Proppant distribution and flowback in off-balance hydraulic fractures

The vast majority of industrial hydraulic fractures propagate in off-balance mode (in which the fracture is not in a single plane), including many shear fractures and branches, and are shorter and narrower than computed by existing models. Proppant transport, deposition, and flowback in these fractures are also substantially different from what occurs in single fractures. Presence of shear fractures leads to the formation of many randomly distributed tight proppant packs. Many of these are inherently unstable and serve as sources for proppant flowback, but they are also the source for fracture conductivity. Gravity causes the formation of proppant beds. These are usually formed at the lower extremities of the fracture. Branches generally trap the proppant and are unlikely sources for flowback. They also add little to the productive capacity of the fracture. Some of the proppant in the fracture is loosely scattered inside and is free to move when sufficient drag is exerted by fluid flow. This paper places special emphasis on proppant flowback and shows that the three requirements for its occurrence are motion initiation, motion maintenance, and infinite conductivity along the return path. Gravity plays a very important role in this process, as does well completion. Tendency for equilibrium between reservoir-fluid velocity and deposited proppant results in a gradual decrease and, eventually, stoppage of proppant return at any given flow rate. This equilibrium can be disrupted by sudden increases in flow rate, which then trigger instability and proppant flowback. Case histories from actual treatments illustrate and reinforce the findings of this paper.