This paper is the result of oil and gas exploration engineering.

Objective To enrich the research on carbon storage in low-porosity and low-permeability depleted hydrocarbon reservoirs in the Ordos Basin, a carbon capture and storage (CCS) project with an annual capacity of 100,000 tons is planned. The project involves 21 injection wells, each with an average injection rate of 0.15 kg/s.

Methods The Chang 6 oil layer of the Yanchang Formation in the Ordos Basin is selected as the target reservoir, characterized by an average porosity of 7%-12% and an average permeability of 0.1-5 mD. Based on the fundamental theories of percolation mechanics, multiphase fluid dynamics, and computational fluid dynamics, both homogeneous and heterogeneous conceptual geological models were constructed. Numerical simulations were employed to comparatively analyze the CO₂ storage characteristics, fluid migration laws, and phase-transition mechanisms under different geological conditions. Furthermore, in combination with field practices, the study investigates the specific features of CO₂ storage in low-porosity and low-permeability reservoirs, the behaviors of fluid migration and transport, and related physical phenomena.

Results The results indicate that during supercritical CO₂ injection, the initial formation pressure equilibrium is disrupted, leading to a rapid pressure increase around the injection wells. In the homogeneous model, the pressure diffuses uniformly in a radial pattern. Conversely, in the heterogeneous model, the pressure front exhibits significant anisotropy and "stepped" distribution characteristics due to permeability contrast. Following the cessation of injection, the formation pressure gradually stabilizes. In the homogeneous model, CO₂ forms a "tongue-like" accumulation zone at the top of the reservoir, with its lateral distribution continuously expanding. In contrast, the heterogeneous model exhibits pronounced "viscous fingering" and jagged edge characteristics.

Conclusions A comparison between the simulated average CO₂ migration velocity during the injection period (0.0157 m/d) and the field-measured velocity following hydraulic fracturing (7.35 m/d) reveals that pre-treating the low-porosity and low-permeability reservoirs in the Ordos Basin with hydraulic fracturing significantly accelerates CO₂ migration and enhances injection capacity.