Analysis of sandstone creep and wellbore instability prevention

• The method to prevent sandstone formation creep is discussed.
• There are critical stresses in all of the three typical creep stages of sandstone.
• The creep mechanism of sandstone has been analyzed.
• The critical stresses of secondary and tertiary creep have been determined.
• The safe mud density window of the creep sandstone formation is determined.

Sandstone of gas reservoir in Miocene Strata of Yinggehai Basin has creep characteristic and many drilling problems caused by wellbore shrinkage have occurred. It is important to determine the suitable mud density to prevent sandstone creep. The sandstone cores of the reservoir are used to do experiments and detect the creep property. The three typical creep stages have their critical stresses respectively, which are demonstrated by the creep experiments. It is important to determine the critical stresses to obtain the safe mud density window. The critical stresses are confirmed by creep mechanism analysis of sandstone. The critical stress of secondary creep is equal to the stress when the transverse wave velocity reaches the peak. The critical stress of tertiary creep is equal to the long term strength. Combined pore pressure, fracture pressure and the critical stresses, the safe mud density window can be determined. Sometimes, the mud density in the window would lead to occurrence of the secondary creep. Viscous-elastic-plastic model are introduced to calculate the creep strain and determine the safe drilling cycle. Through the above methods, the safe mud density window and the safe drilling cycle of the gas reservoir sandstone are calculated and the appropriate mud density is provided.