Weakening of rock salt by water during long-term creep

Abstract

The rheological properties of rock salt are of fundamental importance in predicting the long-term evolution of salt-based radioactive waste repositories and strategic storage caverns, and in modelling the formation of salt diapirs and associated oil traps. The short-term, high-stress rheology of rock salt is well known from laboratory experiments; however, extrapolation to appropriately low stresses fails to predict the rapid flow seen in certain natural structures. Furthermore, experiments have failed to reproduce the recrystallized microstructure of naturally deformed salt. Here we report experiments indicating that the above discrepancies can be explained by taking into account the influence of trace amounts of brine. Trace brine is always present in natural salt but sometimes escapes during experiments. Our tests on dry dilated salt show more or less conventional dislocation creep behaviour, but brine-bearing samples show marked weakening at low strain rates. This is associated with dynamic recrystallization and a change of deformation mechanism to solution transfer creep. Because natural rock salt always contains some brine, these results cast substantial doubt on the validity of presently accepted dislocation creep laws for predicting the long-term rheological behaviour of salt in nature.

Intergranular Pressure Solution in Nacl: Grain-To-Grain Contact Experiments under the Optical Microscope

“…IPS is also recognized as one of the main factors controlling the geological evolution of porosity and permeability, and hence capacity and productivity, of (potential) hydrocarbon reservoirs (Pittman, 1979;Hutcheon‚ 1983;Carrozzi and Von Bergen‚ 1987), as well as a possible mechanism controlling fault creep, fault gouge compaction and fault strength recovery (Rutter and Mainprice, 1978;Angevine et al, 1982;Lehner and Bataille, 1984;Sleep and Blanpied, 1992;Sleep, 1995). In addition, Urai et al (1986) and Spiers et al (1989Spiers et al ( , 1990 have shown that IPS can be an important deformation mechanism in halite rock at low strain rates, which has important implications for the design of waste repositories in rock salt formations, and for understanding salt tectonics. Clearly, then, a fundamental understanding of compaction and deformation by IPS is of substantial interest in petroleum geology, structural/metamorphic geology and tectonophysics.…”

Electrical impedance measurement of plastically deforming halite rocks at 125°C and 50 MPa

“…However, so far it has been poorly understood. In previous studies the fluid distribution has been investigated through microstructures of deformed materials [e.g., Urai et al, 1986a] or ''see-through'' experiments [e.g., Urai, 1987]. Although the structure of fluid phase is mostly preserved after deformation, the fine structure is likely to be modified rapidly.…”

Effect of confining pressure on dilatation, recrystallization, and flow of rock salt at 150°C

“…The conditions investigated in most studies of natural salt cover temperatures in the range 20ø-250øC, strain rates down to around the creep rate probably being controlled by cross slip of screw dislocations at high stresses (i.e., above •-10 MPa at 50ø-150øC [Wawersik and Zeuch, 1986;Skrotzki and Haasen, 1988;Carter et al, 1993]) and by climb of edge dislocations at lower stresses . In addition, experiments on natural rock salt have shown that when brine is present within grain boundaries, dislocation processes can be accompanied by fluid-assisted recrystallization (FARX) [ Urai et al, 1986a;Spiers et al, 1988, irradiation (i.e., precipitation of particulate Na) often reveals evidence for intergranular overgrowth and grain boundary migration, consistent with the operation of both fluid-assisted dynamic recrystallization (FADRX) and pressure solution [Urai et al, 1987;Wanten et al, 1996;Spiers and Carter, 1998]. …”

“…However, no data have been published on the effects of FARX on steady state flow in rock salt. To date, the principal evidence for FARX in salt has been obtained from stress relaxation experiments performed on natural samples (Asse Mine, Germany) with an inherent water content of around 0.05 wt % [see Urai et al, 1986a;Spiers et al, 1988Spiers et al, , 1989. These experiments involved an initial stage of plastic deformation at high strain rate, introducing high dislocation densities and hence a high driving force for FARX.…”