Customizing a fluid system for drilling into the pay zone or working over an existing well requires a thorough understanding of the reservoir. Damage mechanisms while drilling in the pay zone include fine solids migration, clay swelling, drill-in fluid (DIF) incompatibilities with reservoir fluids, and the use of reservoir damaging chemicals that can reduce the average formation permeability and lower production rates. A suitable drill-in fluid system must be chemically nonreactive with the rock minerals and reservoir fluids, and be less invasive to preserve the native state of the rock along with its natural wettability. The selection of the bridging particles type, size distribution, and concentration must be based on reservoir rock morphology and the calculated pore throat size distribution. An adequate selection rapidly establishes a tight, thin filter cake and an efficient bridging mechanism that prevents the invasion of filtrate and fine drilled solids deep into the formation, minimizing damage to the reservoir and the potential for differentially stuck pipe. This paper discusses the extensive laboratory analysis performed to design a DIF system based on the reservoir characteristics. This customized DIF was then successfully field tested with predefined key performance indicators (KPIs) to drill two wells, each with a production interval of approximately 2,000 ft, in the Kingdom of Saudi Arabia. Fluid properties, along with particle size distribution of bridging additives, were continuously monitored and precisely controlled through drilling fluids engineering. The optimized drilling practices and hydraulics analysis also enabled us to improve hole cleaning, prevent influx, and minimize hole instability.