Prediction of Long-Term Proppant Flowback in Weak Rocks

Abstract

Abstract Proppant flowback is an extremely important phenomenon in hydraulic fracturing technology and may cause severe problems for well completion. Various models have been developed to predict the onset of proppant flowback but the physics of the phenomenon has still to be understood to predict the amount of proppant flowback during the life of a well. In particular, determining whether the proppant flowback will stop after a few days of production or will continue at a given rate during the well's life is a key issue when selecting an appropriate completion method. The development of a model allowing a quantitative prediction of this process is therefore a very vital task. In this paper we present a quantitative model to predict proppant flowback. The model is based on treating both the proppant pack and the reservoir as poroelastoplastic media. It allows for solid production from the proppant pack but also from the formation itself in case the fracture was created in a very weak reservoir formation. According to this model, the fluid and rock skeleton are considered as two interpenetrating continua. The Mohr- Coulomb yield criterion with the non-associated plasticity rule written in terms of the Terzaghi effective stress is used to describe the flow of a solid matrix connected with the pore pressure gradient. The influence of proppant properties, rock properties, in-situ stress, fracture width, and flow rate on proppant flowback is studied to determine the controlling parameters. It is shown that when a severe proppant flowback problem occurs, the closure stress around the wellbore is significantly altered. In some cases, the fracture width at the wellbore is decreased leading to the cessation of solid production, while in others continuous production is maintained. It is shown that the selection of proppant properties is an efficient way to control flowback, but obviously the requirement in terms of strength depends on the downhole conditions.