Abstract

The objective of this study was to investigate the role of gas nucleation in the primary production of heavy oils under solution-gas drive. We were particularly interested in determining the effect of gas nucleation on mobility of heavy oils in a porous medium.

A novel apparatus was designed and assembled to carry out simulated solution gas drive experiments in a 200 can long sandpack. Recombined oil was injected into one end of the sand pack at constant pressure (equal to the saturation pressure) and was produced from the other end at constant pressure) and was produced from the other end at constant back pressure. The oil production rate and the pressure profile within the sand pack were measured at various values profile within the sand pack were measured at various values of pressure drawdown. Experiments were conducted using two heavy oils.

The experimental data show that, contrary to some speculations in the literature, the nucleation of gas bubbles under the solution-gas drive adversely affected the bitumen mobility. However, another hither to unaccounted for mechanism was involved: even at the highest pressure drawdown the gas appeared to flow primarily as a dispersed phase Md the apparent trapped gas saturation was very high. phase Md the apparent trapped gas saturation was very high. It is suggested that the heavy oil/gas mixture was flowing in form of an oil continuous foam.

The in-situ formation of M oil continuous foam can lead to recovery of a much higher fraction of the original oil in place under solution gas drive. Therefore this factor should place under solution gas drive. Therefore this factor should be taken into account in developing the strategies for primary production of heavy oil under solution gas drive. production of heavy oil under solution gas drive

Introduction

Several heavy-oil reservoirs in Alberta under the solution-gas drive mechanism, such as Lindbergh and Lloydminster, show "foamy oil" behaviour in wellhead samples, i.e. the produced oil forms an oil continuous foam which is produced oil forms an oil continuous foam which is remarkably stable. Field production data from these reservoirs appear to suggest that production mechanisms are rather complex, and may be quite different from those encountered in the conventional solution-gas drive reservoirs. The productivity of some wells is much more than what can be explained by application of the conventional flow equations in radial geometry. One of the suggested causes of this anomalous behaviour is the nucleation of a very large number of gas micro-bubbles which somehow reduce the apparent viscosity of the oil (1). However, the mechanism of such mobility enhancement is not clear. To address the reasons for anomalously high productivity of some heavy oil wells, a systematic investigation of the role of gas nucleation is required.

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