Inrushes of ground water and the ignition of flammable gases pose risks to workers in deep South African gold mines. Large volumes of water may be stored in solution cavities in dolomitic rocks that overlie the Black Reef (BLR) Formation, while there are several possible sources for methane, namely, coal seams, kerogen found in some gold ore bodies, or methane introduced by igneous intrusions. Potential conduits that may transport water and methane to underground workings were mapped using 3D reflection seismic data. Edge detection attributes successfully identified many faults, some with displacements as small as 10 m. Faults that displace the Ventersdorp Contact Reef (VCR) and the BLR horizons were of special interest, as known occurrences of fissure water and methane in underground workings show a good correlation with faults that were imaged on the VCR and BLR horizons. Because there are uncertainties in determining the linkage of faults with aquifers and methane sources, it is considered prudent to assume that all structures that displace the VCR and BLR horizons are potential conduits.

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As expensive as 3D seismic reflection surveys are, their high cost is justified by improved imaging of certain ore horizons in some of the Witwatersrand basin gold mines. The merged historical 3D seismic reflection data acquired for Kloof and South Deep mines forms an integral part of their Ventersdorp Contact Reef mine planning and development programme. The recent advances in 3D seismic technology have motivated the reprocessing and reinterpretation of the old data sets using the latest algorithms, therefore significantly increasing the signal-to-noise ratio of the data. In particular, the prestack time migration technique has provided better stratigraphic and structural imaging in complex faulted areas, such as the Witwatersrand basin, relative to older poststack migration methods. Interpretation tools such as seismic attributes have been used to identify a number of subtle geologic structures that have direct impact on ore resource evaluation. Other improvements include more accurate mapping of the depths, dip, and strike of the key seismic horizons and auriferous reefs, yielding a better understanding of the interrelationship between fault activity and reef distribution, and the relative chronology of tectonic events. The 3D seismic data, when integrated with underground mapping and borehole data, provide better imaging and modeling of critical major fault systems and zones of reef loss. Many faults resolve as multifault segments that bound unmined blocks leading to the discovery and delineation of resources in faulted areas of the mines.

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Forty-three gel fracture treatments are analyzed in this report -in both Mary Lee/Blue Creek seams and in Black Creek seams. Although 12/20 sand concentrations were added to 10 ppg, there were virtually no screenouts, presumably because pad volumes were so high (almost 50%). The Black Creek fractures are vertical, with substantial height growth, and are characterized by high treating pressures but relatively low fracture propagation pressures. There is conspicuous erosion by 12/20 sand of the fracture "entry region": perforations, perforation/fracture junction, or near-wellbore fracture constriction. It is postulated that erosion is conspicuous because (a) not all perforated zones are taking fluid, (b) the overall perforation/fracture junction may be more complex when subfractures from Black Creek seams join up 'to form the main fracture, or (c) the fluid, sand, and products of erosion are not confined to the coal seams.There are relatively few proppant-induced pressure increases, again possibly due to the fact that the 12/20 sand and products of erosion are not confined to the coal seams. The constant behavior of shut-in pressure with time in the majority of cases is consistent with an absence of any poroelastic effect (although about 25% of cases are consistent with a poroelastic effect). Approximately half of the Mary Lee/Blue Creek fractures are just like the Black Creek fractures and are interpreted similarly. The other half are different and exhibit high fracture propagation pressures. They are probably T-shaped fractures. A T-fracture is confined to a coal seam (there might be a T-fracture in more than one seam). Shut-in pressures measured throughout such fracture treatments are greater than 1 psi/ft, but generally decrease with time. In general the high pressure T·fractures are shallower than the low pressure vertical fractures. They do not show any correlation with a prominent fault block, which contradicts a previous finding.There are more proppant.-induced pressure increases. In the high pressure Mary Lee/Blue Creek cases, the pressure drops at final shut.-in range from References and illustrations at end of paper. 277small (~100 psi) to large (~750 psi). The former appear to be consistent with an elevated fracture tip resistance (or apparent fracture toughness). The latter are consistent with a near-wellbore flow constriction (discrete offsets/obstructions, or multistrands, or tortuous fluid flow path due to T-fracture geometry). The pressure decline after shut-in is not any faster in the high pressure Mary Lee/Blue Creek cases than in the low pressure cases. The most likely explanation is a reduction in coal permeability due t.o (a) high' ambient in-situ stress, (b) damage by the high fracturing pressures. Gas production appears better, by more than 50%, in low pressure Mary Lee/Blue Creek cases t.han in high pressure cases. This is consistent with the notion that a vertical fracture is a more effective stimulation than a T·fracture.