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Numerical simulation codes were exercised on a suite of eight test problems that address CO2 disposal into geologic storage reservoirs, including depleted oil and gas reservoirs, and brine aquifers. Processes investigated include single-and multi-phase flow, gas diffusion, partitioning of CO2 into aqueous and oil phases, chemical interactions of CO2 with aqueous fluids and rock minerals, and mechanical changes due to changes in fluid pressures. Representation of fluid properties was also examined. In most cases results obtained from different simulation codes were in satisfactory agreement, providing confidence in the ability of current numerical simulation approaches to handle the physical and chemical processes that would be induced by CO2 disposal in geologic reservoirs. Some discrepancies were also identified and can be traced to differences in fluid property correlations, and space and time discretization. iv v EXECUTIVE SUMMARYMathematical models and numerical simulation codes are playing an important role in evaluating the feasibility of geologic disposal of greenhouse gases, and they will be necessary tools for designing and operating future disposal systems. In order to serve these functions, simulation codes must be tested to demonstrate that they can adequately represent the physical and chemical processes that would be induced by injection of CO2 and other gases into geologic formations.The present code intercomparison study aimed at such testing and demonstration. The study was initiated and designed by LBNL in the framework of the GeoSeq project. The overall approach was as follows. In a first step, we designed a number of test problems that would probe major issues relating to geologic disposal of greenhouse gases. Actual field applications will involve threedimensional flows in media with multi-scale hydrologic and chemical heterogeneity, and coupled processes involving fluid dynamics, chemical reactions, mechanical deformation, and thermal effects. It was considered that establishing confidence in the capabilities of numerical simulators would be an iterative process, proceeding from simple to complex. Accordingly, the test problems posed for the present study were intentionally designed to be simplified prototypes of actual field problems.The main issues addressed in this work are as follows. Do we understand the fundamental physical and chemical processes that would play a role in geologic disposal of greenhouse gases? Do we have valid mathematical models for them? Can currently available numerical simulators obtain reliable and accurate numerical solutions for conditions and parameters of practical interest?As to the actual execution of the study, the initiators decided that worldwide participation would be sought, and that participants would work with their own funding and using codes available to them.It was hoped that the study would provide a win-win opportunity where all participants could benefit by testing and comparing their codes, learn from one another, and identify areas wh...

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