(640d) Investigations of the Nuclear Magnetic Relaxation Rates and Structural Features of Kerogen

This study elucidates correlations between nuclear magnetic resonance relaxation rates and kerogen structure or composition. The extraction of hydrocarbons from unconventional reservoirs depends on the pore network and chemical composition of complex sedimentary organic formations. Of main interest, kerogen is the insoluble organic portion in shale rocks, majorly accounting for pores and adsorption sites for the trapped hydrocarbons. Therefore, the properties of kerogen may dictate fluid behavior and are especially relevant for the determination of production capabilities. The petroleum output window in shale reserves is generally estimated by the vitrinite reflectance (% VR_o) of the source rock. In addition to being restricted to the presence of vitrinites, it may be insufficient to probe the complexity of the kerogen structure. On-field reservoir potential can be determined via Nuclear Magnetic Resonance (NMR) logging, where porosity and several other characteristics are investigated using a fluid distribution map. Recent technological advancements have contributed to signal resolution. However, given the complex mixture of fluids and solids, calibration is required to accurately interpret NMR logging data.

A convenient and non-destructive technique, NMR relaxometry can reveal the mechanisms of molecular mobility based on the behavior of certain nuclei, such as ¹H. In NMR experiments, system properties are investigated via characteristic relaxation times T₁ and T₂, associated with the magnetization recovery in the longitudinal axis and the loss of coherence in the transverse plane. In bulk fluids, the ratio of T₁/T₂ ≈ 1, but in fluid-saturated media, confinement effects and fluid-surface interactions lead to enhanced relaxation rates. Since T₂ is more sensitive to changes in translational and rotational molecular motion, it is commonly chosen for relaxometry studies. Given the procedural equivalency to NMR logging, relaxometry information can be directly used as an input to interpret reservoir maps. Under adequate diffusion conditions, a proportionality coefficient named surface relaxivity (ρ₂) can be derived from transverse relaxation rate data. Thus, ρ₂ indicates the characteristic surface-solvent affinity and may be used by petrophysicists to obtain the pore size distribution (PSD) in unconventional reservoirs. Using lab-based experiments, ρ₂ can be obtained for a variety of well-controlled kerogen and other carbonaceous samples. Upon meticulous sample characterization, the ρ₂ values obtained from particle suspensions can be utilized for PSD characterization in NMR logging.

In the present study, the chemical and physical properties of isolated type-II (marine) kerogens (% VR_o 0.55 – 2.75) were determined using a series of well-established characterization techniques. Through ¹³C Solid State Nuclear Magnetic Resonance (SS-NMR), the sp²/sp³ carbon hybridization ratios were found to consistently increase with % VR_o, in agreement with the expected transition to graphitic structures as kerogen matures. Additionally, X-ray Photoelectron Spectroscopy (XPS) trends indicate a decrease in the heteroatom content, especially oxygen, at both the kerogen surface (5 nm depth) and after ion milling (50 nm depth). Those observations are supported by Fourier Transform Infrared Spectroscopy (FT-IR), where an inverse trend was observed for the intensity of characteristic wavenumbers associated with aliphatic and aromatic C-H bonds. Peaks associated with oxygen-bearing moieties were also found to lose intensity as maturation progresses. Lastly, a positive trend between specific surface area and thermal maturity was observed using nitrogen adsorption. Notably, the volume associated with small pore diameters, e.g. ~20 Å, was found to increase with respect to maturity. Characterization of structure / composition trends provided a solid base for the interpretation of the NMR relaxometry data.

The transverse NMR relaxation rates were collected for kerogen and reference materials of known structure suspended in several solvents using a benchtop low-field NMR instrument at 0.47 T and 20 MHz for ¹H (Bruker, Germany). The reference materials were chosen to assess the influence of a variety of functional groups commonly reported for kerogen. Additionally, the chosen solvents may serve as a probe for the surface interactions of kerogen with polar fluids (water and acetone), normal alkanes (n-decane), and aromatics (toluene). Even though particle suspensions of polymers could not be obtained for acetone and toluene, the relaxivity trends of water and n-decane indicate a predominant influence of heteroatoms (N, O) in enhanced solvent relaxation. This behavior can be attributed to strong intermolecular interactions, e.g. hydrogen bonding, promoted by the higher electronegativity of N and O, in comparison to C. For the case of kerogen, the observed ρ₂ values suggest that the relaxation phenomenon of each tested fluid is subjected to several kerogen properties. Heteroatom content predominantly influenced the relaxivity of acetone and toluene, with a certain degree of agreement with the control samples. However, the responses from water and n-decane were found to vary along with the tested maturity ranges. Therefore, the conventional vitrinite reflectance should be examined with caution when drawing conclusions related to hydrocarbon adsorption strength.

In summary, NMR logging characterization can indicate important characteristics of oil and gas reserves using relaxation time maps but require calibration for increased accuracy. Here, the surface-solvent interactions, a fundamental aspect of saturated porous media, were investigated. The chemical and physical characterization of kerogen samples at different maturities was accomplished via a supplementary multi-technique approach. Then, using NMR transverse relaxometry studies, their surface relaxivities were obtained for different solvents and compared with carbonaceous materials of known surface chemistry. Common factors, such as the presence of heteroatoms, were found to influence the observed relaxivities. However, solvent relaxation enhancement in kerogen may be part of a more complex set of interactions and will be investigated in further studies.