Shale rocks are multi-component materials with complex interactions among different components, also it contains multi-scale micro-nano pores, making flow mechanism of fracturing fluid in microscopic pores of shale and the interactions between fracturing fluid and shale very complicated. In this study, a new experimental apparatus for spontaneous imbibition of fracturing fluid under the conditions of formation temperature and confining pressure was designed, the three-dimensional expansion law of shale hydration induced micro-fractures was monitored online with nano-CT scanning technology, and the process of formation and expansion of induced micro-cracks was visualized. Secondly, the fixed-point observation technique was used to analyze the relationship between induced micro-fractures and mineral components. Finally, the influence of hydration on the changes of shale physical properties was analyzed. The test results of shale sample from the Longmaxi Formation (LF) of the Sichuan Basin indicated that the micro-fractures induced by shale hydration are mainly shale bedding fractures, with obvious directionality. The micro-fractures extend along the weak surface of mineral and rock cementation. Clay minerals play an important role in the formation and expansion of induced fractures. The novel experimental apparatus is helpful for making clear the micro mechanism of the action between fracturing fluid and shale reservoirs.
Shale gas revolution has not only accelerated the U.S energy independence but also brought about the wave of energy revolution across the world [1–3]. Horizontal well and multi-stage hydraulic fracturing is currently the most effective method to exploit the unconventional resources [4–6]. A large amount of field data shows that the many shale gas wells have low flowback efficiency and negative correlation between the flowback efficiency and production after the hydraulic fracturing [7–9]. One of the important reason is that physical and chemical reactions happen between fracturing fluid and shale [10–12]. At present, the question if the shale gas well needs to be shut in after hydraulic fracturing hinder the continuous and efficient development of shale gas reservoirs.