Impact of chemical osmosis on water leakoff and flowback behavior from hydraulically fractured gas shale

In this paper, the development of a comprehensive multi-mechanistic multi-porosity water/gas/salt flow model to investigate the leakoff and flowback behavior of the fracturing fluid from hydraulically fractured shale gas wells is presented. The multi-mechanistic model takes into account water transport induced by hydraulic pressure driven convection, osmosis pressure driven convection and capillary imbibition, gas transport induced by both hydraulic pressure driven convection and desorption, and salt transport induced by advection and concentration driven diffusion. In the multi-porosity model, hydraulic fractures are considered as a interconnected continuum embedded in shale matrix, where organic shale is interspersed within vast inorganic shale. The organic matrix is thus considered disconnected in the entire reservoir. The water saturation profiles for chemical osmosis-induced, capillary pressure-induced and hydraulic pressure-induced cases are compared, revealing a region of saturation that effectively is immobile even though irreducible saturation has not been reached. In sensitivity analyses, cases with different hydraulic pressure, injected fluid salinity and salt diffusion coefficient are considered.The results indicate that chemical osmosis intensifies water leakoff and hinders water flowback. Further, chemical osmosis is a key mechanism for water retention after the treatment of hydraulic fracturing and should not be ignored especially in flowback data analysis of hydraulically fractured shale gas wells.