This paper describes the experimental study of formation damage in naturally fractured reservoir during conventional drilling operations. Dynamic formation damage (DFD) apparatus of NTNU is designed to simulate the drilling fluid circulation process at the formation face in the well bore face under bottom hole condtions. Chalk core samples are selected to study as a representative of very fine-grained limestone formations. A uniform fracture was made entirely along the conventional core. A series of overbalanced leak-off test were conducted with different drilling fluids under different reservoir conditions on the samples. Dynamic leak-off test was performed on the core samples for 4 hours and then it was followed by static leak-off test for 16 hours. Cummulative invasion of mud into the core during leak-off test is measured and then return permeability test and drawdown pressure effects are employed. The experiment results clearly show that drilling moods can cause large irreversible damage to fractured formation and dramatically reduce the productivity of wells producing from natural fracture network. The results present the significant impact of overbalanced pressure on invasion profile. The use of bridging agent such as CaCO3 and polymer play key role in reduction of solid and filtrate invasion in fractured core sample. Return permeability test indicates that cleanup and remove all solids and particles during production is not completed as well.Therfore the experiments recommend the use of drawdown pressure effects to clean-up of solids and particles during production.

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