Evaluation of Low Resistivity Beds Using Nuclear Magnetic Resonance Log

ABSTRACT. The combination of conventional logs such as density, neutron and resistivity logs is proven to be very effective in the evaluation of normal reservoirs. For low resistivity reservoirs, however, an accurate determination of the petrophysical parameters with the conventional logs is very difficult. This paper presents some cases of low resistivity reservoirs and low contrast resistivity reservoirs where conventional logs fail to determine the petrophysical properties of formation. The problems of these reservoirs are that conventional logging interpretation shows high water saturation zones, but water free hydrocarbon would be produced. In case of low resistivity contrast reservoirs it is difficult to determine oil-water contact with resistivity logs. Nuclear magnetic resonance (NMR) log has been only available as a supplement tool to provide additional information on the producibility of the reservoirs. The main limitations of NMR logging, however, has been the cost and long time of acquiring data. This paper shows that, in the case of low resistivity reservoirs, NMR logging is a very effective tool, which helps to accurately determine the reservoir rock petrophysical properties. For the analysis of NMR data, several aspects of NMR technique have been used; 1) T1/T2 ratio, for fluid identification, 2) the difference between NMR derived porosity and total porosity, to determine the types of clay minerals, 3) NMR relaxation properties, to identify fluids nature and rock properties of low contrast / low resistivity reservoirs. This paper presents three examples of low resistivity reservoirs. Analysis of NMR data of low resistivity zones has helped to identify the producibility of these reservoirs zones, to determine lithology independent porosity and to distinguish between bound and free water. In case of low contrast resistivity reservoirs where there was little resistivity contrast between water bearing formation and oil