Effect of paramagnetic mineral content and distribution on nuclear magnetic resonance surface relaxivity in organic-rich Niobrara and Haynesville shales

Surface relaxivity is required to convert nuclear magnetic resonance (NMR) relaxation times to pore size distributions (PSD). In current practice, a constant value of surface relaxivity is used for an entire well, formation, or rock type, regardless of compositional and textural variations. The presence and distribution of paramagnetic minerals can affect surface relaxivity and, in turn, result in significant errors in PSD calculations. We present surface relaxivity calculations for two organic-rich formations, namely the Niobrara and the Haynesville formations. Surface relaxivity was calculated from surface to volume ratio and comparison of average pore radius measurements. We measured the transverse (T-2) relaxation time with a low field (2 MHz) NMR instrument, porosity and PSD using nitrogen adsorption (N-2) data, total specific surface area from N-2 data and from cation exchange capacity (CEC), and mineralogy from X-ray diffraction (XRD). We find that surface relaxivity is linearly correlated with paramagnetic clay content, specifically chlorite and illite-smectite distributed in the matrix. The presence of smectite clay minerals increases uncertainty in surface to volume ratio and consequently in surface relaxivity calculations. Our surface relaxivity - clay correlations could be applied for surface relaxivity calculations from mineralogy and to invert NMR logs and laboratory data to PSD.