Experimental study on microscopic mechanisms and displacement efficiency of N2 flooding in deep-buried clastic reservoirs

Abstract

Current clastic reservoirs with burial depth of more than 5000 m are important energy sources. And microscopic initiation mechanism of residual oil after water flooding has been regarded as the key to improving oil recovery by gas flooding. In this study, an investigation was conducted in the occurrence states and distribution of residual oil of deep-buried reservoirs from microscopic pore scale. We innovatively designed a microscopic visualization experimental equipment for the first time, which broke through the limitations of visualization accuracy, temperature and pressure of the conventional experimental devices. Microscopic visualization experiments of gas injection after waterflooding at 115 °C and 55 MPa were carried out. According to the permeability and pore-throat 0 structure characteristics of deep-buried clastic reservoir, a two-dimensional glass etching micromodel was designed and fabricated. Meanwhile, the occurrence state and formation mechanism of multi-type residual oil were quantitatively characterized by image processing technology. The results showed that there were five kinds of residual oil which were corner, droplet, membranous, columnar and cluster after water flooding. In addition, it was generally concentrated in the fine pore-throats with greater resistance perpendicular to the main stream line. Two mechanisms, immiscible flooding and miscible flooding, were also found in N 2 flooding. Moreover, the injected gas carried, squeezed and dispersed the continuous clusters of residual oil, transforming them into dispersive shapes such as corner and membranous. And a certain amount of microscopic residual oil after water flooding was effectively mobilized by removing the water locking effect. The oil recovery could be improved by 11.49% in low-permeability zone, which was the highest, since the continuous cluster residual oil was decreased significantly. Membranous, columnar and cluster residual oil were well mobilized in the medium-permeability zone, and oil recovery was increased by 8.31%. In comparison, the oil recovery in high-permeability zone was only increased by 2.53%, and there was no cluster oil. Our study highlights the migration characteristic and initiation mechanism of residual oil in porous media under N 2 injection after waterflooding from the perspective of microscopic visualization, which may help to promote the development of deep-buried clastic reservoirs. • Microscopic experimental device was built to simulate 55 MPa and 115 °C. • Micromodels suitable for deep-buried clastic reservoir are designed and made. • A quantitative characterization method of micro-remaining oil occurrence state is established. • The migration characteristics and initiation mechanism during gas injection after waterflooding are clarified.