Primary production of heavy oil from unconsolidated sand reservoirs is a process that is not well understood. Laboratory depletion experiments of sand packs, which mimic the field situation at a laboratory scale, show that production behavior of heavy oils under solution gas drive depend in a complex way on diverse parameter such as: depletion rate1,2, fluid properties 3, porous medium properties45, etc.

This work focuses on understanding the effect of the clay fraction in the sand pack on the recovery process. Experimental results from depletion tests at constant total volumetric production rate are presented. The porous media consisted of synthetic sands with different proportions of clay, except for one of the experiments, where reservoir sand was used. The oil was extra heavy crude (8 ºAPI) from the Venezuelan Orinoco Belt.

Pressure, pressure drop over the core, and oil and gas production were measured as a function of time. It was found that critical gas saturation and recovery factor increase with increasing clay content, whereas the supersaturation or the deviation from thermodynamic equilibrium decreases. This is interpreted in terms of an increasing number of nucleation sites at increasing clay concentration. More activated nucleation sites lead to more and smaller bubbles that allow a lesser mobility for the gas phase. Gas then becomes significantly mobile at larger gas saturations. This hypothesis is supported by previously published data from micromodel experiments doped with clay particles6.

The fact that critical gas saturation and recovery factor depend on the composition of the reservoir sand may strongly affect the evaluation of reservoir production capacity, because large regional heterogeneities related to clay content are often present in oil-producing "sands".

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