Multi-stage hydraulic fracturing for stimulating tight reservoirs requires a significant volume of fracturing fluid, which is usually water-base. Water recovery is critical due to environmental considerations, water shortage, and formation damage. This work aims at investigating the role of gravity, fingering, and capillarity on water drainage in propped fractures.

We conduct two sets of drainage experiments. In the first set, we investigate the effect of size, size distribution, and wetting properties of proppants on water recovery. In the second set, we conduct drainage experiments in a physical fracture model. In this model, the proppants are packed in the space between two sheets of glass, and the pack is saturated with the fracturing fluid. This simulates the initial condition before opening the well for flowback. For simulating the water drainage in the fractures above, and below the horizontal well, gas is injected from the top and bottom of the cell, respectively.

For gravity-stable tests where water drains downward, the recovery is controlled by capillary forces, which depend on interfacial tension and proppant size. For gravity-unstable tests where water drains upward, the recovery is poor due to the formation of gas fingers. Increasing the injection pressure does not enhance water recovery in the upward displacement. We conclude that a large fraction of non-recovered fracturing fluid resides in the fractures below the horizontal well.

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