Core And Log Nmr Measurements Of An Iron-Rich, Glauconitic Sandstone Reservoir

NMR porosity and relaxation time measurements from an iron-rich. glauconitic sandstone reservoir show quantifiable effects of mineral iron content on NMR T2 relaxation times. This result has significant impact upon measuring irreducible water pore volume where the surface relaxation mechanism is nonconstant. Centrifuge air/brine drainage capillary pressure measurements show that the standard 30 msec T2 cutoff must be lowered to calibrate irreducible water saturation computed from NMR. Although the effects of iron are observable on T2 distributions, permeability estimation from NIMR, using either the Coates or Schlumberger relationships, show excellent agreement to permeability on core plugs. Quantitative mineral composition on core plugs using both XRD and XRF, show iron-rich glauconite to vary from 3 to 31 weight percent. The bulk rock total iron oxide content ranges from 1 to 17 weight percent. High iron content within this reservoir raised concern that NMR surface relaxation would be affected, leading to errors in irreducible water saturation and producible porosity derived from NMR measurements. NMR measurements were acquired using a pulsed field gradient logging tool operating at 530 kHz and on core plugs with a 1000 kHz laboratory spectrometer. Homogenous field NMR core plug measurements are used to show the accuracy of the logging tool to measure NMR porosity, and permeability.