Displacement of water by gas in propped fractures: Combined effects of gravity, surface tension, and wettability

Inefficient recovery of fracturing water used in multi-stage hydraulic fracturing operations is a growing industrial concern. Non-recovered water can be trapped in the tight rock matrix and/or in the complex fracture network. This paper reports results of various drainage experiments conducted to identify the factors controlling water displacement in proppant-filled hydraulic fractures. Experiments were conducted to investigate the displacement of water and isopropanol–water solution by gas (Nitrogen). The displacement direction relative to gravity is changed to investigate the gravity effect on the displacement pattern and ultimate fluid recovery. The visual images of displacement patterns were obtained by taking high-resolution pictures of the porous medium model during displacement experiments. Results plotted in the form of normalized water recovery versus dimensionless time show three distinctly different clusters of data corresponding to vertical upward, vertical downward and horizontal displacements directions. The lowest water recovery was observed during the upward vertical displacements, which could be explained by the formation of gas fingers observed in the images. Reducing the surface tension and using treated hydrophobic proppants considerably improved the sweep efficiency and in turn the normalized water recovery. The effect of changing the wettability by using hydrophobic sand was more pronounced, and could be explained by the formation of thicker fingers in the upward displacement. The results of this study suggest that a significant portion of fracture fluid could be retained in vertical hydraulic fractures below the horizontal well due to the formation of gas fingers and poor sweep efficiency, which in turn are the results of adverse Mobility ratio and gravity segregation.