The reservoir saturation of oil and water changes during the long-term injection and production. Thus, the fracture toughness of rocks changes, which affects the extension of the hydraulic fracture. In order to research the fracture toughness of rocks with different water-oil saturations, the authors put forward a new method to saturate the samples, and a series of experiments on sandstone with different saturations ranging from 100 percent oil to 100 percent water were carried out, with the modified three-point bending test. After the experimental works, several interesting conclusions were reached: (1) the fracture toughness of specimen containing water and oil decreases by 30-50 percent compared with the dry specimen; (2) when there is only one kind saturated fluid, the fracture toughness of the oil-saturated rock is higher than that of water-saturated; (3) in the saturant fluid of water-oil two-phase system, as water saturation goes up the rock fracture toughness decreases drastically. Basing on experimental data, the authors give the theoretical analysis from two aspects: energy conservation and chemical reactions.

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