Hydraulic fracturing is a method employed by the petroleum industry to increase the performance of oil and gas reservoirs. Modeling this process requires accurate characterization of fluid rheology which is significantly affected by the proppant concentration. Various models for slurry viscosity and settling velocity have been reviewed. While many viscosity expressions exist, it was found that many may be inaccurate. The most valuable viscosity correlations are those derived from empirical data based on materials that are commonly used in the hydraulic fracturing industry.

 The fracking process requires accurate characterization of fluid rheology, which is affected considerably by the proppant concentration. Errors in modeling this process can result in less than optimal well performance at a significant monetary cost. A review of the effects of proppant concentrations on the rheology of hydraulic fracturing fluids has been presented. The literature shows that many expressions for slurry viscosity exist. Some expressions were derived theoretically and others empirically, while others were developed by modifying existing equations.

Hydraulic fracturing can be a specialized undertaking; each slurry may be unique in its composition of chemicals and proppants. Therefore, an all-encompassing viscosity expression which takes into account temperature, gel concentration, and proppant concentration and size will not be accurately determined by modifying existing equations that were never intended to account for so many parameters. It is evident that empirical data that relies on materials that are commonly used in frac jobs will provide the most valuable correlations. This review has considered the effects of proppant concentration on proppant transport. Other parameters that should be considered include fracture width and fluid elasticity.