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Abstract
Geological sequestration of CO2 in deep saline reservoirs is one of the ways to
reduce its continuous emission into the atmosphere to mitigate the greenhouse
effect. The selection among prospective saline reservoirs can be expedited by
developing some analytical correlations which can be used in place of reservoir
simulation study for each and every saline reservoir. Such correlations can
reduce the cost and time for commissioning a geological site for CO2
sequestration.
The efficiency of a CO2 sequestration operation depends on risks associated
with storage, several of which can be estimated by i) the time the plume takes
to reach the top seal; ii) maximum lateral extent of the plume and iii) the
percentage of mobile CO2 present at any time. A database has been created from
a large number of compositional reservoir simulations for different reservoir
parameters including porosity, permeability, permeability anisotropy, reservoir
depth, thickness, dip and perforation interval. We use a dimensionless ratio of
gravity to viscous forces to formulate different correlations with the factors
that contribute to sequestration efficiency. We update a previously reported
correlation for time to hit the top seal and develop a new correlation for the
maximum lateral extent of the plume using a newly created database for
different reservoir and operating properties. A correlation for percentage of
mobile CO2 during the buoyancy dominated post injection period is also
developed.
We find that normalizing the maximum lateral extent by a characteristic length
yields a reasonable correlation with the gravity number. This characteristic
length is determined as the maximum lateral distance traveled by plume at any
time assuming constant sand face velocity. The correlation confirms that low
gravity number allows the plume to travel laterally due to high viscous forces
while a high gravity number allows it to move faster in vertical direction due
to strong gravity forces. The change in mobile CO2 after injection ends also
correlates well with gravity number. We normalize the change in mobile CO2
fraction (or, equivalently, the change in trapped CO2 fraction) after the end
of injection by a characteristic CO2 saturation. The characteristic saturation
is obtained by considering the volume filled by vertical, buoyancy-driven
movement through the area associated with the maximum plume extent.
The correlations reproduce almost all simulation results within a factor of
two, and this is adequate for rapid ranking or screening of prospective storage
reservoirs.
Introduction
The continuous emission of CO2 into the atmosphere has increased its
concentration from 280 ppm by volume in pre-industrial times (1970) to 387 ppm
by volume at present. Geological sequestration of CO2 in deep saline reservoirs
is a viable option to restrict this rapidly increasing CO2 concentration in the
atmosphere. Large deep saline aquifers, which are not underground sources of
potable water, are present in different sedimentary basins around the world.
The current paradigm for geologic sequestration of CO2 envisions injection in
supercritical state to reduce the volume needed for storage and to avoid
adverse
effects of CO2 separating into liquid and gas phases in the injection system.
The trapping mechanisms which contribute to secure CO2 storage include
structural or stratigraphic trapping beneath the top seal, residual phase
trapping in which CO2 is trapped as residual gas saturation when water imbibes
back into CO2 plume after the injection, local capillary trapping when CO2 is
trapped at or above residual saturation in regions within a heterogeneous
formation (Saadatpoor et al., 2010), dissolution into formation brine
(solubility trapping) and precipitation of carbonate minerals (mineral
trapping) .
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