SQUEEZE-FILM RHEOMETRY OF NON-UNIFORM MUDCAKES

The standard lubrication analysis for a squeeze flow of a Bingham fluid is reviewed, and then extended to the case of a fluid in which the yield stress varies as a function of depth. These analyses are used to obtain the yield stress τ0 from squeeze-film measurements made on uniform mudcakes formed by filtration of a typical bentonite mud. Over the solids volume fraction range 0.09 < φ < 0.6 we find τ0 (bar) = 3.9φ1.9 for this mud. If the cake remains under an applied differential pressure p∞ for a sufficiently long time, the volume fraction φ eventually reaches a limiting equilibrium value, and the yield stress may then be expressed in terms of p∞ as ln (φ0 (Pa)) = 0.72 ln (p∞ (bar)) + 9.3. Non-uniform mudcakes are also investigated, both directly, by the squeeze-film technique, and also indirectly, by combining measured filtercake concentration profiles with results for τ0(φ) obtained from uniform cakes. There is some agreement between the two sets of results, though further work would have to be done to make the direct measurement technique useful. Once deformation ceases, the relaxation of the stress within the cake is monitored; the relaxation time increases as the bentonite concentration within the cake increases.