Design and performance evaluation of a downhole oil-water separation and surface low-pressure water injection scheme for well A

China's major oilfields have entered the ultra-high water-cut development phase, characterized by rising water content and extensive ineffective water cycling. This not only significantly increases water injection volumes but also continuously escalates the cost of produced fluid treatment, pushing oilfield development close to economic limits. Reducing ineffective cycling and produced water reinjection has become a critical challenge for sustaining effective development in ultra-high water-cut oilfields. The downhole oil-water separation and production-injection technology addresses this by directly separating reservoir-produced fluids downhole. The separated water is re-injected into the injection zone, while the lowwater-cut crude is lifted to the surface, enabling simultaneous production and injection within a single wellbore. This dualpurpose well design alleviates water treatment burdens, reduces water injection energy consumption, and lowers surface system operating costs, effectively extending the economic production life of reservoirs in ultra-high water-cut stages. In 2022, a field trial was conducted in Block Z, achieving favorable initial results. However, due to the lack of external surface water sources, formation pressure gradually declined during development, leading to reduced production-injection rates and negatively impacting block performance. To address this, a surface low-pressure water replenishment program was implemented to restore formation pressure, stabilize production-injection volumes, and ensure optimal development outcomes in the pilot area. This scheme combines downhole oil-water separation with surface low-pressure water injection, achieving both efficient development and energy-saving environmental benefits. It demonstrates significant technical and economic advantages and is recommended for application in similar oilfields. The results of field tests show that after the implementation of surface low-pressure water injection in Well A, the injection-production ratio of the connected wells has been significantly improved. The liquid production has increased by 30 ton, the oil production has increased by 0.65 ton, and the water cut has decreased. The thin and poor layers have been effectively exploited. The surface low-pressure water injection work can effectively improve the development effect of wells with poor performance, providing new ideas for the economic exploitation of injection-production development of downhole oil-water separation wells, and it has broad application value.