Experimental Investigation of Incompatible Deformation Characteristics of Ultra-Deep Composite Rock Salt

Rock salt formation has a worldwide distribution. In China, a typical area for rock salt formation is in Tarim Basin, Xinjiang Uygur Autonomous Region, Northwest of China. Because of the ultra-deep depth and tectonic movement, the rock salt is interbedded with mudstone and other components. As a result, the creep behavior and mechanism remain uncertain, which brings a big challenge for safe drilling and wellbore integrity. In this paper, we compared different types of rock salt, including fine-grain size pure salt outcrop, composite rock salt outcrop with coarse grain size, and ultra-deep halite core with mudstone interlayer from Tarim basin. The result reveals that the halite core is mainly comprised of salt and clay minerals, highly transparent, and the "hardest" among all the samples. It is hard to define the grain size of the core since the salt crystal seems to be in a whole piece. Besides, we notice distinct fluctuations with composite rock salt in creep tests, which is missed in the fine grain pure rock salt. The halite core is less sensitive to temperature compared with the outcrop. In addition, a finer grain size means a faster creep rate. DIC (Digital Image Correlation) analysis shows that a prominent strain area near the composite interface due to the deformability difference. In addition, it validated the creep fluctuation phenomenon since we could observe it intuitively. The substance of creep doesn't attribute to crystal deformation but fine grain dislocation.