Interpretation and numerical modeling of Nuclear Magnetic Resonance in complex reservoirs




Tandon, Saurabh

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Interpretation of Nuclear Magnetic Resonance (NMR) measurements in complex rocks such as shaly sands, mixed-wet rocks, and organic-rich mudrocks has been a challenge for petrophyscists. The major challenges in interpretation of NMR responses in such complex rocks include complex rock composition, presence of clay minerals and organic content such as kerogen, and uncertainties about proton relaxation mechanisms in kerogen and clay minerals. These challenges lead to extensive need for calibration efforts for reliable interpretation of NMR measurements for assessment of petrophysical properties. In this research, I use a combination of experimental and simulation-based approaches to tackle the aforementioned challenges. First, I modify an existing finite-volume simulator to numerically solve Bloch-Torrey equations. This simulator takes pore-scale images of rocks and bulk and surface properties of fluids and grains as inputs. This simulator outputs magnetization decays obtained for different pulse sequences. I apply this simulator to model NMR responses in synthetic and actual pore-scale images of sandstones and carbonates. Next, I use this simulator to quantify the influence of shale topology and internal magnetic gradients on NMR responses obtained in shaly sands. To improve NMR interpretation in mixed-wet rocks, I develop a new NMR-based wettability index. This new index can be applied to mixed-wet rocks with complex pore-structures at any fluid saturation level. I validate this new wettability index in pore-scale and core-scale domains using the developed numerical simulator and experimental measurements documented in a recent publication. Furthermore, I develop a new kerogen surface relaxivity model that accounts for different coupling mechanisms in kerogen pores. I use experimental measurements on kerogen isolates to validate this surface relaxivity model and quantify the influence of inter-molecular coupling on kerogen surface relaxivities. Finally, I develop a new model to quantify clay-bound water relaxation that assimilates the influence of hydrated cations and surface-bound water molecules. The newly-developed kerogen and clay relaxation models are used as inputs for modeling of NMR responses in pore-scale images of organic-rich mudrocks at different reservoir temperatures and Larmor frequencies. The new methods proposed in this dissertation have been applied for forward modeling NMR responses in complex reservoirs. Such analytical and numerical modeling can help in quantifying the impact of experimental parameters and petrophysical properties on NMR responses which can enhance formation evaluation in complex reservoirs


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