Reservoir geomechanical modeling: In-situ stress, pore pressure, and mud design

This paper, demonstrated a scientific approach to prepare a geomechanical model for Bangestan reservoir in an oil well in the south-west of Iran. Bangestan is one of the most important oil reservoirs in the Middle-East that its geomechanical characters are not perfectly known. Moreover, unexpected problems such as casing collapse, production drop and land subsidence occurred in this reservoir that made it necessary to conduct a comprehensive geomechanical model. According to borehole failure observations from a formation micro scanner (FMS) image log, the direction of minimum and maximum horizontal in-situ stresses (S-h and S-H) were N35E and N55W, respectively. Also, using poro-elastic formulations, in-situ stress regime was found to be strike-slip (i.e. S-H > S-w> S-h). Besides, the optimum mud weight for safe drilling at borehole breakout based on Mohr-Coulomb and Mogi-Coulomb criteria were found 85.12 and 80.42 pcf, respectively, while 79 pcf had been applied in practice. Finally, the precision of the geomechanical model was evaluated using natural fracture observations in the FMS image logs. The FMS logs revealed a total of 510 natural fractures in five clusters. In case of all fracture intervals, petrophysical properties and in-situ stresses decreased and therefore, the model was adequately sensitive to the fractures.