Investigating the creep behavior some gypsum and anhydrite rock samples of Gachsaran formation under room andsealrock temperature conditions.

It is very important to know the rock creep behavior in the evaporitic cap rocks in order to investigate the convergence in oil wells. In general, among evaporitic rocks, the creep behavior of gypsum and anhydrite has been less investigated compared to halite, which are investigated in this research. Since accessibg to the cores of oil wells drilled in Gachsaran Formation is very difficult and expensive, in this research, sampling was done from the surface outcrops of section 1 of Gachsaran formation. For this purpose, blocks were prepared from gypsum and anhydrite and then cored from them in the laboratory. First, unconfined compressive strength (UCS) tests were performed on cores to determine the amount of initial load for uniaxial multi-stage creep test. Next, based on the UCS results and the approximate temperature conditions of cap rock, creep tests were performed at two temperature conditions of the laboratory and cap rock environment. Then, based on the data obtained from the creep tests, fitting the exponential and Powerlaw creep equations was done on gypsum and anhydrite. The results show the creep yield stress of 12 MPa for gypsum and 20 MPa for anhydrite. Also the creep tests was performed at two levels of initial stress 0.1 yield stress and 0.75 yield stress in UCS test. Based on these tests, the linear viscous limit for creep of gypsum and anhydrite was obtained as 5.5 to 6.7 MPa and 9.8 to 15.15 MPa, respectively. In addition, gypsum shows more uniform plasticity and creep under temperature compared to anhydrite. This is while in the anhydrite, the threshold of accelerated creep is lowered and the creep time is reduced.