A New Index to Evaluate the Seepage Capacity of Water-Wet Sandstone Reservoirs

The water-wet sandstone reservoirs in the Liushagang Formation of the Beibuwan Basin has tremendous exploration potential for oil reserves in the South China Sea. However, owing to the weak correlations between petrophysical parameters and oil productivity, the production performance and seepage capacity showed significant differences in various reservoirs. The production performance of the field was evaluated by characterizing the mobility of two-phase flow through the concept of classical global mobility. However, global mobility still does not consider differential displacement pressure (DDP) and movable oil saturation (MOS). Therefore, a new index based on the Li-Horne model was established to simultaneously evaluate the seepage capacity of water-wet sandstone reservoirs, accounting for DDP and MOS. The detailed work stemming from the model included selecting water-wet sandstones with different lithologies and comparing the displacement effects of different flooding systems, oil viscosities, injection water salinities, pressure conditions, and MOS in multiple sets of parallel core samples. Finally, the new index was validated. A strong correlation was found between the new index and oil productivity per-unit differential pressure through displacement analysis of rock/oil/water, rock/oil/gas, and rock/oil/polymer systems on several core samples. The developed index for evaluating seepage capacity is suitable for different sandstones, flooding systems, oil viscosities, injection water salinities, pressure conditions, and productivity test data, thus providing insight into the production performance and seepage capacity parameters of water-wet sandstone reservoirs.