Innovative methods for the characterisation of fractured rock aquifers

[摘要] Bulk flow is regional flow. The word region is used in two ways viz; i) A region may be ahydrogeologically and geographically distinct area. Ex: the Karoo basin. ii) A regionmaybe discontinuous but widespread encompassing related non adjacent aquifersystems such as surficial aquifers, coastal aquifers or as in our research project studycase, some selected fractured rock aquifers in South Africa. In case (ii) regions, topicalinvestigations are optimized for regional applications. In such investigations, focus is onprocesses 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 beendeveloped to determine inclined and horizontal fracture apertures b, in fractured rockaquifers. These methods are; i) The SLUG-TRACER (ST) TEST; ii) The NAPLENTRY PRESSURE (NEP) TEST/ NAPL INJECTION PRESSURE (NIP) TEST.Mathematical formulations were developed from laboratory experimentation usingtransparent Perspex parallel plate physical models and 27 apertures of 0.008 mm to 6mm, created by using aluminum foil and thickness gauges between 20 mm thickclamped 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 observationboreholes. The NEP test uses a NAPL (Sunflower oil) hydraulic head and transducersto get the entry pressure. Using these mathematical formulae, fracture apertures arethen determined for horizontal and inclined apertures. The NIP test uses the entrypressure recorded by transducers, of a NAPL (Sunflower oil) by injection and its volumeto determine the fracture aperture for horizontal and inclined fractures. Results fromsmooth and rough (Buffed to 10x20 microns) fracture surfaces gave accurate results for96-98 % 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 bodydivided into three chambers, mounted on a ten liter capacity water reservoir, with apump. Three types of samples can be used; Consolidated (in-situ), loose/friable (insitu),and unconsolidated samples (Drill/auger cuttings, Mine tailings/ash etc.). Theapparatus was used to determine hydraulic conductivities of samples ranging fromcoarse gravel to very fine clayey dam tailings. The values ranged from 2.81 E-03 to4.32E+03 (m/d). The results were reproducible and compared well with those of othermethods. 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 (0.16rrr'): Rapid results (Complete determination for a sample within tens of minutes); Thisapparatus is particularly suited to determine the hydraulic conductivity of clasticformations for non-confining flow under atmospheric conditions.Laboratory experiments on the small (cm) scale aimed at determining the effectof variable thickness of formations on the hydraulic conductivity, determine the effect ofcomposition, layering ,spatial disposition and develop a tool for predicting bulk hydraulicconductivity in phreatic aquifers were carried out. From these, the partial hydraulicconductivity formulation was developed empirically, to determine the bulk hydraulicconductivity of the samples, irrespective of the spatial disposition. With geologic insight,the bulk hydraulic conductivities were determined using the partial hydraulic conductivitytheory. When the thicknesses of the layered sequences varied, the laws of compositionbroke down.The Trigger-tube is an apparatus developed for mixing solutes and tracers forinjection tests in boreholes. It is a simple cap-trigger tube segment and the techniquemixes solutes in boreholes in two minutes. Solutes are introduced into the well and thetrigger 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 thismethod 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 testsgave a Darcy velocity of 4.06 m/day and natural velocity of 123 m/day using NaCI forthe same fracture at 21m in borehole U05. This apparatus takes comparatively ashorter time to carry out SWIW tests than using the pump mixing method. Field testsgave 13 minutes for the trigger-tube method and 25 minutes for the pump mixingmethod for a point dilution test using NaCI. This apparatus can be used for any test thatneeds the introduction of a homogenous mixture in single well tests.The thermal dilution test is a test developed to determine the position, numberand groundwater (Darcy) velocity of fractures found in a single borehole drilled into afractured 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 warmergroundwater flows into the well is measured as the change in temperature and used todetermine flow zones, the position of fractures, their depths and the Darcy velocity ofthe various lithologies and fractures with flow present, from top to bottom of theborehole the method was used in a single well test on borehole U05 to determinefractures at 14m, 15m, 16.8m, 18m, 19.4m, 21m, 22.4m, 24.2m,26m and 27.5m belowthe 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 wasconfirmed by acoustic scan and borehole camera images of the borehole. This methodis very useful to determine the hydraulic properties of fractures and formations undernatural conditions (Without pumping) using a single well.

[发布机构] University of the Free State