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[1] Carbon dioxide (CO 2 ) is often used for enhanced oil recovery in depleted petroleum reservoirs, and its behavior in rock is also of interest in CO 2 capture and storage projects. CO 2 usually becomes supercritical (SC-CO 2 ) at depths greater than 1,000 m, while it is liquid (L-CO 2 ) at low temperatures. The viscosity of L-CO 2 is one order lower than that of normal liquid water, and that of SC-CO 2 is much lower still. To clarify fracture behavior induced with injection of the low viscosity fluids, we conducted hydraulic fracturing experiments using 17 cm cubic granite blocks. The AE sources with the SC-and L-CO 2 injections tend to distribute in a larger area than those with water injection, and furthermore, SC-CO 2 tended to generate cracks extending more three dimensionally rather than along a flat plane than L-CO 2 . It was also found that the breakdown pressures for SC-and L-CO2 injections are expected to be considerably lower than for water.

Cubical granite specimens were fractured by borehole pressurization of 1 cP water, 80 cP oil and via a urethane sleeve. Viscous oil tends to generate thick and planar cracks with few branches, while water tends to generate thin and wavelike cracks with many secondary branches. While penetrating fluids extended cracks rapidly, pressurization via a urethane sleeve led to stepwise crack extension. Fault-plane solutions of AE (Acoustic Emission) events indicated that shear-type mechanisms were dominant during water injection and sleeve pressurization, whereas tensile-type mechanisms were dominant during oil injection. These results could be helpful in optimizing stimulation treatments in the petroleum industry.

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