Compaction of fine granular calcite by pressure solution at high temperature and stress with and without organic additive in pore fluid

Master's thesis in Industrial economicsPressure solution is a compaction mechanism that reduces both porosity and permeability. It is a common and important water-rock interaction process and it is a widespread phenomenon that influences many essential natural processes. It is an important mechanism, but it is poorly understood. Especially when there are organic molecules present. It often leads to materials having a stronger and more resilient structure and texture, compared to the absence of the organic molecules. Obtaining a good understanding of the mechanism and the rate of compaction in calcite under upper crust conditions are therefore of major interest. The objective of this study has been to make artificial cores out of calcite powder and performing uniaxial strain tests with loading to an axial stress of 30 MPa at 130 C, where basic quantities like stress-strain relationship and axial creep over time were measured. Two different brines were used to flood the artificial cores, one with and one without organic additive, too see how this affected the results. Also, two tests were performed at lower stress, to see how the cores and grains behaved with a smaller amount of stress applied. Chemical analysis and scanning electron microscope (SEM) analysis were conducted to investigate the ion concentrations and their potential changes. A novel contribution in this thesis compared to previous work is that we used the SEM images to quantify the grain size distribution, and changes in grain size distribution when the various tests were performed. All tests performed with an axial stress of 30 MPa showed a similar behavior for both the axial strain loading phase with values laying between 12.7-15.5% and for the creep phase with values between 8.2-12.0%. A minor difference was seen on the creep, as the presence of oxalic acid seemed to make the cores stronger. This was supported by SEM investigations, the presence of oxalic acid made the occurrence of tiny grain size almost nonexistent compared to the cores being flooded without an organic additive. Comparing the high pressure tests to the low pressure tests, the grain distribution in low-pressure tests were more evenly distributed throughout the cores, with small variations. For the chemical analysis in the tests, there were only observed changes for the Ca2+ concentrations. A loss in calcium throughout the test durations was observed, and there were indications that the loss seemed to be greater with the presence of oxalic acid. This study revealed or at least point to differences in the mechanical behavior and microstructure in the cores flooded with and without organic additives, and with and without overburden pressure. In order to further quantify the differences and draw some solid conclusions regarding the nature of pressure solutions, we believe that the tests should have been performed for longer period of times. However, the results in this thesis indicates that the presence of oxalic acid preserves the grain distribution to a larger degree compared to the cores flooded without oxalic acid, which should be investigated further