The Latest Developments of Laboratory NMR Techniques in Unconventional Shale Characterization
Authors:   Core Laboratories LP, Xie Z. Harry , Reffell Omar ,   Core Laboratories LP
Published: November 2024

The complexity of the microstructure and fluids in unconventional reservoirs presents a significant challenge to the traditional approaches to the evaluation of geological formations and petrophysical properties due to the low porosity, ultralow permeability, complex lithology, and fluid composition. Nuclear magnetic resonance (NMR) techniques have been playing major roles in unconventional shale characterization in the last decades as NMR can provide critical information about the reservoirs for quantifying their petrophysical parameters and fluid properties and estimating productivity. Laboratory higher frequency (HF), e.g., 23-MHz NMR techniques, especially two-dimensional (2D) T1-T2 mapping, and applications have been essential for the noninvasive characterization of tight rock samples for identifying kerogen, bitumen, heavy or light hydrocarbons, and bound or capillary water. The traditional T2 cutoffs need new definitions to reflect the inferences from water and hydrocarbons separately. The legendary crushed rock analysis method, as applied to unconventional formations, has shown great success in evaluating total porosity and water saturation but suffers from inconsistency in results due to desiccation and solvent effects. The industry has witnessed significant development of HF-NMR techniques that couple advances in petrophysics, petroleum engineering, and geochemistry with a broad range of applications. This article will summarize key advances in laboratory NMR applications in unconventional shale characterization, including monitoring processes of liquids equilibrium, desiccation, and imbibition in fresh shale samples, determination of activation energy of hydrocarbons in shales, monitoring changes in a shale sample during liquid flooding experiment, and direct measurements on kerogen. Future NMR applications, such as in EOR, gas condensation, and saturation profile, will be discussed. In this article, we will review the following techniques: laboratory NMR, especially 23 MHz, techniques for unconventional shale sample measurements, 2D NMR T1-T2 mapping with an inter-echo spacing time of 0.7 ms at various sample temperatures, and early-time NMR signals acquired using the solid-echo pulse sequence together with pyrolysis results in kerogen studies. We will review the necessaries and advantages of the HF-NMR techniques and applications, explain the importance of the inter-echo spacing time in shale NMR experiments, give examples of monitoring liquid redistribution and desiccation in fresh shale samples, compare the results from NMR, Dean-Stark, and pyrolysis, examine the multidiscipline approaches as better tools to solve petrophysical problems. Examples include kerogen study using the solid-type NMR measurement combined with geochemical pyrolysis and NMR combined with capillary pressure. There has been significant advancement of NMR techniques in unconventional shale evaluation over the last decade. It is time for us to summarize such technological advances and draw conclusions to help us in planning unconventional core analysis programs.