Large Scale Experiments on the Tightness of Boreholes under Cyclic Loading

The transition from nuclear and fossil energy to renewable energy leads to higher fluctuations in energy supply – but storage for power is negligible so far. In underground gas storages (UGS) huge amounts of TWh can be stored to meet the demand consistently. However, this results in increasing injection and extraction frequencies, leading to faster pressure and stress changes and therefore posing additional challenges for reservoir rock, cap rock and technical components. To evaluate the effects of additional cyclic loading on the rock-cement-steel-compound of the UGS infrastructure, we use an autoclave system on a realistic scale. Mainly abandoned drillings are simulated, the system therefore consists of a 2 m long cemented steel casing with an autoclave chamber at each end and surrounding heating mats. To simulate injection and extraction, gas pressure (N2) is applied and released on both ends. Additionally, temperature can be raised to 100 °C. Between loading cycles, permeability can be measured to determine the effect of pressure and temperature variation on the tightness of the system. We present results from the analysis of three cemented casings. Since the hardened cement isn’t connected to the steel casing after experiments, we assume an annular gap as main gas path. This gap is modelled and fitted to the experimental data. After pressure variations between 0 bar and 60 bar, tightness of the system decreased in every experiment, which leads to an increased modelled annular gap width. Temperature variations between 30 °C and 70 °C tend to increased tightness slightly.