Creep and Consolidation in Pierre Shale - Identification of how time-dependent effects are influenced by alteration in pore fluid pH and changes in stress conditions
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The Petroleum Safety Authority of Norway (PSA) has now acknowledged the use of shale as an annular barrier for permanent plug and abandonment (PP&A) operations in the oil industry. Shale can exhibit a natural self-sealing effect; a result from delayed deformation of the rock. It is believed that creep in combination with consolidation plays important roles in the process when shale closes in and forms a tight barrier around a well. For using shale as a permanent annular barrier it is important to comprehend how these time-dependent mechanisms are affected by externally induced parameters, like stress, temperature and chemical alterations of the pore fluid. The purpose of this master thesis is to investigate creep characteristics and consolidation effects in Pierre shale. Two literature studies have been completed and several laboratory experiments have been carried out on small size core plugs drilled normal to the bedding plane. Axial strain-time curves have been obtained by performing uniaxial compression tests with step loading at a temperature around 30oC. The main objectives have been to identify how the creep and consolidation processes are affected by different stress levels and variations in the pore fluid pH. Eventually, the laboratory work resulted in four successful creep experiments. The obtained data showed that Pierre shale exhibits considerable time-dependent deformation in response to applied stress. Changing the stress conditions affected the amount of deformation in periods with constant stress and it appeared like the shale demonstrated different creep behavior before and after a certain test dependent stress level. It was also seen that the temperature variation increased with extended test time for the conducted experiments and the experimental results implied that p-wave velocities are related to strain as well as stress. In addition, the creep tests indicated a correlation between p-wave velocities and temperature. Finally, it was found that the pH value of the pore fluid had an impact on consolidation and creep properties. A lower pH value reduced the overall stiffness and strength of the samples, resulting in more consolidation. More creep deformation and increased creep rates were seen compared to a near-neutral pH environment.