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This paper summarises the results of a benchmark study that compares a number of mathematical and numerical models applied to specific problems in the context of carbon dioxide (CO 2 ) storage in geologic formations. The processes modelled comprise ad-H. Class (B) · A. Ebigbo · R. Helmig · M. Darcis · B. Flemisch vective multi-phase flow, compositional effects due to dissolution of CO 2 into the ambient brine and nonisothermal effects due to temperature gradients and the Joule-Thompson effect. The problems deal with leakage through a leaky well, methane recovery enhanced P. Audigane BRGM, French Geological Survey, 410 Comput Geosci (2009) 13:409-434 by CO 2 injection and a reservoir-scale injection scenario into a heterogeneous formation. We give a description of the benchmark problems then briefly introduce the participating codes and finally present and discuss the results of the benchmark study.

In a natural analog study of risks associated with carbon sequestration, impacts of CO 2 on shallow groundwater quality have been measured in a sandstone aquifer in New Mexico, USA. Despite relatively high levels of dissolved CO 2 , originating from depth and producing geysering at one well, pH depression and consequent trace element mobility are relatively minor effects due to the buffering capacity of the aquifer. However, local contamination due to influx of brackish waters in a subset of wells is significant. Geochemical modeling of major ion concentrations suggests that high alkalinity and carbonate mineral dissolution buffers pH changes due to CO 2 influx. Analysis of trends in dissolved trace elements, chloride, and CO 2 reveal no evidence of in situ trace element mobilization. There is clear evidence, however, that As, U, and Pb are locally co-transported into the aquifer with CO 2 -rich brackish water. This study illustrates the role that local geochemical conditions will play in determining the effectiveness of monitoring strategies for CO 2 leakage. For example, if buffering is significant, pH monitoring may not effectively detect CO 2 leakage. This study also highlights potential complications that CO 2 carrier fluids, such as brackish waters, pose in monitoring impacts of geologic sequestration.

CO 2 -enhanced oil recovery (CO 2 -EOR) is a technique for commercially producing oil from depleted reservoirs by injecting CO 2 along with water. Because a large portion of the injected CO 2 remains in place, CO 2 -EOR is an option for permanently sequestering CO 2 . This study develops a generic integrated framework for optimizing CO 2 sequestration and enhanced oil recovery based on known parameter distributions for a depleted oil reservoir in Texas. The framework consists of a multiphase reservoir simulator coupled with geologic and statistical models. An integrated simulation of CO 2 − water−oil flow and reactive transport is conducted, followed by a global sensitivity and response surface analysis, for optimizing the CO 2 -EOR process. The results indicate that the reservoir permeability, porosity, thickness, and depth are the major intrinsic reservoir parameters that control net CO 2 injection/storage and oil/gas recovery rates. The distance between injection and production wells and the sequence of alternating CO 2 and water injection are the significant operational parameters for designing a five-spot CO 2 -EOR pattern that efficiently produces oil while storing CO 2 . The results from this study provide useful insights for understanding the potential and uncertainty of commercial-scale CO 2 sequestrations with a utilization component.

In the context of geologic sequestration of carbon dioxide in saline aquifers, much interest has been focused on the process of density-driven convection resulting from dissolution of CO2 in brine in the underlying medium. Recent investigations have studied the time and length scales characteristic of the onset of convection based on the framework of linear stability theory. It is well known that the non-autonomous nature of the resulting matrix does not allow a normal mode analysis and previous researchers have either used a quasi-static approximation or solved the initial-value problem with arbitrary initial conditions. In this manuscript, we describe and use the recently developed non-modal stability theory to compute maximum amplifications possible, optimized over all possible initial perturbations. Non-modal stability theory also provides us with the structure of the most-amplified (or optimal) perturbations. We also present the details of three-dimensional spectral calculations of the governing equations. The results of the amplifications predicted by non-modal theory compare well to those obtained from the spectral calculations.