Modern system of multiphase flow in porous media and its development trend
2017
Fluid flow in porous media is the key scientific problem in the
development of oil and gas reservoirs. The traditional mechanics of
fluid flow in porous media which based on the continuum hypothesis
and Darcy′s law plays an important role in developing conventional
oil and gas resources. In recent years, unconventional reservoirs
are drawing more and more attention all over the world, therefore
the development theory and technology, especially the corresponding
flow mechanisms have become the hot research issues. The unconventional
reservoirs exhibit distinct multiscale characteristics, even with
six orders of magnitude difference. In addition, the application of
massive multi-stage hydraulic fracturing can induce strong stress
interactions. Therefore, the traditional theory of fluid flow in porous
media cannot accurately describe the flow characteristics in unconventional
reservoirs. In essence, the development of unconventional oil and
gas resources involves multiphase fluids (e.g. oil, water and gas)
flow in multi-scale porous media with multi-field coupling and various
flow patterns. Therefore, the concept of modern system of multiphase
flow in porous media is proposed, which means multiphase fluids flowing
in multi-scale porous media with multi-field coupling and various
flow patterns. The research status and development tendency are reviewed
from the aspects of: (1) micro- and nanoscale oil and gas flow simulation;
(2) upscaling for reservoir simulation, (3) macroscale flow simulation
of unconventional oil and gas reservoirs; (4) simulation of flow in
large scale fractured and vuggy carbonate reservoirs and (5) physical
simulation of hydrocarbon transport in porous media. More specifically,
in nanoscale the density functional theory and molecular simulation
method can be used to study the interfacial phenomena to understand
the hydrocarbon transport behavior in nanopores and provide key parameters
for mesoscale flow simulation. The current study of nanoscale simulation
mainly focuses on developing more realistic molecular structure model
to represent the heterogeneous shale samples. Microscale simulation
methods involve pore network model, lattice Boltzmann method, direct
simulation of Navies-Stokes equation, level-set method and smoothed
particle hydrodynamics, etc. Digital core and pore network model are
the fundamental research platforms. Various methods can be used to
reconstruct digital cores with multiscale pore structures and mineral
compositions. The complex physicochemical phenomena namely adsorption/desorption,
wettability change and boundary effect should be considered in the
microscale flow simulations and extensive works have been done in
microscale gas flow simulations. The future work on microscale simulation
should focus on the multiphase flow mechanisms with multi-field coupling.
The multiscale characteristics of unconventional reservoirs indicate
the necessity of upscaling process to introduce the microscale flow
mechanisms to macroscale. Homogenization theory and volume averaging
method are the main upscaling approaches. Current upscaling methods
are mostly based on the periodic boundary condition and are unreliable
to be used in complex oil and gas reservoirs, which needs further
study. In addition, more research needs to be conducted on the upscaling
from molecular scale to mesoscale. In macroscale simulations of unconventional
oil and gas reservoirs, the fluid-structure interaction should be
considered and high efficiency numerical algorithm needs to be established.
For large scale fractured and vuggy carbonate reservoirs, the non-Darcy
flow characteristics and different flow regimes in vugs and fractures
should be taken into account during flow simulation. Physical simulations
of hydrocarbon transport in porous media are conducted at two scales:
macroscale, nano- and microscale. Macroscale physical simulations
aim at monitoring the dynamic saturation and pressure fields change
under the realistic reservoir conditions. Nano- and microscale physical
simulations are mainly applied to study the fluid transport mechanisms
in single pore or throat. In summary, the proposed theory of multiphase
fluids flowing in multi-scale porous media with multi-field coupling
and various flow patterns can be applied to study the fluid flow problems
in unconventional oil and gas industries.
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