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Permeability of Luhui granite with 200mm in diameter and 400mm long was measured at triaxial stresses and elevated temperature using 600℃20MN servo-controlled triaxial rock mechanics testing machine. It is found that 300℃ is the threshold temperature (Tc) of permeability change with temperature in thermally cracked granite. The magnitude of permeability is 10-7D with a low increase below Tc and the permeability whose magnitude is 10-6D increases drastically with high amplitude at 300℃-400℃ while the magnitude is 10-5D at 400℃. Simultaneously, the change of micro-crack quantity in thermally cracked Luhui granite at temperature up to 400℃ was quantificationally investigated by MPV micro-photometer. Two peaks of micro-crack quantity that the length is more than 5μm and 10μm respectively exist at temperature up to 400℃. The temperature scope where micro-crack quantity peak appears in length>5μm is less than that in length>10μm. The quantity of micro-crack whose length is more than 10μm increases sharply at the rate of one per ten centigrade at temperature above 300℃. The drastic increase of micro crack above 300℃ is the main reason that permeability increases sharply at temperature above 300℃ in thermally cracked granite. Compared with preheated results, thermal and mechanical coupled effect can more heavily affect granite permeability than only thermal effect.
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