CO2 storage in deep saline aquifers is one of the most promising options for reducing the atmospheric emissions of CO2 and the geomechanical issues associated with this approach, such as injection-induced stress-and-strain changes, ground surface deformation, cap-rock and well-bore integrity, existing fault reactivation and potential notable seismic events, have received increasing attention from researchers, policy-makers and the general public. In order to address these challenges, many analytical methods, including analytical analyses, numerical modeling and response surface strategies, have been proposed and implemented. In particular, numerical modeling is a widely used approach because of its feasibility for the modeling of large-scale and complex geometry. Thus, this paper provides a review of the numerical modeling of the geomechanics associated with CO2 storage in deep saline aquifers. The paper first provides a brief introduction of coupled Thermo-Hydro- Mechanical-Chemical (THMC) processes in porous media and the common constitutive models that are used in that simulation followed by the existing categories of coupling methods for a numerical simulation. Secondly, the geomechanical aspects of CO2 storage in deep saline aquifers that result from numerical modeling are summarized in detail. Finally, some of the weaknesses that exist in the current simulators and simulation approaches are discussed and improvement measures are recommended.

1 Introduction

Geological CO2 storage (GCS), particularly in deep saline aquifers, is considered to be a promising technology to reduce the CO2 emissions into the atmosphere due to the burning of fossil fuels. Large-scale and long-term CO2 sequestration is a significantly coupled thermal-hydraulic-mechanicalchemical (THMC) process, which may cause a series of mechanical issues, including ground surface deformation, reduced integrity of the cap-rock and wellbore and the reactivation of fault or seismic events (Rutqvist 2012, Li et al. 2006). For example, in a GCS project in Salah, Algeria, an uplift of 2.5 cm at a rate of 5mm per year has been observed. In addition, at Weyburn, approximately 100 micro-seismic events with magnitudes ranging from -1 to -3 have been detected since 2004. Fortunately, no grave security issues associated with CO2 storage systems have yet been reported. However, serious accidents caused by the injection of fluid into a deep geologic formation have been reported. For instance, a geothermal project in Basel was shut down after a 3.4 magnitude seismic event was induced by a water injection. Water injection is similar to CO2 injection; hence, the potential for a large-scale CO2 injection to cause serious accidents in the future cannot be ruled out. Thus, it is vital to comprehensively assess the geomechanical issues associated with CO2 sequestration (Rutqvist 2012).

The approaches for analyzing the THMC processes in CO2 sequestration can be summarized into three categories: analytical analysis, numerical modeling and response surface strategy (Song & Zhang 2013). Numerical modeling is the most widely used method because of its feasibility at large-scales and with complex geometry. Thus, this review focuses on the numerical modeling of the geomechanical issues associated with CO2 sequestration systems and provides a brief introduction to the coupled THMC process and numerical modeling methods. In particular, a more detailed review of the geomechanics of CO2 storage that results from numerical modeling is performed. Finally, the defects of the present numerical modeling methods are described and measures for improvement are recommended.

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