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Abstract
To find suitable locations to permanently sequester CO2 is crucially important for carbon storage. Deep saline formations are thought to be good candidates for CO2 sequestration due to their large capacity potential. On the other hand, cases have shown that saline formations can be naturally fractured to some extent, from small individual fractures to large-scale fracture clusters. 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.

Modeling of CO2 flow in fractured systems remains a challenge. In this work, Discrete Fracture Modeling (DFM) that represents fractures individually and explicitly is applied to simulate CO2 transport 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 a connection list based simulator. Several examples, including injection to a formation with or without fractures and with different hydraulic fracture length, have been simulated based on data from an actual CCS project.

Results have shown that the existence of mudstone layers could prevent injected CO2 from leaking outside the reservoir when no fractures are present. While vertical fractures intersecting with mudstone layers will cause significant leakage increase as the fractures forms extremely preferential pathways for CO2 transport. On the other hand, if fractures are far enough away from the CO2 plume, it could alleviate dramatic pressure buildup caused by CO2 injection in the formation and thus help expedite CO2 propagation.

This work presents a systematic way of modeling CO2 sequestration in saline formations with natural or hydraulic fractures. The main application is to help improve evaluation and planning of possible CO2 storage location selection to identify and quantify possible leakage risks through existing or induced fractures in the injection process.

Introduction
One of the main concerns for CO2 geological storage is the potential leakage from the target saline aquifer into other places including the caprock, buffer aquifers, potable water sources or worstly the atmosphere. Quantification on each of the above consequences is important to risk evaluation and for decision makers to propose critical remedies in case of leakage.