Proppant Distribution and Flowback in Off-Balance Hydraulic Fractures

Abstract The vast majority of industrial hydraulic fractures propagate in off-balance mode, where the fracture is not in a single plane, includes many shear fractures and branches, and is shorter and narrower than computed by existing models. Proppant transport, deposition and flowback in these fractures are also substantially different than for single fractures. Presence of shear fractures leads to 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 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 productive capacity of the fracture. And, some of the proppant in the fracture is loosely scattered inside it and is free to move when sufficient drag is exerted by fluid flow. The 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 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 triggers instability and proppant flowback. Case histories from actual treatments illustrate and re-enforce the findings of the paper.