Micro Bubbles in Solution–Gas Drive in Heavy Oil: Their Existence and Importance

Abstract

This work investigates the existence and importance of the micro bubbles in heavy oil subjected to solution–gas drive. The term ‘micro bubble’ will be used to refer to the free-gas phase that flows with the oil, no faster and no slower. Two types of experiments are reported here; slow and fast experiments. These experiments were previously reported by Sheikha and Pooladi-Darvish (SPE Res Eval Eng 12(3):390–398, 2009) and were used to investigate the effects of pressure gradient and depletion rate of oil recovery. In this work, we investigate the nature of two-phase flow, and find that in the slow runs, the flow was characterized by single-phase flow of oil until a gas saturation of 2±1% was reached. Above this gas saturation, bulk flow of gas was observed at mobilities much higher than that of micro bubbles. Recovery factor of the slow tests was below 4%. In the fast runs, flow of bubbles is observed shortly after they are formed in porous media. The gas mobility and fractional flow remain low until a gas saturation of 7±1% is reached. Flow of gas between approximately 2 and 7% gas saturation is consistent with that of micro bubbles. Gas fractional flow increases sharply at gas saturations above approximately 7%. The results indicate that the attainment of high recovery values (12–14%) observed in the fast experiments is partially as a result of low mobility of micro-gas bubbles. The pressure decline rate of each flow experiment was varied independent of its respective withdrawal rate. This did not alter the difference in recovery and mobility behaviour of the fast and slow experiments; the fast experiments exhibited a significant period of low mobility gas flow consistent with flow of micro bubbles. Regardless of the pressure decline rate, the slow experiments did not exhibit this period of low mobility gas flow.