The rock physics of carbonate-water interactions : laboratory induced decarbonation and microstructural manipulation measurements using 3D printing
Keywords: Geophysics.

This thesis explores the changes in rock microstructure and bulk physical properties driven by a specific type of rock-fluid interaction, and a use-case for a new technology to connect changes in microstructure to changes in bulk physical properties. Rock-fluid interactions are very important to understand because fluids are ubiquitous in the crust. While they have historically been treated as only interacting with porous media mechanically, they have the ability to drive physical changes in rocks through Thermo-Hydro-Mechanical-Chemical (THMC) alteration. Improved lab characterization of these processes can provide the necessary data to improve remote characterization and thus modeling and interpretation of rocks altered by rock-fluid interactions. Ultimately, I hope that the work here can provide a better understanding of one THMC process, a template for how to investigate other THMC processes in the lab, and a future outlook for the use of new technology to link microstructural changes to changes in bulk physical properties. Inducing THMC reaction processes in the laboratory can be challenging, but it is necessary to allow for separately evolving pore and confining pressure systems in order to mimic subsurface conditions. I augmented the SRPL HTHP reactor vessel to enable flow during reactions, which allowed me to induce decarbonation in natural carbonate rock samples. The modification consisted of an upstream pump and a downstream, metered, automated pressure release valve, and it enabled episodic to quasi-continuous flow. The modified equipment is also now capable of inducing other reactive flow processes in natural rock samples. Generally, decarbonation is any reaction which releases CO2. Specifically, I induced an exchange reaction which reacts calcium carbonate and silica to produce wollastonite and release CO2. This reaction was induced under confining stress, in the presence of water, with pulsing flow. After undergoing decarbonation, the samples showed dramatically reduced elastic stiffness and dramatically enhanced elastic stiffness sensitivity to pressure, but a very small increase in the connected effective porosity. I interpreted these changes to be caused by small, randomly oriented and distributed microcracks, an interpretation I worked to confirm using SEM images of polished thin sections. Because cutting vertical transects to make thin sections of the interior of the sample is destructive, I was only able to use that technique to quantify the relative difference in microcrack density between an unreacted and a decarbonated sample from the same sample set. To confirm the crack density interpretation in a different way, I used a penny-shaped crack model that converts the stress sensitivity of the dynamic elastic moduli to a crack density. Then, I extracted the effective aspect ratio of the added cracks using a simple Differential Effective Medium (DEM) model. These two methods demonstrated that a very small amount of low aspect ratio (soft, microcrack) porosity was able to account for the dramatic loss of elastic stiffness with a very small increase in connected porosity. Finally, all the changes that I measured were within material that was being uniformly decarbonated. In nature, this process might take place heterogeneously, and therefore uniform elastic softening without a loss of competence in the laboratory could result in a different behavior in outcrop. For example, the elastic softening could lead to failure and/or the development of preferential flow paths in a heterogeneous outcrop setting or a setting undergoing different stresses. Regardless, these large changes in the elastic property of the rock, particularly the large decrease in shear stiffness, cannot be accounted for through a traditional fluid substitution model. This means that the rock-fluid interactions lead to a need for a change in modeling methodology in monitored areas undergoing this type of THMC alteration. Using traditional rock physics techniques, I was able to provide a hypothesis about how microscale features (microcracks) were driving the loss of elastic stiffness without dramatically enhancing the connected porosity in the decarbonated samples. Despite the substantial evidence, these techniques are still not able to directly connect a change in microstructure with a change in bulk physical properties. Even the current very high-resolution imaging capabilities are either focused on a small field of view and thus a small subsample of a laboratory core sample, or image at a larger scale but lack the resolution to identify small features such as microcracks. I presented work that explored the preliminary potential for 3D printing technology to allow for direct microstructural manipulation digitally, followed by direct physical measurement of the resulting changes in bulk properties using the printed models. While I did identify technological limitations in both printer resolution and material, I also found that 3D printing shows a good potential to enable direct connection across different scales of measurement. I utilized printers that produce a plastic material, so I only quantified bulk flow properties (not bulk elastic properties). However, recent innovation in the time since our publication has shown that 3D printing now has the ability to print in geologically relevant materials including sands and gypsum - another exciting development for future utility. 3D printing could enhance experimental repeatability and our ability to directly connect physical properties to microstructural changes, including changes in surface area or tortuosity which have been traditionally difficult or impossible to quantify. I hope that this technique will enable future characterization and measurement of the effects of challenging microstructures, such as microcrack networks, and a better understanding of the connections between those microstructural changes and changes in bulk physical properties