Nuclear Magnetic Resonance: A Useful Compliment to Reservoir Characterisation of Brunei Deltaic Sequences
Authors: Warrilow I.M. , Ugueto-Cabrera G.A. , Nawawi Hj. , Sibbit Alan 
Published: December 2007

Routine reservoir characterisation in Brunei Darussalam is hampered by the preponderance of shaly sands. This has made routine formation evaluation dependent upon the Waxman-Smits shaly-sand relationship. Currently available wireline tools, used to probe the geological sub-surface formations, enable reliable estimates of both total porosity and hydrocarbon saturation, averaged over the volume of formation investigated by the tools. The Waxman-Smits relationship, however, does not take into account the nature in which shale is distributed throughout any geological layer. From the point of view of determining hydrocarbon saturation this is not too significant. It is well known, however, that the nature of shale distribution becomes very important when we are required to assess hydrocarbon producibility. For this we must calculate the so-called effective porosity and effective hydrocarbon saturation.

Some fifty years ago the benefits of NMR (Nuclear Magnetic Resonance) were extolled as a means of characterising materials by monitoring the relaxation behaviour of hydrogen protons in a fluid saturated rock. Apart from this, NMR techniques have proven to possess the ability to investigate properties of materials not directly accessible with other methods. This has resulted in the technique being well established today in both the medical and scientific industry at large. Nuclear magnetic resonance has been used by the oil industry as a standard laboratory measurement to obtain a greater insight into the intrinsic formation fabric (i.e. pore size distribution, moveable fluid volume), effective porosity and producibility. Recent technology developments have seen rapid contractor developments in the area of downhole NMR wireline measurements.

In this paper we will discuss a recent acquisition of wireline NMR data from Brunei's oldest field, the Seria Field. From this, a direct estimate of effective porosity and moveable fluids comparable to estimated values from traditional methods were obtained. This provided for the first time, using a completely independent technique, a check on assumptions made with the Waxman-Smits shaley-sand relationship. To further ratify the performance of the wireline NMR measurement, a comparison between the wireline NMR and equivalent independent measurements made on core material, obtained from the well, will be shown.

Introduction

The deltaic environment of Brunei Darussalam has a preponderance of shaly sands making reservoir characterisation dependent on such models as the Waxman-Smits shaly-sand relationship to determine hydrocarbon saturation. In short, this model states that the conductivity behaviour of a shaly sand is equivalent to a hypothetical clean sand having the same porosity and pore-geometrical parameters, except that the conductivity of the bulk water appears to be greater than the salinity of the formation water would indicate. This excess conductivity is due to the presence of excess cations (quantified by the parameter Qv - in units of meq/ml) in the bulk water surrounding the clay particles and compensating for their negative charge. Front its inception the Waxman-Smits relationship represented a major breakthrough in the reservoir characterisation of the shaly sands encountered in Brunei Darussalam. For the first time it provided a means to quantify the under-estimate in hydrocarbon saturation, due to the presence of conductive shales found in all our deltaic sequences. It is equally important, however, that we remember that the Waxman-Smits model will deliver the same estimate of hydrocarbon saturation independent of the manner in which the shale is distributed throughout the formation.

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