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Abstract
Injection of CO2 into West Sak reservoir of Alaska was studied. The study was
conducted with the objective to store CO2 and enhance the oil recovery. Due to
low reservoir temperature, mixing of possible injected mixtures, including pure
CO2 and HC-enriched CO2, and the in-place hydrocarbon will result in
development of three hydrocarbon phases, gas, oil-rich and CO2-rich phases, at
certain pressures encountered in reservoir. Commercially available simulators
are incapable of appropriate identification of this new phase and using them
will result in unrealistic results and wrong predictions.
A compositional EOS simulator, capable of handling up to four phases, including
two HC liquid phases, was used. A three dimensional simplified model of the
reservoir was built, based on reservoir core data, to study the oil
displacement by various injected fluids including pure CO2. Sensitivity of the
results to the relative permeability parameters of the second liquid phase was
studied. Effect of using WAG with different water to gas ratios on the oil
recovery and CO2 sequestration was also investigated. Various simulations runs
resulting from combining different water-CO2 injection plans, well completion
schemes, well spacing and production constraint were conducted to propose an
injection-production strategy that can optimize the oil recovery and CO2
storage.
Results show that the second liquid phase significantly affects both
breakthrough time and total oil recovery. The sensitivity analysis of relative
permeability parameters shows its profound effect on the results, emphasizes
the need to use accurately measured relative permeability data to produce
accurate results.WAG scheme improves the sweep efficiency but decreases the
amount of stored CO2.Within the range investigated, 880 -1240 ft, we found
higher recovery rate for smaller well spacing but negligible change in ultimate
recovery.
West Sak contains 7 to 9 billion barrels of original oil in place. Its high
viscosity makes the primary recovery scheme inefficient. Although waterflooding
was successful to a certain degree but CO2 injection can be a successful option
to increase recovery and store considerable mass of CO2 that might be produced
from coal-bed gasification in Alaska. Although CO2 cannot develop full
miscibility with West Sak oil; however, it causes the oil to swell, oil
viscosity to decrease, and recovery to increase.
Introduction
Industrialization and population growth have increased our energy needs
considerably compared to past century. With exception of a small fraction,
almost all of it comes from fossil resources such as coal, oil and gas. The
process of burning, mainly for electricity generation, and processing of these
resources, for purification and producing new products, generates considerable
mass of undesirable components mostly in gaseous form. Those gaseous
components, including CO2 which is mainly accounted for global warming, are
emitted into atmosphere. Different approaches have been proposed to decrease
CO2 emission. Among all the options, subsurface injection of CO2 into
geological formations is considered as the best and possibly safest method, in
the long-term, to store large quantities of CO2 (Bachu 2000).For a successful
underground sequestration, presence of an upper seal, cap rock, with reliable
integrity and also sufficient and connected pore volume inside formation, are
the requirements. In line with this approach are injections into:
- Deep saline aquifers
- Mature oil reservoirs as Enhanced Oil Recovery (EOR)
- Depleted oil and gas reservoirs
- Enhanced coal bed methane
- Shale gas
- Basalt formations
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