Improved Analysis of NMR Measurements in Organic-Rich Mudrocks Through Quantifying Hydrocarbon-Kerogen Interfacial Relaxation Mechanisms
- Saurabh Tandon (The University of Texas at Austin) | Zoya Heidari (The University of Texas at Austin)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- SPWLA 60th Annual Logging Symposium, 15-19 June, The Woodlands, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2019. held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors
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Nuclear Magnetic Resonance (NMR) measurements have become a popular choice for estimating hydrocarbon saturations in organic-rich mudrocks. Previous publications have shown that the dominant mechanism for surface relaxation during NMR measurements in organic pores is intra-molecular dipolar coupling among hydrocarbon protons. However, the influence of kerogen-hydrocarbon inter-molecular interactions and kerogen thermal maturity on the surface relaxivity has not been reliably quantified. The objectives of this paper are to (i) experimentally quantify the influence of inter-molecular coupling on kerogen surface relaxivities, (ii) compare the experimentally determined surface relaxivities with those obtained from our previously published analytical model, and (iii) quantify the impact of inter-molecular coupling on estimates of adsorbed hydrocarbon phase 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°C 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 thermally mature kerogen samples were then saturated with protonated and partially-deuterated chloroform mixtures. Afterwards, we performed longitudinal (T1) and transverse (T2) measurements on the kerogen-chloroform mixtures. Then, we compared the surface relaxivity values estimated from T1-T2 measurements to those predicted by a theoretical model derived from generalized adsorption theory. Finally, we performed a sensitivity study demonstrating the impact of inter-molecular dipolar coupling on estimates of adsorbed hydrocarbon volume by modeling kerogen pores as simple synthetic spherical objects.
The results indicate that synthetic maturation of kerogen samples increased their specific surface areas relatively by up to 97.1%. When chloroform deuteration is kept constant and kerogen samples were heat-treated from temperatures of 25°C to 450°C, the T1 and T2 surface relaxivities relatively decreased by up to 70.1% and 80.3%, respectively. The previously derived analytical model was able to reliably quantify the kerogen surface relaxivities estimated from experimental measurements with relative error of less than 30%. The results of the sensitivity analysis showed that improved assessment of kerogen surface relaxivity by including inter-molecular coupling enhanced the NMR-based adsorbed hydrocarbon volume estimates relatively by up to 28.1% when kerogen pores were modeled as spherical objects. The results of experimental measurements support the observations of the analytical developed surface relaxivity model derived from generalized adsorption theory. Accurately quantifying the mechanism contributing to surface relaxation helps in providing accurate temperature corrections for T2 and T1-T2 cut-off values. Such cut-off values can then be extended to insitu conditions improving down-hole estimates of NMR-based hydrocarbon saturations in organic-rich mudrocks.
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