Volume 57

Enhanced Gas Recovery and Dominant Geological Factors for CO2 Injection in Offshore Gas Reservoirs with Weak Edge-Bottom Aquifer Bao Cao, Yuqiang Zha, Nan Zhao, Lianzhi Zhang, Runfu Xiong, Ronghui Lai

Abstract

CO2 injection into gas reservoirs offers dual benefits of enhancing gas recovery and achieving carbon storage. However, the effectiveness of this process varies significantly across different geological settings, making it essential to identify the primary geological factors. Using the DF gas field with a weak edge-bottom aquifer in the South China Sea as a case study, a numerical model was established for CO2-enhanced gas recovery (CO2-EGR), which incorporated key mechanisms such as phase behavior, CO2 diffusion, and dissolution. Various geological factors were systematically evaluated to determine their influence on EGR. Results indicate that reservoir heterogeneity, thickness and edge-bottom aquifer are the dominant controlling factors, whereas permeability, rhythmic type, and dip angle (0~5°) exhibit comparatively minor effects. Horizontal heterogeneity, in particular, exerts the strongest impact on recovery performance. Reservoirs with thin layers (<10 m), large aquifers (5~10 times), or low permeability (<10 mD) show higher potential for EGR, primarily due to their lower recovery under depletion development. Composite and reverse rhythmic reservoirs are more favorable for CO2 injection, with improved recovery observed when CO2 is injected into lower structural positions. Well placement and pattern design for CO2 injection should prioritize reservoir heterogeneity, thickness, and aquifer characteristics. In the DF gas field, a configuration with three CO2 injection wells is projected to enhance gas recovery by 12% and sequester over one million tons of CO2 annually, supporting the green and low-carbon development of offshore gas fields.

Keywords CO2 injection, enhance gas recovery, geological factors, reservoir heterogeneity, edge-bottom aquifer