Issue 27

X. Ran et alii, Frattura ed Integrità Strutturale, 27 (2014) 74-82; DOI: 10.3221/IGF-ESIS.27.09 80 Figure 7: The distribution of displacements. - Pore pressure. The distribution of pore pressure of well wall rock at different time after drilling is shown in Fig. 8. Due to the non-uniform ground stress, the maximum pore pressure is occurred at the direction of minimum horizontal ground stress, and the minimum pore pressure is occurred in the direction of maximum horizontal ground stress. With the passage of time, the pore pressure is dissipated gradually. After ten days, it tends to uniform in any direction. It is equal to the well fluid pressure at the well wall and to the initial pore pressure in the distance. It presents linear distribution between of them. (a) (b) (c) (d) Figure 8 : The pore pressure at different time (unit: Pa). (a) 100s; (b) 1h; (c) 10h; (d) 10d. D IFFERENT DRILLING FLUID DENSITY ig. 9 shows the distribution of damage when m  =1.98 g/cm 3 . Both the direction of maximum and minimum ground stress exist damage near the wall. The range and value of damage in the direction of maximum ground stress are larger than the direction of minimum ground stress. The hoop stress (   ) and radial stress ( r  ) in the direction of the maximum ground stress is indicated in Fig. 10. It is found that both hoop stress and radial stress are tensile stress. So there is a tensile damage in this direction. It can be deduced that the rock fracture when tensile damage reached to its limit ( max t D ). F