Two-dimensional nuclear magnetic resonance (NMR) T1-T2 maps are fast becoming the industry standard for fluid typing in unconventional shale rocks due to their sensitivity to molecular mobility (Kausik et al., 2011, 2014, 2016; Rylander et al., 2013). The increasing mobility of the different components of unconventional plays—ranging from solid kerogen to the fluid components of viscous bitumen, clay-associated water, oil in oil-wet organic pores to fluids (oil and water) in the mixed-wet inorganic pores and natural fractures—is measured by this methodology to determine the fluid types and their confining environments for the construction of universal 2D maps of different wells. One of the biggest challenges for the universal application of this methodology is that the impact of variation in the temperature between different basins, wells, or even multiple depths within a well, on the 2D NMR T1-T2 maps needs to be well understood.
The main objective of this paper is to understand the changes in molecular mobility of the different fluids in shale rocks as a function of temperature and their influence on 2D NMR T1-T2 maps. For this purpose, we performed NMR relaxation experiments on the extracted bulk components of shale rocks, such as kerogen, bitumen, and light oil, and also investigated them under confinement, such as bitumen and oil in organic kerogen pores, oil and water in inorganic pores, other than clay-associated water. This enabled us to obtain a universal picture of the different fluids in both bulk state and under oil-wet or water-wet confinement, with different pore sizes and surface relaxivity.
The multi-temperature NMR relaxometry experiments were conducted on both low- and high-frequency NMR systems to enable comparison with the logs and to obtain the highest-resolution data, respectively. It has been demonstrated that the relaxation-time dependence of light oil is proportional to viscosity over temperature. The temperature dependence of bitumen or heavy viscous oil relaxation times is proportional to viscosity over temperature with a power law of –0.45. Both the heavy oil and light oil in the oil-wet organic pores of the kerogen show a much weaker dependence on temperature. This can be explained by the fact that fluids in the wetting environments are constrained in their motions by the surface interactions (and residence times), and if these are not significantly changed by increases in temperature. We also compare this relaxation behavior with that of the oil or water in mixed-wet inorganic matrix, where a much stronger temperature dependence due to the lower reduction in the bulk phase mobility can be observed.
In conclusion, we investigated the 2D NMR T1-T2 response of different shale rock components such as bitumen, oil, and water in a bulk state as well as in oil-wet (organic kerogen) or mixed-wet (mineral matrix) confinements. The hybrid Bakken petroleum system was used as the template for these experiments, with the behavior of the upper Bakken organic mudstone interval contrasted with that of the middle Bakken inorganic matrix. Based on these studies we propose a more universal understanding of fluid typing based on 2D NMR T1-T2 logging.
