Relationships between observed pore and pore-throat geometries, measured porosity and permeability, and indirect measures of pore volume by nuclear magnetic resonance

摘要

Carbonate reservoirs are a network of pores and connecting pore-throats thatcontain at least half of the world's oil. Genetic classification of carbonate pores enablesone to map the pore types that have greatest influence on reservoir performance.Though NMR logging has been used to estimate pore sizes, it has not been used toidentify genetic pore types or to aid in determinations of reservoir quality for differentpore assemblages.Five genetic pore types identified in 40 carbonate and 7 sandstone samples weresubjected to NMR measurements. Results reveal close correspondence between NMRderivedpore volumes and 2-D pore size and shape gleaned from petrographic imageanalysis. Comparisons of real and synthetic pore shapes showed that shapes of all poretypes in the medium size range of 0.02-0.5mm can be reliably compared with syntheticvarieties, but such comparisons were unreliable for vuggy pores smaller than 0.5mm. T2relaxation times for depositional pores exhibit low amplitude, narrow wavelengthresponses. Moldic pores produced medium amplitude, asymmetrical wavelength responses, and intercrystalline pores show high amplitude, narrow wavelengthresponses. NMR-derived pore volumes on pores with ferroan dolomite interiorsunderestimated pore diameter by up to 3 orders of magnitude. Calculated pore-throatsizes from MICP data correlate strongly with measured permeability.Samples with high, intermediate, or low poroperm values displayed characteristicT2 curves confirming that reservoir quality can be estimated from NMR measurements.Future work is expected to show that NMR logging can estimate reservoir quality atfield scale and aid in mapping flow units in compartmentalized reservoirs.