hillsborough Environmental Commission

April 25, 1994

Ground Water Associates, Inc.

Peter M. Demicco

.

PROBLEM OF SCALE

.l Major fractures on one scale becoming minor

fractures on large scale@ Connected network of fractures on one scale

becomes an isolated cluster on a large scale ,

0 Observations limited to specific scales.

Se = f&L,Cl,fP,t) .

where Se = Scale at which continuum approach canbe useda

I = Fracture intensityL =- Fracture lengthC I = Degree of interconnectionfPJ = Orientat on distribution

Dershowitz and Doe (1988)

c

-180

.,

NESTED STRUCTURE -OF HETEROGENEITY

( .:

AQUIFER-TEST ANALYSISIN FRACTURED-ROCK

AQUIFERS

a

e

Radial flow

Non-radial (linear) flow

l Anisotropic flow

Double-Do rositv aquifer -

. : a Radial flow.

1

. *..*.

Center of welf

0 = 240 g p m (1 .310 ma/day) --4?,=7OOR(213m) q

.-

f6 n (1.8 m)

1--

Figure 9.7. Changes in radius and depth of cone aldepression after equal intervals of time, at constantpumping rate.

24

OtHafWe from pumped wdl (ft)

10.0

5.0

3.0

2.0E

‘=3g 1.0

s

Time since pump started (mln)

: ,Figure 9.47. Data given in Table 9.1, pbtted on brrithmk graph paper, define I curve similnt tr, shapeto the type curve in Figiy~n 9.45.

b * .

‘.:

. ‘ : . . * . .

- LINEAR FLOW** Flow in some fractured-rock aquifers may be linear

rather than radial .Vertical fracture acts asan extended well; fIow islinear to the fracturerather than radial to theweIl

0 Row lines are parallel anddrawdown is a function ofdistance from the fractureand not the pumped well

#-t .

from Jenkins and Prentice (1982)

l Linear flow equations have been developed fordrawdown in the pumped Well (Gringarten and Ramey,1974) and observation wells (Sen, 1986) assuming: l

Aquifer is homogeneous,isotropic and confined andfully penetrated by a singlevertical fractureResistance to flow in thefracture and storage in thefracture is negligible

from Kruseman and deRidder (1990) _I

l Similar situation, flow to a fblly penetrating drain in aporous median,‘was defined by Ferris and others

.L

(1962)

-579

24I 1PumPedWd

f 2.

from Kruseman and deRidder (1990)

* Early-time flow to anextended well is linear andcharacterized by a straighthe with l/z unit slope onlog-log piot

l Late-time flow to anextended we11 and flow atlarge distances is pseudo.radial and classic Theismethods can be used todescribe drawdown *

\I L

-v-

.- - -.---- - . . .

II Pumped well iI t

I vI I I? I I1 I

III

I!IIII

Fig. 1, Conceptual modal af a linaar flow wsten,,-w

Flowlines

x

.

fig. 8. Semi-log plots of dtawdown (s) versus time It) andresidual dtawdown (s’) versus t/t’ in obssrvation weh EGH-1

and EGWZ, Silver City, New Mexico (aftsr fraugar andtavery, t 976 1.

anisotropic and that the usual methods of welltest analysis are not applicable.

Sicking (1979) identified linear flow as the

Fig. 10. Arithmetic plot of drawdown (s) versus Jt inpumped well (EGH) and observation wells (EGW-I andEGW-2). Silver Civ, New Mexico (after Sicking, 7979).

cause of the anomalous results. A single straightline cm be passed through the rfrawdosvn data ofthe two observation wtils when plotred as sversus Jt (Figure 10). By correcting for encranctlosses, Sicking showed that the drawdown in the

SPECIAL CASE OF DIFFERENT DIFFUSWTYON EACH SIDE OF THE FRACTURE

If tfre diffusivity on each side of the fracnrrc isdifferent, the flux through chc production surfacesof the two sides will be different. Hence, if we labcione side as Side A and the ocher side as Side B, thenSide A contributes QA to the total discharge andSide B contributes Qs. Clearly:

drawdown in these wciIs is a function of time ratherthan distance, suggesting that the pumped wcil andthe observation weifs penetrate the same fr;lcturtsystem within which the resistance to flow isnegligible. A unique value for hydraulic diffikvityof the system cannot be determined because all ofthe weiIs are in the fracture and no obscntations inthe aquifer art available. The vaiuc of Lmdetermined from equation787,600 ftVday”.

