Dynamic properties of anhydrites, marls and salts of the Gachsaran evaporitic formation, Iran

A large carbonate oil field in Iran is suffering from severe casing collapses and related operational problems in anhydrite, marl and salt sequences of the Gachsaran cap rock formation. To investigate the causes and cures of operational problems, specifically casing collapse, knowing geomechanical properties of anhydrite, marl and salt of this formation is a prerequisite. However, taking cores in this formation is virtually impossible due to high solubility and weakness of the rocks. Moreover, there are insufficient well log data in this formation and the only available running well log is Vp. In this paper, in order to obtain the dynamic parameters of the Gachsaran formation, Vp, Vs and ρb in anhydrite, marl and salt cores, which had been taken from depths up to 300 m, were measured. Moreover, Vp and Vs in salt cores under different triaxial and hydrostatic stress conditions were obtained. The Vp–Vs, (Vp/Vs)–Vp and Vp–ρb relations in anhydrite, marl and salt were investigated. The established relations in these anhydrite samples were verified by the data derived from limited cores which were taken from 3600 m depth. The relations between dynamic properties of salt with lateral and hydrostatic stresses were investigated. In conclusion, Vs, ρb and the ratio of Vp/Vs in anhydrite and marl can be estimated through the established relations and having Vp logs in the Gachsaran formation. As a result, the dynamic properties of these rocks can be calculated in different depths of this evaporitic formation. Furthermore, the dynamic properties of salt rock seem to be constant in various depths and under differing triaxial and hydrostatic stress conditions. (paper)