Characteristics of pore-throat structures in volatile oil reservoirs and strategies for optimal development

The third member of Shahejie Formation (also referred to as Sha 3 Member) in Dongpu Depression, China, a volatile, low-permeability oil reservoir with complex fluid compositions, is subjected to high temperature and high pressure (HPHT), which poses significant challenges to conventional water injection. To elucidate flow mechanisms and optimize development strategies, this study integrates constant-rate mercury injection (CRMI), nuclear magnetic resonance (NMR), and HPHT three-phase oil/gas/water relative permeability experiments to analyze pore-throat structures, movable fluid characteristics, and relative permeability. The CRMI results indicate that the reservoir exhibits low porosity and low permeability, with dominant throat radius ranging from 0.6 to 5.0 μm, and mean pore-throat radius ratio ranging from 40.303 to 278.320, demonstrating significant microscopic heterogeneity. The NMR results reveal that water-alternating-gas (WAG) injection enhances oil recovery by 16.28 % (Sample W1) and 13.52 % (Sample W2) compared to conventional water injection, primarily due to the gas phase's low viscosity and high mobility, enabling access to micropores unreachable by water phases. The HPHT three-phase relative permeability tests demonstrate positive correlations between saturation and relative permeability, with oil permeability significantly influenced by three-phase saturation and rock wettability. These findings establish a microscopic seepage model for optimizing enhanced oil recovery (EOR) strategies in volatile reservoirs.