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Abstract A new method based on actual data to determine geostress is introduced. Without the traditional assumption that wellbore is cylindrical，the relationship among wellbore deformation，rock mechanical properties，geostress，pore pressure and mud density is established by using dual media elastic mechanics. Using this relationship，geostress and pore pressure are inverted from mud log and conventional log. Then collapse pressure and fracture pressure are calculated. The results show that the calculated fracture pressure is in good agreement with the actually measured value，and the inverted pore pressure is in agreement with the practical mud density. The agreement verifies the rationality of this new method. It is a breakthrough to determine earth pressure and pore pressure from actual mud log and conventional log. Due to the basis of rigorous mechanical model and actual data，this new method has high degree of accuracy. With measurements while drilling，this method can predict pore pressure，geostress and wellbore stability at real time. So the method may be wildly used.
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Received: 12 November 2008
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