Quantification of surface and bulk fluid induced proton relaxation mechanisms for low field nuclear magnetic resonance

Abstract

Characterization of downhole formation properties by interpreting physical responses of logging tools is the basis of petrophysical studies. Amongst the wide array of instruments utilized in these studies, nuclear magnetic resonance (NMR) is the only measurement capable of determining rock pore size distribution, formation porosity, and distinguishing between fluid type and relative fluid mobility in the subsurface environment. NMR is also performed in the laboratory to help constrain parameters from downhole measurements at higher signal to noise ratios and finer resolution than logging tools. These measurements are complicated by temperature-pressure effects of in-situ fluids and fluid interaction with complex grain surfaces that affect proton relaxation in unconventional reservoir systems. In this thesis, I measured low-field (2 MHz) NMR longitudinal (T1) and transverse (T2) proton (hydrogen) responses in fluid-saturated unconventional formations with complex mineralogy and on bulk fluids under pressure. I first calculate surface relaxivity in shales for relevance with increased mineral variety and complications from differing thermal maturities in organic-rich formations. Surface relaxivity calculations show that NMR analysis is improved by using appropriate laboratory methods to determine surface area and volume of the pore space dependent on mineralogy for comparison with NMR derived pore size distribution. I find that the level of paramagnetism, the concentration, and the distribution of minerals affect surface relaxivity. I then implement T1-T2 mapping to determine fluid mobility and fluid-surface interaction for effects on NMR response of representative mineralogies in the Permian Basin unconventional petroleum system. Fluid saturation history and formation wettability result in differing moveable fluid cutoff values in the same formation. These values are below commonly used values from literature with mobile fluids at NMR times as low as 0.1 ms. I find that T1-T2 ratio is not effective at distinguishing fluid type in low viscosity crude oil-brine systems in fast relaxing formations. Then I quantify the effects of paramagnetic oxygen on NMR relaxometry in both bulk fluid and surface relaxation mechanisms. Enhanced relaxation occurs in pressurized NMR experiments with oxygen presence, resulting in relaxation times in bulk fluids that can be as fast as confined pores in unconventional formation types. Variations of 61% in T2LM of bulk water can occur depending on global location, and variations in pore relaxation of up to 36% in simple porous media can occur with changes in O2 partial pressure of less than 3 psi. I establish a quantitative relationship between O2 concentration in solution and NMR relaxation times. This empirical relation allows for more accurate predictions of pore space properties using low-field NMR relaxometry in laboratory environments. My results show the effects of surface and bulk fluid relaxation on NMR response for complex systems and correct for oxygen introduced relaxation in the laboratory environment. Paramagnetic impurities in the form of surface clay minerals or oxygen in solution result in wide variation of NMR times on time scales that overlap. Since these impurities are often both present in laboratory studies of porous media, characterization of surface and bulk fluid effects is needed for accurate petrophysical analysis using NMR.