With estimated shale gas resources greater than
those of US and Canada combined, China has been
embarking on an ambitious shale development program.
However, nearly 30 years of American experience in shale
hydrocarbon exploration and production indicates a low
total recovery of shale gas at 12 %–30 % and tight oil at
5 %–10 %. One of the main barriers to sustainable development
of shale resources, namely the pore structure (geometry
and connectivity) of the nanopores for storing and
transporting hydrocarbons, is rarely investigated. In this
study, we collected samples from a variety of leading
hydrocarbon-producing shale formations in US and China.
These formations have different ages and geologic characteristics
(e.g., porosity, permeability, mineralogy, total
organic content, and thermal maturation). We studied their
pore structure characteristics, imbibition and saturated
diffusion, edge-accessible porosity, and wettability with
four complementary tests: mercury intrusion porosimetry,
fluid and tracer imbibition into initially dry shale, tracer
diffusion into fluid-saturated shale, and high-pressure
Wood’s metal intrusion followed with imaging and elemental
mapping. The imbibition and diffusion tests use
tracer-bearing wettability fluids (API brine or n-decane) to
examine the association of tracers with mineral or organic
matter phases, using a sensitive and micro-scale elemental
laser ablation ICP-MS mapping technique. For two
molecular tracers in n-decane fluid with the estimated sizes
of 1.39 nm 9 0.29 nm 9 0.18 nm for 1-iododecane and
1.27 nm 9 0.92 nm 9 0.78 nm for trichlorooxobis (triphenylphosphine)
rhenium, much less penetration was
observed for larger molecules of organic rhenium in shales
with median pore-throat sizes of several nanometers. This
indicates the probable entanglement of sub-nano-sized
molecules in shales with nano-sized pore-throats. Overall
findings from the above innovative approaches indicate the
limited accessibility (several millimeters from sample
edge) and connectivity of tortuous nanopore spaces in
shales with spatial wettability, which could lead to the low
overall hydrocarbon recovery because of the limited fracture–
matrix connection and migration of hydrocarbon
molecules from the shale matrix to the stimulated fracture
network.