Two dimensional (2D) Nuclear Magnetic Resonance (NMR) measurements can enhance the quantification of petrophysical properties in organic-rich mudrocks. However, the interpretation of these measurements is still challenging as the mechanisms of proton surface relaxation in organic pores are not well understood. The objectives of this paper are to verify the existence of different coupling mechanisms using 2D NMR responses, to develop a new surface relaxivity model, and to apply the new model for simulation of NMR responses in organic-rich mudrocks. The first step includes extracting pure kerogen from different samples of organic-rich mudrocks. Then we perform transverse relaxation (72), longitudinal and transverse relaxation (T1-T2), and T2-T2 measurements on the extracted kerogen samples saturated with polar (water) and non-polar (decane) solvents. We use the results of NMR measurements to confirm the prevalence of both inter- and intra- molecular dipolar coupling in kerogen-fluid interactions. We develop a new relaxivity model by including the effects of the aforementioned coupling mechanisms, internal molecular motion, and surface averaging of relaxation time on kerogen surface relaxivity. The new surface relaxivity model is used to simulate NMR T2 measurements in a pedagogical ellipsoidal pore and in pore-scale images of organic-rich mudrocks using our previously developed three dimensional (3D) finite-volume numerical simulator. The results of NMR simulations are used to quantify the sensitivity of different coupling mechanisms on NMR T2 measurements and NMR-based estimates of adsorbed hydrocarbon volume.
We performed NMR measurements on two kerogen samples at different thermal maturities, saturated with water and decane. T2 measurements in the kerogen samples reveal multiple peaks with dominant T2 values between 10–250 milliseconds for both decane and water. The dominant peak in T1-T2 measurements present T2/T2 ratios of up to 7 demonstrating the impact of intra-molecular dipolar coupling on surface relaxation in kerogen pores. The T1-T2 measurements yield up to 4 off-diagonal peaks indicating the influence of spin diffusion and inter-molecular dipolar coupling during surface relaxation in organic pores. The results of numerical simulations demonstrate that misidentifying the coupling mechanisms could cause a relative error of up to 80.6% and 24.2% in the estimation of adsorbed hydrocarbon volume in ellipsoidal pores and mudrock samples, respectively. The outcomes of this paper can potentially enhance the interpretation of NMR responses in organic-rich mudrocks by quantifying the influence different coupling mechanisms on kerogen surface relaxivity, NMR T2 measurements, and NMR-based assessment of adsorbed hydrocarbon volume. The developed surface relaxivity model is more reliable than the previously published relaxivity models as it includes the effects of both inter- and intra-molecular dipolar coupling on surface relaxation in organic pores. The new model can be extended for quantifying surface relaxivity at higher temperatures and fluid viscosities which enables interpretation of NMR logs in-situ condition. Enhanced quantification of surface relaxivity also enhances NMR-based reservoir characterization, and helps to improve estimates of hydrocarbon reserves in organic-rich mudrocks.