Fractured Aquifer Testing (NT)
FRACTURED AQUIFER TESTING, NON-TECHNICAL
(For a technical web page on fractured aquifer testing, click here.)
By Darrel Dunn, Ph.D., PG , Hydrogeologist. (Professional Synopsis)
Fractured aquifers are layers of water-bearing rock that are permeable enough to yield useful amounts of water to wells from fractures. In addition to water supply, fractured aquifers are also of interest in mine drainage, waste water disposal via injection wells, geothermal resource evaluation, and tunnel construction. Fractured aquifers may be confined aquifers sandwiched between low-permeable layers, or they may be water table aquifers at the ground surface. Water wells pump water from the fractures. When water is pumped from the fractures in confined aquifers, the water pressure in the fractures is reduced and seepage into the fractures is induced from the blocks of rock that make up the bulk of the rock between the fractures. This seepage affects how much water a well in a fractured aquifer can produce. One way to estimate how much water can be produced from such aquifers by a water well is to perform a constant rate pumping test. As the well is pumped at a constant rate, the water level in the well declines to draw water from the aquifer. The rate of decline is determined by making successive measurements of water level in the pumped well. This decline is called drawdown. It is also useful to measure drawdown in nearby observation wells that are not being pumped.
Analysis of pumping test data involves comparing the observed drawdown in pumped wells and observation wells in real aquifers with unknown properties to calculated drawdown for simplified idealized hypothetical aquifers with known properties. The unknown properties include (1) the ability of the fractures to transmit water toward the well (aquifer transmissivity or permeability), (2) the amount of water produced by the aquifer due to compression of the fractures as pressure in the water declines, (3) the ability of the rock between the fractures to transmit water toward the fractures (matrix hydraulic conductivity), (4) the resistance to flow into the fractures due to mineral deposits on the fracture walls (fracture skin), and (5) the amount of water yielded to the fractures due to compression of the rock between the fractures (matrix specific storage). The construction of the pumped well can affect the pumping test results and the productivity of the well. The diameter of the well casing affects the amount of water yielded from storage in the well as the water level declines, which can significantly affect test results. Also, any restriction to inflow to the well caused by well construction (well skin effect) can affect the productivity of the well. The aquifer pumping test provides estimates of all of these unknowns if the analytical method used is sufficient to do so. A computer program (DP_LAQ) developed by the United States Geological Survey has this capability. Less detailed methods do not have this capability, although some are special simplified cases that can be treated by (DP_LAQ).
Detailed methods of pumping test analysis, such as DP_LAQ, involve successive calculations of drawdown in hypothetical wells and aquifers with trial values of fractured aquifer properties, inter-fracture rock properties. and pumped well skin. The parameters describing these properties are adjusted until the calculated drawdown in the pumped well and observation wells approximates (matches) the actual drawdowns. This match yields estimated values of the aquifer parameters (such as transmissivity and storativity), fracture skin effect, and inter-fracture rock properties (matrix hydraulic conductivity and specific storage coefficient).
Figure 1 is an example of a DP_LAQ "match" between calculated drawdowns and actual drawdowns in two observation wells in a fractured limestone aquifer where the fractures have been enlarged when groundwater moving through the fractures dissolved some of the limestone to form fissures. The matched curves include one for an observation well that penetrated fissures (ZS13), and one for an observation well that only penetrated rock with no fissures (ZS-1). This is an especially challenging situation, and DP_LAQ performed fairly well. However, as discussed on the technical page, the observed and calculated curves depart significantly after the 16 hours shown in Figure 1. This departure illustrates the effect of areal variability of hydraulic properties of the fractured aquifer system on pumping test results. Due to this variability, pumping tests in a fractured aquifer may only yield useful values for properties (like permeability) of the aquifer near the well and possibly yield indications of the nature of the change in properties farther from the well. It is unsafe to extrapolate a fractured aquifer pumping test water level trend into the future. Suitable allowance for uncertainty in such extrapolation should be made. The cost-effectiveness of using computer modeling methods (e.g. MODFLOW) that can use data from surrounding wells to include the effect of variability of the fractured aquifer system should be considered. Considerable professional knowledge of mathematics, computer software, groundwater hydrology, and water well technology is needed for effective use of advanced pumping test analysis software such as DP_LAQ and modeling techniques such as MODFLOW.