mechanisms in micropores benefit permeability forecasting,
resulting in a value greater than the Darcy prediction. Additional research is needed to better understand permeability’s
role in diffusion’s oil-recovery efficiency.
Acknowledgment
The authors thank the Institute of Drilling Engineering and
Fluid Mining of Freiberg University of Technology and Ji-angsu Ke Petroleum Instrument Co. Ltd. for assisting with
the experiments.
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which agrees with the anticipated contribution of microporosity and macroporosity to permeability.
Dominant CO2 flows in porous media change from Darcy
flow to diffusion with the reduction of pore sizes from macro to micro. CO2 preferentially flows through the connected
macropores by Darcy flow, producing oil through viscous
flow during CO2 flooding. Oil in micropores is bypassed unless enough time exists for diffusion to provide contact and
subsequent mixing between oil and CO2.
Residual oil saturation in heterogeneous carbonates is related to relative percentages of macropores and micropores.
Residual-oil saturations of 20-30% were found in micropores compared with 10% in macropores.
The CO2 flow regime in micropores does not contribute to
Darcy permeability. Diffusion is a dominant mechanism for
oil recovery from micropores and other bypassed regions. 10
Fig. 7 shows different mechanisms of oil recovery from
micro- and macropores by CO2 injection in fractured tight
reservoirs.
CO2 injection
Table 3 shows water saturation results for Indiana 1 and Indiana 3 samples after CO2 flooding experiments. Temperatures and pressures for all experiments were kept below
critical conditions to ensure immiscibility.
Table 4 lists results of the core flooding experiments with
CO2 injection. The first shows oil recovery before CO2 breakthrough in the outlet and the second oil recovery represents
recovery after 12 hr of soaking.
Equations 3-4 show that the 12-hr diffusion length of
CO2 in Indiana 1 and Indiana 3 was enough for samples
with diameters up to 25 mm.
Macropores controlled permeability and produced oil
during first-stage viscous flow. Micropores provided low
permeability, with diffusion acting as the oil recovery mechanism in the second stage. Fig. 8 shows first-oil recovery
in the higher-permeability sample as higher than the lower-permeability sample.
Low-permeability samples realized higher total oil recovery than the higher-permeability samples. Indiana 3 demonstrated higher second-oil recovery than first-oil recovery.
Comparison of the two samples shows diffusion-driven oil
recovery from micropores as better in low-permeability environments than high permeability.
Oil production from low-permeability samples depends
on effective flow mechanisms through porous media. Flow
in low-permeability samples can be quite different from flow
in high-permeability samples. When pore sizes decrease to
nanometer range, free gas molecules follow a mean free path
guided by flow-channel diameters. Molecules strike and slip
the pore walls frequently. Knudsen theory helps evaluate
molecular-pore wall collisions’ effect on flow and transport.
Darcy’s law helps determine flow capacity in high-permeability samples. Knudsen diffusion and other diffusive flow
