Comparison of surface and near-surface geochemical methods for detection of gas microseepage from carbon dioxide sequestration

A summary of geochemical techniques for purposes of determination of a baseline condition and subsequent monitoring of potential gas microseepage from CO2-EOR or geologic sequestration is presented. The methods include: above ground open-atmosphere techniques, measurements at the land surface-atmosphere interface, and shallow subsurface sampling and measurements. The advantages and disadvantages are presented based on previously published measurements utilizing these techniques. The monitoring for seepage from a CO2-EOR or geologic sequestration project might logically focus on measurements of CO2. Carbon dioxide measurements, particularly in the atmosphere will suffer some quantifiable disadvantages; relatively high atmospheric concentration for a tracer, high environmental variance, multiple natural sources, and solubility/reactivity with water. These factors combine to suggest that CO2 may not effectively serve as an early warning tracer or surface indicator of seepage. Other methods and tracers that are discussed focus on gas exchange measured at the surface and shallow soil gas measurements utilizing a variety of tracers. These parameters include; CO2, CH4 and light hydrocarbons, stable carbon isotopic measurements of carbon-containing gases, carbon-14 content of CO2, inert gases, and artificial tracers entrained in the injected CO2. Use of secondary carbonates in the soil or upper portion of the sedimentary column and their readily measured stable isotopic ratios, and composition of shallow groundwater are included. These materials can be useful in the baseline characterization and provide evidence for fossil gas leakage from depth. (C) 2011 Elsevier Ltd. All rights reserved.