ABSTRACT:

The paper evaluates the reliability of the most common empirical correlations used for determining reservoir fluid properties whenever laboratory PVT data are not available: bubblepoint pressure. solution GOR. bubblepoint OFVF, isothermal compressibility, dead-oil viscosity, gas-saturated oil viscosity and undersaturated oil viscosity.

The reliability has been evaluated against a set of about 65 heavy and extra-heavy oil samples. About 1200 measured data points have been collected and investigated. All measured data points are reported in the paper. For all the correlations, the following statistical parameters have been calculated:

  • relative deviation between estimated and experimental values,

  • average absolute percent error,

  • standard deviation.

Oil samples have been divided in two different API gravity classes: extra-heavy oils for APl< 10, heavy oils for 10< API< 22.3. The best correlations for each class of API gravity have been evaluated for each oil-property.

The functional forms of the correlations that gave the best results for each oil property have been used for finding a better correlation with errors reduced, on average, by 10%. In particular, for extra-heavy oils, since no correlations are available in literature (except for viscosity), a special investigation has been performed and new equations are proposed.

Introduction

The calculation of reserves in an oil reservoir or the determination of its performance and economics, requires a good knowledge of the fluid's physical properties. Bubblepoint pressure, GOR, OFVF and compressibility are of primary importance in material balance calculation, whereas viscosity plays an important role in production test interpretation and in well problem analysis. Ideally, these properties are determined from laboratory studies on samples collected from the bottom of the wellbore or from the surface. Such experimental data are however not always available because of one or more of these reasons:

  • samples collected are not reliable,

  • samples have not been taken because of cost saving,

  • PVT analyses are not available when needed.

This situation often occurs in production-test interpretation in exploration wells.

In such cases PVT properties must be determined by using empirical derived correlations. Obviously the accuracy of such correlations is critical for the above mentioned calculations and it is not often known in advance.

Despite the great number of work performed in the past 50 years on PVT correlations. each of them seems to be applicable with a good reliability only in a well-defined range of reservoir fluid characteristics. This is due to the fact that each correlation has been developed by using samples belonging to a restricted geographical area, with similar fluid compositions and API gravity. In particular for oils with gravity less than 22 API the literature is very poor and nearly absent for oils with gravity less than 10 API.

This work is aimed at analysing the reliability of literature correlations, listed in table 1, relevant to heavy and extra-heavy Agip's reservoir fluid samples. shown in table 2.

This will make it possible to evaluate the use of some correlations in ranges of API gravity in which no correlations have been proposed yet (except for viscosity): for oils with density lower than 10 API.

LITERATURE REVIEW

The following presents a review of the most known correlations published in literature. The range of input data used by each Author in developing his correlation are provided in tables 3 and 4.

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