The effect of clay-swelling induced cracks on shale permeability during liquid imbibition and diffusion

• Published in: Journal of natural gas science and engineering 2020-11, Vol.83, p.103514, Article 103514
• Main Authors: Meng, Mianmo, Ge, Hongkui, Shen, Yinghao, Li, Longlong, Tian, Tonghui, Chao, Jing
• Format: Article
• Language: English
• Subjects: Clay-swelling induced cracks; hale gas reservoir; Liquid diffusion; Permeability; Spontaneous imbibition
• ISSN: 1875-5100
• DOI: 10.1016/j.jngse.2020.103514

Shale gas wells with high production rates usually have large amounts of hydraulic liquid retention, which can cause aqueous phase trapping (APT). However, the APT can be auto-relieved after a well shut-in for a certain period. To clarify its mechanism, the experiments of shale permeability during liquid imbibition and diffusion were conducted. Then, theoretical analysis was interpreted. Samples include the Longmaxi Formation (LMX Fm) from Sichuan province and the Niutitang Formations (NTT Fm) from Sichuan and Guizhou provinces. Permeability during imbibition in Sichuan LMX Fm has decreasing and increasing stages. Increase in permeability is caused by new cracks induced by clay-swelling. Only the decreasing stage of permeability occurs during imbibition in NTT Fm, tight sand, and tight volcanic rock because imbibed liquid reduces the gas-flow channel. The relative permeability of gas recovers during liquid diffusion among all samples. The permeability recovers quickly initially, which is caused by liquid migrating from large pores and cracks into small pores. The large pores and cracks have more contribution to permeability than the small pores. Subsequently, permeability nearly keeps constant because liquid migrates among the small pores, which have less influence on permeability. Sichuan LMX Fm has induced cracks, and its gas relative permeability can exceed 1 after liquid diffusion. In theoretical analysis, four types of gas-flow channels are considered, namely, organic pores, inorganic pores, organic cracks, and inorganic cracks. Initial cracks are mainly water wet. Induced cracks include both organic cracks and inorganic cracks. The ratio between inorganic induced cracks and total induced cracks influences permeability. The lower the ratio is, the more evident the permeability increase is during imbibition. The denser the cracks are, the higher the permeability recovery is during liquid diffusion. Overall, the induced cracks can contribute to the relief of APT in shale gas reservoirs. •The influence of clay-swelling induced cracks on shale permeability during liquid imbibition and diffusion was researched.•Shale with induced cracks has increasing stage of permeability during imbibition and permeability recovers in process of liquid diffusion.•Theoretical analysis shows that shale permeability recovery during liquid imbibition and diffusion has positive relationship with the number of induced cracks.