Innovative methods for the characterisation of fractured rock aquifers
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Date
2008-12
Authors
Akoachere, Richard Ayuk II
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
Bulk flow is regional flow. The word region is used in two ways viz; i) A region may be a
hydrogeologically and geographically distinct area. Ex: the Karoo basin. ii) A region
maybe discontinuous but widespread encompassing related non adjacent aquifer
systems such as surficial aquifers, coastal aquifers or as in our research project study
case, some selected fractured rock aquifers in South Africa. In case (ii) regions, topical
investigations are optimized for regional applications. In such investigations, focus is on
processes rather than properties of specific aquifers (Groundwater science).
Characterization tends towards common processes (drivers of the various processes)
rather than geographical locations and particularities. Two new methods have been
developed to determine inclined and horizontal fracture apertures b, in fractured rock
aquifers. These methods are; i) The SLUG-TRACER (ST) TEST; ii) The NAPL
ENTRY PRESSURE (NEP) TEST/ NAPL INJECTION PRESSURE (NIP) TEST.
Mathematical formulations were developed from laboratory experimentation using
transparent Perspex parallel plate physical models and 27 apertures of 0.008 mm to 6
mm, created by using aluminum foil and thickness gauges between 20 mm thick
clamped Perspex plates. The ST test uses a slug in which is added NaCI as tracer
(500mg-5000 mg/I) and an EC meter is used to detect breakthrough in the observation
boreholes. The NEP test uses a NAPL (Sunflower oil) hydraulic head and transducers
to get the entry pressure. Using these mathematical formulae, fracture apertures are
then determined for horizontal and inclined apertures. The NIP test uses the entry
pressure recorded by transducers, of a NAPL (Sunflower oil) by injection and its volume
to determine the fracture aperture for horizontal and inclined fractures. Results from
smooth and rough (Buffed to 10x20 microns) fracture surfaces gave accurate results for
96-98 % aperture determinations of twenty six (26) apertures from 0, 04 mm to 6.3 mm.
The Phreatic Hydraulic Conductivity (PHC) apparatus was developed to measure
the hydraulic head gradient of samples. The PHC apparatus was made of a solid body
divided into three chambers, mounted on a ten liter capacity water reservoir, with a
pump. Three types of samples can be used; Consolidated (in-situ), loose/friable (insitu),
and unconsolidated samples (Drill/auger cuttings, Mine tailings/ash etc.). The
apparatus was used to determine hydraulic conductivities of samples ranging from
coarse gravel to very fine clayey dam tailings. The values ranged from 2.81 E-03 to
4.32E+03 (m/d). The results were reproducible and compared well with those of other
methods. The PHC apparatus' advantages are: Can be used in the field and laboratory
(compact); Simple to use and needs limited maintenance (Three components);
Economical, needing small volumes of water (ten liters); Light (6kg) and compact (0.16
rrr'): Rapid results (Complete determination for a sample within tens of minutes); This
apparatus is particularly suited to determine the hydraulic conductivity of clastic
formations for non-confining flow under atmospheric conditions.
Laboratory experiments on the small (cm) scale aimed at determining the effect
of variable thickness of formations on the hydraulic conductivity, determine the effect of
composition, layering ,spatial disposition and develop a tool for predicting bulk hydraulic
conductivity in phreatic aquifers were carried out. From these, the partial hydraulic
conductivity formulation was developed empirically, to determine the bulk hydraulic
conductivity of the samples, irrespective of the spatial disposition. With geologic insight,
the bulk hydraulic conductivities were determined using the partial hydraulic conductivity
theory. When the thicknesses of the layered sequences varied, the laws of composition
broke down.
The Trigger-tube is an apparatus developed for mixing solutes and tracers for
injection tests in boreholes. It is a simple cap-trigger tube segment and the technique
mixes solutes in boreholes in two minutes. Solutes are introduced into the well and the
trigger is released. The tube is withdrawn and the solute mixes instantaneously to give a homogeneous mixture of solute with the borehole groundwater. Field tests using this
method and apparatus for point dilution tests gave a Darcy velocity of 4.06 m/day,
Seepage velocity of 122.89 m/day and effective porosity of 0.33. Natural gradient tests
gave a Darcy velocity of 4.06 m/day and natural velocity of 123 m/day using NaCI for
the same fracture at 21m in borehole U05. This apparatus takes comparatively a
shorter time to carry out SWIW tests than using the pump mixing method. Field tests
gave 13 minutes for the trigger-tube method and 25 minutes for the pump mixing
method for a point dilution test using NaCI. This apparatus can be used for any test that
needs the introduction of a homogenous mixture in single well tests.
The thermal dilution test is a test developed to determine the position, number
and groundwater (Darcy) velocity of fractures found in a single borehole drilled into a
fractured rock aquifer using temperature as a tracer. Using a trigger-tube apparatus,
cold at 2 degrees Celsius is introduced into a single well. The rate at which warmer
groundwater flows into the well is measured as the change in temperature and used to
determine flow zones, the position of fractures, their depths and the Darcy velocity of
the various lithologies and fractures with flow present, from top to bottom of the
borehole the method was used in a single well test on borehole U05 to determine
fractures at 14m, 15m, 16.8m, 18m, 19.4m, 21m, 22.4m, 24.2m,26m and 27.5m below
the surface. These fractures had Darcy velocities ranging from 1.54m/day t04.17m/day,
with the largest fracture contributing to flow in the borehole being that at 21 m. This was
confirmed by acoustic scan and borehole camera images of the borehole. This method
is very useful to determine the hydraulic properties of fractures and formations under
natural conditions (Without pumping) using a single well.
Description
Keywords
Aquifers -- South Africa -- Bloemfontein, Hydrogeology -- Methodology, Groundwater -- South Africa -- Bloemfontein, Thesis (Ph.D. (Institute for Groundwater Studies))--University of the Free State, 2008