A study is presented of the dynamic fluid loss of hydraulic fracturing fluids under realistic shear conditions. During a hydraulic fracturing treatment, a polymeric solution is pumped under pressure down the well to create and propagate a fracture. Part of the fluid leaks into the rock formation, leaving a skin layer of polymer, or a polymer filter cake at the rock surface or in the pore space. The fluid-loss behavior depends on the extent to which the polymer invades the porous rock and the thickness of the polymer filter cake, which is limited by the shear stress that the flowing fracturing fluid exerts on it. In this paper the effect is studied of shear rate on the dynamic fluid-loss behavior of linear gels and crosslinked borate guars using cores of permeabilities ranging from 0.5 to 70 md, pressure drops of 1000 psi and temperatures between 75°F and 150°F. The effect of shear on the formation of external filter cake and on the effectiveness of particulate fluid-loss additives was also studied. A shear rate history representative of that experienced by a rock segment in the fracture was used. In this history the shear rate drops from a very high rate of 380 s-1 down to 40 s-1 due to the widening of the fracture width as time advances.
Constant shear rate results show that high shear rates eliminate external filter cakes. In the absence of external cakes, the formation of internal filter cakes controls the fluid loss, especially in high-permeability cores. The effectiveness of particulate fluid-loss additives increases with the permeability of the rock and decreases with the shear rate and viscosity of the fluid.