Imbibition Mechanisms of Fracturing Fluid in Shale Oil Formation: A Review from the Multiscale Perspective

The breakthrough of shale oil in North America led toa worldwideenergy revolution. Shale reservoirs show multiscale properties anda large proportion of nanopores compared to conventional reservoirs,which makes the imbibition mechanisms of fracturing fluids withinshale reservoirs exceptionally complicated. For instance, the fracturingfluid shows a distinct imbibition behavior at different scales. Traditionalmodels face significant challenges in imbibition characterization.Understanding the imbibition mechanisms in shale oil formations froma multiscale perspective can help to make full use of the positiveeffect and enhance oil recovery. We review imbibition mechanisms inshale reservoirs from the nanoscale to the reservoir scale. The imbibitionprinciples in a single nanopore and the improvement of the traditionalcharacterization models are clarified. Then, at the pore scale, weelaborate on the imbibition laws obtained from the microfluidic chip,pore network model, and direct simulations. We also outline the imbibitiondynamics at the core scale, the influences of capillary forces andosmotic pressure, and the variations of the pore structure. Finally,the imbibition behavior at the reservoir scale and field examplesconsidering the imbibition effect are introduced. We analyze the existingproblems on each scale and discuss the future trends in this area.This work is expected to help readers systematically understand themechanisms and behavior of fracturing fluid imbibition in shale oilreservoirs at different scales and accelerate the exploitation ofshale resources.