Multiscale characterization of the impacts of geochemistry and reservoir conditions on the wettability of kerogen and organic-rich mudrocks




Jagadisan, Archana

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The geochemistry of organic-rich mudrock reservoirs impacts the interfacial properties of the pores and the distribution of fluids in the pore space. It can also affect borehole geophysical measurements such as resistivity logs, multiphase fluid flow, and hydrocarbon recovery. A significant fraction of organic-rich mudrocks is composed of kerogen (i.e., insoluble organic matter), and therefore, the properties of kerogen can considerably affect the mudrock properties. The chemical structure and composition of kerogen can vary as a function of kerogen type and thermal maturity, due to which the oxygen, hydrogen, and aromatic carbon contents are affected. Kerogen is often considered as hydrocarbon-wet in reservoir characterization. However, the impact of geochemistry on the wettability of kerogen is not fully understood and quantified. In addition, the moisture content in kerogen, which can vary with kerogen geochemistry, can decrease the methane adsorption capacity and lead to unreliable gas-in-place estimates. Changes in reservoir temperature also alter surface interfacial forces and affect kerogen properties. Furthermore, hydrophilicity and hydrophobicity of pore surfaces impact the fluid flux and apparent permeability. Fundamental research is yet needed to quantify the impacts of kerogen geochemistry and reservoir conditions on the physical properties, such as wettability and adsorption capacity, and dynamic properties such as fluid flux and permeability. In this dissertation, I combine experimental and molecular simulation-based methods to understand the impacts of geochemistry and reservoir temperature on the wettability of kerogen and organic-rich mudrocks, methane and water adsorption capacities of kerogen, and fluid flux in kerogen pores. The results showed that the water wettability of kerogen varies with thermal maturity, kerogen type, and temperature. Kerogen was found to be water-wet at low thermal maturity, and the water wettability decreases with increasing thermal maturity. The kerogen wettability had a measurable impact on the wettability of organic-rich mudrocks, where the water wettability of organic-rich mudrock samples decreased with an increase in the thermal maturity of kerogen. The adsorption capacity of kerogen was impacted by kerogen geochemistry and molecular structure, where methane adsorption capacity increases with an increase in thermal maturity, and water adsorption capacity decreases with an increase in thermal maturity. Finally, an increase in kerogen hydrophilicity is shown to decrease the apparent permeability of water in kerogen pores. These results highlight the importance of considering the geochemistry of organic content in formation evaluation and reservoir modeling for the reliable estimation of static and dynamic reservoir properties in organic-rich mudrocks. The outcomes of this dissertation can also potentially provide insights into water production concerns through an understanding of fluid-flow mechanisms in organic-rich mudrocks.


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