Nuclear magnetic resonance relaxation measurements in shales
Martínez, Gabriela Alejandra
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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 (NMR) 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 estabhshes that shale petrophysical information is accessible using standard NMR lab techniques employed in the oil industry. Even for shale seals, porosity and pseudo-capillary 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. About fifi;een sets of samples were analyzed in this research. The samples were mainly black shales composed of clays, quartz, with minor proportions of feldspars and calcite. Two different saturation techniques were employed to restitute the fluid content of the shales. Since the saturation driven by pressure difference fragmented some of the samples, a controlled saturation in a humidity chamber was performed to ensure the integrity of the shales. NMR measurements were performed in a low field spectrometer primarily on the shales characterizing seals. Multiexponential T2 and Ti relaxation rates were determined from CPMG and inversion recovery experiments, respectively. Mean log T2 values of the untreated samples were approximately 0.4 milhseconds. The T2 relaxation times increased afl;er the shales were saturated but remained below 2 milhseconds. 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 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. The data displayed a power law relationship with respect to the capillary pressure obtained by mercury injection.