A Laboratory Investigation into the Kinetics of Supercritical Carbon Dioxide With Crushed, Water Saturated, Madison Limestone.

摘要

Subterranean injection of carbon dioxide into limestone geologic formations is occurring right now as a method of reducing carbon dioxide levels from the atmosphere and yet all the geochemical ramifications of doing so are still unknown. CO2 is only one of a handful of greenhouse gases; those gases in our atmosphere that make the earth temperate and habitable. But an atmospheric surplus of greenhouse gases results in global warming whose effects, such as rising sea levels and weather system intensification, are extremely slow to materialize. Anthropogenic CO2, or CO2 resulting from human activity, levels in our atmosphere have been slowly rising since industrialization began over one hundred years ago, and the majority of that rise has occurred in the past three decades. The result has been a subtle uptick in worldwide temperatures over that time frame. Among the other greenhouse gases, CO2 has been implicated as the largest contributor of the global warming trend and therefore warrants the most focus toward reduction. CO2 is a byproduct of the combustion of fossil fuels. The technology to capture and contain CO2 as it is being produced exists, however the idea of capturing emissions from mobile CO 2 emitting sources such as automobiles is unrealistic, unless someone develops something akin to a colostomy bag for cars. So the focus is on capturing CO2 emissions from immobile sources such as coal burning electricity generating plants. Once the CO2 has been captured and contained it is injected down existing wellbores into geologic formations where it will supposedly be trapped for perpetuity. One concern associated with this method of CO2 disposal arises from the fact that as CO2 dissolves in water, carbonic acid is formed which in turn dissolves the limestone, which is generally referred to as the acid-carbonate system. This system of acid-carbonate interaction and subsequent dissolution occurs naturally in karst regions, where the resulting features include caves, sinkholes, and underground streams. If in the dissolution process the structural integrity of the limestone matrix is jeopardized and that translates into a precarious foundation for buildings, then current injection projects must be questioned, reconsidered, and possibly terminated. The concerns are heightened because geothermal and hydrostatic gradients present in most formations will ensure that the CO2 will be in a supercritical state, where the boundary between the liquid and gas phase no longer exists. The result is a fluid that transports mass like a liquid and has the dissolving capabilities of a strong solvent, yet it can permeate even the smallest pores and diffuse like a gas. This research will investigate the kinetics of the dissolution of limestone by supercritical CO2-induced carbonic acid to provide one more facet of understanding in the long term ramifications of sequestering CO2 in limestone formations.