Permeability Model Calibration and Pore Throat Radius Determination Using Core Analysis and Nuclear Magnetic Resonance Data in Mixed-Lithology Reservoirs, South Western Venezuela

Abstract

Abstract In complex geological sequences composed by clastic and carbonatic rocks, where the matrix can be formed by several minerals, such as calcium carbonate, silica and magnesium carbonate, the determination of reservoir properties and rock quality represents a major challenge. Using conventional logs to calculate total and effective porosity in this type of reservoirs can lead to underestimation or overestimation of pore space in the rock and do not provide information about pore size and porosity distribution associated with irreducible water saturation that possibly will impact of reservoir permeability. To address these problems the response of the Nuclear Magnetic Resonance Log was calibrated to the petrophysical and geological properties measured in core. Correlation models were generated for total porosity, permeability and pore throat radius; which could be used in future wells of the area, where core data is not available. A relation between the predominant pore throat radius calculated with the Pittman R45 equation (1992) and the irreducible water saturation from the Nuclear Magnetic Resonance Log was obtained. From this analysis five petrofacies were obtained: megaporous, macroporous, mesoporous, microporous and nanoporous, being predominant the macroporous, mesoporous, and the nanoporous. Total Porosity from the Nuclear Magnetic Resonance correlates very well with the total porosity data from core analysis. To calibrate permeability values derived from NMR data two different T2 cutoffs and Coates-Timur equation constants were used. Showing a relation with sedimentological facies described in macroscopic whole core analysis. Seven sedimentary facies type were identified: Limey Sand, Sandy Limestone, Glauconitic Sand, Muddy Sand, Fine-grained Sand, Coarse-grained Sand and Shale. The best types of rocks (megaporous and macroporous) correspond to the best facies (Fine-grained Sand and Coarse-grained Sand). Limey Sand, Muddy Sand and Sandy Limestone were identified in mesoporous, microporous and nanoporous rocks, and most of the glauconitic sand intervals correspond to microporous and nanoporous petrofacies.