A NMR investigation of spontaneous and forced imbibition of shale under different flow and confinement conditions

In the development of shale gas reservoirs, hydraulic fracturing is followed by an imbibition (or soaking) stage, during which the fracturing wetting fluid migrates into the reservoir matrix. As a consequence, laboratory imbibition experiments have been performed in shale samples. However, these tests were generally conducted at atmospheric pressure and thus only involved spontaneous imbibition, which does not correspond to in-situ reservoir conditions. This study addresses this limitation by conducting forced imbibition experiments in shale samples at different flow and confinement conditions while measuring the nuclear magnetic resonance T₂ relaxation spectra at regularly increasing times. It was observed that increasing the initial pressure difference between the upstream and downstream ends of the sample (hereafter called the differential pressure) significantly improved gas displacement efficiency by promoting greater water migration into the shale pore space. Moreover, it was found that decreasing the confinement (i.e., by lowering the effective pressure) further enhanced the imbibition displacement efficiency, which reaches a maximum when the effective pressure approaches zero and spontaneous imbibition occurs. Reducing the effective pressure leaded to a substantial increase in the water intake and the formation of micro-cracks, as confirmed by post-mortem scanning electron microscopy images. These results emphasize that the differential pressure and effective pressure are key factors influencing the imbibition efficiency and the related microstructural changes in shale rocks. The study highlights the importance of replicating in-situ pressure conditions in future research and provides valuable insights for optimizing gas recovery strategies in shale gas reservoirs.