Hydrocarbon Detection in Low-Contrast Resistivity Pay Zones, Capillary Pressure and ROS determination with NMR logging in Saudi Arabia
Authors: Hassoun T.H. , Zainalabedin K. , Minh C. Cao 
Published: December 2007

Abstract
NMR logging is primarily used to determine porosity independent of lithology and permeability. It might also be used to address some unsolved log interpretation problems with conventional Resistivity-Nuclear logs. We investigate three separate applications:Hydrocarbon identification in low contrast resistivity pay zones. Central Arabia sandstones reservoirs are often difficult to interpret because of unknown water salinities. There is little resistivity contrast between water-bearing reservoirs and oil-bearing reservoirs. The water-bearing reservoirs contain relatively fresh water, and thus show relatively high resistivity readings. The resistivity in the oil-bearing reservoirs is variable because the reservoirs contain fresh or salty water. Often, the oil-bearing reservoirs show a high level of irreducible water saturation that depresses further the resistivity reading, thus making the pay identification from the resistivity log extremely difficult.We used NMR logging with a modified Differential Spectrum Method (DSM) to isolate the hydrocarbon signal from the water signal. The modified DSM has a third pass at very short Wait Time (WT) which allows the computation of three DSM spectra instead of one from conventional two pass DSM. Pay zones are those that show mostly the hydrocarbon signal after removing the water component from the total signal. The technique works well in this so-called "low-contrast resistivity", but "high-contrast NMR relaxation" environment.Capillary pressure and permeability determination. Capillary pressure depends on pore throat size which is often a function of pore size in clastic-like sediments. NMR log gives pore size distribution that might be used to derive capillary pressure curves. Capillary pressure curves are the key to understand NMR permeability. We show that NMR permeability is lithology dependent, and that T2 cutoff can be derived from NMR log and conventional core analysis of permeability. T2 cutoff is essential to determine bound fluid (BF) and free fluid (FF) volumes to predict water production. No other conventional log gives this information.Residual Oil Saturation (ROS) determination. ROS determination using NMR logging technique has been performed in a carbonate reservoir. We used a two-pass technique: a first pass to determine the total fluid volume (water+oil), followed by a second pass to determine the oil volume after doping the mud with Manganese Chloride to kill the water signal. ROS is calculated as the ratio of the oil volume over the total fluid volume.

Introduction
NMR logging technique is well publicized and was introduced in the kingdom of Saudi Arabia in May 1995. Shortly thereafter, Saudi Aramco started the evaluation of NMR logging because of its potential to resolve many unanswered log interpretation problems. We have reviewed the published papers on direct hydrocarbon typing, permeability relationship with capillary pressure data, and residual oil saturation (ROS) determination using NMR logging in view of applying the techniques in the kingdom. The above applications are important and are unique to NMR logging. In retrospective, we have improved the techniques and applied them to our reservoirs which comprise fresh-water shaly sandstones as well as complex carbonates. This paper summarizes our findings and contribution to the field of NMR applications from the viewpoint of the reservoir engineer.

Hydrocarbon Detection in Low-Contrast Resistivity Pays
Central Arabia sandstones reservoirs are often difficult to interpret because of unknown water salinities. There is little resistivity contrast between the fresh water-bearing reservoirs and the oil-bearing reservoirs.
We used NMR logging with a modified Differential Spectrum Method (DSM) to isolate the hydrocarbon signal from the water signal. P. 127^