Drainage Against Gravity: Factors Impacting the Load Recovery in Fractures

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Abstract
Horizontal wells in the tight reservoirs are stimulated using multistage hydraulic fracturing. The fracturing fluid used in this operation is generally water based. A very low percentage of fracturing water is recovered during the flowback operation. Non-recovered water can block the gas flow and damage the reservoir. The water recovery in propped fractures is controlled by microscopic displacement efficiency and fracture areal sweep efficiency. Microscopic displacement efficiency is mainly controlled by capillarity, and has been studied extensively. Areal sweep efficiency during water drainage in propped fractures is controlled by mobility ratio and displacement direction relative to gravity direction, which needs further investigation. This paper reports various fracture drainage experiments to understand the effect of gravity, surface properties, and viscosity on fracture sweep efficiency and total load recovery. A visual cell was built to model a propped fracture. Glass beads were packed in the space between two glass plates, and the pack was then saturated with the fracturing fluid. This simulates the initial conditions before opening the well for flowback. For simulating the water drainage against gravity, representing a fracture element below the horizontal well, the gas was injected at the bottom of the cell. Four sets of experiments were conducted to study the effect of injection pressure, interfacial tension, wettability, and fluid rheology on sweep efficiency and load recovery. Glass beads were chemically treated to alter their wettability from hydrophilic to hydrophobic. For each test, normalized fluid recovery was obtained by measuring the mass of produced fluid and using the material balance. The cell was kept in a dark room in front of a light box, and pictures were taken at regular time intervals. These images were later used to compare the drainage patterns. The results suggest that water recovery in an upward displacement very low mainly due to poor sweep efficiency. Increasing the injection pressure does not improve water recovery. Reducing surface tension and using treated hydrophobic sand improves the sweep efficiency and in turn the load recovery.

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