Microscopic Mechanism of SCCO2 Displacing CH4 in Deep Carbonate Gas Reservoirs Based on X-CT Scanning: A Case Study of the Dengying Formation, Penglai Gas Area, Sichuan Basin

  Deep carbonate gas reservoirs represent a crucial frontier in natural gas exploration. However, their strong heterogeneity and complex pore structures often lead to technical challenges and low recovery rates. The Dengying Formation in the Penglai gas area, Sichuan Basin, characterized by typical vuggy, fracture-vuggy, and porous reservoir types, serves as an ideal focus for addressing these issues. Nevertheless, conventional core flooding experiments lack in-situ visualization and real-time monitoring capabilities, making it difficult to characterize dynamic fluid migration at the microscopic scale. Therefore, establishing a new experimental methodology is urgently needed.Moreover,This experiment employed CO2 as the displacement gas. The injection of CO2 into deep carbonate formations enables the underground storage of greenhouse gases, realizing carbon sequestration with substantial environmental benefits.

  In this study, typical carbonate samples from the Dengying Formation were selected to conduct high-temperature and high-pressure (HTHP) physical simulation experiments of SCCO2 displacing methane (CH4) using X-ray Computed Tomography (X-CT). A complete experimental workflow covering "formation water saturation, gas charging, and SCCO2 displacement" was established, along with a quantitative parameter system. Through real-time online monitoring, fluid migration patterns and displacement characteristics were quantitatively analyzed based on CT images and CT number variations.

  The results indicate that: (1) Fracture-vuggy reservoirs exhibit the best displacement performance under high pressure, with the sweep volume of SCCO2 expanding progressively over time. (2) In fracture-dominated reservoirs, SCCO2 tends to migrate along preferential "fracture-vug" pathways under high pressure, leading to gas channeling (fingering) and low sweep efficiency; optimizing the pressure differential (reducing displacement rate) can effectively mitigate channeling and improve matrix mobilization. (3) Vuggy reservoirs have a high mobilization threshold, requiring a higher pressure gradient and longer displacement duration, with the sweep zone expanding gradually. (4) Porous (tight-matrix) reservoirs show the poorest performance; due to narrow throats and poor connectivity, high seepage resistance prevents significant saturation changes or displacement fronts from being observed in CT images.

  This study reveals the microscopic mechanisms of SCCO2 displacing gas under different carbonate pore structures and clarifies the control of heterogeneity on displacement efficiency, providing theoretical support for Enhanced Gas Recovery (EGR) and CO2 sequestration in deep carbonate reservoirs.

Keywords: Deep carbonate reservoir; X-CT scanning; SCCO2-EGR; Physical simulation; Dengying Formation