Date of Award

January 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Petroleum Engineering

First Advisor

Hui Pu

Second Advisor

Vamegh Rasouli

Abstract

The world is currently experiencing a higher demand for low-carbon technologies, and carbon capture utilization and storage (CCUS) is an important CO2 emissions reduction technology. Studies have shown the potential for CCUS in shale formations either as a caprock or as a reservoir storage unit. However, there is limited knowledge of the changes in shale reservoirs after long-term exposure to CO2, and the resulting impacts on storage permanence. This study coupled experimental and numerical modeling methods to investigate how long-term CO2 storage in shale formations might alter pore structure and minerals, which might cause a resultant change in chemical and mechanical attributes at various scales of measurement. To conduct these investigations, samples from the Bakken Formation (Upper, Middle and Lower Members) were used. The samples were saturated with supercritical CO2 for a range of periods (0, 3, 8, 16, 30 and 60 days). To assess the effects of CO2 on the microstructural, geochemical, and geomechanical properties after 60 days of saturation at several stages (0, 3, 8, 16, 30 and 60 days). X-ray diffraction (XRD) analyses were conducted out to study the changes in the mineral compositions. Field emission scanning electron microscopy (FESEM) was implemented to visually characterize the pore structure evolution. Gas adsorption (N2) isotherm measurement and fractal theory were used to gain insight into the changes of the pore structure. Triaxial tests and a nanoindentation method were performed to identify changes in mechanical properties at macro and nano scale, respectively. After saturation, the pore size distributions of the saturated samples were consistently lower at all pore size scales of the samples in CO2, indicating that the pores decreased because of the reaction. Fractal dimension has an increasing trend as the samples were exposed to CO2, where the roughness of the pore surface and the complexity of pore structure increased after 8 – 16 days of CO2 saturation because of dissolution. The roughness of the pore surface and the complexity of the pore structure decreased after 30 – 60 days of saturation because of precipitation. Furthermore, the correlation of the fractal dimensions and the mineral composition from the XRD results showed the clay mineral dissolution enhances the pore volume of micropores, and carbonate dissolution increases the specific surface area of mesopores. Rock physics modeling were applied to the nanoindentation results. The homogenization of the elastic properties from the rock physics model were compared with triaxial test properties to assess the degree of correlation between the numerical model predictions and experimental results. The results showed a high degree of correlation between the modeling predictions and the experimental results. Also, the results showed soft minerals such as clay were precipitating, whereas brittle minerals such as the carbonates were dissolving. The slope of hardness and elastic moduli cross plots increase post−CO2 saturation also showed the samples becoming more ductile post saturation. This study provides experimental evidence to further test the mechanisms of geologic CO2 storage in organic-rich self-sourced plays. The study also provides valuable insight for a CCUS project in the Bakken formation for understanding long-term CO2 storage, CO2-induced alterations, migration pathways due to CO2 injection, reservoir characterization, caprock integrity assessment, and prediction of trap evolution. Furthermore, this study can play a significant role in long-term CO2 storage security, and an accurate reservoir-scale prediction through modeling to ensure safe and successful CCUS project in other unconventional reservoirs.

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