Numerical and laboratory investigations of thermally induced fractures in rock salt

Abstract

In order to widen the basis for the dimensioning of storage caverns in rock salt, some special attention must be paid to the temperature developments within the cavity during its operation. Passed numerical investigations have shown that a temperature change in the storage medium, such as natural gas or hydrogen, can cause stress differences of several MPa in the rock mass surrounding the cavern. If, as a result, the difference between horizontal and vertical stresses is increased, the rock being additionally loaded by a fluid pressure can fracture orthogonally to the lowest principal stress. The knowledge of temperature conditions and heat flows in the cavern surrounding salt rock are therefore of great importance. A wide variety of factors must be considered when investigating such mechanisms. The novel test facility at the Institute of Geotechnical Engineering of the Leibniz University Hannover is used to simulate the situation that occurs at the caverns edge when storage medium is withdrawn. This means, the cooling and the gas pressure inside the cavity are reconstructed on hollow cylindrical test samples made of rock salt from various locations to draw conclusions about the propagation of fractures and thus the safety of salt caverns under gas-loading. Before inducing any mechanical loads by means of a triaxial cell, a comprehensive investigation of the temperature field, resulting from the locally limited, artificially induced cooling of a sample is carried out with the new testing facility. While local temperatures in the sample drop by about 20 Kelvin, it is free to contract inwards, causing thermal stresses to decrease significantly. Therefore, the facility needs to be optimized and the testing scheme is changed. Instead of reducing stresses thermally, one of the mechanical loadings is diminished while the gas pressure inside the borehole is kept constant. As a result, the material visibly fractures while mechanical loadings are at a fully-compressive state.