Mechanisms of imbibition during hydraulic fracturing in shale formations

Hydraulic fracturing technology has been proven to significantly increase production from shale gas and oil formations. However, during a hydraulic fracturing treatment a large percentage of the fracturing fluid usually remains unrecovered. Therefore, the reasons for this low fracturing fluid recovery have become the focus of many studies. Imbibition of fracturing fluid in the shale is believed to be one of the explanations for the low amount of the fracture fluid recovery. The fluid is imbibed by the shale matrix and trapped inside the rock. Capillarity has generally been the primary mechanism considered during imbibition in conventional formations, such as sandstone and carbonate formations. In shale formations osmosis diffusion also exists and cannot be ignored because the clay in the shale rock functions similarly to that of a membrane. It is believed that both capillarity and osmosis diffusion work together to result in imbibition during hydraulic fracturing in shale formations. This paper investigates the effects of both capillarity and osmosis diffusion as the key mechanisms in fluid imbibition through simultaneous imbibition experiments. The results of these tests illustrate that the imbibition process is dominated by both capillarity and osmosis diffusion. This domination is based on the change of water saturation in shale rocks. In addition, the capillary and osmotic pressures, which influence the imbibed rate, can be qualitatively determined by the contact angle and salinity, respectively. Higher capillary and osmotic pressures correlate to faster rates of imbibition. This study, which examines the mechanisms of imbibition and their influences, can improve the understanding of fluid behavior when imbibition occurs during hydraulic fracturing in shale formations. The understanding of this behavior is useful for further simulation research. (C) 2016 Elsevier B.V. All rights reserved.