Bubble nucleation, growth and mobilisation of gas are important phenomena encounter in oil production by the depressurisation process. The drive energy for oil production during pressure depletion is supplied initially by oil expansion but mainly by gas evolution from solution and expansion of reservoir fluids. Some heavy oil reservoirs in Venezuela and Canada show a high recovery factor during primary production under the solution gas drive process. Factors responsible for high oil recovery in heavy oil reservoirs are not well understood to allow reliable predictions to be made for economic evaluation of the process. A series of flow visualisation tests at the pore level was conducted using a high-pressure glass micromodel to identify the key features of the process.

A heavy crude oil, with a viscosity of 2500 cp at the bubble point pressure and API of around 10, was used to perform the tests at reservoir conditions. Micromodels with realistic pore pattern and different wettability characteristics, including oil-wet, water-wet and mixed-wet were used in the tests. A series of experiments was performed to study the effect of depletion rate, saturation history and water presence on the nucleation process, gas evolution and hydrocarbon movements.

Our observations highlighted the significance of test conditions, particularly saturation history and operational conditions on the nucleation process and bubble formation. Laboratory tests can produce a large number of bubbles formed by pre-existing micro bubbles activation, or a limited number of bubbles due to bulk nucleation, resulting in widely different depressurisation results. Hence, interpretation of laboratory data and its application to field performance would require particular considerations as identified in this study.

Critical gas saturation and subsequent gas/oil production are strongly affected by the critical value of supersaturation and the number of bubbles formed during depressurisation. Two generalised correlations have been developed to predict the above parameters at realistic reservoir conditions.

The experimental data generated in this study and interpretation of the results provide information on gas nucleation, critical supersaturation, bubble density and critical gas saturation, which are essential in field development planning and estimation of oil and gas recovery by depressurisation. The study also clears a number of ambiguities in the literature on prevailing mechanisms in depletion of heavy oil and contradictions between laboratory and field results.

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