Abstract
Many workers have recognised the link between Nuclear Magnetic Resonance (NMR) derived T2 distributions and pore size distributions in reservoir rocks. This property has been used to develop models to obtain primary drainage capillary pressure curves from T2 distributions. These models often assume that the rocks pore space resembles a simple bundle of capillary tubes. They do not consider the existence of multiple pore body connections and pore body restrictions/throats. The most successful models utilise variable scaling factors to convert T2 times to pore diameters and hence capillary pressure. The variable scaling factor approach recognises the existence of variable surface relaxivity throughout the pore space due to variations in mineralogy and pore topography. This investigation uses SCAL data from the ART NMR Sandstone Rock Catalogue to obtain core calibrated variable scaling factors for 174 reservoir sandstone samples. The depositional environments for these samples include; aeolian, fluvial, coastal and shallow and deep marine. The samples used have a wide variety of mineralogy, diagenetic overprints and cover six orders of magnitude in absolute permeability. Three different methods for obtaining the scaling factors are presented and the relative merits of each discussed. A global model to predict capillary pressure from NMR T2 distributions in reservoir sandstones has been developed using correlations between the variable scaling factors and permeability.
Highlights
Nuclear Magnetic Resonance (NMR) measurements have been used extensively to characterise reservoir rock pore geometry using logging and lab-based NMR spectrometers [1]
Knowing the height above free water level allows us to use the capillary pressure curves to estimate water saturation at each depth without the need to revert to a T2 cutoff or irreducible water saturations concepts
We propose a new simplified method based on matching NMR time domain data and reconstructed Mercury injection capillary pressure (MICP) time domain data which removes some of the issues associated with other matching techniques
Summary
Nuclear Magnetic Resonance (NMR) measurements have been used extensively to characterise reservoir rock pore geometry using logging and lab-based NMR spectrometers [1]. The sensitive of the NMR measurement to pore-size distribution potentially enables capillary pressure to be modelled from NMR data. Several workers have proposed methods to derive drainage capillary pressure vs saturation curves from NMR data [24]. This paper expands this theory and model types. In a water-wet rock, relaxation of hydrogen nuclei in the water occupying the smallest pores occurs, because of interaction with the pore surfaces. Using a T2 cut-off value to define capillary bound water for NMR logs will assume that all rocks are at an irreducible water saturation. To use NMR logs to accurately define water saturations it is beneficial to convert the NMR T2 distributions to capillary pressure curves at each depth. Knowing the height above free water level allows us to use the capillary pressure curves to estimate water saturation at each depth without the need to revert to a T2 cutoff or irreducible water saturations concepts
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