The Mechanical Stability of Propped Hydraulic Fractures: A Numerical Study

Abstract

Summary Proppant is sometimes produced along with hydrocarbons in hydraulically fractured petroleum wells. Sometimes 10% to 20% of the proppant is backproduced, which can lead to damaged equipment and downtime. Furthermore, proppant flowback can lead to a substantial loss of fracture conductivity. A numerical study was conducted to help understand what conditions are likely to lead to proppant flowback. In the simulations, the mechanical interaction of a large number (several thousand) individual proppant grains was modeled with a distinct-element-type code. The numerical simulations show that hydraulic fractures propped with cohesionless, unbonded proppant fail under closure stress at a critical ratio of mean grain diameter to fracture width. This is consistent with published laboratory studies. The simulations identify the mechanism (arch failure) that triggers the mechanical instability and also show that the primary way that drawdowns (less than ˜75 psi/ft) affect proppant flowback is to transport loose proppant grains in front of the stable arch to the wellbore. Drawdowns >75 psi/ft are sufficient to destabilize the arch and to cause progressive failure of the propped fractures. Introduction Proppant back production can significantly affect well economics because of (1) loss of fracture conductivity [unpropped sections of fractures tend to close under stress (Figs.1 and 2)], (2) the damage it inflicts on equipment (i.e., abrasion of pumps, casing, and wellhead), and (3) downtime and expense required to clean up the wellbore. Refs. 1 through 3 address technical issues involved in the flowback of proppant. Ref. 1 presents experimental data that show how various properties and conditions (e.g., closure stress, fracture width, proppant embedment in the fracture walls, grain-size distribution, relative inclination of the fracture walls, and drawdown) affect the mechanical stability of cohesionless, unbonded proppant packs. It also raises fundamental questions regarding the process of proppant production (e.g., why does the transition from stability to instability occur over a narrow range of fracture widths and why do proppant packs in the laboratory fail at much smaller fracture widths than are known to exist in the field). A numerical study presented in this paper addresses some of these fundamental questions. The objectives of the study are to help understand the mechanical process of proppant back production and to show how various field conditions and proppant properties affect the propensity for proppant flowback. This is accomplished by means of numerical simulations of proppant on a grain-size scale.