Experimental study of the osmotic effect on shale matrix imbibition process in gas reservoirs

Abstract Speaking of imbibition, it usually refers to the displacement of the non-wetting phase by the wetting phase in a porous medium by capillary action. Most of the focuses in the technical literature of this area have been given to the factors related to capillarity and viscous forces with or without gravity effect. However, solute (ions/salt)-rock interaction has been overlooked for long, which is also a critical imbibition mechanism as solvent (water)-rock interaction. Moreover, different from conventional sandstone and limestone, shale gas formations exhibited semi-permeable properties due to high clay content and unique pore structure, which result in the osmotic effect. This may explain the low amount of fracturing fluid flowback and the osmosis diffusion-enhanced imbibition could be a critical recovery mechanism from the low permeability rock. Hence, it is important to investigate the imbibition behavior in the fracture network of stimulated reservoir volume which created by hydraulic fracturing. In this study, shale samples from Horn River shale gas formation were used to conduct spontaneous imbibition tests to study the effect of capillarity and osmosis diffusion. To simulate the process during hydraulic fracturing, the fluids with different salinities were used in the experiment. The experimental results showed that the clay content in shale samples determined how capillarity and osmosis diffusion influenced the imbibition process. The linear relationship between imbibition mass and square root of time was observed only in the lower clay content shale samples. The imbibition process in these samples was dominated only by capillarity. That relationship was used to predict the imbibition mass during the water flooding work. On the other hand, in the higher clay content shale samples, due to osmotic effect, the relationship between imbibition mass and square root of time was non-linear. The results of this study can help people understand the imbibition process of the fracturing fluid when fluid contacts with the shale rock during hydraulic fracturing. Adjusting the salinity of the fracturing fluid to change osmotic effect in the shale formation can be used to control fracturing fluid imbibition.