Dynamic Fluid Loss in Hydraulic Fracturing Under Realistic Shear Conditions in High-Permeability Rocks

Abstract

Summary A study of the dynamic fluid loss of hydraulic fracturing fluids under realistic shear conditions is presented. 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 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. The effect of shear rate on the dynamic fluid-loss behavior of linear gels and crosslinked borate guars was studied using cores of permeabilities ranging from 0.5 to 70 md, pressure drops of 1,000 psi, and temperatures between 75 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 sec−1 down to 40 sec−1 because of 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 fluid loss, especially in high-permeability cores. The effectiveness of particulate fluid-loss additives increases with the permeability of the rock and decreases with shear rate and viscosity of the fluid. Shear history results show that the highest leakoff rates and volumes occur during the first 10 minutes of leakoff, i.e., when the rock surface is exposed to high shear rates. Once the shear rate drops below a critical shear value, polymer filter-cake deposition begins and the rate of leakoff slows down. The effect on fluid loss is that a majority of the fluid is lost at high shear rates during a fracturing treatment (i.e., when no polymer filter cake is formed), especially at the beginning of the treatment. To have a major effect on reducing fluid loss, the emphasis has to be placed on plugging the pore throats at the surface of the rock.