( 18) is approximately

pumped wtil also plotted along the tine which passesthrough the observation wcfl data. Therefore, tfic

Q=QA +QB (20)

where Q is the total discharge.If an observation wcfl is present in each side,

the diffusivity values can be found for each sidefrom equation (11) which is independent of thedischarge.

Ltt TA, SA# and TB, SB bt the nansmissivityand storage coefficients of Side A and Side Brespectively.

Suppose now that at some time (t), the

Fig. 9. Log-log plot of drawdown Is) ven;us t/t’ in O~SWV*tion wells EGH-t and EGH-2. Silver City, New Mmieo.

drawdowns sA and ss arc measured for the weffsin Sides A and B and that the distances from theextended wtil are xA and xg, respectively. 1Thtnthe discharge QA and Qs can be found fromequation (8) to be:

Qg=Q-QA (221

c

.

. Anisotropic flow .

As an examination d the drawdown equations indicates, for a given time, lines ’of equal drawdown around a well pumping from an anisotropic aquifer have the formof concentric ellipses (fig. 1) with transverse axes along the maximum transmtibiiity

.

axis 6 and conjugate axes along the minimum transmissibility axh 7.

t s=0.0001 Q=S,46iitets/~8C

.

Fig. 5

A semilogarithmic plot of the data (fig. 5) shows that the straightline method ofanalysis is appkabfe. The slope of the lines drawn thtougfi the latter part of the datais the same for all three lines and hzu the value of 1.15 meters per log cycle. Thet-intercepts are: -

FRACTURED-ROCK AQUIFERS

SINGLE-POROSITYAQUIFERS

c-y .l -

h

e

e

l

DOUBLE-POROSITY AQUIFERS. .. .

Mkrofractures Primary-PorosityMatrix

from Kmseman and deRidder (1990)

In double-porosity aquifers, flow through fractures isaugmented by flow from matrix blocks

Two overlapping mediaLow-permeability, high-s torage matrix blocksHigh-permeability, low-s torage fractures

Barenb’iatt and others (1960), Warren and Root (1963),and Bourdet and Gringarten (1980) assumed flow frommatrix blocks to fractures under pseudo-steady-stateconditions; spatial variation in head in the matrixblocks is ignored

I

-35

WELLBORE STORAGE

0. cn

AB Wellbore storageBC Radial flow

‘og ‘D .

@ Wellbore-storage effects commonly appears asunit dope on log-log plots

l T will be underestimated andestimated using Theis cuwe

S will be over-. .

es after the end of0 Curve match 1 to I .5 log cyc Iunit Slope

l Best way to avoid welisolate test zones; not

Ibore-storage: always pract 0

I

effects is tocal so

tinalytical methods have been developed toaccount for wellbore storage

WELLBORE STORAGE. .

l Many bedrock aquifers contain highly trammissivefractures of considerable lateral extent

* .

l Howe&, the combined compressibility of thenonfract&e-d rock mass and fractures may contributea relatively small amount of water

,* Therefore, aquifer tests in fractured bedrockcommonly are strongly affected by wellbore storage

. .2

* WeUbore storage in the pumping well delays It----- .a.-ItransGission of the stress horn the well to the aquifer

l* Wellbore storage in the observation wells delay

.

responie of the fluid in the wells to head changes in .the aquifers

- - - -s log .

+

//

I + t fog

Effect of wellbore storage in a pumped -well on drawdown in an observation t‘Iwell (Kruseman and deRidder, 1990)

8

~VELLE~~RE stm

0 Wellbore damage- (permeability reduction)and vvellbore improvement (permeabilityenhancement)

ORAWUCIW N

.

- ..

I l Positive skin (permeability reduction) effect.appears a drop in hydraulic head across athin zone between the wellbore and aquifer

l Positive skin shifts the type cures to latertime values and acts to extend the periodeffected by wellbore storage

.* Negative skin (permeability enhancement)

effect is much less than positive skin; shiftsthe type curves to early time values

I

. t-. ‘-

0

Y

L150 260

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