Experimental analysis of dynamic and static mechanical properties of deep thick anhydrite cap rocks under high-stress conditions

Abstract

Across the globe, the formation of large oil and gas fields is always associated with a wide range of anhydrite cap rocks. Thus, an accurate analysis of the dynamic and static mechanical properties of anhydrite rocks is required. A deep thick anhydrite cap rock is developed in the Paleogene formation of the southwestern Tarim Basin, NW China. In this paper, we designed mechanical and acoustic synchronization tests and acquired the corresponding parameters. The effects of stress and mineral composition on the dynamic and static mechanical properties of the anhydrite rock were analyzed systematically. The results indicate that under uniaxial conditions, the anhydrite rock showed obvious brittle rupture characteristics, while under triaxial conditions, the shear rupture characteristics were obvious. The stress–strain curves of the anhydrite rocks under triaxial conditions can be divided into five stages: initial compression stage, linear elasticity stage, nonlinear steady extension stage, nonlinear unsteady extension and failure stage and post-peak stress stage. An increase in the anhydrite content can delay the weakening time of the anhydrite rocks, greatly improving the rock strength. Under high-stress conditions, the anhydrite mineral particles can accumulate large amounts of energy, exhibit a long period of creep behavior and produce significant additional stresses. The anhydrite composition can improve the strength and rigidity of the anhydrite rocks. However, anhydrite rock with a high anhydrite content will also have strong plasticity. The greater the stress environment that the anhydrite rock bears, the stronger its plasticity will be. The anhydrite component can improve the ability of the anhydrite rocks to resist shear rupture. Compared to the anhydrite mudrocks (AMR), argillaceous anhydrite rocks (AR) have higher VP, VS and VP/VS values. There is a very good positive correlation between VP/VS and the anhydrite content. A regression model of the longitudinal and shear wave time differences of the anhydrite rocks is obtained, which can be used for the prediction of the shear wave time difference. The dynamic Young’s modulus, Poisson’s ratio and shear modulus of the AR are higher than those of the AMR. The regression models of dynamic and static mechanical parameters of the anhydrite rocks are also obtained. These models will be valuable in the evaluation of mechanical characteristics for similar types of anhydrite rocks worldwide.