Risks posed to drinking water aquifers due to leakage of dissolved CO2 in improperly abandoned wellbores.

摘要

In order to ensure safe long-term storage of carbon dioxide in geologic formations, the risks posed by improperly abandoned wells must be understood and minimalized. In addition to supercritical and gaseous CO2, brine containing dissolved CO2 poses a leakage risk. CO2 dissolution in brine leads to denser brine and better long-term storage security, but its leakage risk is not zero. Under specific circumstances with formation overpressure or overlying aquifer drawdown, dissolved brine can flow up improperly abandoned wells where it can potentially enter and contaminate drinking water aquifers. The possibility that depressurization in the wellbore may cause CO2 exsolution from brine to form a separate buoyant gas phase is of primary concern. Analytical as well as numerical models are used to evaluate these effects in wellbores as well as to examine the effects of system parameters on brine leakage rates through wellbores.;A simple analytical model for uniform density flow is used to evaluate the effects of physical parameters on fluid leakage. It is a useful screening tool for estimating leading order effects of system parameters on leakage of CO2 laden brine. The TOUGH2-ECO2N simulator is also used to evaluate wellbore leakage of dissolved CO2 considering gas exsolution due to pressure, temperature, phase, and salinity changes.;Simulations identify the conditions under which a separate gas phase exsolves in a wellbore during CO2 laden brine leakage. Up to 20% of the dissolved brine is found to exsolve in the numerical simulations. This gas accumulates along the top of a drinking water aquifer as a buoyant phase. Simulations also show that the degree of leakage is constrained by the properties of the well, with the permeability of the well being of chief importance. However, at high well permeabilities, simulations show that the geologic formations provide more resistance to flow than the well and constrain leakage rates. Additional analyses are performed in order to see how dissolved CO2 may leak from a wellbore in a geologic system of stratified permeable layers. It is found that the presence of stratigraphy limits the possibility of upward migration of dissolved CO2, whether through overpressure of drawdown.