Abstract

A detailed physical characterization of tar from a carbonate reservoir in Saudi Arabia was made to evaluate its mobility and ways of establishing contact between the lighter oil and its aquifer. Density and viscosity measurements were carried out on several tar samples, under simulated reservoir conditions of pressure and temperature. Other physical parameters such as simulated distillation, pour point and penetration index were also experimentally determined.

Tar physical properties were found to vary with depth and area within the same field. The obtained experimental results showed a gradual increase in density and viscosity from the tar/oil contact towards the tar/water contact. This increase was much more pronounced in the neighborhood of the tar/water contact. Density and viscosity of tar diluted with toluene were in excellent agreement with those of pure tar.

The density of non preserved tar varied between 0.956 g/cc at 200°F and 1.008 g/cc at 76°F while that of preserved tar varied between 0.944 g/cc at 200°F and 0. 991 g/cc at 76°F. The tar samples analyzed appear to behave as Newtonian fluids.

Introduction

The present paper discusses detailed physical characterization of several extracted and RFT bottom hole tar samples obtained from a carbonate reservoir in Saudi Arabia. The chemical aspect has already been presented elsewhere [1]. Tar is defined as extra heavy oil with a gravity ranging between 29 and 9°API albeit the distinction between heavy oil and tar is rather shady.

Tar mat is present in abundance in the Middle East, Africa and elsewhere. In recent publications Kaufman et al. [2] mentionned the presence of a tar zone at the water/oil contact in Burgan field. This has been known for a long time. However, to ascertain lateral and vertical delineation of the tar zones appear to be still unresolved. Thick tar zones identified through visual observation and Latroscan analyses of weathered core samples were reported in the Raudhatain field [3].

A tar mat is generally a thick column laying between an aquifer underneath and a much lighter oil reservoir above. This peculiar location poses a host of challenging problems for an efficient management of the lighter oil and ultimately for a proper management of the tar zone itself [4].

In this study, density of tar was measured with a digital Anton Parr densiometer having a maximum temperature range of 300°F and a pressure limit of 6000 psi. Based on these measurements, specific gravity and API gravity were determined.

A rolling ball viscometer was used to measure tars viscosity at elevated pressures and various temperatures. The effect of visbreaking or permanent viscosity reduction due to thermal alteration has also been examined.

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