The Effect of Yield Stress on Fracture Fluid Cleanup Nolte, SPE, Dowell
Authors: May E.A. , Britt L.K. , Nolte K.G. 
Published: December 2007

Abstract
This paper evaluates polymer fluid cleanup in general and more specifically why Cotton Valley fracture treatments recover less than one half of the injected polymer. The inefficient cleanup is expected to explain why post-fracture well testing often indicates a smaller fracture length and conductivity than anticipated. Numerical simulations were performed on a typical fracture treatment using a compositional reservoir model and considered a range of yield values for the rheology of the fluid remaining after leakoff. The yield value range was based on that provided by an independent laboratory. Simulation results compare favorably to actual field production and well tests.
The yield value for the polymer residue was introduced into the simulator by newly derived flow relations based on the Herschel-Bulkley yield power-law model. The paper presents these relations and the method for inclusion into a compositional reservoir model. Simulations indicate that the fracture only cleans up to a length governed by the yield stress of the fluid and multiphase effects. Without a yield value, the complete length would eventually clean-up if nominal values of dimensionless conductivity are provided. This study represents the first attempt to investigate the effect of yield stress in fracture fluid cleanup. The results suggest that yield stress and relative permeability effects are the dominant mechanisms controlling the cleanup of the fracture and that the cleanup length is determined before gas breakthrough. A curve is presented which relates effective fracture length to polymer yield stress. This type of relation, developed for specific reservoir conditions, can be used to adjust the predicted fracture length and permit more accurate project evaluations without the need for detailed simulation of every fracture treatment.

Introduction
Fracture fluid cleanup is one of the most important aspects of a fracturing treatment. For many years the relationship between load recovery and post-fracture productivity has been known but only recently have the various mechanisms affecting cleanup been characterized.
Research to date has identified five major contributing mechanisms to fracture cleanup and proppant pack permeability. These mechanisms are (1) the effects of time and temperature on proppants, (2) gel residue and its damage to the proppant pack, (3) viscous fingering through the proppant pack, (4) the effects of unbroken fluid on proppant pack permeability, (5) non-Darcy and multiphase fluid flow effects, and (6) capillary pressure (water block) effects.
Pope. et al studied guar removal after hydraulic fracturing in the Codell formation of Colorado. This report shows that load recovery is not the key driver in fracture fluid cleanup. They suggest that a better gauge of cleanup would be to measure the amount of polymer recovered, because polymer recovery and load recovery are not always proportional. Willberg, et al performed a similar analysis of cleanup in the Cotton Valley formation of East Texas. They reach similar conclusions as Pope, however their work better characterizes the entire flowback period. They found that in formations which produce significant formation water, that formation water production begins almost immediately upon flowback and that this occurs before gas breakthrough. In addition, they noted that the flowback polymer concentration is generally less than or equal to the average pump-in polymer concentration. Also, they found that only about 35% of the total guar pumped is recovered, with the majority being returned in the initial flowback period, and that the flowback rate does not effect the amount of polymer recovery when formation water is also produced.
In another study, Pope, et al studied the viscous fingering of fluids through the proppant pack. They observed that the permeability reduction in the proppant pack caused by viscous fingering can be attributed to a reduction in the effective porosity of the pack.
P. 517^