The lost circulation is a primary consideration while drilling through fractured carbonate formations. Uncontrolled lost circulation may result in high nonproductive drilling time and cost, stuck pipe, side-tracks, blowouts and occasionally, the abandonment of expensive wells depending upon the severity of the loss. Additionally, drill solids entering the reservoir as a result of lost circulation may plug the pore throats, leading to a significant decrease in production.In the industry, there are two approaches to struggle with lost circulation; to mitigate (control and stop) losses after they occur, or alternatively strengthen the loss zones to prevent losses. Preventive methods, known as wellbore strengthening methods, aim to both alter stresses around wellbore and minimize fluid loss. They are effective not only on natural fractures but also induced fractures which occurs during drilling. The objective of this study is to determine the optimum concentration and particle size distribution for fractured reservoir zones. A polymer-based reservoir drill-in fluid supported by wellbore strengthening materials (WSM) was used in this study. Sized ground marble (GM) was chosen as a WSM because of its hydrochloric acid solubility and reservoir non-damaging nature. Sized GM was used as a WSM in different concentrations and in different particle sizes range. The experiments were conducted by using Permeability Plugging Apparatus (PPA). Fractured formations were simulated by using metal slotted disks with fracture width of 400, 800 and 1200 microns. Tests were conducted at room temperature. During the study, a total of 269 tests were run to investigate the effect of different particle size distribution, concentration, and fracture width. The results have been compared according to the maximum sealing time required to reach predetermined pressure (2000 psi) and fluid lost volumes, therefore, optimum compositions have been determined.