The Impact of Proppant Retardation on Propped Fracture Lengths

Abstract

Abstract A model for the velocity of proppant particles in slot flow is presented. The proppant is either retarded or accelerated relative to the fluid depending on the ratio of the proppant size to the fracture width. It has been found that when this ratio is small, the proppant travels faster than the average fluid velocity at that location because the proppant tends to be confined to the center of the flow channel where the fluid velocity is higher. As the proppant size increases, the effect of the fracture walls becomes more important and the proppant is retarded by the walls. The retardation of particle relative to the fluid is greater for larger particles and greater proximities to the fracture walls due to the hydrodynamic stress exerted on the sphere by the walls in the narrow gap. A higher proppant concentration restricts the area available to flow and increases the drag forces on the particles. A model is presented for the effect of fracture walls and proppant concentration on proppant transport. The effect of this increased drag force is accounted for by modifying the wall - particle interaction. The influence of the surrounding proppant spheres on the drag force on a particle is estimated from the effect of a wall on the drag force acting on a single particle. The equivalent hydraulic diameter is then used to determine the proppant retardation. The effects of wall roughness and fluid leakoff are discussed. Models are suggested that capture these first order effects. The new model for proppant retardation has been incorporated into a 3D fracture simulator. Results show that the proppant placement is substantially different when proppant retardation/acceleration is considered. Comparisons of propped fracture lengths obtained with the new model agree much better with propped and effective fracture lengths reported in the field. 1.Introduction Hydraulic fracturing is a commonly used stimulation technique. Proppant transport is a key factor in determining the productivity of these fractured wells. Water fracs are common stimulation treatments for low permeability gas reservoirs. These treatments use low viscosity Newtonian fluids to create long narrow fractures in the reservoir, without the excessive height growth that is often seen with cross-linked fluids. The low viscosity fluid and the narrow fractures introduce some significant challenges for proper proppant placement. The low viscosity of the carrying fluid leads to high settling velocities for the proppant. The narrow fractures created can have widths comparable to the diameter of the proppant and can alter proppant transport significantly due to the hydrodynamic forces acting on the proppant because of the fracture walls. Other proppant particles create additional hydrodynamic drag forces leading to retardation. Fracture diagnostic studies that have been reported in the literature have observed that the effective propped lengths for both water fracs and conventional gelled fracs are sometimes significantly different than those predicted by fracture models. Designed and created fracture lengths are usually much longer than the effective fracture lengths obtained from post production analysis[1–4]. They can sometimes be an order of magnitude lower. Proppant transport is a key factor determining the effective propped lengths and therefore the productivity of these fractured wells. In current hydraulic fracture models, the proppant is assumed to flow with the fluid in the direction of fracture propagation. It is shown in this paper that the proppant usually flows at a different velocity than the fluid, particularly in narrow fractures.It is important to develop reliable models to predict proppant transport. A detailed model for proppant settling in water fracs was presented earlier by the authors5. Several correlations for modeling proppant settling in water fracs were presented. These UTFRAC correlations allow fracture models to correct the settling velocity for inertial effects, proppant concentration, fracture width and turbulence. The models were implemented in a 3-D hydraulic fracture simulator and results showed that propped fracture lengths could vary significantly when settling was properly accounted for.