Abstract: The imbibition of shale samples from the Sichuan Basin are studied under different pressure cycles using the technology of NMR in order to have some insight into the influence of invading fluids in shale gas reservoirs. The results show that the T₂ curve of sample S1 (confining pressure 0) gradually expands during the process of spontaneous imbibition and changes from a single peak to bimodal ones, indicating that the amount of imbibition continuously increases and a new pore-fracture system is formed, which is observed in microstructure experiments. Meanwhile, the T₂ curve of sample S2 (confining pressure 8 MPa) has single peak and almost remain unchanged; when the confining pressure on sample S2 is unloaded, the T₂ curve turns into the bimodal ones, which is consistent with the results of sample S1, illustrating that the stress on the shale sample has an effect on the imbibition of shale. Therefore, it is suggested that the effect of stress sensitivity on shale imbibition should be considered in future study and application.

Key words: imbibition, stress sensitivity, nuclear magnetic resonance, shale rock, pore structure