Occurrence state and forming mechanism of microscopic remaining oil controlled by dynamic and static factors

Abstract

In order to reveal the occurrence state and forming mechanism of multi-type microscopic remaining oil controlled by both static reservoir factors and dynamic development factors, cores of the alluvial fan reservoir in the eastern steep slope zone of the Cangdong sag, the Bohai Bay Basin, China was selected, and then the occurrence state of multi-type microscopic remaining oil was quantitatively characterized and the forming process and mechanism of multi-type microscopic remaining oil was discussed based on waterflooding experiments on glass etching models and CT scanning on waterflooding process in real cores, and the basic classification of microscopic remaining oil. The results show that the two-dimensional microscopic remaining oil as individual oil blocks can be divided into two movable types -- isolated and semi-filled in pores, two adhering types – to pore walls and pore corners, and two retained types –in pores and throats. The three-dimensional microscopic remaining oil as accumulated oil blocks can be divided into three types, including cluster remaining oil, droplet remaining oil and flocculent remaining oil. The greater the permeability, the weaker the heterogeneity, the greater the waterflooding rate and the more displacement along the paleocurrent direction, the more the retained remaining oil tends to be movable. Changing the measures of dynamic water injection, the remaining oil retained in pores is the easiest to be recovered, followed by the remaining oil retained in throats and that semi-filled in pores. The movable remaining oil is formed when oil blocks are dispersed or gradually peeled off after being swept by injected water; the adhering remaining oil is formed when the part between oil block and pore wall or corner does not overcome its friction and capillary force and adheres to pore wall or corner; the retained remaining oil is the result that oil blocks are not swept or driven because of the bypassing injected water. The microscopic remaining oil as accumulated oil blocks is the aggregation of the microscopic remaining oil as independent oil blocks in a three-dimensional space.