ABSTRACT
Previous work on rocksalt at room temperature and a strain rate of 10−4 s−1 has demonstrated a decrease in plastic flow strength with increasing pressure for both single crystal and polycrystalline NaCl (tested up to 1000 MPa), suggesting constriction controlled cross slip (Aladag et al., 1970, Auten et al., 1973). Conrad & Yang (1999) also reported unconfined creep behaviour consistent with cross slip control, up to temperatures of 264°C. Above 264°C, dislocation climb took over control. However, the effect of pressure was not investigated. None of these previous studies has resulted in a rigorously verified dislocation mechanism for polycrystalline halite at moderate temperatures. Moreover, the high pressure experiments were all conducted using “unjacketed” samples, i.e., samples not sealed from the confining medium. The samples were in effect open to penetration by the confining fluid, hence were potentially
1 INTRODUCTION
Despite the large body of data that already exists, important aspects of the rheological and transport properties of rocksalt remain insufficiently understood to allow reliable modelling of the long term geomechanical and geohydrological behaviour of salt bodies. We focus on one key question regarding the behaviour of dense halite rock, namely: what microphysical mechanisms govern plastic flow of natural rocksalt by dislocation motion at temperatures in the range 100-200°C. Many different mechanisms have already been proposed. Some authors suggest that dislocation creep at these temperatures is climb controlled (Senseny 1992, Carter et al., 1993), while others conclude control by dislocation cross slip or glide (Auten et al., 1973, Skrotzki & Haasen 1981, Wawersik & Zeuch 1986, Conrad & Yang 1999). Since these mechanisms are characterized by quite different constitutive equations, reliable extrapolation of laboratory creep data to in-situ strain rates is possible only if an appropriate mechanism-based flow law for the range 100-200°C is defined.
