Effect of Relative Humidity on the Creep Rate of Rock Salt at Intermediate–Low Stresses

Abstract

Understanding and accurately modeling the creep behavior of rock salt is of great interest for its relevance to geological storage of nuclear wastes, oil, and gases. Many experimental studies on the creep of salt samples are performed at a deviatoric stress above 5 MPa. The creep rates at smaller stress levels are usually estimated by extrapolating from data at higher stress levels. However, recent low-stress creep tests conducted at below 1 MPa suggest that such extrapolation can significantly underestimate the creep rate of rock salt, as the controlling micro-mechanisms are very different in low- and high-stress regimes. Meanwhile, field observations suggest that salt creep is accelerated under higher ambient relative humidity (i.e., the so-called Joffe effect). The effect of humidity on salt creep in the pressure-solution dominating low-stress regime is even less understood at present. In this context, we conducted a series of long-term creep tests at intermediate–low uniaxial stresses of 1, 3, and 5 MPa on Avery Island salt samples. In order to quantify the effect of moisture on salt creep, a combined triaxial cell and air circulation system is designed to permit control over the environmental humidity. Three relative humidity levels (33%, 55% and 77%) are selected and are achieved through the vapor equilibrium technique. Our study confirmed that the creep rate at low stress level is indeed higher than those extrapolated from high-stress creep tests. For the same deviatoric stress, higher ambient humidity produces faster steady-state creep rate of salt samples. Finally, a modified creep law that incorporates the humidity dependency is proposed and validated against the new experimental data.

Highlights

• The influence of ambient humidity on creep behavior of rock salt at intermediate–low stress regime was experimentally investigated.

• A novel experimental setup was developed to provide stable humidity- and stress-control during long-term uniaxial creep tests.

• Humidity-dependent scaling functions inspired by existing water retention and adsorption models were introduced in the steady-state creep model for rock salt.