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Abstract
The first and foremost task for carbon storage is to find proper locations to permanently store CO2 which are injected into deep saline aquifers. However, fractures in the reservoir as well as channels along the wells form preferential pathways for CO2 transport to make it easily escapes from the reservoir. Various scientific studies, pilot CCS programs and commercial CCS projects have shown that leakage problems will be detrimental to the environment and safety to human race.
Modeling CO2 flow in fractures along the well remains a challenge. In this work, several CO2 pathways are characterized, including fractures in cement, casing and the rock. Discrete Fracture Modeling (DFM), which represents fractures individually and explicitly, is applied to simulate CO2 movement in a saline aquifer. This requires unstructured gridding of the saline formation using Delaunay triangulation and transmissibility evaluation between each pair of adjacent cells. Simulations have been done using General Purpose Reservoir Simulator with a non-neighbor connection list.

Several examples including flows through wellbore failures, sloped layers as well as Hydraulic fractures are presented. Through the simulation results, it is found that near well fractures act as extremely preferential flow paths for CO2 transport. Fracturing would help CO2 retained in the target aquifer.

The main application of the framework presented in this paper is to help those involved in evaluation and planning of possible CO2 storage location selection to identify and quantify possible leakage risks through drilling and completion induced fractures.

Introduction
Carbon Capture and Storage (CCS) comes to the most promising wedge to alleviate the world’s greenhouse gas emission with the developing concern over greenhouse effects. One of the main concerns for CO2 geological storage is the potential leakage along wellbores.

Oil and gas exploration and production activities have resulted in millions of wells, especially in the high oil production areas, such as Texas in US and Middle East. High well density will cause significant leakage problems that CO2 plume will intersect a number of existing wells. Wells help provide a direct path from the target aquifer to the buffer aquifers and even to the surface, which are more severe than the fractures crossing the layers. Abandoned, production, injection and monitering wells with integrity issues will lead to high potential leakage of CO2. If the integrity of the wells is not warranted, CO2 will leak along the existing wells to the upper aquifer and even pollute the potable water. Locations of some of the abandoned wells are even unknown, which also raise the level of leakage potential (Watson and Bachu, 2007). In some cases, wells are even orphaned. Current well closure and abandonment technology are not mature enough to prevent the future leakage if CO2 is injected. Aquifer saturated with CO2 will also undermine the integrity of the closed well and form pathway to the upper aquifers.