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Abstract
Natural or injection induced fractures, and abandoned wells with no complete
seals are the main sources for CO2 leakage after sequestration. Based on
research from various scientific studies, pilot CCS programs and commercial CCS
projects, formations and/or cap rocks with fractures may have significant
leakage problems thus degrade CO2 sequestration results. Modeling and
Simulation of CO2 flow in fractured systems is crucially important for CO2
storage location selection and leakage risk evaluation.
With continuous improvement in fracture characterization, Discrete Fracture
Modeling (DFM) is becoming more and more widely used as it can effectively
resolve the fracture geometry with much less cells. Recent works from Gong et.
al have presented successful application of DFM to large-scale compositional
simulations for a gigantic carbonate reservoir. In this work, DFM work flow is
applied for simulations on CO2 sequestration for the first time. Fractures are
represented explicitly and individually as planar surfaces under an
unstructured grid system. Transmissibility between each pair of adjacent cells
is calculated and flow simulations are performed by a general connection-list
based reservoir simulator.
Several examples including several 2D systems with or without fractures and a
system with strong capillary pressure effects are demonstrated. Simulations on
systems with fractures in CO2 injection formation versus in cap rocks are
compared. Results have shown that the existence of mudstone layers could
prevent injected CO2 from leaking outside the aquifer when no fractures are
present. While vertical fractures intersecting with mudstone layers will cause
significant leakage increase as the fractures form extremely preferential
pathways for CO2 transport. Capillary forces also enable us to store CO2 in
target aquifer for a long
timeframe as it could help trap the residual phase CO2. In comparison with
conventional simulators using structured grids, this work provides typical
speedups of 3-10.
Introduction
With the world’s developing concern over greenhouse effect and more and more
carbon dioxide being emitted into the atmosphere, Carbon Capture and Storage
(CCS) comes to the most promising wedge to alleviate the world greenhouse gas
emission. There are fundamentally five main mechanisms that help sequestrate
CO2 in saline reservoirs.
i. Large scale trapping beneath a seal or cap rock;
ii. CO2 dissolution into the saline aqueous phase;
iii. Residual CO2 trapping and capillary force;
iv. Adsorption onto organic matter in shale or sandstone;
v. Mineralization.
In a relatively short timescale such as field development, 20 years in Sleipner
for instance, the first three mechanisms have considerable impact than the last
two. In this paper, we only consider the first three mechanisms.
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