Experimental investigation on imbibition characteristics of shale with highly developed bedding fractures

Abstract Imbibition is a vital factor leading to the low water recovery rate (WRR) in shale, and it is also the critical mechanism causing formation damage. However, many shale gas wells in Fuling can reach well gas yields under ultra-low WRR, and the corresponding mechanism is currently unclear. In this paper, multiple types of imbibition experiments were conducted on shale samples from typical low WRR but high yield wells in Fuling. Combined with pore structure and nuclear magnetic resonance (NMR) analyses, the forming mechanisms of low WRR and unaffected gas yields in Fuling were detailly analyzed and discussed. The results indicate that highly developed bedding fractures in Fuling play the key roles. Bedding fractures are directionally arranged and are of much wider aperture than randomly distributed microfractures in Fuling. Consequently, the permeability along the bedding direction is two orders of magnitude larger than that in the vertical direction, and the contact area between the fracturing fluid and the reservoir matrix also increased significantly. Both factors increased the imbibition rate and leading more water imbibed into the samples. However, due to the much wider aperture, less tortuosity and more complex fracture surface morphology, water imbibed in bedding fractures are much easier to be drained, and the flow channels for gas are effectively retained. Besides bedding fractures, mineral constituents, formation pressure also plays important roles. Thus, the ultra-low WRR but well gas yield wells in Fuling get its uniqueness, and it may not necessarily occur in other shales even with highly developed bedding fractures. Overall, this work helps understand the mechanism of the ultra-low WRR but high gas yield phenomenon in gas shale, which is significant for the post fracturing flowback optimization.