Petrophysical evaluation of low-resistivity sandstone reservoirs with nuclear magnetic resonance log

摘要

The combination of conventional logs, such as density, neutron and resistivity logs, is proven to be very effective in the :valuation of normal reservoirs. For low-resistivity reservoirs, however, an accurate determination of the petrophysical Jarameters with the conventionallog reservoirs is very difficult. This paper presents two cases of low-resistivity reservoirs md low-contrast resistivity reservoirs, where conventional logs fail to determine the petrophysical properties of reservoirs, nainly, low-resistivity and low-contrast resistivity reservoirs. The problems of these reservoirs are that conventional logging nterpretation shows high water saturation zones, but water-free hydrocarbon would be produced. In the case of low-resistiv- tY contrast reservoirs, it is very hard to determine water hydrocarbon contact with resistivity logs. Nuclear magnetic 'esonance (NMR) has only been available as a supplementary tool to provide additional information on the producibility of he reservoir. The main limitations of NMR have been the cost and time of acquiring data. This paper shows that in the case of low-resistivity reservoirs, NMR is a very cost-effective tool and is of help in Iccurately determining the reservoir rock petrophysical properties. In the analysis of NMR data, several aspects of NMR echnique have been used: (1) Tl/T2 ratio for fluid identification, (2) the difference between NMR-derived porosity and otal porosity to determine the types of clay minerals, (3) NMR relaxation properties to identify fluids composition and rock Jroperties. This paper presents four examples of low-resistivity reservoirs. Analysis of the NMR data of low-resistivity .eservoirs has helped identify the producibility of these zones, determine lithology-independent porosity and distinguish Jetween bound and free water. For the case of low-contrast resistivity reservoir, where there was little resistivity contrast Jetween water-bearing formation and oil-bearing formation, NMR has been able to identify the fluid composition of the two formations, as well as the height of the oil column. This was based mainly on the high contrast of NMR relaxation parameters.