Laboratory testing of gas shales in unsaturated conditions

Water uptake of gas shales is commonly considered one of the most important factors responsible for fluid loss during flowback operations after hydraulic fracturing. Imbibition experiments cover a key role in this context to analyze the impact of several factors that contribute to the water uptake in these unconventional reservoirs. The aim of this study is the quantification of the impact of the volumetric response (swelling and shrinkage) of gas shales on the water uptake during wetting and drying processes. An experimental method-ology based on the control of total suction is developed to quantify the volumetric response in both free and under-stress conditions. Results from experiments performed through both vapour diffusion and direct flooding with deionized water on two gas shale core samples extracted from different plays are provided. Obtained results clearly highlight the mutual influence between the volumetric response and the water uptake. Specimens tested in free stress conditions exhibited a response much more pronounced than specimens tested under stress. These features provide clear evidence that the volumetric swelling of gas shales cannot be neglected for the quantification of the water uptake in imbibition tests; these aspects are expected to influence significantly the upscaling of the experimental data to estimate fluid loss at field scale.