ABSTRACT:

Ordovician carbonate rocks make up the main reservoir stratum in the Tuofutai region of the Tahe Oilfield. The rocks exhibit extensive formations of the faults, fractures and caves which results in a rather complex stress distribution. To determine the variation in the stress field and the rotation of the principal stress, fracture distribution was obtained by logging data and FRACMAN numerical analysis was conducted. The results showed that: 1) two conjugate faults and fractures were recognized by FMI images and logging data, and the trends of the fractures were about NNW343° and NNE28°, which means the trend of maximum horizontal principle stress is about NNE10. 2) The vertical stress is the maximum principle stress at about154-170MPa, the maximum horizontal principle stress is about 122-135MPa and the minimum horizontal principle stress is about 89-95MPa when the depth is 6200-6700 m. 3) The value of each principle stress decreased about 5-12MPa in the fault zone and the degradation is about 8-10%. The trend of maximum horizontal principle stress varied from NNE5° to NNE30° at the ends of the fault. 4) The value of each principle stress decreased about 3-5MPa with fracture development and the degradation is about 3-5%. The trends usually held steady; 5) The value of maximum horizontal principle stress decreased about 20-30MPa around the caves and the minimum principle stress decreased about 10-20MPa; and the maximum horizontal principle stress varied from NNW282° to NNE70°. 6) The stress field in the surrounding rock was mainly concentrated within the distance of 0.5 times of the cave radius. The rotation of the principal stress mainly occurred at the end of faults and around the caves.

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