Applications of Two-Dimensional Laboratory Higher-Frequency NMR in Unconventional Shale Characterization

The complexity of the microstructure and fluids in unconventional reservoirs presents challenges 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 NMR techniques at higher frequency (HF), e.g., 23 MHz, especially two-dimensional (2D) T1-T2 mapping, and their 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. Traditional T2 cutoffs, established with low frequency (LF) NMR, no longer apply and need new definitions to reflect the inferences from water and hydrocarbons separately. The crushed rock analysis method, as applied to unconventional formations, has been successful in evaluating total porosity and water saturation but also suffers from inconsistency in results due to desiccation and solvent effects. In the past decade, the oil and gas industry has witnessed significant development of HF NMR techniques that couple advances in petrophysics, petroleum engineering, and geochemistry with a broad range of applications. It is necessary to review such technological advances and draw conclusions to benefit unconventional core analysis programs. 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 experiments, and direct measurements on kerogen.