Summary

Over the past year or so, a new generation of nuclear magnetic resonance tools has been introduced in the Middle East. These tools, in contrast to the previous logging tool, the NML* introduced by Schlumberger more than 15 years ago, no longer require mud doping to kill the borehole signal. This greatly increases the number of candidate wells for this type of measurement.

In Abu Dhabi, a number of logs have been recorded in the Thamama, Shuaiba and Arab formations. These logs proved very challenging to interpret because very few nuclear magnetic resonance measurements have been made in carbonates until now. In particular, the relaxation times measured in Abu Dhabi were generally found to be longer than elsewhere. This was confirmed by extensive laboratory measurements on cores. This in turn means that it is extremely difficult to get a complete description of pore size distribution in continuous logging mode. However, experience has also shown that a representative value of the so-called "bound fluid volume", sometimes referred to as "microporosity volume", can be obtained even at "standard" logging speeds, i.e. 600 to 900 fph.

The microporosity volume is a very important piece of information in the carbonates of Abu Dhabi, because it can be related to the volume of irreducible water and therefore help in predicting whether a zone, even one with high water saturation, can produce water-free oil. If such information can be reliably and routinely derived from nuclear magnetic resonance data, it may help reduce the amount of well testing necessary in the appraisal of many wells.

In this paper we briefly review the basic physics of nuclear magnetic resonance measurements, discuss the effect that mud filtrate can have on the measurement, and present an example from the Arab formation where zones with high water saturation were perforated and produced oil without water.

1. Introduction

In 1995, a comprehensive campaign of nuclear magnetic resonance measurements was conducted in eight wells in Abu Dhabi, for four different operating companies. The main purpose of this campaign was to evaluate the nuclear magnetic resonance response of the Cretaceous and Jurassic carbonates, which constitute most of the oil reservoirs in the region. In parallel to the logging campaign, which took place over a period of a few months, core analysis was also conducted on a number of samples from five different wells. Some of the cores were taken in wells where no nuclear magnetic resonance log was available, as the core study was initiated prior to the arrival of the downhole logging tool in Abu Dhabi.

Although much of the log and core data is still under evaluation and unreleased, some of the results obtained on cores were presented in 1995 (Kenyon et al.) and one very interesting example of nuclear magnetic resonance logging applications was released and will be discussed in this paper.

The potential applications of nuclear magnetic resonance to formation evaluation are numerous. In the literature, reference is often made to permeability determination, lithology - independent porosity, hydrocarbon typing, even diffusion coefficient determination and so on. Some of these applications may be valid in clastics, but experience in Abu Dhabi has shown that attempting to tackle them at this early stage of the introduction of the measurement would be like trying to run before being able to walk. Indeed, the data acquired so far has demonstrated that the carbonates of Abu Dhabi have relaxation characteristics which are different from most of the areas where prior experience was available. In particular the transverse relaxation times T2 have been found to be somewhat longer than most previously logged carbonates. For this reason, this early work concentrates primarily on getting a representative value of capillary bound fluid volume to help in predicting what fluid will flow from any given zone.

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