In oilfield cementing, it is common to displace one viscoplastic fluid by another fluid along some sort of channel (e.g., a pore space or the annulus during cementing). Whether the resident fluid is displaced completely depends largely on fluid rheologies and chemical design of the fluids. Results of recent research are, in many respects, counter to accepted oilfield intuition. The direct application of these results is in the design of spacer and cement-slurry properties for effective mud removal during primary cementing.