Abstract
Numerous observations have shown that shale formations may creep in and close the annulus between rock and casing in deep wells, thus effectively sealing off the well sections in a natural way. This movement of the rock is partly controlled by strain and partly by stress, and parts of the rock may be in a post-peak state while other parts are in a pre-peak state. The effect of pre-peak versus post-peak conditions on creep characteristics was studied in a set of stress relaxation tests on Pierre II shale. It was found that the stress relaxation process can be described with great precision both by a logarithmic function and a power-law function. However, only the power-law function fulfills the expected asymptotic behavior for infinite time. Temperature fluctuations can be accurately accounted for by an Arrhenius-type factor. The shear stress relaxation appears to be insensitive to pore pressure alterations. There is an indication that the relaxation rate is slightly higher when the rock is in a strain hardening state than in a strain softening state. No significant effect of exposure to NaOH was seen on the relaxation process.