Greensites and brownsites: Implications for CO2 sequestration characterization, risk assessment, and monitoring

Abstract Proposed CO 2 sequestration storage sites will require different approaches in characterization, risk assessment, and monitoring, given prior site history and land use. Those sites lacking previous subsurface development are defined as greensites , whereas sites where the subsurface is developed, particularly for hydrocarbon production, are defined as brownsites . Greensite CO 2 injection is specifically for storage of CO 2 . Most CO 2 enhanced oil recovery operations using incidental storage would be characterized as a brownsite. Application of monitoring approaches developed for greensites is inadequate when applied to brownsites because intrinsically different uncertainties may lead to investment in ineffective monitoring. Subsurface data are sparse at greensites. Characterization and monitoring must ensure that the subsurface can accept and retain CO 2 in large volumes and that the confining system will isolate CO 2 from the atmosphere over extended time frames. Brownsites will have extensive data on capacity, injectivity, and fluid retention from past operations. However, brownsite confining-system integrity may be compromised by the nature of past development. Greensite pore fluids are typically unperturbed and relatively stable, providing a simpler environment for characterization and monitoring. Brownsite reservoir fluids, however, may have been modified by resource recovery. Geologic processes commonly have introduced trace hydrocarbons into the shallow subsurface. At the surface, brownsite legacy infrastructure and contamination must be evaluated so that preinjection transients do not mimic or mask leakage signals. To be effective, policies and regulations need to recognize inherent differences between greensites and brownsites during CO 2 storage project development.