25th International Meeting on Organic Geochemistry IMOG 2011

P-317
Laboratory simulation of vertical hydrocarbon microseepage
from the reservior to the surface using a 3-D model
Guojian Wang 1,2 , Tongjing Cheng 1,2 , Ming Fan 1,2 , Li Lu 1,2
1 Sinopec Research Institute of Petroleum Exploration and Production, Beijing, China, 2 Wuxi Institute of
Petroleum Geology, Sinopec Research Institute of Petroleum Exploration and Production, Wuxi, China
(corresponding author:kingdomjian@126.com)
Surface Geochemistry Exploration (SGE) based on
hydrocarbon microseepage theory is still called
‗unconventional‘ and not widely accepted. One of the
primary impediments to acceptance of SGE methods
in the petroleum industry has been the lack of
experimental verification of the transport of gases
(microseepage) from an oil and/or gas reservoir to the
surface without significant dilution and dispersion.
Understanding of the processes and mechanisms of
hydrocarbon microseepage is thought to result in
acceptance of SGE and improving its applying effect.
it is necessary to conduct laboratory simulation of
vertical hydrocarbon microseepage.
Based on a conceptual model of hydrocarbon
microseepage, an experimental 3-D model was
constructed, which includes a point gas source, an
injecting water system, a temperature controlling
system, and a 3-D sampling sites. A cuboid which is a
mixture of cement, quartz sand, and water is used as
simulated caprock and its overlying strata. The cuboid
is 100 cm long, 100 cm wide and 120 cm high. The
soil profile on the top of model is 30 cm thick. Gas
composition of the point gas source used in the
experiments is composed of light hydrocarbons
according to the composition of the real wet gas in
one gas accumulation in a petroliferous basin in
China, which includes 89% methane, 4.4% ethane,
2.3% propane, 0.66% i-butane, 1.04% n-butane,
0.096% i-pentane, 0.104% n-pentane, 2.4% Nitrogen.
The 13 C value of methane is -33.68‰, typical of
thermogenic gas. Controlling temperature system at
the bottom of the model can simulate temperature
field. Injecting water system at the bottom of the
model can infiltrate through the cuboid and keep its
pores wet. The porosity and permeability of the
simulated caprock is in accordance with that of some
mudstone caprock of natural gas. The soil profile is
similar to Quaternary sediments.
The experiments were operated at a room
temperature of 25ºC. The bottom temperature of the
cuboid was held at 65ºC, similar to the temperature at
a depth of about 1330m. A stable temperature
gradient was formed from bottom to top in the
simulated rock cuboid after a week of temperature
control. The pressure of the point-gas source was
controlled at 0.2 MPa (~2 atmospheres). Water was
injected under 0.2 MPa pressure into the bottom of
simulated rock cuboid. The natural gas composition
was injected from the point source for a period of 102
days. Hydrocarbon concentrations at each sampling
site were then determined in order to explore the
dynamics of hydrocarbon microseepage.
Results of our experiment study indicate that on
the path of hydrocarbon microseepage, the
distribution of hydrocarbon concentration is fanshaped;
differential adsorption of alkanes by the
simulated caprock and its overlying strata results in
the occurrence of a chromatographic effect, which the
ratio C1/C2 increased gradually from base to top over
point gas source, but this is not the case for vertical
detecting lines far from the point gas source. Different
migrating patterns within simulated caprock are
shown by the ratio of i-butane to n-butane (iC4/nC4),
following diffusion and infiltration (buoyant)
mechanisms. An upwelling gas flow in the form of a
plume was found during hydrocarbon microseepage.
A notable question is that the 13 C value of methane in
the sampling site near point gas source became -
15.94‰. It may be indicated an aerobic environment
where 12 C is preferentially oxidized by aerobic
bacteria.
The dispersive halo of microseeping hydrocarbons in
the subsurface is adequately discribed with the 3-D
model used in this simulation experiment. The
features and pathways of hydrocarbon microseepage
with depth that connect the source with the surface,
and the possible formation process of anomalies in
SGE are revealed. ―Vapor infiltration along fracture
driven by pressure‖ has been proposed as the
possible main formation mechanism of surface
geochemical anomaly. Hydrocarbon micro-seep
always along the predominant pathways, such as
faults, fractures, micro-fractures. The concentration
location of active state hydrocarbon microseepage is
dominated by the predominant pathways.
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