Quantification of Imbibed Heptane in Shale Rocks Determined by Edited T₁–T₂ Nuclear Magnetic Resonance Relaxation Experiments at High Magnetic Field

Abstract

Typification and quantification of organic matter and fluids in shale rocks are prime topics for the characterization and geochemical evaluation of unconventional oil reservoirs. High field 1 H nuclear magnetic resonance is a fast and reliable technique that requires a small amount of sample. The acquisition of T 1 −T 2 relaxation correlation maps provide a rich variety of information, where signals arising from solid organic matter may be clearly distinguished from that of water or hydrocarbon. However, quantification is hampered due to the finite echo times used in the multipulse sequence used to determine the T 2 distribution, where even for the shortest available echo times partial relaxation is unavoidable. In this work, we apply a variation of a sequence developed for fluid typing at low fields in conventional reservoirs to shale mudstones at a high magnetic field. The addition of a Hahn echo with variable echo time applied before the T 2 determination renders a data set from which extrapolation to time zero is more accurate than with previous methods. We applied this sequence to a sample saturated with a varying degree of heptane and compare our results with gravimetric ones. Furthermore, the evaporation kinetics show a change from a funicular to a pendular regime when the amount of heptane evaporating from inorganic pores reaches a critical level.

NMR Measurements for Gas Adsorption Characterization on Shale: State of the Art and Perspectives

“…It is assumed that the fast-relaxing components in the shale sample are less affected by the internal gradient, so the FID signal still reflects its intrinsic relaxation time . IR-solid-echo , and IR-magic-echo are used to better recover the magnetization of the solid component which dephases through homonuclear dipolar coupling, while others choose to filter the fast-relaxing solid signals using an extra Hahn echo or binomial pulse . Since the T 1 measuring part can be very time-consuming, T 1 *– T 2 * and PIR-CPMG sequences are proposed to accelerate the T 1 encoding process.…”

Application of Gaussian and Exponential Inversion for 2D Relaxation Maps in Source Rock Oil and Gas Reservoirs

“…20,29 For even higher magnetic fields, ranging from 100 to 400 MHz, the separation of signals becomes more complex as the relaxation times of both fluids and solids decrease significantly. 30,31 Another approach is the acquisition of a free induction decay (FID) following a single excitation pulse, which is the most direct manner of acquiring a signal decay. In this case, the FID relaxation time T 2 * is composed of both the intrinsic system's dynamics and the dephasing due to magnetic field inhomogeneities.…”

“…For low-field instruments, with 1 H resonant frequencies of 2 MHz, the resonant circuit ring down after the radiofrequency pulses is a stringent condition on the echo time, which is around 100–200 μs, thus blind to solid-like signals . For higher magnetic fields such as 20 MHz, solid and liquid signals have been successfully detected. , For even higher magnetic fields, ranging from 100 to 400 MHz, the separation of signals becomes more complex as the relaxation times of both fluids and solids decrease significantly. , …”