INJECTION AND FLOWMETER TESTS Injection Tests

With the aim of determining the hydraulic conductivity before monsoon with different methods and to characterize the hydraulic properties of the fractured aquifer in the vertical direction (weathered and fissured zones), a new hydraulic tests campaign was carried out between May and July 2002 in the Maheshwaram watershed. In the continuation of slug tests campaigns (March 2001 and March-April 2002), eight flowmeter tests have been coupled with injection tests of short duration. The 2003 campaign was carried out during monsoon period on the seven wells drilled by NGRI during that period.

Injection tests were interpreted according to Theis method with ISAPE software. Concerning flowmeter tests, the determination of hydraulic conductivities of the wells and each located fractures was based on the Dupuit equation.

Principle

Since flowmeter measurements require a water flow (pumping or injection test) into the bore-well, injection tests of short duration have been coupled with flowmeter tests. For each well, the field procedures were identical: the water was available from water tanker of 6 m3. Water from the tanker is injected into the bore-well with a constant discharge and the experiment is carried out for a period of 25 to 75 minutes depending on the flow rate.

The main problem during these tests was to maintain a constant discharge. At the head of installation, the valve of water tanker wasn’t reliable. Therefore, a drum, which controlled a permanent load, was installed in the middle of the water circuit. The obtained permanent load in the drum has given a constant discharge through the pipe inside the bore-well.

Water level in the bore-wells were recorded with the help of Madofil (automatic water level recorder) and with a follow-up of manual measurements for helping in calibration and checking up of the Madofil data. Each recording includes a buildup of water level during the injection of water and then a drawdown that begins when the injected flow is stopped.

Data Processing and Interpretation

Injection tests of short duration are interpreted by Theis method, which is based on the principle of unsteady state radial flow to the well, in confined or unconfined aquifer (if hydraulic head variations are low). The other assumptions include that the aquifer is homogeneous and isotropic with infinite extent and constant thickness.

Analyses of Aquifer Parameters from Different Hydraulic Tests 119 Theis equation h(r, t) = 1 u u 0 4  S

Ô

where W(u) is the Theis’ well function, i.e., exponential integral of order 1 (E₁(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 [m3/s], T: transmissivity [m2/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

120 J.C. Maréchal et al.

bottom of the aquifer was chosen at the top of the massive granite layer. Flowmeter tests show that transmissive fractures only exist above this fresh basement.

Flowmeter Tests

The location of hydraulically conductive fissures was obtained using vertical profiles of flowmeter measurements. In order to increase the vertical extension of the investigated zone, the measurements are done during injection tests and not pumping. A water tanker is brought to the site and water is directed to a drum connected to the well through a pipe with a valve. A constant head is maintained in the drum thanks to overflowing water flow. This allows injecting constant flow rate. During the injection at constant rate Q when a pseudo steady state is reached, the flowmeter, which measures the vertical flow within the screen, is lowered close to the bottom of the bore-well and a measurement of velocity is obtained. The flow meter is then raised a few decimetres, another reading taken, raised another few decimetres, and so on. As illustrated in the lower portion of Fig. 6, the final result is a series of data points giving vertical discharge Q(z) within the screen as a function of depth

z. Just above the top of the screen the meter reading should be equal to Q,

the steady injection rate that is measured independently on the surface with a water meter. The procedure may be repeated several times to ascertain that readings are stable.

As usually assumed, one considers the aquifer as a series of horizontal layers. The difference between two successive meter readings constitutes the net radial flow (q_rj) entering each layer of the aquifer:

q_rj= i 1 i

Q Q

e

 

(4) where e is thickness of the layer.

Then, the Dupuit formula for a confined aquifer and horizontal flow to a well is applied to each layer, leading to the estimation of the layer hydraulic conductivity: K_i= ln 2 rj q R h r S ' (5)

where 'h is the drawdown, R the investigation radius and r the well radius. The aggregated permeability of the profile is obtained:

K_p= 1 1 n j j j n j j K e e

Ç

Ç

Analyses of Aquifer Parameters from Different Hydraulic Tests 121

Interpretations of long duration pumping test carried out in the area have shown that aquifers are unconfined. Consequently, flowmeter conductivities, calculated for confined condition, need to be corrected. A correction factor is applied to the obtained conductivities in order that the aggregated conductivity of the well is equal to the global conductivity, K, using the Dupuit formula for unconfined aquifer for the injection tests.

K = ln( / ) (2 ) R r Q h h H S ' '  (7)

122 J.C. Maréchal et al.

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APGWD, 1977. Studies on hydrologic parameters of groundwater recharge in water balance computations, Andhra Pradesh. Government of Andhra Pradesh Ground Water Department, Hyderabad, Research series no. 6, pp. 151

De Marsily, G., 1986. Quantitative Hydrogeology. Academic Press, Orlando. FAO, 1990. Irrigation and Drainage Paper no. 56, Crop Evapotranspiration,

Chapter 6.

Kruseman, G.P. and de Ridder, N.A., 1994. Analysis and Evaluation of Pumping Test Data, 2nd Edition, Publication 47, International Institute for Land Recla- mation and Improvement, PO Box 45, 6700 AA Wageningen, The Netherlands. Lachassagne, P., Wyns, R., Bérard, P., Bruel, T., Chéry, L., Coutand, T., Desprats, J.F. and Le Strat, P., 2001. Exploitation of High-yield in Hard-rock Aquifers: Downscaling Methodology Combining GIS and Multicriteria Analysis to Delin- eate Field Prospecting Zones, Ground Water, 39(4): 568-581.

Monteih J.L., Huda, A.K.S. and Midya, D., 1989. Modelling Sorghum and Pearl Millet—Rescap: A Resource Capture Model for Sorghum and Pearl Millet. In: Modelling the growth and development of Sorghum and Millet. Research Bul- letin no. 12. International Crop Research Institute for the semi-Arid Tropics, pp. 30-40.

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Upscaling of Slug Test Hydraulic Conductivity Using DFN Modelling 123