Kausik, Ravinath (Schlumberger-Doll Research) | Freed, Denise (Schlumberger-Doll Research) | Fellah, Kamilla (Schlumberger-Doll Research) | Feng, Ling (Schlumberger-Doll Research) | Ling, Yanchun (Schlumberger-Doll Research) | Simpson, Gary (Independent Consultant)
Two-dimensional nuclear magnetic resonance (NMR) T1-T2 maps are fast becoming a routine methodology for fluid typing in unconventional shales due to their sensitivity to molecular mobility. The differences in the 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 the need for an understanding of the impact of the variation in frequency on the applications of different wireline, LWD, wellsite and laboratory tools which work at different Larmor frequencies, and of the variation in temperature between different basins, wells, or even multiple depths within a well. The main objective of this paper is to understand the changes in molecular mobility of the different fluids in shales as a function of temperature and their influence on 2D NMR T1-T2 maps measured at different frequencies.
For this purpose, we performed NMR relaxation experiments on the extracted bulk components of shales, 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. The experiments were carried out as a function of frequency, ranging from 10 kHz to 20 MHz, and temperature, ranging from 30 to 90ºC. This enabled us to obtain a fundamental understanding of the 2D NMR T1-T2 maps for different fluids in both the bulk state and under oil-wet or water-wet confinement, with different pore sizes and surface relaxivity. The theoretical model we present shows how the frequency dependence and temperature dependence arise from the same description of the molecular motions of the fluids.