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

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 H-1 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.

Automated summary

This article discusses the importance of typifying and quantifying organic matter and fluids in shale rocks for the characterization and evaluation of unconventional oil reservoirs. The use of high field H-1 nuclear magnetic resonance is introduced as a fast and reliable technique with minimal sample requirements. A modified sequence is proposed to improve the accuracy of data extrapolation for T-2 determination. The study also examines the evaporation kinetics of heptane in shale mudstones and identifies a critical level at which a transition from a funicular to a pendular regime occurs.