Research on the damage mechanism and reservoir protection drilling fluid system of Jurassic reservoir in Zhenbei oilfield.

Low permeability oil and gas reservoirs play an increasingly important role in China's hydrocarbon exploration and development, serving as critical targets for reserve expansion, and the Jurassic reservoirs of Yanchang formation in the Ordos Basin exemplify such reservoirs, hosting substantial resources. During this study, the mechanism of drilling fluid-induced damage in Jurassic reservoirs is investigated via comprehensive analysis of core physical parameters, mineralogy, pore structure, and formation sensitivity. Results indicate that the reservoirs feature favorable overall petrophysical properties, characterized by intergranular and dissolution pores, low displacement pressure, low porosity, and low permeability. However, the invasion by foreign solids or fluids readily induces particle migration within pore throats, leading to blockage and significant formation damage, which consistent with previous research. Secondly, through the optimization of oil-soluble temporary plugging agents and filter loss reducing agents, combined with experimental methods, such as particle size analysis and scanning electron microscopy, a drilling fluid system for Jurassic reservoir protection in Zhenbei Oilfield was obtained, which includes temporary plugging agent G325 and self-degradable filter loss reducing agent. The formulated system exhibits a medium-pressure fluid loss of 4.5 mL, achieves an average sealing efficiency of 71.06%, and delivers an average core permeability recovery rate of 97.80%. This demonstrates that the developed drilling fluid system with superior sealing capacity and effective formation damage control. Specifically, as oil development progress in the later stages, the temporary plugging layer gradually dissolves and self-degrades, restoring unobstructed pore throats and achieving the goal of reservoir protection.