Impacts of elevated temperature on the performance of compacted bentonite as a sealing material

  • Linhua He

Student thesis: Phd

Abstract

Compacted bentonite clays as engineered barrier can be exposed to elevated temperature (i.e., due to the decay heat from waste in HLW disposal, and the geothermal around oil and gas wells). Understanding the temperature effects on hydro-mechanical behaviour of bentonite is important for the design and performance assessment of engineered barrier systems and plugging and sealing the wells and boreholes. The clay barrier is expected to maintain a high swelling pressure, low hydraulic conductivity, and long-term integrity under elevated temperatures. This thesis presents an investigation of temperature effects on swelling pressure development, hydraulic conductivity, and erosion behaviour of compacted bentonite at a range of dry densities. A bespoke high-pressure, high-temperature constant rate of strain system (CRS) was designed and constructed to conduct two series of investigations on: i) the hydro-mechanical properties of Volclay bentonite compacted at different temperature ranging from 20°C to 80°C during and after saturation; and ii) the erosion behaviour at elevated temperatures. The experiments of the first series were carried out on a range of compacted samples (dry densities 1.1, 1.4 and 1.7 Mg/m3). The results showed that with the increase of temperature, the swelling pressure decreases whilst the hydraulic conductivity increased. Furthermore, the rate of swelling pressure development was found to be considerably enhanced at elevated temperatures. The results indicated that the hydro-mechanical response of the clay to elevated temperature is strongly linked with the exchange of pore water in clay microstructure, which is facilitated by the reduced retention capacity of water in the clay system and by changes to the viscosity and density of water. The diffuse double layer theory (DDL) was revisited to include micro/macro aspect of clay system for the calculation of swelling pressure. A model for swelling pressure of compacted bentonite at different temperatures was proposed. It combined the regular solid-solution theory and the diffuse double layer theory into a thermodynamic framework for the evolution of hydrous and anhydrous smectite at different temperatures. This new model was assessed by comparing the predicted swelling pressure and experimental data for calcium bentonite and sodium bentonite. The results showed that the consideration of temperature effects on pore evolution and distance between clay aggregates improves considerably the prediction of the swelling pressure by DDL at elevated temperature. The research presented offers an improved method for assessing the swelling pressure development of compacted bentonite, which incorporates the evolution of micro/macro pore at elevated temperature. The experimental results of erosion of compacted bentonite at elevated temperature showed that the concentration of the eroded bentonite particles increased with increasing temperature. The results indicated that bentonite erosion does not significantly reduce the physical stability of a generic bentonite plugs for applications in plugging and abandonments of wells. However, the temperature effects on chemical transport processes in compacted clay requires further studies to underpin safety assessment of bentonite plugs.
Date of Award1 Aug 2022
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorMojgan Hadi Mosleh (Supervisor), Andrey Jivkov (Supervisor) & Majid Sedighi (Supervisor)

Keywords

  • plugging and abandonment of wells
  • erosio behaviour
  • elevated temperature
  • diffuse double layer
  • HLW disposal
  • engineered barrier
  • compacted bentonite
  • swelling pressure

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