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NIRE Annual Report
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1997

Chemically Induced Weakening of Rock Strength

Fracture Mechanics and Explosives Division
Safety Engineering Department
Objectives
Chemical alteration of the strength of sandstone has been investigated to establish the fundamental knowledge for chemically assisted fracturing. If the rock strength can be chemically lowered intentionally, this technology would be useful to raise the fracturing efficiency.
To evaluate chemically induced change of rock strength, Brazilian tests and multi-stage triaxial compression tests have been performed on saturated sandstones with solutions of polyethylene oxide (PEO) and dodecyltrimethyl ammonium bromide (DTAB). And, zeta()-potential, which is an electric potential produced across solid-liquid interface, was also measured to investigate the relation between the strengths and -potential.
Results
The tensile strength of the sandstone varies significantly with concentration of chemicals, and can be markedly influenced by the zeta potential at the rock-liquid surface. The maximum tensile strength reduction relative to the water-saturated specimen are 23 % at the DTAB concentration of 1.0 × 10-3mol/l, and 33 % in the PEO concentration range from 20-100 ppm. Fig.1 shows the variation of tensile strength of sandstone with concentration of DTAB solution, with the graph showing the zeta-potential. The tensile strength is significantly reduced when the zeta-potential approaches zero. On the contrary, the compressive strength did not vary significantly with the chemical's concentration. The reduction of the strength was quite little compared with its strength in distilled water even at the concentration where the tensile strength is the lowest. The possible explanation for the experimental result is that the chemical solution produce an effect on the strength of the sandstone only when the failure mechanism is dominated by tensile mode, for example hydraulic fracturing, rock cutting, and rock grinding.
The acoustic emission differs from dry specimen to the saturated specimen with water or chemical solutions. The AE activity is the most active in dry condition, and the least AE activity is found in DTAB solution. The surface hardness of the materials became greatest when zeta-potential was zero. The results may suggest that the deformation behavior changes to brittle manner, and that the fracture toughness is lowered by chemicals.


Selected Publications
1) Effect of chemical solutions on the strength of Gosford sandstone, Proc. 7th ANZ Conf. on Geomechanics, 1996, 216-221
2) The effect of chemical solutions on strength and acoustic emission behavior of Gosford sandstone, Proc. Int. Symp. on Mining Sci. and Tech., 1996, 131-136

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