Volume 56

EOR Mechanisms and Influencing Factors of CO2-WAG Immiscible Displacement in Heterogeneous Sandy Conglomerate Reservoirs Using Nuclear Magnetic Resonance Technology Dengfeng Zhang, Junrong Liu, Shuyang Liu, Rundong Gong, Hangyu Li,

Abstract

Sandy conglomerate reservoirs are known for their tight and highly heterogeneous nature. CO2-WAG (Water-Alternating-Gas) has been identified an effective method to enhance oil recovery of such reservoirs, while also achieving a certain amount of CO2 storage. However, the enhanced oil recovery (EOR) mechanisms and the main controlling factors of this technology remain unclear, posing challenges in providing clear guidance for field practices.
In this paper, the CO2-WAG displacement experiments were conducted using cores from different layers in parallel to simulate a heterogeneous reservoir and address these challenges. Nuclear Magnetic Resonance (NMR) technology was used to monitor the dynamic distribution of residual oil during the displacement process. This approach explored the EOR mechanisms of CO2-WAG across multiple scales, including the reservoir, layer, and pore levels. Additionally, the influence of water-gas ratio and injection rates on the oil recovery at multiple scales were investigated.
The experimental results show that the overall recovery factor of CO2-WAG flooding reaches about 24%, representing a 41.18% increase compared to pure CO2 flooding. Analysis of residual oil changes in different layers and pore types reveals that CO2-WAG flooding effectively inhibited early breakthrough in high-permeability layers, improved displacement efficiency in medium and low-permeability layers, and enhanced micro-displacement efficiency in medium and small pores. Moreover, the study demonstrates that increasing the water-gas ratio gradually enhances the oil recovery, primarily attributed to increased oil recovery in low-permeability layers and micropores. However, as the injection rate gradually increases, the overall oil recovery progressively decreases. This is attributed to higher injection rates resulting in earlier gas breakthrough in high-permeability layers, thereby hindering CO2 sweep through the medium- and low-permeability layers. Conversely, the increase in displacement pressure resulting from high injection rate leads to enhanced oil recovery in the micropores.

Keywords EOR, NMR, Heterogeneous, CO2-WAG, Sandy conglomerate reservoir