Petrophysical Measurements on Shales Using NMR

Abstract Shales are important constituents of petroleum systems, and it is necessary to study their petrophysical properties as both reservoir components and as seals. Nuclear magnetic resonance has proven to be a good technique for measuring the reservoir engineering properties of rocks. This paper presents measurements of NMR relaxation in shales. It establishes that shale petrophysical information is accessible using standard NMR lab techniques employed in the oil industry. Even for good seals, shale porosity and pseudocapillary pressure curves can be derived from NMR relaxation data. This opens the question as to whether NMR logging can be used to ascertain seal quality for oil and gas storage reservoirs and for CO2 disposal reservoirs. NMR measurements were performed in a low field spectrometer primarily on shales characterizing seals. Multiexponential T2 and T1 relaxation rates were determined from CPMG and inversion recovery experiments, respectively. Mean log T2 values of the undersaturated samples were about 0.4 ms. The T2 relaxation times increased after the shales were saturated but remained below 2 ms. Porosity for each sample was derived from the T2 magnetization and calibrated against a standard. Since NMR detects total porosity, the porosity was generally larger than that determined from laboratory gas flow-saturation techniques. The T2 relaxation rate distributions were normalized using the total magnetization calibration and integrated from the larger times to yield pseudo-capillary pressure curves.