Abstract: Oil based drilling fluid applied to Weizhou 12-1 Oilfield in the South China Sea could solve the collapse of the hard and brittle shale, increase drilling rate and reduce borehole problems. However, oil based drilling fluid containing cuttings will firmly adhere to the casing wall after cementing operations, and will directly affect the oil reservoir protection in perforating operations. Therefore, clean out system as well as cleaning fluid has been developed in indoor experiment, which could not only clean out the oil film on the casing wall, but also prevent the secondary pollution by oil flecked fluid. It's proved the system could effectively protect reservoirs and the cleaning ratio is over 90 %.