ABSTRACT:

During the hydraulic fracturing process, the fracturing fluids can infiltrate into the shale porous matrix surrounding the fractures by spontaneous imbibition. However, the pores in shale oil reservoirs mainly range from 2 to 200 nm. For the nanoscale pore network, the fluid-solid (water-wall and oil-wall) molecular interactions, which can result in the nanoscale effects of the boundary slip and the varying interfacial fluid viscosity, will make the fluid flow behaviors more complex and difficult to characterize. Additionally, the change in pore size due to the stress sensitivity could consequently affect the imbibition processes. In this work, we firstly establish a new generalized imbibition length model in shale nanopores and nanoporous matrix by considering the effects of pore dimensions, tortuosity, nanoscale effects and stress sensitivity. Further, based on the imbibition model, the enhancement factor model is conducted to characterize the nanoscale effects on the imbibition capacity. Finally, the influence of the multi-mechanisms on the imbibition volume and enhancement factor is discussed. Based on discussions, this study can provide microscopic basics into water imbibing into nanopores and nanoporous matrix, and provide basic theoretical models for some pore-scale simulations, such as pore network models, etc.

1. Introduction

Shale oil is currently one of the most important unconventional resources in petroleum exploitation, and due to the production of horizontal drilling and multistage fracturing, it can be effectively and economically explored (Hongjun et al., 2016). During the hydraulic fracturing process, the fracturing fluids can be imbibed into the porous matrix surrounding the fracture surface by spontaneous imbibition, as shown in Fig. 1, and it has a great effect on the enhanced shale oil recovery and the formation damage (Li et al., 2019). In shale reservoirs, the pore size is nanoscale, which mainly ranges 2 nm–200 nm (Saraji and Piri, 2015; Yuan et al., 2017). For the nanoscale pore network, the fluid-solid (water-wall and oil-wall) molecular interactions, which can result in the nanoscale effects of the boundary slip and the varying interfacial fluid viscosity, will make the fluid flow behaviors more complicated and difficult to characterize (Wang et al., 2019a; Wu et al., 2017). Additionally, the change in pore size due to the stress sensitivity could consequently affect the imbibition processes (Shen et al., 2017; Sheng et al., 2018). Therefore, clarifying the imbibition of fracturing fluids into the nanoporous matrix considering the nanoscale and compressibility effects is essential for the enhanced shale oil recovery, and there are few researches about that.

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