The breakthrough of shale oil in North America led to
a worldwide
energy revolution. Shale reservoirs show multiscale properties and
a large proportion of nanopores compared to conventional reservoirs,
which makes the imbibition mechanisms of fracturing fluids within
shale reservoirs exceptionally complicated. For instance, the fracturing
fluid shows a distinct imbibition behavior at different scales. Traditional
models face significant challenges in imbibition characterization.
Understanding the imbibition mechanisms in shale oil formations from
a multiscale perspective can help to make full use of the positive
effect and enhance oil recovery. We review imbibition mechanisms in
shale reservoirs from the nanoscale to the reservoir scale. The imbibition
principles in a single nanopore and the improvement of the traditional
characterization models are clarified. Then, at the pore scale, we
elaborate on the imbibition laws obtained from the microfluidic chip,
pore network model, and direct simulations. We also outline the imbibition
dynamics at the core scale, the influences of capillary forces and
osmotic pressure, and the variations of the pore structure. Finally,
the imbibition behavior at the reservoir scale and field examples
considering the imbibition effect are introduced. We analyze the existing
problems on each scale and discuss the future trends in this area.
This work is expected to help readers systematically understand the
mechanisms and behavior of fracturing fluid imbibition in shale oil
reservoirs at different scales and accelerate the exploitation of
shale resources.