Modeling Proppant Settling in Water-Fracs

Abstract

Water-fracs, consisting of proppant pumped with un-gelled fluid are the type of stimulation used in many low-permeability reservoirs throughout the United States. The use of low viscosity, Newtonian, fluids allows the creation of long narrow fractures in the reservoir without the excessive height growth often seen with cross-linked fluids. Proppant transport is a central issue in all these treatments because of the low viscosity of the fracturing fluid. New models for proppant transport and settling in hydraulic fractures were developed and implemented in a 3-d hydraulic fracturing code. It is shown that a simple Stokes settling model is grossly inadequate. The proppant settling models developed in this paper account for the effects of fracture walls, changes in settling velocities and rheology caused by changes in proppant concentration, turbulence effects due to high fluid velocities and inertial effects associated with large relative velocities between the proppant and the fluid. Narrower fractures, higher proppant concentration and smaller proppant size reduce settling whereas turbulence leads to an increase in settling. Results are presented to show how the settling velocities are impacted by fluid velocity, proppant size, fluid rheology and fracture width. In most instances settling velocities differ significantly from the Stokes settling velocity. The new proppant settling model was incorporated into a 3-D hydraulic fracture simulator (UTFRAC-3D). Simulation results show that when settling is accounted for, significantly shorter propped lengths are obtained. The narrow fractures associated with water-fracs alter settling and thereby alter the proppant placement significantly. Although increasing fluid viscosity can reduce settling rates, increased height growth reduces the distance to which proppant can be placed. This clearly suggests a need to optimize fluid rheology. The improved fracture simulator can be used to better design fracture treatments (fluid rheology, injection rates, proppant concentration and size) for better proppant placement under a given set of in-situ stress conditions. Introduction Water-fracs are commonly applied in low permeability gas reservoirs. These treatments involve pumping low viscosity ungelled fracture fluids. The low viscosity of the slick water leads to long created fracture lengths. However, due to high settling velocities of the proppant in the low viscosity fluid, the propped lengths achieved can be very small. Modifications to the water-frac stimulation design are needed to transport proppant further out into the fracture. This requires suspending the proppant until the fracture closes without generating excessive fracture height. Proppant transport clearly is a central issue in all these treatments. An improved proppant transport model is presented that can accurately model proppant transport when either un-gelled or cross-linked fluids are used to place the proppant. The use of this proppant transport model will allow engineers to customize treatment designs for individual wells. The complete model for proppant transport in hydraulic fractures was incorporated into UTFRAC-3D, a fully three-dimensional hydraulic fracture simulator. The proppant transport equations were solved on an adaptive finite element mesh. The settling of the proppant was modeled taking into account the change in settling velocities and rheology due to changes in proppant concentration, turbulence effects due to high fluid velocities, and inertial effects associated with large relative velocities between the proppant and the fluid. Inertial effects become significant at high settling velocities (Rep>2) and are discussed in the next section. The effects of particle concentration, fracture width and turbulence are discussed in the following sections. An example calculation is shown to demonstrate the importance of each of the correlation factors applied to the Stoke's settling velocity. Finally, the settling correlations are incorporated into a proppant transport model in a fully 3-D fracture simulator (UTFRAC-3D). Results from the model are discussed in the last section. The project is co-funded by the US Department of Energy - National Energy Technology Laboratory.

A new model for simulating particle transport in a low‐viscosity fluid for fluid‐driven fracturing

“…Eulerian‐Eulerian methods mean both the fluid and proppant are solved in Eulerian grids. Several models using Eulerian‐Eulerian method were built to simulate the proppant transport in petroleum engineering . In these models, the pressure is solved by mass balance equation for the slurry.…”

Evaluation of numerical schemes for capturing shock waves in modeling proppant transport in fractures

“…Later other models have been proposed to simulate proppant transport numerically (Friehauf 2009;Gadde et al 2004;Liu 2006;Ouyang 1994;Sharma and Gadde 2005). The main focus of these models was simulating transport in multi-phase and multi-component slurries.…”

“…Little attention was given to the efficiency of the numerical scheme in solving the proppant transport equations. Gadde et al (2004), Liu and Sharama (2005), Liu (2006) and Friehauf (2009) used the Perkins-Kern-Nordgren (PKN) fracture geometry and included some of the experimental works in the literature related to proppant transport in their numerical model. The finite difference scheme was employed to solve the transport equation similarly with little attention to the efficiency of the numerical scheme used.…”

Transport and Distribution of Proppant in Multistage Fractured Horizontal Wells: A CFD Simulation Approach

“…A CFD study by Tsai et al (2012) reveals advances in numerical simulation of proppant transport in hydraulically stimulated fractures for oil and gas production. Additional literature (Gadde et al 2004) discuss the effect of fracture walls, changes to settling velocities, rheology caused by changes in proppant concentration, and turbulence effects attributed to high fluid velocities and inertial effects associated with large relative velocities between the proppant and the fluid.…”