A potentially important component of the total pressure drop from a fracture into a formation during hydraulic fracture stimulation is that across the polymer invaded zone. Following experimental work with long cores (up to 60 cm-long) and with permeabilities up to 5 md it was determined that the polymer invasion for a zirconate- crosslinked HPG-system has not particular significance and is thus negligible compared to the very dominant filtercake. The observed early- and late-time filtration phenomena were described using a numerical cake layer model and verified by experimental measurements. It is shown that the early-time "hyperbola" is not a result of polymer invasion but is caused by a changing pressure differential across the filtercake at the beginning of the filtration experiment.
Fluid leakoff during hydraulic fracturing is controlled by a pressure difference, which is the sum of the pressure drops across the filtercake, across the polymer invaded zone, through the filtrate invaded zone (if displacement effects are significant) and in the reservoir. The latter is controlled by the transients of linear filtrate flow.
In the field the pressure drop components in the vicinity of the fracture face can be treated as a skin effect. These near face components are the filtercake and the polymer invaded zone. (Displacement effects are not considered here.) Their rate, time and stress-dependent properties are especially important since the skin effect component is likely to change accordingly during the fracturing operation and may have an effect on the pressure response. Their contribution can be thoroughly investigated in the laboratory, and thus, they may be included in the transient solution for varying injection (leakoff) rates through an infinite-conductivity fracture during a fracture calibration treatment.
The component filtercake was analyzed in a series of fluid-loss experiments by Mayerhofer, Economides and Nolte for zirconate-crosslinked HPG fracturing fluids and by Zeilinger, Mayerhofer and Economides for borate-crosslinked fluids. Especially, the stress-sensitivity of crosslinked polymer filtercakes was investigated in detail by imitating the pumping and closure phase of a fracture treatment by continuously increasing and, subsequently, decreasing the pressure. These varying pressure differentials must account for the superposition of additional cake deposition and influence the stress-sensitive properties of the already deposited cake. The concept of hydraulic filtercake resistance was introduced. It was found that viscoelastic theory can provide an adequate background to explain the observations of pressure-dependent leakoff.
The second component, the influence of polymer invasion at the fracture face for crosslinked fracturing fluids was not considered adequately in the literature, although there is some indication of possible effects. Additionally, it was shown in Ref. 3 that for non-crosslinked HPG-fluids there exists a limit core permeability between filtercake-controlled filtration and a non-Newtonian filtration. The decoupling of polymer invasion and its contribution to the overall flow resistance for long cores between 1 to 5 md permeability for a zirconate-crosslinked HPG fluid was the scope of this work. It was done using permeability measurements and pressure transducers positioned near the core face.