Difficulties in laboratory characterization of weak sandstone

Rock mechanical properties of formation rocks play an important role in understanding the behavior of the formation rocks under stress, avoid wellbore instability, and optimizing the production of hydrocarbons from underground reservoirs. Characterizing young shallow soft rocks is very important to avoid rock instability in many engineering applications including, civil, electrical, and nuclear facility construction. In this study, cylindrical samples were cored from bulk weak sandstones collected from an outcrop area South of Riyadh, Saudi Arabia. The samples were prepared and then tested in the laboratory where many difficulties were encountered. Difficulties in characterizing the soft rock ranged from basic core measurement to triaxial mechanical testing. For example measuring permeability using gas permeameter has been difficult as the high permeability is out of the range of most of the measuring equipment. Another example is the difficulty of precisely identifying the failure situation due to two reasons; firstly, is the absence of sharp break of the rock and gradual failure of the core samples, secondly is the compaction of the sand grains when loading is continued after failure.