Geology Reference
In-Depth Information
Theis equation
1
u
Qe
du
Tu
u
4
Tt
rS
h
(
r
,
t
)=
,
u
=
(2)
#
2
4
0
Q
h
(
r
,
t
)=
Wu
()
(3)
4
T
where
W
(
u
) is the Theis' well function, i.e., exponential integral of order 1
(
E
1
(1/
u
)), which is known and tabulated. The “Theis curve” is drawn as a
function of the parameter
u
. The
h
is hydraulic head [m],
r
: well radius [m],
t
: time [s],
Q
: pumping flow rate [m
3
/s],
T
: transmissivity [m
2
/s], and
S
:
storage coefficient [-].
Storage coefficient cannot be evaluated when there is no observation well
and if the test duration is short. In fact, the storage coefficient is more
representative of the well effects and storage of fractures in the vicinity of
the bore-well than the real storage of aquifer if the test is of a short duration.
Thus, in the report, storage values are not presented. Concerning
transmissivity, values are valid in the nearby area of the well and characterize
locally the fractures that are intercepted by the bore-well.
Properties and Functions of ISAPE Software
One of the advantages of ISAPE software is to consider variable injection
flow rates and depletion effect (the injection pipe empties back to the well
once the injection is stopped) allowing a more accurate determination of
transmissivity and storage coefficient.
Principle is to fit a theoretical curve, calculated from Theis well function
(equation 3) on the experimental curve (Fig. 6) determined from the field.
In most of the cases, transmissivity is determined from the buildup (during
injection) and the drawdown (after injection), but when discharge control
was difficult at the beginning of the test (variations of water level), the
aspect of the buildup curve doesn't allow fitting the theoretical model and
the fit on the drawdown has always been privileged.
In an easy way, injection tests have been interpreted like pumping tests
transforming buildup in drawdown and drawdown in recovery;
T
and
S
values are not affected. The three required parameters to be controlled are
the flow rate, the well radius and the buildup recording.
Hydraulic Conductivity
Injection tests help in determining the transmissivity of the aquifer, while
flowmeter tests give a measure of the hydraulic conductivity. For having a
detailed comparison the results of these two methods, hydraulic conductivity
must be determined from results of injection tests, using the thickness of
aquifer evaluated from geological observations during well drilling. The
