Abstract: In shale gas drilling, the residues of oil base drilling fluid on the borehole wall and the surface of the casing strings have long been a problem. The residual oil base drilling fluid will inevitably be mixed with and therefore render contamination to cement slurry. The surface of casing and the borehole wall, which are long in contact with oil base drilling fluid, can become lipophilic or hydrophobic, thus bringing about many problems, such as difficulties in displacing the oil base drilling fluid and cleansing the casing string, poor rheology of the mixed slurry, reduced strength of the contaminated cement slurry, and poor bonding of the interfaces between cement sheath and casing string and the interfaces between cement sheath and the borehole wall. Laboratory studies have been conducted on surfactants and spacers to resolve the problems mentioned above. The studies included the following:1) investigating the mechanisms of the contact contamination between oil base drilling fluid and cement slurry; 2) verifying the flushing efficiency of surfactants by measuring surface tension; and 3) measuring the flushing efficiency of surfactant solution and spacer using simulated casing flushing experiment. The results of the studies showed that use of surfactant remarkably decreased the oil-water interface tension, hence enhanced the flushing efficiency. A compounding surfactant spacer, having a flushing efficiency of 92.86%, was formulated with three surfactants in a ratio of LAS:JFC-6:AOS=1:1:1. The spacer was able to enhance the bonding quality of the interfaces between casing and cement sheath and the interfaces between cement sheath and borehole wall, and was helpful in improving the quality of shale gas well cementing.