Investigation of Physical Properties of Hydrocarbons in Unconventional Mudstones Using Two-Dimensional NMR Relaxometry

Understanding of organic matter properties is crucial in characterization of unconventional plays. It is always a challenge for petrophysicists to differentiate and quantify mobile and immobile hydrocarbons in unconventional mudstones. High frequency (22MHz) NMR for unconventional rock core analysis has gained industry acceptance for its high efficiency and high sensitivity to measure very small volumes of fluids and solid hydrocarbons in tight rocks. Previous work has revealed that one-dimensional (1D) NMR T₂ method is insufficient to study organic matters in fresh core samples due to overlapping T₂ signals from both hydrocarbons and water. Co-existence of structurally bound water and solid hydrocarbons in shales leads to short T₂'s in microseconds range, and further complicates the situation. In this work, we present the first detailed analysis method using the two-dimensional (2D) NMR T₁-T₂ mapping techniques to study physical properties of hydrocarbons in various shale rocks. Combined NMR pulse sequences were used to acquire signals from solids and liquids that contain hydrogen. The 2D T₁-T₂ maps were further processed by removing the map regions which are from water to obtain 1D T₂ distributions for hydrocarbons only. Measurements on mudstone core samples at various temperatures, from 22 °C to 90 °C, show that the relaxation time T₂'s of hydrocarbon components increase with temperature due to molecular mobility increasing but at different rates, implying that hydrocarbons present in different environments (e.g., organic and inorganic pores) within these tight core samples are undergoing different thermal dynamics processes. T₂ of each hydrocarbon component as a function of temperature were analyzed to calculate the activation energy (Ea) based on the Arrhenius equation for molecular kinetics, and producibility is then estimated accordingly. This NMR method provides petrophysicists with a powerful way to study hydrocarbons that are confined in organic matter such as bitumen and kerogen to understand the mechanism of enhanced oil recovery (EOR) in unconventional reservoirs. Furthermore, results from such NMR relaxometry analysis and multiple-heating rate pyrolysis indicate that the combined techniques are very promising for investigating producible estimates from the free/adsorbed hydrocarbons present in source-reservoir intervals. Such an alternative approach leads to an analytical protocol for the determination of NMR cut-offs to quantify mobile and immobile hydrocarbons.