Improved Analysis of Nuclear-Magnetic-Resonance Measurements in Organic-Rich Mudrocks Through Experimental Quantification of the Hydrocarbon/Kerogen Intermolecular-Interfacial-Relaxation Mechanism

Abstract

Summary Nuclear-magnetic-resonance (NMR) measurements have become a popular choice for estimating hydrocarbon saturations in organic-rich mudrock reservoirs. Previous publications have shown that the dominant mechanism for surface relaxation during NMR measurements in organic pores is intramolecular dipolar coupling among hydrocarbon protons. However, the influence of kerogen/hydrocarbon intermolecular interactions and kerogen thermal maturity on the surface relaxivity has not been reliably quantified. The objectives of this paper are to experimentally quantify the influence of intermolecular coupling on kerogen surface relaxivities; compare the experimentally determined surface relaxivities with those obtained from our previously published analytical model; and quantify the effect of intermolecular coupling on estimates of the adsorbed-hydrocarbon phase volume in simple geometries. First, we selected two organic-rich mudrock formations with different kerogen thermal maturities and extracted pure kerogen from them. The extracted-kerogen samples were synthetically matured by increasing the temperature at 4°C/min from 25 to 450°C under a controlled environment. The petrophysical properties of kerogen samples at different thermal maturities were quantified using pyrolysis and Brunauer-Emmett-Teller (BET) measurements. The untreated and thermally mature kerogen samples were then saturated with protonated and partially deuterated chloroform mixtures. Consequently, we performed longitudinal (T1) and transverse (T2) measurements on the kerogen/chloroform mixtures. Then, we compared the surface relaxivities estimated from T1/T2 and BET surface-area measurements with those predicted by a previously published theoretical model derived from generalized adsorption theory. Finally, we performed a sensitivity study demonstrating the effect of intermolecular dipolar coupling on estimates of adsorbed-hydrocarbon volume by modeling kerogen pores as synthetic spherical objects. Results indicate that synthetic maturation of kerogen samples relatively increased their specific surface areas by up to 97.1%. When chloroform deuteration is kept constant and kerogen samples were heat treated from temperatures of 25 to 450°C, the T1 and T2 surface relaxivities relatively decreased by up to 70.1 and 80.3%, respectively. Our recently introduced analytical model was able to reliably quantify the kerogen surface relaxivities estimated from experimental measurements with a relative error of 30.5%. The results of the sensitivity analysis showed that improved assessment of kerogen surface relaxivity by including intermolecular coupling enhanced the NMR-based adsorbed-hydrocarbon-volume estimates relatively by up to 41.9% when kerogen pores were modeled as synthetic spherical objects. The results of the experimental measurements support the observations of the analytically developed surface-relaxivity model derived from the generalized adsorption theory. Accurately quantifying the mechanism contributing to surface relaxation helps in providing accurate temperature and frequency corrections for T2 and T1/T2 cutoff values. Such cutoff values can then be extended to in-situ conditions improving downhole estimates of NMR-based hydrocarbon saturations in organic-rich mudrocks.