A General Correlation for Proppant Settling in VES Fluids

Polymer-free viscoelastic surfactant-based (VES) fluid systems are used to minimize damage to the proppant pack and to efficiently transport proppants into fractures. Proper selection of proppants and fracturing fluids for maximum propped fracture length requires reliable data and correlations for the impact of fracture fluid viscoelasticity and the effect of fracture walls on proppant settling. Current models and correlations neglect the important influence of fluid elasticity on proppant transport. This paper presents an experimental study that investigates the impact of fluid elasticity and fracture width and presents a general correlation for proppant settling in VES fluids.

Proppant settling experiments are performed in shear thinning VES fluids. Experimental data is presented to show that fluid elasticity plays an important role in controlling the settling rate of proppants. Increasing fluid elasticity can either increase or decrease the settling velocity depending on the rheological properties of the fluid and the properties of the proppants. A new experimental correlation is presented to quantify the settling velocity of proppants in VES fluids as a function of the fluid rheology and proppant size. It is shown that the VES fluids should be designed such that the relaxation time is greater than the critical relaxation time (Tcrit).

The productivity of fractured wells depends strongly on proper placement of proppants in the fracture. Experimental data/correlations are presented for the first time to show that the settling velocity of proppants is significantly impacted by the fracture width and in VES fluids this dependence is different than for the non-elastic fluids. Data is presented to show that settling velocity is reduced as proppant size becomes comparable to the fracture width. Results show that elasticity reduces the retardation effect caused by fracture walls. An experimental correlation to quantify the retardation effect due to fracture walls is presented. Proposed correlations highlight the advantages and limitations of using VES fluids for efficient proppant transport. These correlations can be directly used in fracture simulators for proppant selection and for the design of fracturing fluids.