A common explanation for disappointing production results from fracturing treatments in tight-gas sand reservoirs is gel filtercake damage. Gel filtercake rheology is poorly understood, which makes modeling efforts difficult. The current method of describing gel filtercake movement is by measuring the "pressure drop to initiate flow" in special laboratory tests conducted at room temperature and atmospheric pressure. Typical filtercake concentrations are considered to be 0.6 to 0.8 lbm/gal, but can become much higher on fracture closure in low sand-concentration proppant packs. Laboratory experiments have been completed that measured the rheology of concentrated depolymerized (broken) hydroxypropyl guar (HPG) across a molecular weight (MW) range of 26 to 580 Kg/mole, a temperature range of 60 to 280°F, a concentration range of 0.25 to 3 lbm/gal, and shear rates of 0.001 to 1000 1/sec. Results from these experiments have been applied to calculate apparent pressure drops to initiate flow of decrosslinked filtercakes across a broad range of conditions.
The rheology of the noncrosslinked fluids was found to have Carreau type behavior. The Flory-Huggins equation was used to match the behavior of the infinite and zero shear rate viscosities at a reference temperature of 62°F. The remaining Carreau parameters were determined as a function of temperature and MW. The complete rheology model is presented. The rheology model was combined with flow-through porous media equations to successfully determine apparent pressure drops to initiate flow according to the currently accepted experimental criteria. This work significantly advances the understanding of the rheology of decrosslinked filtercakes, demonstrates that the Carreau behavior can still exhibit an apparent pressure drop to initiate flow in porous media, and provides a useful rheology model for further fracture-cleanup modeling.